Title
Thermal hydrolysis and thermal alkaline pretreatment
of waste activated sludge: Comparison of effects on
biogas yield, dewaterability, sludge liquor
and cost-benefit analysis
Vorgelegt von
M.Sc.
Vahid Toutian
ORCID: 0000-0002-1328-8504
an der Fakultät VI - Planen Bauen Umwelt -
der Technischen Universität Berlin
zur Erlangung des akademischen Grades
Doktor der Ingenieurwissenschaften
- Dr.-Ing. -
genehmigte Dissertation
Promotionsausschuss:
Vorsitzender: Prof. Dr.-Ing. Reinhard Hinkelmann
Gutachter: Prof. Dr.-Ing. Matthias Barjenbruch
Gutachter: Prof. Dr.-Ing. Jens Tränckner
Tag der wissenschaftlichen Aussprache: 11. Mai 2023
Berlin 2023
i
Printed and/or published with the support of the German Academic Exchange Service
ii
Acknowledgment
First and foremost, I would like to express my sincere gratitude to my university supervisor Prof.
Matthias Barjenbruch at technical university of Berlin (TUB). I am very grateful for his trust and
support through scholarship application. My special thanks are for allowing me to use laboratory
facilities and equipment at department of urban water management during my Ph.D. His scientific
input along my entire Ph.D. through multiple meetings are very much acknowledged, too.
My exclusive and boundless acknowledgement goes to Dr. Christian Remy, my supervisor at
Berlin center of competence for water (KWB). He was my partner in crime through my time at
KWB. I cannot count the number of discussions and meetings we had together. It was a true
pleasure for me to share an office room with him. I sincerely thank him for all his scientific and
emotional support through ups and downs of this adventurous journey. My special thank is for
all his valuable input on my journal articles drafts.
I would like to thank Dr. Christian Loderer, who was our project manager at KWB. He was a
good listener and helped me practically very much during experimental phase of my Ph.D. which
was accompanied by many challenges along the way. Dr. Ulf Miehe of KWB is also
acknowledged for his trust in me in first place and supporting me through my scholarship
application.
I am also very grateful to my bachelor and master students, Kerstin Gerundt, Tina Unger, Jonas
Hunsicker, Paul Hebbe, Minh Anh Pham and Jann Müller, who helped me enormously during
experiments. I would also like to thank Fabian Kraus, Lea Conzelmann and all my colleagues at
KWB for all their scientific discussions and funny moments in the office.
My gratitude goes also to Dr. Alexander Wriege-Bechtold, Elke Dalman, Tayebeh Zinati Shoa
and all my friends and colleagues from urban water management department at TUB for their
support and help as well as the interesting annual scientific meetings.
Andreas Lengemann and Mathias Mittelstedt of Berliner Wasser Betriebe (BWB) are appreciated
very much for all their technical support on Waßmannsdorf wastewater treatment plant. I am also
grateful to all operating personnel of Waßmannsdorf as well as other BWB staff who helped us
sincerely throughout the project. Holger Stählke is also appreciated for providing data of Rostock
sewage treatment work.
This dissertation was written in frame of “E-VENT” project within KWB, which was financed by
BWB and Senate of Berlin. All three are acknowledged for their financial support of this project.
I would like to especially thank German Academic Exchange Service (DAAD) for their financial
support via NaWaM 2016 scholarship program during my entire Ph.D., the very useful German
language course and all their invitations to numerous attractive meetings and tours around
Germany. This financial support helped me to focus on my studies and make the best out of it.
I also acknowledge Prof. Jens Tränckner for accepting to be my dissertation reviewer and Prof.
Reinhard Hinkelmann for chairing my defense session.
Last but not least, I dedicate my deepest gratitude to my family and friends for always being there
for me during hardships and challenges. Most of all, I am thankful to my lovely mother, who
inspired me all my life and revitalized my motivation in times of hurdles.
Vahid Toutian
March 2022
iii
Abstract
Aim of this dissertation was to investigate effect of process conditions (T, pH) of thermal
alkaline pretreatment (TAP) and thermal hydrolysis (TH) of waste activated sludge
(WAS) on biogas yield increase, dewatering potential of digestate, return load increase
and refractory COD using same sludge. Moreover, it was intended to compare economic
viability of TAP and TH for an individual wastewater treatment plant (WWTP) with fully
comparable conditions.
Higher initial anaerobic biodegradability of WAS leads to lower increase in biogas after
pretreatment. Based on WAS initial biodegradability, 22-97% increase in biogas can be
expected. Treatment temperatures below 100°C can be as effective as temperatures above
100°C in terms of biogas increase. Alkali dosage and resulting initial pH play a significant
role on biogas increase. It is recommended to keep NaOH dosage in range of 40-60 mg
per g total solids (TS) in sludge. Treatment time of 1.5-5.0 h suffices, and longer treatment
times show no extra positive impact on biogas yield. Effects of TAP on dewatering
potential and polymer consumption are still ambiguous and more systematic
investigations using standard methods or full-scale dewatering equipment are needed.
Low temperature TAP of WAS in pilot-scale showed a sinusoidal trend for biogas
production throughout the year. Biogas yield increased maximum +42% in summer and
minimum +3% in winter resulting an average yearly increase of +20% (by digestion of
primary sludge+WAS). Ammonium and orthophosphate in sludge dewatering liquor
increased by +34.6%, and +17.0%, respectively. Dewatering potential showed no
improvement in terms of TS of centrifuged digested sludge. Aerobic biodegradability test
of sludge dewatering liquor showed 30.3% increase in refractory COD. Modeling via data
from six WWTPs in Berlin showed that TAP leads to 0.8-1.1 mg per L increase in effluent
COD.
TH of WAS in temperature range of 130-170°C in lab-scale showed an increase of 17-
27% in biogas. Dewatering potential tests via lab-scale centrifugation demonstrated 12-
30% relative increase in TS of dewatered cake (+4% in absolute TS). Test of aerobic
biodegradability indicated that refractory COD in sludge dewatering liquors can increase
by 50-200%. It was shown that with reducing temperature from 170 to 160°C, refractory
COD is halved, while 19% increase in biogas and 20% relative increase in TS of
dewatered cake is still achievable. Modeling via data of six WWTPs in Berlin showed
that effluent COD increase of 2-15 mg per L after TH is expected.
Mass and energy balances of an individual WWTP showed TH results in increase of
digestion capacity, while TAP has no effect on it. Furthermore, TH results in a slightly
higher total biogas production than TAP. On the other hand, increase of refractory COD
and consequently increase of effluent COD of WWTP is higher by TH and low to
negligible by TAP. Moreover, TH results in higher reduction of disposal costs than TAP
due to improvement of dewatering potential. On the contrary, polymer costs increase by
TH. TAP and TH both enhance electricity and heat balance of WWTP. Capital costs of
TH are 4-5 times higher than TAP. Both TAP and TH yield high revenues due to reduction
of total operating costs. Specially for lower biogas increase, TAP yields higher revenue
than TH. With increase of biogas potential due to pretreatment, TH slightly surpasses
TAP in yielding revenue. Finally, TAP and TH are both profitable and other key factors
such as capital costs, refractory COD formation, digestion capacity increase or simplicity
of process can be considered by decision making.
iv
Zusammenfassung
Ziel dieser Dissertation war die Untersuchung des Einflusses der Prozessparameter (T,
pH) bei thermisch-alkalischer Hydrolyse (TAH) und Thermo-Druck-Hydrolyse (TDH)
von Überschussschlamm (ÜS), speziell auf Anstieg des Biogasertrags,
Entwässerungsgrad des ausgefaulten Schlamms sowie Anstieg der Rückbelastung und
des refraktären CSB. Darüber hinaus wurde die Wirtschaftlichkeit von TAH und TDH
für ein spezifisches Klärwerk mit vollständig vergleichbaren Bedingungen untersucht.
Bei guter anaerober Abbaubarkeit des unbehandelten ÜS führt die Vorbehandlung nur zu
geringem Anstieg des Biogasertrags. Relativ zum unbehandelten ÜS wurde ein Anstieg
von 22-97% im Biogasertrag gefunden. Temperaturen unter 100°C sind ebenso effektiv
wie Temperaturen über 100°C in puncto Biogasertrag. Die Alkalidosis und der
resultierende pH-Wert spielen eine signifikante Rolle für den Anstieg des Biogasertrags.
Es wird empfohlen, eine Dosis im Bereich von 40-60 mg NaOH pro g Trockenrückstand
(TR) des Schlamms anzuwenden. Eine Vorbehandlungszeit von 1,5-5,0 h genügt, da
längere Retentionszeiten keine weitere Steigerung des Biogasertrags bringen. Die Effekte
der TAH auf Entwässerungsgrad und Polymerverbrauch sind immer noch unklar, und
weitere systematische Untersuchungen mittels standardisierter Methoden oder
Entwässerungsaggregate im Großmaßstab sind erforderlich.
Die TAH des ÜS bei niedriger Temperatur zeigte im Pilotbetrieb einen sinusförmigen
Trend bezüglich des Biogasertrags im Verlauf eines Jahres. Der Biogasertrag stieg
maximal +42% im Sommer und minimal +3% im Winter an, die mittlere Steigerung lag
bei +20% für das gesamte Jahr bei Faulung von Primärschlamm und ÜS. Ammonium und
Orthophosphat im Schlammwasser wiesen jeweils einen Anstieg von +34,6% and
+17,0% auf. Der Entwässerungsgrad zeigte keine Verbesserung im TR des zentrifugierten
ausgefaulten Schlamms auf. Im aeroben Bioabbaubarkeitstest des ausgefaulten
Schlammwassers wurde ein Anstieg von 30,3% beim refraktären CSB ermittelt. Eine
Modellierung mit Daten der sechs Klärwerke Berlins zeigte auf, dass die TAH dort zu
einem Anstieg von 0,8-1,1 mg pro L CSB im Klärwerksablauf führen würde.
Die TDH des ÜS im Temperaturbereich von 130-170°C zeigte im Laborversuch einen
Anstieg des Biogasertrags um 17-27%. Beim Test des Entwässerungsgrads mittels
Laborzentrifuge wurde ein relativer Anstieg von 12-30% beim TR des ausgefaulten
zentrifugierten Kuchens ermittelt, das entspricht +4% im absoluten TR. Darüber hinaus
wurde im aeroben Bioabbaubarkeitstest ein Anstieg des refraktären CSB um 50-200% im
Schlammwasser aufgezeigt. Mit einer Reduzierung der Temperatur von 170°C auf 160°C
kann die Bildung des refraktären CSB halbiert werden. Mit 160°C kann noch ein Anstieg
des Biogasertrags um 19% und ein relativer Anstieg von 20% beim TR des Dickschlamms
erreicht werden. Hier zeigte die Modellierung für sechs Klärwerke Berlins einen Anstieg
um 2-15 mg pro L CSB im Klärwerksablauf nach TDH.
Die Massen- und Energiebilanzierung eines individuellen Klärwerks zeigt, dass die TDH
die Kapazität des Faulturms erhöht, während die TAH hier keinen Einfluss hat. Darüber
hinaus erreicht die TDH leicht höhere Biogasproduktion als die TAH. Andererseits ist der
Anstieg des refraktären CSB and daraus resultierend auch der CSB im Klärwerksablauf
höher bei TDH, während dieser Effekt bei TAH sehr gering ist. Des Weiteren führt TDH
zu deutlich geringeren Schlammentsorgungskosten als TAH aufgrund des verbesserten
Entwässerungsgrads. Andererseits werden mit TDH die Polymerkosten erhöht. TDH und
TAH verbessern beide die Strom- und Wärmebilanz des Klärwerks. Die Kapitalkosten
der TDH sind 4-5 mal so hoch wie für TAH. TDH und TAH erwirtschaften beide positive
Erträge aufgrund reduzierter Betriebskosten. Besonders wenn der Anstieg des
Biogasertrags niedrig ist, erwirtschaftet TAH mehr Erträge als TDH. Wenn der Anstieg
v
des Biogasertrags nach Vorbehandlung hoch ist, erwirtschaftet TDH leicht höhere Erträge
als TAH. Letztlich sind TAH und TDH beide wirtschaftlich zu betreiben. Damit sind auch
andere Schlüsselfaktoren wie Kapitalkosten, Bildung von refraktärem CSB, Erhöhung
der Kapazität der Faulung und Einfachheit des Verfahrens bei der Auswahl des passenden
Verfahrens zu berücksichtigen.
vi
Table of Contents
Title ................................................................................................................................... i
Acknowledgment ............................................................................................................ ii
Abstract .......................................................................................................................... iii
Zusammenfassung ......................................................................................................... iv
Table of Contents ........................................................................................................... vi
List of Figures .................................................................................................................. x
List of Tables ................................................................................................................ xiv
List of Abbreviations and Symbols ............................................................................ xvi
1 Motivations, aims and outline .................................................................................... 1
1.1 Motivations and aims ......................................................................................... 1
1.2 Outline ............................................................................................................... 1
2 Introduction ................................................................................................................ 3
2.1 Sewage sludge treatment on WWTPs ................................................................ 3
2.1.1 Production and disposal of sludge .............................................................. 3
2.1.2 Anaerobic digestion .................................................................................... 4
2.1.3 Dewatering.................................................................................................. 5
2.1.4 Reject water ................................................................................................ 5
2.2 Sludge pretreatment ........................................................................................... 6
2.2.1 Pretreatment techniques overview .............................................................. 6
2.2.2 Thermal hydrolysis ..................................................................................... 7
2.2.3 Thermal-alkaline pretreatment ................................................................... 8
2.2.4 Comparison of thermal hydrolysis and thermal alkaline pretreatment ...... 9
3 Impact of process parameters of thermal alkaline pretreatment on biogas yield and
dewaterability of waste activated sludge ......................................................................... 12
Highlights ........................................................................................................................ 12
Abstract ............................................................................................................................ 13
Keywords ......................................................................................................................... 13
3.1 Introduction ...................................................................................................... 13
3.2 Anaerobic digestion ......................................................................................... 16
3.2.1 Biomethane yield increase ........................................................................ 16
3.2.1.1 Effect of initial biodegradability of WAS ......................................... 17
3.2.1.2 Effect of treatment temperature......................................................... 18
3.2.1.3 Effect of alkali type and dosage ........................................................ 19
3.2.1.4 Effect of treatment time..................................................................... 23
3.2.2 Composition of biogas .............................................................................. 25
3.2.3 Kinetics of biomethane production........................................................... 25
vii
3.2.4 Other issues .............................................................................................. 26
3.3 Dewaterability and sludge liquor quality ......................................................... 27
3.3.1 Dewatering potential................................................................................. 27
3.3.2 Sludge liquor quality ................................................................................ 28
3.4 Important process design parameters ............................................................... 31
3.4.1 Total solids of WAS input to TAP ........................................................... 31
3.4.2 Optimum alkali dosage ............................................................................. 33
3.4.3 Alkali addition prior to thermal heating ................................................... 34
3.5 Conclusions ...................................................................................................... 36
References ....................................................................................................................... 36
4 Pilot study of thermal alkaline pretreatment of waste activated sludge: Seasonal
effects on anaerobic digestion and impact on dewaterability and refractory COD ......... 43
Highlights ........................................................................................................................ 43
Abstract ............................................................................................................................ 44
Keywords ......................................................................................................................... 44
4.1 Introduction ...................................................................................................... 44
4.2 Materials and methods ..................................................................................... 46
4.2.1 Characteristics of sludge ........................................................................... 46
4.2.2 Pilot scale thermal alkaline pretreatment.................................................. 46
4.2.3 Transportation and storage of sludge ....................................................... 47
4.2.4 Anaerobic digestion .................................................................................. 47
4.2.5 Stability of digester ................................................................................... 48
4.2.6 Dewaterability test .................................................................................... 48
4.2.7 Zahn-Wellens test ..................................................................................... 49
4.2.8 Analytics ................................................................................................... 49
4.2.9 Calculation methods ................................................................................. 49
4.2.9.1 Solubilization degrees ....................................................................... 49
4.2.9.2 Specific biogas production and volatile solids reduction .................. 50
4.2.9.3 Refractory sCOD after Zahn-Wellens test ........................................ 50
4.2.9.4 Conversion factor to predict refractory soluble COD ....................... 50
4.2.9.5 Modelling effluent soluble COD increase of a WWTP .................... 51
4.3 Results and discussion ..................................................................................... 51
4.3.1 Characteristics of WAS after TAP ........................................................... 51
4.3.2 Specific biogas production and volatile solids reduction ......................... 53
4.3.3 Stability of digester ................................................................................... 55
4.3.4 Dewaterability of digestate ....................................................................... 57
4.3.5 Sludge liquor load increase....................................................................... 58
viii
4.3.6 Refractory soluble COD of sludge liquor ................................................. 59
4.3.7 Effluent soluble COD increase of WWTP ............................................... 61
4.4 Conclusions ...................................................................................................... 61
References ....................................................................................................................... 63
5 Effect of temperature on biogas yield increase and formation of refractory COD
during thermal hydrolysis of waste activated sludge....................................................... 66
Highlights ........................................................................................................................ 66
Abstract ............................................................................................................................ 67
Keywords ......................................................................................................................... 67
5.1 Introduction ...................................................................................................... 67
5.2 Materials and Methods ..................................................................................... 68
5.2.1 Characteristics of sludge ........................................................................... 68
5.2.2 Procedure of the experiments ................................................................... 68
5.2.2.1 High temperature thermal hydrolysis ................................................ 69
5.2.2.2 Biomethane potential test .................................................................. 69
5.2.2.3 Dewaterability test............................................................................. 70
5.2.2.4 Zahn-Wellens test .............................................................................. 70
5.2.3 Analytics ................................................................................................... 70
5.2.4 Calculation methods ................................................................................. 70
5.2.4.1 Solubilization degree ......................................................................... 70
5.2.4.2 Biomethane potential and volatile solids reduction .......................... 71
5.2.4.3 Soluble COD elimination degree in Zahn-Wellens test .................... 71
5.2.4.4 Conversion factor to predict refractory soluble COD ....................... 72
5.2.4.5 Modelling the effect of thermal hydrolysis on the effluent COD of
WWTPs ........................................................................................................... 73
5.3 Results and discussion ..................................................................................... 74
5.3.1 Effect of temperature on solubilization of COD, Orthophosphate and
Ammonium .................................................................................................................. 74
5.3.2 Effect of temperature on the biomethane potential .................................. 76
5.3.3 Effect of temperature on dewaterability of digested sludge ..................... 78
5.3.4 Effect of temperature on refractory soluble COD .................................... 78
5.3.5 Modelling the effect of thermal hydrolysis on effluent COD of six WWTPs
in Berlin .................................................................................................................. 81
5.4 Conclusions ...................................................................................................... 84
References ....................................................................................................................... 84
6 Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline
pretreatment of waste activated sludge before anaerobic digestion ................................ 88
Highlights ........................................................................................................................ 88
ix
Abstract ............................................................................................................................ 89
Keywords ......................................................................................................................... 89
6.1 Introduction ...................................................................................................... 89
6.2 Calculation method .......................................................................................... 90
6.2.1 Characteristics of WWTPs ....................................................................... 90
6.2.2 Definition of scenarios.............................................................................. 90
6.2.2.1 Biogas increase or volatile solids reduction ...................................... 91
6.2.2.2 Dewatering potential and polymer consumption .............................. 92
6.2.2.3 Nitrification of sludge liquor ............................................................. 92
6.2.3 Mass and energy balance parameters assumptions................................... 92
6.2.4 Cost-benefit analysis parameters assumptions ......................................... 93
6.3 Results and discussion ..................................................................................... 94
6.3.1 Mass and energy balance of sludge treatment line in Waßmannsdorf ..... 94
6.3.2 Cost-benefit analysis of Waßmannsdorf .................................................. 99
6.3.3 Mass and energy balance of sludge treatment line in Rostock ............... 101
6.3.4 Cost-benefit analysis of Rostock ............................................................ 106
6.3.5 Refractory COD increase in Effluent ..................................................... 108
6.4 Conclusions .................................................................................................... 109
7 Conclusions and recommendations for future research ......................................... 112
7.1 Conclusions .................................................................................................... 112
7.2 Final statement ............................................................................................... 114
7.3 Recommendations for future research ........................................................... 114
Supplementary Material ................................................................................................ 116
Publication bibliography and contribution of authors ................................................... 133
x
List of Figures
Figure 2.1 Schematic of wastewater and sludge treatment lines in a wastewater
treatment plant with anaerobic digestion .......................................................................... 3
Figure 2.2 Metabolic pathways of anaerobic digestion and involved microorganisms ... 4
Figure 2.3. Categorization of pretreatment techniques of sewage sludge ........................ 6
Figure 2.4. Schematic of thermal hydrolysis process of Cambi® .................................... 8
Figure 2.5. Schematic of thermal alkaline pretreatment process of Pondus® ................. 9
Figure 3.1 Process flow diagram of sludge treatment line including thermal alkaline
pretreatment (TAP) before anaerobic digestion. ............................................................ 16
Figure 3.2 Effect of different ranges of initial biomethane yield on relative biomethane
yield increase after thermal alkaline pretreatment (Lines in box plots represent
minimum, 25%, 50% and 75% quartiles and maximum. x and ° represent average and
outliers, respectively. n is number of data points. List of data references in Table S1 of
supplementary material) ................................................................................................. 17
Figure 3.3 a) Effect of different temperature ranges in thermal alkaline pretreatment on
absolute biomethane yield increase (Lines in box plots represent minimum, 25%, 50%
and 75% quartiles and maximum. x and ° represent average and outliers, respectively. n
is number of data points. List of data references in Table S1 of supplementary material)
b) Effect of increase of temperature on absolute biomethane yield increase from
individual studies (data from (Chi et al., 2011), (Guo et al., 2016), (Kim et al., 2013),
(Campo et al., 2018), (Ruffino et al., 2016), (Heo et al., 2003), (Zhang et al., 2019),
(Valo et al., 2004)). ......................................................................................................... 19
Figure 3.4 Experimental relationship between alkali dosage and pH of real WAS with
two different TS percentages (pH measured after dosing NaOH and mixing). A)
Suggested alkali dosage range by Pondus leading to self-neutralization and TAP
effluent pH of 6.8-7.0 B) Alkali dosage range with further meaningful increase in pH of
WAS after dosing. However, with increase of alkali dosage in this range, self-
neutralization does not suffice which leads to increase in pH of TAP effluent C) alkali
dosage range with no meaningful increase in pH of WAS after dosing and not
economical. ..................................................................................................................... 20
Figure 3.5 a) Effect of different alkali dosage ranges in thermal alkaline pretreatment on
absolute biomethane increase. (Lines in box plots represent minimum, 25%, 50% and
75% quartiles and maximum. x and ° represent average and outliers, respectively. n is
number of data points. List of data references in Table S1 of supplementary material) b)
Effect of pH in thermal alkaline pretreatment on absolute biomethane increase (data
from (Vlyssides, 2004), (Dong et al., 2016), (Xu et al., 2014), (Wang et al., 2016c),
(Abudi et al., 2016), (Liu et al., 2019), (Chen et al., 2020))........................................... 22
Figure 3.6 Effect of treatment time ranges in thermal alkaline pretreatment on absolute
biomethane increase. (Lines in box plots represent minimum, 25%, 50% and 75%
quartiles and maximum. x and ° represent average and outliers, respectively. n is
number of data points. List of data references in Table S1 of supplementary material). 24
Figure 3.7 Schematic illustration of kinetics of biomethane production of WAS with and
without pretreatment and its effect on maximum stabilization time reduction. ............. 26
Figure 3.8 Different linked parameters influencing thermal alkaline pretreatment design
xi
in full-scale. .................................................................................................................... 32
Figure 3.9 Schematic relationship between viscosity and required heat and alkali reagent
for thermal alkaline pretreatment vs total solids percentage in sludge........................... 32
Figure 3.10 Schematic probable illustration of pH of WAS and concentration of
released OH- and H+ after thermal alkaline pretreatment versus alkali dosage .............. 34
Figure 3.11 Schematic probable illustration of importance of carrying out alkaline step
before thermal heating step in thermal alkaline pretreatment to reduce extra alkali and
acid costs. ....................................................................................................................... 35
Figure 4.1 a) Schematic process flow diagram of pilot plant of thermal alkaline
pretreatment. b) Configuration of anaerobic digesters and feedstock containers WAS,
PS and HWAS. ............................................................................................................... 47
Figure 4.2 Monitored parameters of WAS and HWAS. Box-plots show minimum, 25%
quartile, median, 75% quartile and maximum throughout the study. a) Total solids
(n=35) b) Volatile solids (n=38) c) pH (n=36) d) Soluble COD (n=27) e)
Orthophosphate (n=26) f) Soluble organic phosphorus measured as Orthophosphate
(n=24) g) Ammonium (n=24). ........................................................................................ 52
Figure 4.3 a) Gliding specific biogas productions (SBP) with respective relative
increase and temperature in activated sludge tank of reference WWTP b) Gliding
volatile solids reductions (VSR) with respective relative increase. ............................... 54
Figure 4.4 Monitored parameters in fermentates. Left side: Individual data, Right side:
Box plots. a and b for pH (n=117), c and d for VOA (n=116), e and f for TAC (n=116),
and g and h for VOA/TAC (n=116). .............................................................................. 56
Figure 4.5 a) Normalized capillary suction time (NCST) of digestates. b) Box-plot of
NCST of digestates (n=22). c) TS of sludge cakes after centrifugation tests. d) Box-plot
of TS of sludge cakes after centrifugation tests with polymer (n=8). ............................ 58
Figure 4.6 Monitored parameters in sludge liquors. Left side: Individual data, Right
side: Box plots. a and b for Ammonium (n=44), c and d for Orthophosphate (box-plot
drawn for stabilized concentrations after week 13, n=30) and e and f for Sulphate
(n=44). ............................................................................................................................ 59
Figure 4.7 Results of 7-day Zahn-Wellens tests of sludge liquors. Three identically-
marked lines represent results of three separate tests from three successive weeks (each
data point represents average value of duplicate measurements). a) Soluble COD
elimination degrees. b) Absolute soluble COD concentrations ...................................... 60
Figure 5.1 The schematic flow diagram of experiments. Numbers represent steps of
experiments. Solubilization degrees were calculated after thermal hydrolysis. BMPs
were measured during batch anaerobic digestion tests. Conversion factors were
calculated after Zahn-Wellens tests. ............................................................................... 69
Figure 5.2 Different sinks and sources (formation or degradation points) of refractory
sCOD calculation in this study. ...................................................................................... 72
Figure 5.3 Characteristics of WAS and thermally hydrolyzed WAS at different
temperatures. Solubilization degrees are defined in section 5.2.4.1 a) TS, VS and
VS/TS. b) sCOD concentrations and solubilization degrees. c) Orthophosphate
concentrations and solubilization degrees d) Ammonium concentrations and
solubilization degrees. .................................................................................................... 75
xii
Figure 5.4 BMP-tests results of WAS and thermally hydrolyzed WAS at different
temperatures. For more clarity only the results of test number 4 are presented in a and b
(refer to Table 5.2). a) Specific methane production during BMP-test period. b)
Percentage of daily specific methane production to the ultimate methane production
during BMP-test period. c) Absolute BMP values and BMP percentage increases of
thermally hydrolyzed WAS in comparison to not pretreated WAS. d) Absolute VS
reductions and VS reduction percentage increases of thermally hydrolyzed WAS in
comparison to not pretreated WAS. ............................................................................... 77
Figure 5.5 Dewaterability test results of digested WAS and digested thermally
hydrolyzed WAS in temperature range 130-170°C. ....................................................... 78
Figure 5.6 Zahn-Wellens tests results of WAS and thermally hydrolyzed WAS at
different temperatures. For more clarity only the results of test number 4 are presented
in a (refer to Table 5.2). a) sCOD elimination during the test period b) Conversion
factor and percentage increases in conversion factor of thermally hydrolyzed WAS in
comparison to not pretreated WAS. c) Conversion factor percentage increase and BMP
percentage increase versus different TH temperatures. Data are fitted to exponential
models (refer to Table 5.3). d) Conversion factor percentage increase versus BMP
percentage increase. ........................................................................................................ 79
Figure 5.7. Results of refractory sCOD increase based on conversion factors and mass
balances from this study. a) Increase in effluent COD of a WWTP based on
implementation of AD and TH at different temperatures depending on its specific WAS
production (for definition of SWASP refer to section 5.2.4.5). b) Predicted increase in
effluent COD of six WWTPs in Berlin based on implementation of AD and TH at
different temperatures depending on their specific WAS productions. .......................... 82
Figure 6.1. Cake weight and liquor volume of different scenarios with and without
pretreatment for WWTP Waßmannsdorf ....................................................................... 96
Figure 6.2. Biogas production biogas percentage increase of different scenarios with and
without pretreatment for WWTP Waßmannsdorf (TH scenarios with TS+6% and
TS+8% are same as TS+4%) .......................................................................................... 97
Figure 6.3. Electricity consumption or production in different process units of sludge
treatment line with and without pretreatment for WWTP Waßmannsdorf (TH scenarios
with TS+6% and TS+8% are same as TS+4%) .............................................................. 97
Figure 6.4. Heat consumption or production in different process units of sludge
treatment line with and without pretreatment for WWTP Waßmannsdorf. LGH: Low
grade heat, HGH: High grade heat (TH scenarios with TS+6% and TS+8% are same as
TS+4%) ........................................................................................................................... 98
Figure 6.5. Costs and savings of sludge treatment line with and without pretreatment for
WWTP Waßmannsdorf ................................................................................................ 100
Figure 6.6. Return of investment duration, total revenue during plant lifetime and
pretreatment plant installation costs for WWTP Waßmannsdorf ................................. 101
Figure 6.7. Cake weight and liquor volume of different scenarios with and without
pretreatment for WWTP Rostock ................................................................................. 103
Figure 6.8. Biogas production biogas percentage increase of different scenarios with and
without pretreatment for WWTP Rostock (TH scenarios with TS+6% and TS+8% are
same as TS+4%) ........................................................................................................... 104
xiii
Figure 6.9. Electricity consumption or production in different process units of sludge
treatment line with and without pretreatment for WWTP Rostock (TH scenarios with
TS+6% and TS+8% are same as TS+4%) .................................................................... 105
Figure 6.10. Heat consumption or production in different process units of sludge
treatment line with and without pretreatment for WWTP Rostock. LGH: Low grade
heat, HGH: High grade heat (TH scenarios with TS+6% and TS+8% are same as
TS+4%) ......................................................................................................................... 106
Figure 6.11. Costs and savings of sludge treatment line with and without pretreatment
for WWTP Rostock ...................................................................................................... 107
Figure 6.12. Return of investment duration, total revenue during plant lifetime and
pretreatment plant installation costs for WWTP Rostock ............................................ 108
Figure 6.13. Effect of refractory COD formation due to pretreatment of WAS on
increase of effluent COD for WWTP Waßmannsdorf and Rostock ............................ 109
Figure S1. Increase of interest in thermal alkaline pretreatment of sludge showed by
returned number of published peer-reviewed articles by searching terms of ‘thermal
alkaline’ and ‘sludge’ on Web of Science®. ................................................................. 116
Figure S2. Characteristics of waste activated sludge (WAS) and primary sludge (PS)
over 10 days storage time at room temperature. As shown, pH decreases due to
acidification, while Orthophosphate and sCOD increase due to hydrolytic and
biochemical processes. ................................................................................................. 117
Figure S3. The pilot plant of thermal alkaline pretreatment; 1) WAS influent pump 2)
NaOH dosing pump 3) Electrical heater 4) Heat exchanger for heating 5) Thermostat 6)
Reactor 7) Heat exchanger for cooling ......................................................................... 118
Figure S4. Sieve used to protect the influent pump of WAS against course material a)
before use b) after use ................................................................................................... 118
Figure S5. The two parallel-in-work anaerobic digesters (DBI gGmbH, Freiberg,
Germany) ...................................................................................................................... 119
Figure S6. The lab scale thermal hydrolysis test rig. 1) steam generator 2) Sludge input
hopper 3) reactor 4) pressure regulator (to set the temperature) 5) sludge flash tank .. 120
Figure S7. Centrate samples of a) thermally hydrolyzed and b) digested thermally
hydrolyzed WAS at different temperatures filtered with 0.45 µm filter ...................... 121
Figure S8. Results of biomethane yield from lab-scale thermal alkaline pretreatment of
WAS at 70°C and different dosages of NaOH. Increase in biomethane is between 15
and 26%. ....................................................................................................................... 122
Figure S9. Polymer consumption versus total solids of dewatered cake after TH for
WWTP Waßmannsdorf (TS of cake without pretreatment 26.5%) correlated with data
from (Barber, 2020) ...................................................................................................... 123
Figure S10. Polymer consumption versus total solids of dewatered cake after TH for
WWTP Rostock (TS of cake without pretreatment 24.5%) correlated with data from
(Barber, 2020) ............................................................................................................... 123
xiv
List of Tables
Table 2.1. Recent review papers on pretreatment techniques of sewage sludge. ............. 7
Table 2.2. Comparison of process parameters of TH and TAP ........................................ 9
Table 3.1 Advantages and disadvantages of low temperature thermal alkaline
pretreatment before anaerobic digestion......................................................................... 14
Table 3.2 Comparison of low temperature thermal alkaline pretreatment (TAP) with
high temperature thermal hydrolysis (TH). .................................................................... 15
Table 3.3 Process conditions of thermal alkaline pretreatment process of Pondus®
(Pondus Verfahrenstechnik GmbH, Germany). ............................................................. 16
Table 3.4 Summary of literature dewaterability results after thermal alkaline or alkaline
pretreatment of sludge before or after anaerobic digestion. ........................................... 29
Table 4.1 Characteristics of WAS and PS in this study ................................................. 46
Table 4.2 Specifications of the pilot plant of thermal alkaline pretreatment ................. 48
Table 4.3 Characteristics of anaerobic digesters ............................................................ 48
Table 4.4 Characteristics of fermentate in digesters and produced biogas measured
online (average values with standard deviations in parentheses out of 423295 data
points) ............................................................................................................................. 55
Table 4.5 Characteristics of six WWTPs in Berlin and expected effluent sCOD increase
after implementing TAP ................................................................................................. 62
Table 5.1 Characteristics of WAS .................................................................................. 68
Table 5.2 Mass balance of BMP tests and the following Zahn-Wellens tests................ 80
Table 5.3. Curve fitting results of percentage increase in BMP and CF ........................ 81
Table 5.4. Characteristics of six WWTPs in Berlin and expected effluent CODs after
implementing TH at different temperatures ................................................................... 83
Table 6.1. Characteristics of WWTP Waßmannsdorf and Rostock ............................... 90
Table 6.2. Characteristics of scenarios defined for cost-benefit analysis of each WWTP
........................................................................................................................................ 91
Table 6.3. Factors of mass and energy balance used in modeling.................................. 92
Table 6.4. Cost factors used in cost-benefit analysis ...................................................... 93
Table 6.5. Mass balance of sludge treatment line for WWTP Waßmannsdorf .............. 94
Table 6.6. Mass balance of sludge treatment line for WWTP Rostock........................ 102
Table 6.7. Overall comparison of TAP and TH effects on parameters of mass/energy
balance and cost-benefit analysis of sludge treatment line of WWTPs Waßmannsdorf
and Rostock .................................................................................................................. 110
Table S1. Details of data references for Figure 3.2, Figure 3.3a, Figure 3.5a and Figure
3.6. ................................................................................................................................. 124
Table S2. Kruskal-Wallis Test for different initial biomethane yield ranges. .............. 127
Table S3. Dunn's Post Hoc Comparisons for different initial biomethane yield ranges (L
CH4 per kg VSadded). ..................................................................................................... 127
xv
Table S4. Kruskal-Wallis Test for different temperature ranges.................................. 128
Table S5. Dunn's Post Hoc Comparisons for temperature ranges (°C). ....................... 128
Table S6. Kruskal-Wallis Test for different alkali dosage ranges. ............................... 129
Table S7. Dunn's Post Hoc Comparisons for different alkali dosage ranges (mg NaOH
per g TS). ...................................................................................................................... 129
Table S8. Statistical analysis for data in Figure 5b. ..................................................... 130
Table S9. Kruskal-Wallis Test for different treatment time ranges (h). ....................... 130
Table S10. Dunn's Post Hoc Comparisons for different treatment time ranges (h). .... 130
xvi
List of Abbreviations and Symbols
AAD
After anaerobic digestion
AD
Anaerobic digestion
BAD
Before anaerobic digestion
BMP
Biochemical methane potential
BY
Biomethane yield
CF
Conversion factor
CHP
Combined heat and power
COD
Chemical oxygen demand
CST
Capillary suction time
DS(A)
Dewaterability parameter defined by J. Kopp
EBPR
Enhanced biological phosphorus removal
ED
Elimination degree
EPS
Extracellular polymeric substances
HRT
Hydraulic retention time
HWAS
Hydrolyzed waste activated sludge
ISR
Inoculum to substrate ratio
NCST
Normalized capillary suction time
OLR
Organic loading rate
PS
Primary sludge
SBP
Specific biogas production
sCOD
Soluble chemical oxygen demand
sCODref
Soluble refractory chemical oxygen demand
SDCOD
Solubilization degree of chemical oxygen demand
SD𝑁𝐻4
±N
Solubilization degree of ammonium
SDPO4
3−−P
Solubilization degree of orthophosphate
sPorg
Soluble organic phosphorus
SRF
Specific resistance to filtration
sTP
Soluble total phosphorus
SWASP
Specific waste activated sludge production
TAC
Total alkalinity of carbonate
TAN
Total ammonia nitrogen
TAP
Thermal alkaline pretreatment
TCOD
Total chemical oxygen demand
TH
Thermal hydrolysis
TN
Total nitrogen
TP
Total phosphorus
TS
Total solids
VOA
Volatile organic acids
VS
Volatile solids
VSR
Volatile solids reduction
WAS
Waste activated sludge
WWTP
Wastewater treatment plant
WWTPref
Reference wastewater treatment plant
Xorg
Total waste activated sludge organics
XU
Unbiodegradable particulates in waste activated sludge
Chapter 1: Motivations, aims and outline
1
1 Motivations, aims and outline
1.1 Motivations and aims
Berlin as the capital city of Germany possesses six wastewater treatment plants (WWTP),
which account for treating wastewater of 3.7 million population and industries around.
This leads to production of significant amounts of waste activated sludge (WAS) which
need to be handled and disposed, properly. WAS is the byproduct of wastewater treatment
process. Its handling and disposal accounts for up to half the operation costs on a WWTP
(Appels et al., 2011). Modern WWTPs are equipped with anaerobic digestion (AD)
process to reduce amount of organic matter in WAS which needs to be disposed and
produce biogas, simultaneously. However, this is impeded by low biodegradability of
WAS. Besides, dewatering potential of WAS has also shown to be poor. Pretreatment of
WAS through different mechanical and non-mechanical methods have shown to alleviate
these hurdles. Increase of biogas production leads to production of more electricity and
heat. Moreover, less volatile solids must be disposed. This results in reduction of
operational costs. On the other hand, improvement of dewatering potential leads to
reduction of sludge weight, which also contributes to reduction of operational costs.
Increase of biogas production after pretreatment of WAS is mainly due to destruction of
its structure. This leads to increase of released organic matter in aqueous phase. This
aqueous phase returns to head of WWTP and flows through treatment line for retreatment.
Part of the solubilized organic matter is not biologically degradable in activated sludge
process. Consequently, this fraction appears as solubilized refractory chemical oxygen
demand (sCOD) in WWTP effluent. WWTPs in Germany have a general COD limit of
75 mg/L for effluents which discharge into surface waters. However, Berlin has its own
specific limit value which is lower than this general value (65 mg/L). Exceeding these
limits leads to financial fines which must be paid by treatment plants. Pretreatments also
increase inorganic load of return flow (ammonium and orthophosphate) which increase
operational costs of WWTP for their removal.
Among all pretreatment methods, high temperature thermal hydrolysis (TH, T=165°C)
process plants have increased in recent years, worldwide. On the other hand, a handful of
low temperature thermal alkaline process plants (TAP, T=65°C, NaOH) as an alternative
process to TH have been installed in Germany during last decade. Despite numerous
studies in literature on each of these pretreatment techniques, there were no studies which
compared the two of them with same sludge. Therefore, specific objectives of this
dissertation were as following:
• Compare TH and TAP processes with same sludge
• Investigate impact of process conditions of TH and TAP (T, pH, etc) on
pretreatment effects (volatile solids reduction and biogas increase, dewatering
potential, liquor quality, refractory COD)
• Test TAP in long-term pilot study with same process parameters
• Assess economic viability of TH and TAP for full scale WWTPs
1.2 Outline
This dissertation is written in cumulative form. Contents of chapters 3, 4 and 5 are already
published as individual peer-reviewed papers. Information regarding publications is given
on first page of each chapter. These chapters stand alone for themselves and have their
own highlights, abstract, keywords, introduction, materials and methods, results and
Chapter 1: Motivations, aims and outline
2
discussion, conclusions, and references. Abbreviations and symbols used throughout the
dissertation are defined by their first use in each chapter, separately. A brief outline of
dissertation is presented here:
Chapter 2 presents an introduction of sewage sludge treatment line on a WWTP.
Following, sludge pretreatment methods are described. TH and TAP as two pretreatment
techniques which are focus of this dissertation are introduced in more details.
Chapter 3 presents a literature review on TAP of WAS before AD. Its specific focus is
on effects of process parameters on biogas yield increase, dewatering potential of
digestate and sludge liquor quality.
Chapter 4 is on low temperature TAP of WAS before AD. This chapter reports on
experimental results of a long-term pilot plant.
Chapter 5 is on high temperature TH of WAS before AD. This chapter presents lab-scale
experimental results of TH for a range of process temperature.
Chapter 6 is on cost-benefit analysis of TH and TAP for full-scale WWTPs. It shows
using mass and energy balance how TH and TAP compare, economically.
Chapter 7 concludes this dissertation by presenting conclusions and recommendations
for future research.
1.3 References
Appels, L., Lauwers, J., Degrève, J., Helsen, L., Lievens, B., Willems, K., van Impe, J.,
Dewil, R., 2011. Anaerobic digestion in global bio-energy production: Potential and
research challenges. Renewable and Sustainable Energy Reviews 15 (9), 4295–
4301.
Chapter 2: Introduction
3
2 Introduction
2.1 Sewage sludge treatment on WWTPs
2.1.1 Production and disposal of sludge
Global increase of urbanization has led to increase of centralized wastewater treatment
capacities, worldwide. Wastewater treatment process consists of a combination of
physical, chemical and biological treatments (Tchobanoglous, 2014). Physical treatment
accounts for removal of macro impurities such as sand, gravel, toilet paper, tree branches,
fat and grease, etc. Using a sedimentation tank, primary sludge (PS) is collected and
removed from wastewater and stored in another tank for disposal or further treatment.
Using chemicals in processes of flocculation, coagulation and sedimentation, organic and
inorganic load of wastewater is further reduced. In biological treatment, soluble and
particular organic and inorganic load is reduced with help of microorganisms (Grady,
2011). Activated sludge is one of the most established biological processes implemented.
However, biofilm systems (trickling filters, rotating biological contactors, etc.) have also
been widely used (Chen et al., 2020). Living microorganisms in activated sludge consume
organic and inorganic matter in aqueous phase for reproduction and produce more
biomass. Following, produced biomass is separated in a sedimentation tank from effluent
and returned to aerobic tanks to keep them in the process. Since activated sludge is
produced more than needed, part of it must be steadily removed from the process. This
part which is called “waste activated sludge” (WAS) must be next thickened statically or
via equipment to reduce its volume (DWA, 2007). As WAS is a type of biomass, it can
be converted to biogas in an anaerobic digester (Appels et al., 2011). Modern WWTPs
implement anaerobic digestion (AD) in their sludge treatment line to reduce organic
matter and produce biogas (Appels et al., 2008). Biogas produced in a digester can then
be turned into heat and electricity in a combined heat and power plant. Produced heat can
be used to provide needed process temperatures in a digester (35-40°C for mesophilic
digestion). Produced electricity can be used for internal consumption of WWTP units and
the extra amount can be injected into grid. WAS is usually mixed with primary sludge
before sending to AD.
Figure 2.1 Schematic of wastewater and sludge treatment lines in a wastewater treatment plant with
anaerobic digestion
After being digested, sludge is dewatered mechanically (DWA, 2011). Dewatered sludge
cake is then transported away via trucks for further resource use on agricultural fields or
energy recovery in incineration plants. Remained aqueous phase also known as “reject
Chapter 2: Introduction
4
water” must be pumped back to heads of biological nutrient removal step for retreatment
in main-stream line or in a separate side-stream unit. Process flow diagram of a WWTP
is shown in Figure 2.1.
2.1.2 Anaerobic digestion
Anaerobic digestion has been in our surrounding nature on earth since millions of years
ago. This is the process which occurs in swamps and turns decayed organic matter
primarily into methane. AD has been in use by humans since thousands of years ago. A
couple of centuries ago, it attracted attention of scientists to itself. One and half century
ago, first anaerobic digester was built in India. Only a century ago, interest in production
of flammable gas from keeping sewage in a septic tank was awakened in England. Since
then, number of digesters around the world has increased significantly. Nowadays, AD is
a usual process unit in newly built WWTPs. Anaerobic digestion of sludge leads to
reduction of organic solids which must be disposed (DWA, 2014). Simultaneously,
biogas is produced which is a renewable energy source and helps with reduction of CO2
footprint from fossil fuels. Reaction mechanisms of AD have been studied for decades. It
is stated that AD consists of four sequencing biochemical steps which occur
concomitantly in a digester (Rosenwinkel et al., 2015). These four steps include
hydrolysis, acidogenesis, acetogenesis and methanogenesis as shown in Figure 2.2.
Figure 2.2 Metabolic pathways of anaerobic digestion and involved microorganisms
Anaerobic digesters are simple to operate and have low maintenance costs. A digester is
basically a simple tank usually having mixers or recirculating pumps for mixing purposes.
That is what makes them a suitable option for reduction of sludge disposal costs on a
WWTP. Key affecting parameters of anaerobic digesters include pH, volatile organic
acids, alkalinity, temperature, and solid/hydraulic retention time (Appels et al., 2008).
Chapter 2: Introduction
5
Each of these parameters must be carefully monitored and controlled for a stable
anaerobic digestion.
PS has inherently higher anaerobic biodegradability than WAS (DWA, 2014). This is due
to biological structure of WAS which is contained of biomass with resistant cell
membranes. These cells are not easy to attack for microorganisms. That is why
pretreatment of WAS with an external force with purpose of destructing its cell
membranes and release of intracellular matter has attracted attentions since a couple of
decades (DWA, 2016). These pretreatment techniques function as an external pre-
hydrolysis step, easing, improving, and accelerating the biological hydrolysis step for
microorganisms in AD. Pretreatments lead to increase of organic matter reduction
resulting in more biogas production (Carrere et al., 2010).
2.1.3 Dewatering
After being digested in AD, sludge loses part of its organic solid matter. However, it still
possesses a significant amount of water. This water must be removed in a dewatering
unit. Mechanical dewatering helps reduction of water content with use of polymers as
flocculation facilitating material. Polymers play a significant role in operating costs of
sludge treatment line. There are different types of dewatering machines with different
properties, advantages, and disadvantages. Centrifuges, belt filter presses and chamber
filter presses are the most widely used equipment (DWA, 2011).
Primary sludge has a better dewatering potential than WAS. This is due to biochemical
properties of WAS flocs which entrap water molecules. Poor dewatering feature of WAS
leads to increase of disposal costs. That is also another reason, why pretreatment
techniques have gained attentions of researchers since decades ago. Pretreatments are
believed to destruct formidable sludge floc structure and cell membranes causing release
of entrapped water molecules (DWA, 2016). However, increase of solubilized and
particular matter can increase polymer consumption, alleviating benefits of disposal costs
reductions. Therefore, it is needed to investigate both dewatering potential and polymer
consumption after pretreatments for a reasonable cost benefit analysis.
2.1.4 Reject water
As shown in Figure 2.1, reject water from dewatering unit is pumped back to heads of
aerobic tanks for retreatment. Microorganisms in AD attack the sludge and destruct its
structure. This leads to increase of solubilized organic matter in aqueous phase. After
being further degraded, amount of organic and inorganic matter which are final products
of biochemical degradations increase. Part of these nutrients (biodegradables) can be
removed during retreatment of sludge liquor and part of it (non-biodegradables) cannot.
Carbonaceous organic matter which was not degradable by anaerobic microorganisms
could be partly removed via aerobic microorganisms. However, the aerobic non-
biodegradable part flows through treatment line and appears in effluent. This leads to
increase of chemical oxygen demand (COD) in effluent of WWTP. Organic phosphorous
and nitrogen content in sludge liquor go through same story. Inorganic nitrogenous and
phosphorous contents of sludge liquor are in form of Ammonium NH4
+−Nand
Orthophosphate PO4
3− −P, respectively. Both inorganic forms of these nutrients can be
removed in main treatment line. However, this leads to increase of operating costs via
increase in chemicals and electricity consumption. Pretreatments lead to increase of
organic and inorganic matter in reject water.
Chapter 2: Introduction
6
2.2 Sludge pretreatment
2.2.1 Pretreatment techniques overview
Pretreatment techniques fall into two main groups of mechanical and non-mechanical
methods. This categorization and their sub-categories are shown in Figure 2.3.
Figure 2.3. Categorization of pretreatment techniques of sewage sludge
Mechanical pretreatment is use of hydro-cavitation as external disintegration force
(Barjenbruch and Kopplow, 2003). High pressure systems (usually more than 100 bars)
produce desired conditions for disintegration of sludge. After leaving the high-pressure
section, vapor bubbles are produced in sludge due to sudden significant reduction of
pressure. Upon blasting, they damage sludge flocs and cell membranes (DWA, 2016).
Chemical pretreatments are easy to use, since dosing a chemical substance is enough and
no further equipment or process units are needed. Alkalis, acids and oxidation agents such
as hydrogen peroxide have been used in chemical pretreatments. Since effect of chemical
pretreatments alone are not high enough, they are usually combined with other
pretreatment techniques (DWA, 2016). Biological pretreatment is mainly use of external
enzymes which attach to macromolecules in AD and enhance hydrolysis process. Due to
loss of these expensive enzymes biological methods have not attracted much attention for
full-scale installations.
Ultrasonic and electromagnetic radiation methods have been extensively investigated in
literature and have shown to be effective (Bougrier et al., 2006; Jang and Ahn, 2013).
However, number of full-scale installations are not comparable to those of thermal
pretreatments. Thermal pretreatment method has been investigated since decades ago in
different ranges of temperature. High temperature thermal hydrolysis (TH, T=165°C) has
gained much interest in recent years and tens of full-scale plants have been installed,
worldwide (Barber, 2016). Thermal pretreatment can also be practiced in lower
temperatures (T<100°C). However, due to alleviation of external force of disintegration,
efficiency of process decreases. Therefore, it has been combined with chemical
pretreatment to intensify its effects (Zheng et al., 2021). Among chemicals, alkalis have
shown higher disintegration efficiency than others. In next sections, TH and thermal
alkaline pretreatment (TAP) which are focus of this dissertation are introduced in more
details.
Regarding pretreatment techniques, there are very well written review articles in
literature. A list of these articles is presented in Table 2.1. German association for water,
wastewater and waste has also published a very informative guideline on disintegration
Mechanical
Hydrocavtiation
Ball milling
Non-Mechanical
Chemical
Physical
Thermal
Ultrasonic
ElectromagneticBiological
Chapter 2: Introduction
7
of sewage sludge (DWA, 2016).
Table 2.1. Recent review papers on pretreatment techniques of sewage sludge.
Reference
Title
(Khanh Nguyen et al., 2021)
Review on pretreatment techniques to improve
anaerobic digestion of sewage sludge
(Volschan Junior et al., 2020)
A review of sludge pretreatment methods and co-
digestion to boost biogas production and energy self-
sufficiency in wastewater treatment plants
(Elalami et al., 2019)
Pretreatment and co-digestion of wastewater sludge
for biogas production: Recent research advances and
trends
(Zhen et al., 2017)
Overview of pretreatment strategies for enhancing
sewage sludge disintegration and subsequent
anaerobic digestion: Current advances, full-scale
application and future perspectives
(Neumann et al., 2016)
Developments in pre-treatment methods to improve
anaerobic digestion of sewage sludge
(Carrere et al., 2016)
Review of feedstock pretreatment strategies for
improved anaerobic digestion: From lab-scale
research to full-scale application
(Harris and McCabe, 2015)
Review of pre-treatments used in anaerobic digestion
and their potential application in high-fat cattle
slaughterhouse wastewater
(Cano et al., 2015)
Energy feasibility study of sludge pretreatments: A
review
(Guo et al., 2013)
Minimization of excess sludge production by in-situ
activated sludge treatment processes--a
comprehensive review
(Carlsson et al., 2012)
The effects of substrate pre-treatment on anaerobic
digestion systems: a review
(Tyagi and Lo, 2011)
Application of physico-chemical pretreatment
methods to enhance the sludge disintegration and
subsequent anaerobic digestion: an up to date review
(Carrere et al., 2010)
Pretreatment methods to improve sludge anaerobic
degradability: a review
2.2.2 Thermal hydrolysis
History of thermal hydrolysis dates to a couple of decades ago, when its main goal was
to improve dewatering potential of sludge to reduce disposal costs (Barber, 2016). After
several trials, interest decreased due to high costs of required energy and harsh operational
conditions of high temperature and pressure. With increase of oil price more attentions
were paid towards renewable sources of energy such as biogas from anaerobic digestion
of sewage sludge. Due to concerns regarding climate change more attentions have been
attracted to renewable energy resources like biogas from sewage sludge which contribute
to decrease of fossil fuels CO2 footprint (Appels et al., 2011). Therefore, interests in
thermal pretreatment of sludge has increased, again. Technology of high temperature
thermal pretreatment is usually called “thermal hydrolysis”. Thermal pretreatment has
Chapter 2: Introduction
8
been practiced in different temperature ranges. However, it has been shown that
temperatures higher than 180°C led to significant formation of refractory organic matter
and with further increase, even anaerobic biodegradability decreased (Bougrier et al.,
2008). Since dewatering potential increased with temperature, temperature range of 160-
180°C was proposed for optimal increase in biogas production and dewatering potential
enhancement.
Figure 2.4. Schematic of thermal hydrolysis process of Cambi®
There are already more than 70 plant installed or planned to be installed, worldwide
(Cambi, 2019). Cambi, Veolia, Haarslev, Eliquo Stulz and Sustec are well-known thermal
hydrolysis technology providers. Veolia offers both batch and continuous system and
Eliquo Stulz used thermal oil as heating medium instead of injecting vapor. TH used to
be installed before anaerobic digestion to increase biodegradability. However, post-
treatment of digested sludge using TH has also been practiced and offered by technology
providers (Barber, 2020; Svensson et al., 2018). In a newer effort, TH has been installed
as an intermediate unit between two series digesters (Bjerg-Nielsen et al., 2018). Each
configuration has its own features and effects on solids reduction and biogas production,
dewatering potential, digester capacity, energy balance, solid cakes quality and needed
capacity of TH plant. Each can be applied depending on desired purposes. Subject of this
study is limited to use of TH before anaerobic digestion as pretreatment. Schematic of
Cambi is presented in Figure 2.4.
2.2.3 Thermal-alkaline pretreatment
Thermal alkaline pretreatment has attracted attentions mainly due to its lower operating
temperature and simpler conditions. Alkaline is used as intensifier of disintegration
effects. Several TAP plants have been installed in Germany and Pondus® is the well-
known technology provider. Schematic of Pondus is depicted in Figure 2.5. More
information regarding TAP is presented in Chapter 3 which is already published as a
review article.
Chapter 2: Introduction
9
Figure 2.5. Schematic of thermal alkaline pretreatment process of Pondus®
2.2.4 Comparison of thermal hydrolysis and thermal alkaline pretreatment
A brief comparison of process parameters of TH and TAP are presented in Table 2.2. Due
to different process parameters, different operating and capital costs are expected.
Therefore, a cost-benefit analysis is needed to see which process is more economical for
a WWTP. Differences of the two processes are discussed in more details in chapter 3
(Table 2.2).
Table 2.2. Comparison of process parameters of TH and TAP
Temperature
Pressure
Chemicals
Thermal hydrolysis
170°C
6-8 bar
No
Thermal alkaline pretreatment
70°C
Atmospheric
Yes
2.3 References
Appels, L., Baeyens, J., Degrève, J., Dewil, R., 2008. Principles and potential of the
anaerobic digestion of waste-activated sludge. Progress in Energy and Combustion
Science 34 (6), 755–781.
Appels, L., Lauwers, J., Degrève, J., Helsen, L., Lievens, B., Willems, K., van Impe, J.,
Dewil, R., 2011. Anaerobic digestion in global bio-energy production: Potential and
research challenges. Renewable and Sustainable Energy Reviews 15 (9), 4295–
4301.
Barber, B., 2020. Sludge thermal hydrolysis: Application and potential / Bill Barber.
IWA Publishing, London.
Barber, W.P., 2016. Thermal hydrolysis for sewage treatment: A critical review. Water
research 104, 53–71.
Barjenbruch, M., Kopplow, O., 2003. Enzymatic, mechanical and thermal pre-treatment
of surplus sludge. Advances in Environmental Research 7 (3), 715–720.
Bjerg-Nielsen, M., Ward, A.J., Moller, H.B., Ottosen, L.D.M., 2018. Influence on
anaerobic digestion by intermediate thermal hydrolysis of waste activated sludge
and co-digested wheat straw. Waste management 72, 186–192.
Bougrier, C., Albasi, C., Delgenès, J.P., Carrère, H., 2006. Effect of ultrasonic, thermal
and ozone pre-treatments on waste activated sludge solubilisation and anaerobic
biodegradability. Chemical Engineering and Processing: Process Intensification 45
(8), 711–718.
Bougrier, C., Delgenès, J.P., Carrère, H., 2008. Effects of thermal treatments on five
Chapter 2: Introduction
10
different waste activated sludge samples solubilisation, physical properties and
anaerobic digestion. Chemical Engineering Journal 139 (2), 236–244.
Cambi, 2019. https://www.cambi.com/what-we-do/thermal-hydrolysis/.
Cano, R., Pérez-Elvira, S.I., Fdz-Polanco, F., 2015. Energy feasibility study of sludge
pretreatments: A review. Applied Energy 149, 176–185.
Carlsson, M., Lagerkvist, A., Morgan-Sagastume, F., 2012. The effects of substrate pre-
treatment on anaerobic digestion systems: a review. Waste management 32 (9),
1634–1650.
Carrere, H., Antonopoulou, G., Affes, R., Passos, F., Battimelli, A., Lyberatos, G.,
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Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability
of waste activated sludge
12
3 Impact of process parameters of thermal alkaline
pretreatment on biogas yield and dewaterability of waste
activated sludge
Highlights
• Thermal alkaline pretreatment (TAP) of WAS increases biomethane yield (BY)
by 22–97%.
• Higher initial biodegradability of WAS leads to lower increase in BY after TAP.
• T < 100 °C is as effective as T > 100 °C in terms of BY increase in TAP.
• Optimum temperature, alkali dose and treatment time for highest BY were
determined.
• Effect of TAP (T < 100 °C) on dewaterability of digestate needs more research.
Contents of this chapter were published in Water Research journal
Volume 202, 1 September 2021, 117465
Received 6 April 2021
Revised 18 July 2021
Accepted 23 July 2021
Available online 29 July 2021
https://doi.org/10.1016/j.watres.2021.117465
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability
of waste activated sludge
13
Abstract
Thermal alkaline pretreatment (TAP) of waste activated sludge (WAS) before anaerobic
digestion (AD) was reviewed. Focus of the review was on impact of TAP process
parameters on biomethane yield (BY) and kinetics of AD and downstream dewatering.
With higher initial biodegradability of untreated WAS, effect of TAP on BY decreases.
Depending on initial biodegradability, BY increase of 22-97% is expected. Treatment
temperatures below 100°C showed to be as effective as temperatures higher than 100°C
in terms of BY increase. Alkali dosage and resulting initial pH have a significant effect
on BY increase and showed to have an optimum range of 40-60 mg NaOH per g total
solids (TS) of sludge. It is advised that alkali is dosed based on solids content in WAS
and monitored by pH. Treatment time of 1.5-5h is sufficient for an effective low
temperature TAP (T<100°C), with longer treatment times showing no positive impact on
BY increase. Load of sludge liquor with organics and nutrients increases with more
intensive TAP conditions. Despite kinetic enhancement of hydrolysis step in AD, more
research is needed to clarify if TAP improves kinetics of entire AD process which
determines required digester volume. Impact of TAP on dewaterability of digestate is
ambiguous and needs more investigation using standardized methods, also with regards
to potential effects on polymer demand. Findings of experimental studies were reflected
against available data from commercialized TAP process of Pondus®, throughout review.
Finally, important process design parameters of TAP such as input TS and point of alkali
dosage are discussed and recommendations for future research are presented.
Keywords
Wastewater treatment plant; Pondus; Thermal hydrolysis; Biodegradability; Anaerobic
digestion; Dewatering;
3.1 Introduction
Waste activated sludge (WAS) is the unfavorable byproduct of sewage treatment process,
which accounts for up to 50% of treatment costs on a wastewater treatment plant (WWTP)
(Appels et al., 2008). Therefore, its reduction can lead to significant savings in disposal
costs. Fresh WAS from clarifiers typically consists of around 99% water and 1% solids
(organic and inorganic). To reduce sludge handling costs, it is favorable to reduce these
three parts (water, organic and inorganic solids) as much as possible in most economical
ways. Thickening and dewatering units in a WWTP are responsible for reduction in water
content. Anaerobic digestion (AD) turns part of organic matter in WAS into biogas, which
leads to renewable energy production and sludge solids reduction, simultaneously. The
inorganic solids in WAS are mostly untouched and remain in final dewatered sludge and
must be disposed.
Due to limited and kinetically slow organic solids reduction of WAS in AD (30-45%) and
its poor dewatering potential (typically not more than 15-20% total solids (TS) can be
reached), pretreatment techniques have been broadly investigated in the last decades to
improve both aspects (Carrere et al., 2010; DWA, 2008, 2014). They include mechanical
and non-mechanical (thermal, chemical, physical, and biological) techniques or a
combination of these which primarily target the low and slowly biodegradable structure
of WAS leading to solubilization of solids and release of organic matter into the liquid
phase. This consequently accelerates the rate limiting hydrolysis stage of AD and
potentially increases biogas yield. These techniques differ in efficiency, effects,
complexity, costs of operation, etc. Among all, thermal pretreatments have gained much
interest in terms of full-scale installations, due to their proven effects and energy
integration potentials on a WWTP. High temperature thermal pretreatment (T~170°C)
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability
of waste activated sludge
14
known as thermal hydrolysis (TH) has proven many advantages regarding sludge
viscosity reduction, organic loading rate increase in AD, biogas yield increase, solids
reduction, and dewatering improvements (Barber, 2016). As an alternative to TH, thermal
alkaline pretreatment (TAP) is a combined technique which exploits potentials of both
thermal and alkaline pretreatments, together. It has been tested in high (T>100°C) as well
as low (T<100°C) temperature ranges. However, high temperature TAP is subject to
unfavorable issues associated with high temperatures plus their adverse combined effects
with alkali addition (Delgenès et al., 2000; Penaud et al., 1999; Penaud et al., 2000;
Stuckey and McCarty, 1984). On the contrary, low temperature TAP has gathered much
more attention due to its simpler operating conditions and less intensive drawbacks
(DWA, 2016). Advantages and disadvantages of low temperature TAP before AD are
listed in Table 3.1. Moreover, Table 3.2 summarizes a brief comparison of low
temperature TAP with high temperature TH. Process flow schematic of sludge treatment
line including TAP before AD is shown in Figure 3.1. Primary sludge (PS) has inherently
better dewatering potential and higher anaerobic biodegradability than WAS. Therefore,
it is more meaningful to leave PS out of pretreatment and use it for cooling and
neutralizing pretreated WAS, instead.
Table 3.1 Advantages and disadvantages of low temperature thermal alkaline pretreatment before anaerobic
digestion.
Advantages
References
Significant solubilization of particulate organic matter
(proteins and carbohydrates) leading to increased kinetics
of hydrolysis step of AD.
(Campo et al., 2018; Liu
et al., 2019; Zawieja,
2019)
Significant viscosity decrease facilitating pumping and
mixing leading to better mass transfer and kinetics
enhancement of AD.
(Li et al., 2017b; Ruiz-
Hernando et al., 2014;
Wang et al., 2016b)
Biogas yield increase and subsequently less organic solids
to dispose.
(Campo et al., 2018; Heo
et al., 2003; Nagler et al.,
2016; Ruffino et al.,
2016)
Potential of foaming suppression in digester.
-
Potential of hygienization (when primary sludge (PS) is
also hydrolyzed).
-
Disadvantages
References
Return load increase (particulates and soluble organics and
nutrients) leading to more capacity and costs needed to re-
treat sludge liquors in main- or side-stream.
(Guo et al., 2017; Park et
al., 2014; Toutian et al.,
2020a)
TAP of sewage sludge has been extensively studied in last decades in experimental
studies, and research interest has increased significantly during recent years (Figure S1).
Nevertheless, there was no comprehensive literature review on this potential technique.
Reported studies from literature apply various methods for TAP trials, use different
process conditions, and investigate the process parameters using different methods for
analytics and measurement. In addition, sludge quality as important input material differs
from study to study. This diversity makes it very difficult to compare results of different
studies to each other and validate or cross-check future findings between individual
studies. Therefore, the aim of this review was to extract, summarize, analyze, and interpret
the results of studies in literature to report on current knowledge and identify topics for
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability
of waste activated sludge
15
future research. The focus of this review is on effects of TAP process parameters on AD
regarding biomethane yield (BY) increase, composition of biogas and kinetics of
biomethane production. Subsequently, effects of TAP on digestate dewaterability and
sludge liquor quality as two important economic factors are discussed. For all parameters,
findings of experimental studies are reflected against available data from full-scale
installations of TAP based on the Pondus® process. This commercially available TAP
process has been installed in several full-scale WWTPs and is operated with specific
process conditions (specifications in Table 3.3, process configuration in Figure 3.1).
Finally, important TAP process design parameters are discussed and recommendations
for future research are presented.
Table 3.2 Comparison of low temperature thermal alkaline pretreatment (TAP) with high temperature
thermal hydrolysis (TH).
Parameter
TH (T~170°C)a
TAP (T<100°C)b
Pre-dewatering before
pretreatment
Requires pre-dewatering to 16-
18% to be energy efficient
(higher polymer demand)
Suffices thickening to 6-8%
(lower polymer demand)
Heat demand
High heat demand
Low heat demand
Energy source
Requires boiler to produce high
temperature steam or hot oil
Does not require a boiler,
works well with hot water
from a combined heat and
power plant (CHP)
Viscosity reduction
Significant
Significant
Digestion capacity
increase
Increase of digestion capacity
up to two times due to viscosity
reduction and pre-dewatering
of WAS plus possible capacity
increase due to kinetics
enhancement of AD
Possible capacity increase
due to kinetics
enhancement of AD
Biogas yield increase
and organic solids
reduction
Potential increase in biogas
leading to decrease in organic
solids
Potential increase in biogas
leading to decrease in
organic solids
Digestate dewatering
and polymer demand
Increases dewaterability with
increase in polymer demand
Needs more research (to be
discussed in Section 3.3)
Sludge liquor quality
Increase in loads of organics
and nutrients
Increase in loads of
organics and nutrients
Complexity of
operation
More complex
(high temperature and pressure
handling)
Less complex
(highly concentrated alkali
handling)
Flexibility
Requires stable performance of
pre-dewatering equipment in
delivering sludge with 16-18%
TS
No specific complexities
leading to flexibility
complications
Capital costs
Requires higher capital costs
due to multi-stage reactors,
robust construction material
and complex controlling
system needed for high
Less capital costs due to
simpler construction
material and controlling
system owing to ambient
pressure and low
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability
of waste activated sludge
16
pressure and high temperature
conditions, off gases handling,
(or clean in place for processes
with thermal oil heat
exchanger)
temperature conditions, no
off-gas handling
Operational costs
Requires skilled operating
staff, extra fuel to produce
required steam
Does not require skilled
operating staff, requires
permanent supply of alkali
Full scale installations
More than 70 since 1995
Less than 10 since 2004
Well known suppliers
(heating medium)
Cambi (vapor), Veolia (vapor),
Sustec (vapor), Haarslev
(vapor), ELIQUO STULZ (hot
oil)
Pondus® (hot water),
Lystek
areferences: (Barber, 2016), (DWA, 2016), operational manuals
breferences: (DWA, 2016), operational manual
Figure 3.1 Process flow diagram of sludge treatment line including thermal alkaline pretreatment (TAP)
before anaerobic digestion.
Table 3.3 Process conditions of thermal alkaline pretreatment process of Pondus® (Pondus
Verfahrenstechnik GmbH, Germany).
Parameter
Unit
Value
Temperature
°C
65-70
Alkali type
-
NaOH (50% w/w)
Alkali dosage
mg NaOHa per g TS of WAS
12-32
Reaction time
hour
2.0-2.5
TS of input WAS
%
6-8
pH of WAS after TAP
-
6.8-7.0
a100% purity
3.2 Anaerobic digestion
3.2.1 Biomethane yield increase
In this section, effect of initial biodegradability of sludge on BY increase is clarified, first.
Afterwards, to determine the effect of different ranges of temperature, alkali dosage and
treatment time on BY increase and consequently finding optimum conditions, absolute
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability
of waste activated sludge
17
BY increase is used as the target parameter. During collection of data, it was noticed that
researchers use different test methods and various units to report biogas production data.
This makes collection and interpretation of data challenging, while there are already
standardized test methods for biomethane potential measurement. Accordingly, it is
highly recommended that researchers use recognized standard methods for batch and
continuous AD tests to make it easier for authentic cross-checking of results from
different studies. (Holliger et al., 2016) has proposed a procedure for a standardized
biomethane potential test. It contains necessary information regarding different steps of
batch biogas measurement from substrate preparation to analysis ,validation and reporting
of data (Holliger et al., 2021). Moreover, guideline of association of German engineers
for fermentation of organic material is another source which describes all steps of a
standard biogas measurement system for batch and continuous digestion processes (VDI,
2016).
3.2.1.1 Effect of initial biodegradability of WAS
One of the main incentives of TAP is enhancing BY which directly relates to reduction
in organic solids. Regarding relative (%) BY increase after TAP of WAS, there is a wide
range of results in literature. Effect of different ranges of initial biodegradability on
relative BY increase is shown in Figure 3.2. Non-parametric statistical test of Kruskal-
Wallis (Table S2) showed a significant difference between average relative BY increase
of different initial biodegradability ranges (p<0.05). Dunn’s post-hoc comparison test
(Table S3) between different groups showed that range of 0-50 L CH4 per kg added
volatile solids (VSadded) is significantly different (p<0.05) from ranges of 50-100, 200-
250 and 250-300 L CH4 per kg VSadded. Moreover, range of 100-150 L CH4 per kg VSadded
has a significantly different average from range of 200-250 L CH4 per kg VSadded. All
other inter-group differences were insignificant (p>0.05).
Figure 3.2 Effect of different ranges of initial biomethane yield on relative biomethane yield increase after
thermal alkaline pretreatment (Lines in box plots represent minimum, 25%, 50% and 75% quartiles and
maximum. x and ° represent average and outliers, respectively. n is number of data points. List of data
references in Table S1 of supplementary material)
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability
of waste activated sludge
18
The significant difference between average relative BY increase related to lower and
upper end ranges of initial biodegradability implies that there is a decreasing trend in
between. This has also been previously reported in thermal hydrolysis by Carrere et al.
(2008). In their study, they observed a significant decreasing trend in BY with increase
in initial biodegradability of WAS from six WWTPs. Initial anaerobic biodegradability
of WAS is per se dependent on various parameters including raw wastewater
characteristics, types and configurations of treatment processes, and operational
conditions of WWTP (e.g., sludge age). Another point of Figure 3.2 is that upper ends of
box plots are more dependent on initial biodegradability than lower ends.
Overall, average relative BY increase was between 22-97% depending on initial
biodegradability of sludge (Figure 3.2). Researchers sometimes reference their individual
results to relative BY increases in literature for comparison or validation purposes, but
this direct comparison needs more careful considerations. Therefore, it is strongly
recommended, that researchers also compare initial biodegradability of sludge, when
comparing results of their relative BY increase after TAP to those of other studies with
different source of sludge. It should be noted that data in Figure 3.2 are from TAPs of all
ranges of temperature, alkali dosage and treatment time.
3.2.1.2 Effect of treatment temperature
As mentioned earlier, to examine effect of temperature ranges on BY, absolute BY
increase after TAP was chosen as target parameter. Effect of temperature on absolute BY
increase of TAP studies has been shown in Figure 3.3a. Kruskal-Wallis test (Table S4)
showed that there was no significant difference between means of different temperature
ranges (p>0.05). Accordingly, in terms of absolute BY increase, temperatures below
100°C showed to have comparable results to temperatures above 100°C. This means
synergies between chemical and thermal effects seemed to show no further improvement
for temperatures above 100°C. This is not the same as with TH process, which has been
shown to have an optimum temperature range around 160-180°C regarding BY increase
(Bougrier et al., 2008). Above these temperatures, formation of refractory organic matter
enhances, which leads to decreased anaerobic biodegradability of sludge (Carrere et al.,
2010). Adding alkali to the process, increases the possibility of these unwanted organic
matter (Delgenès et al., 2000; Penaud et al., 1999; Penaud et al., 2000). Below these
temperatures, BY decreases due to reduced sludge disintegration and solubilization effect
(Devos et al., 2020).
To take a closer look at the effect of temperature in TAP, individual BY increase
(measurement points) of multiple studies are separately shown in Figure 3.3b. As it can
be seen, BY enhances as temperature increases in each study. However, all these studies
investigated temperature ranges either >100°C or <100°C, except for one study by (Zhang
et al., 2019). They compared low temperature thermal pretreatment (80°C, 12 h) with low
temperature TAP (80°C, 154 mg NaOH per g TS, 1.3 h) and high temperature TH (170,
30 min). BY increase of 16.1%, 22.8% and 20.4% was reported for these three
pretreatments, respectively. This study showed that low temperature TAP can yield even
more biogas than energy intensive high temperature TH. (Toutian et al., 2018) has also
shown that low temperature TAP (80 mg NaOH per g TS, 70°C, 2 h) and high temperature
TH (170°C, 30 min) show comparable BY increases in batch tests (+25% und +26%,
respectively). This is an advantage for low temperature TAP in comparison to high
temperature TH in terms of energy consumption (DWA, 2016). Moreover, safety issues
regarding high temperatures and pressures are omitted, which promotes its popularity for
full scale installations on WWTPs. However, it should be noted that low temperature TAP
is associated with hazardous chemical handling (transport, storage, dosing, etc.), while in
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability
of waste activated sludge
19
TH no chemicals are used. Nevertheless, more systematic research investigating BY
increase after low temperature TAP and high temperature TH with same sludge still
enriches available data in literature.
Figure 3.3 a) Effect of different temperature ranges in thermal alkaline pretreatment on absolute biomethane
yield increase (Lines in box plots represent minimum, 25%, 50% and 75% quartiles and maximum. x and
° represent average and outliers, respectively. n is number of data points. List of data references in Table
S1 of supplementary material) b) Effect of increase of temperature on absolute biomethane yield increase
from individual studies (data from (Chi et al., 2011), (Guo et al., 2016), (Kim et al., 2013), (Campo et al.,
2018), (Ruffino et al., 2016), (Heo et al., 2003), (Zhang et al., 2019), (Valo et al., 2004)).
3.2.1.3 Effect of alkali type and dosage
Regarding alkali type, there was no research study, which compared the effect of different
alkali types on BY, directly. This might be due to the fact that, Ca(OH)2 and Mg(OH)2
have shown to be less efficient than KOH and NaOH in solubilization degrees after TAP
(Campo et al., 2018; Huang et al., 2016; Li et al., 2008; Mancuso et al., 2019; Penaud et
al., 1999; Ruffino et al., 2016; Wang et al., 2016a). Moreover, alkalis with divalent cations
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability
of waste activated sludge
20
such as Ca(OH)2 and Mg(OH)2 are preferably avoided due to promoting precipitation of
struvite, brucite, etc. in digesters and downstream pipeline and equipment (Heinzmann
and Engel, 2006). Finally, Between KOH and NaOH, the latter is preferred due to lower
chemical costs and lower resource value of sodium in comparison to potassium.
Apart from alkali type, alkali dosage also plays a significant role on the economics of a
TAP process as an operational cost factor. While some researchers have chosen to dose
alkali based on pH as target parameter to study the effect of different pHs on BY (Dong
et al., 2016; Kavitha et al., 2017; Li et al., 2017a; Liu et al., 2019; Nagle et al., 1992;
Wang et al., 2018; Xiao et al., 2017), some others have investigated the effect of alkali
dosage based on TS of sludge (Campo et al., 2018; Demir, 2018; Guo et al., 2016; Nagler
et al., 2016; Ruffino et al., 2016; Zawieja, 2019; Zhang et al., 2015). This makes
comparison of these studies difficult, as pH of sludge reduces with proceeding of TAP
due to release of organic acids from sludge. Organic acids are produced from VS which
is itself part of TS. Therefore, it is recommended that researchers use alkali dosage based
on VS of sludge as a more fixed and unified parameter, beside pH. This makes it easier
for comparison of alkali consumption among different studies, too. However, TS has been
used throughout this study due to being more usual parameter used in literature.
To understand the effect of alkali dosage on BY, it is firstly needed to clarify the
relationship between pH and alkali dosage. As per theoretical relationship between pH
and alkali dosage for pure water (assumed that water has 7%TS with zero buffer capacity
and no release of organic acids from its solid content), nearly 6-60 mg NaOH per g TS is
needed to increase pH of water to 12-13 (for an effective disintegration as discussed later).
To see how these ranges compare to ranges for real WAS, experimental relationship
between alkali dosage and pH of real sludge with two different TS percentages is shown
in Figure 3.4 (not published data by authors; measurement method description in
supplementary material, but briefly: pH measured after adding alkali to WAS and
mixing). For WAS with 1.7% and 7.7% TS, the alkali dosage to increase pH of sludge to
12 was at least 50 and 70 mg NaOH per g TS, respectively, which is higher than the
mentioned ranges for pure water.
Figure 3.4 Experimental relationship between alkali dosage and pH of real WAS with two different TS
percentages (pH measured after dosing NaOH and mixing). A) Suggested alkali dosage range by Pondus
leading to self-neutralization and TAP effluent pH of 6.8-7.0 B) Alkali dosage range with further
meaningful increase in pH of WAS after dosing. However, with increase of alkali dosage in this range, self-
neutralization does not suffice which leads to increase in pH of TAP effluent C) alkali dosage range with
no meaningful increase in pH of WAS after dosing and not economical.
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability
of waste activated sludge
21
This is to be expected, since buffer capacity of WAS (e.g. bicarbonate) and abrupt release
of organic acids consume part of dosed alkali. Pondus with 12-32 mg NaOH per g TS
suggests a less alkali dosage range for a well-designed TAP (Table 3.3 and Figure 3.4) in
order to benefit from self-neutralization effect. With initiation of TAP, alkali addition
increases pH of WAS. Following, release of organic acids due to disintegration of
microbial cells neutralizes WAS (self-neutralization effect) during TAP. This is
especially essential as sludge needs to be within neutral pH zone before sending to AD
process. When alkali dosage surpasses a certain point an extra neutralization step with
acids is needed after completion of TAP and before AD. This worsens economics of TAP
(mechanisms are discussed in detail in section 3.4).
Majority of TAP studies needed to neutralize pretreated sludge with extra acids before
AD (Chen et al., 2020; Kim et al., 2003; Kim et al., 2013; Liu et al., 2019; Park et al.,
2005; Wang et al., 2018). Only a few studies did not need to neutralize sludge after
pretreatment with acids (self-neutralization sufficed). These studies specifically used low
levels of alkali dosage (<40 mg NaOH per g TS). These included (Toutian et al., 2020a)
and (Li et al., 2017b) who practiced Pondus® in pilot and full-scale, respectively (12-32
mg NaOH per g TS). Moreover, (Nagler et al., 2016) and (Campo et al., 2018) reported
on direct AD of pretreated WAS after TAP without neutralization with 32 and 40 mg
NaOH per g TS. Beyond the alkali dosage range proposed by Pondus®, i.e. 32 up to 80
mg NaOH per g TS, pH of WAS still increases meaningfully (Figure 3.4). However, effect
of self-neutralization starts to decrease with increase of alkali dosage in this range. This
is due to limited amount of releasable organic acids in WAS (more details in section
3.4.2). Therefore, as shown in Figure 3.4, for alkali dosage between 0-80, it should be
tested to see which dosage leads to self-neutralization of pretreated WAS in TAP effluent,
whilst achieving high enough pH right after dosing alkali. Dosing alkali beyond 80 mg
NaOH per g TS has no significant effect on final pH and should be avoided to limit
chemical costs.
Effect of different alkali dosage ranges in TAP on absolute BY increase is shown in
Figure 3.5a. Kruskal-Wallis test (Table S6) showed a statistically significant difference
between alkali dosage ranges (p<0.001). Dunn’s post-hoc comparison test (Table S7)
between different groups showed that only alkali dosage of 40-60 mg NaOH per g TS is
significantly different (p<0.05) from dosages of 0-20, 20-40 and 400-700 mg NaOH per
g TS. All other inter-group differences were insignificant (p>0.05). As shown in Figure
3.5a, as alkali dosage increases from 0-20 up to 20-40 and 40-60 mg NaOH per g TS, BY
also increases to its maximum values. This increase is due to solubilization enhancement
of carbohydrates and proteins as alkali dosage increases (Kim et al., 2003; Li et al., 2008;
Rani et al., 2012; Shehu et al., 2012). Afterwards, with further increase in alkali dosage,
BY is negatively affected and starts to decrease. It should be noted that error bars of
ranges of 40-60 (for upper and lower end), 60-100 (for upper end) and 100-200 mg NaOH
per g TS (for lower end) are wider than others. This indicates the variability of data for
these ranges which might attribute to impact of other parameters (e.g. temperature, which
is not considered in Figure 3.5a and Figure 3.5b). Thus, above-mentioned interpretations
should be considered with caution. Decrease in BY in higher alkali dosages is due to
formation of refractory organic matter known as melanoidins (recognizable from
produced brown color and aroma), which are the products of Maillard reactions
(Echavarría et al., 2012). These complex set of reactions, long known and researched for
their application in food industry are initiated by reaction of carbonyl and amine
compounds (e.g. sugars and amino acids) after heating (Chung et al., 1986). According
to Figure 3.4, rate of pH increase falls beyond alkali dosage of 80 mg NaOH per g TS.
Therefore, decrease in anaerobic biodegradability and formation of melanoidins is not
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability
of waste activated sludge
22
dependent of pH in high alkali dosages. It might be possible, that reactions and
mechanisms leading to formation of these matter vary with alkali dosage increase. This
interpretation needs more fundamental research to clarify background mechanisms.
Nevertheless, high alkali dosage leads to promotion of melanoidins formation (Ajandouz
et al., 2001; Ajandouz et al., 2008), thus reducing anaerobic biodegradability (Delgenès
et al., 2000; Penaud et al., 1999; Penaud et al., 2000). Therefore, it is recommended to
keep the alkali dosage below 80 mg NaOH per g TS to avoid decreasing anaerobic
biodegradability of WAS and possible increase of recalcitrant matter, beside economic
savings.
Figure 3.5 a) Effect of different alkali dosage ranges in thermal alkaline pretreatment on absolute
biomethane increase. (Lines in box plots represent minimum, 25%, 50% and 75% quartiles and maximum.
x and ° represent average and outliers, respectively. n is number of data points. List of data references in
Table S1 of supplementary material) b) Effect of pH in thermal alkaline pretreatment on absolute
biomethane increase (data from (Vlyssides, 2004), (Dong et al., 2016), (Xu et al., 2014), (Wang et al.,
2016c), (Abudi et al., 2016), (Liu et al., 2019), (Chen et al., 2020)).
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability
of waste activated sludge
23
Effect of pH on absolute BY increase is shown in Figure 3.5b. pH has a significant
increasing effect (p=0.005) on BY (Table S8). This clearly shows that it is required to
increase the pH of sludge to high levels (11-12) in the beginning of TAP to maximize the
effect on solubilization of organic matter leading to biodegradability enhancement in AD.
This corresponds to alkali dosage range of 50-70 mg NaOH per g TS (for WAS with
7.7%TS) according to Figure 3.4. It should be noted that as the effect of high pH on
disintegration of sludge is abrupt (Navia et al., 2002), it is not needed to keep this pH up
for a certain time by constant dosing. Since this has no meaningful effect on disintegration
efficiency and leads to worsening economics of TAP (through both extra alkali and acid
consumption for neutralization), if not failure of AD (Wang et al., 2018).
For mechanisms of hydrolysis reactions there are two speculations. First, by adding alkali
agent (such as NaOH) to WAS, and increasing pH high enough (>12), concertation of
hydroxide ion OH- goes extremely high. The abrupt strong concentration difference of
OH- around the semi-permeable cell membrane causes an osmotic shock. Subsequently,
water molecules exit the cell through membrane to compensate for concentration
difference leading to decrease of turgor pressure and eventual destruction of cell
membrane. Finally, intracellular organic macromolecules release into liquid phase.
Second, cell membranes in WAS microorganisms are composed of phospholipid bilayers,
proteins, and carbohydrates. Lipid (or fats), which are esters of glycerol, and three long
chain carboxylic acids (fatty acids) are attacked by hydroxide ions. This process
(Saponification) is a chain of reactions in which fats are converted to alcohol and soap
when imposed to a strong alkali agent such as NaOH and heat. Therefore, it is indeed pH
which should be maximized (without introducing too much alkali) to maximize the
disintegration effect. Alkali dosage is used as a parameter to indicate alkali consumption
to reach this pH and not overdosing sludge (self-neutralization failure).
In conclusion, targeted pH and alkali dosage both have an important effect on anaerobic
biodegradability of WAS. With an optimized alkali dosage (below 80 mg NaOH per g
TS) for a maximum targeted pH through proper design of TAP (details in Section 3.4),
alkali consumption is reduced significantly, while removing further step of neutralization
of sludge with acids. Moreover, anaerobic biodegradability is maximized, and formation
of recalcitrant organic matter is reduced. Process conditions of Pondus® has shown to be
well designed to achieve this goal (Table 3.3 and Figure 3.1).
3.2.1.4 Effect of treatment time
Treatment time (or reaction time) is one of the main factors in capital costs of a
pretreatment process which determines required reactor volume. Although for some
treatment techniques such as microwave or ultrasound, it also has a direct effect on energy
consumption as an operating cost factor. For thermal and chemical pretreatments, its main
effect is on reactor volume, when the reactor is thermally insulated. It is extensively
reported that for high temperature pretreatments such as TH, treatment times around 30
min are enough for increase of anaerobic biodegradability (Donoso-Bravo et al., 2011; Li
and Noike, 1992; Perez-Elvira et al., 2015; Sapkaite et al., 2017). Increasing treatment
time beyond this threshold enhances solubilization of organic matter but has no
significant effect on BY increase. Moreover, it also leads to increase of solubilized
recalcitrant organic matter. However, as temperature decreases (specifically for
T<100°C), the efficiency of pretreatment declines, thus demanding longer treatment times
for compensation (Pilli et al., 2014). Therefore, for low temperature thermal
pretreatments, treatment times up to several hours have also been investigated. (Ferrer et
al., 2009) showed that 9 h thermal pretreatment of raw sludge at 70°C leads to 30%
increase in BY in thermophilic AD which was more than BY values of 24 h, 48 h and 72
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability
of waste activated sludge
24
h pretreatment times. This indicates that there is a limit in pretreatment time for low
temperature thermal pretreatment. On the other hand, as alkali compensates for low
temperatures in TAP in relation to solubilization efficiency, treatment time could be
lessened more with NaOH dosing. (Zhang et al., 2019) showed that TAP (80°C, with
NaOH) for 1.3 h increased relative BY up to 56%, while only thermal pretreatment (80°C)
for 12 h led to 40% increase in relative BY. Hence, treatment time could be reduced by a
factor of 10 after adding NaOH to reach comparable BY increase effect.
Figure 3.6 shows the effect of treatment time on absolute BY increase. Kruskal-Wallis
test (Table S9) showed there was no significant difference between means of different
treatment times (p>0.05). However, while treatment time equal or less than 0.5 h has been
dominantly practiced for high temperature TAPs, treatment time range of 1.5-5.0 h is the
optimal range for low temperature TAPs, in terms of maximum BY and reaching self-
neutralization point of pretreated sludge before AD. After fixing temperature and alkali
dosage, treatment time could be varied in this range to determine self-neutralization point
of sludge. Pondus® as a low temperature TAP (70°C) demands treatment time of 2.0-2.5
h, which falls within this range. In similar TAP studies (70°C), which did not demand
neutralization of WAS after TAP, treatment time was 1 h (Nagler et al., 2016) and 1.5 h
(Campo et al., 2018). Nevertheless, even at low temperatures, treatment time should be
as low as possible while maximizing BY to reduce formation of recalcitrant organic
matter.
Figure 3.6 Effect of treatment time ranges in thermal alkaline pretreatment on absolute biomethane increase.
(Lines in box plots represent minimum, 25%, 50% and 75% quartiles and maximum. x and ° represent
average and outliers, respectively. n is number of data points. List of data references in Table S1 of
supplementary material).
To conclude section 3.2.1, optimum conditions of TAP for maximum BY increase of
WAS have shown to be temperatures below 100°, alkali dosages below 80 mg NaOH per
g TS and treatment time of 1.5-5.0 hours. Furthermore, the goal should be achieving
neutralization of sludge after TAP (without using acids) to improve economics of process.
These limited ranges also help reduce formation of refractory organic matter. Best
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability
of waste activated sludge
25
combination of conditions within these ranges should be further determined
experimentally through optimization methods for each sludge sample.
3.2.2 Composition of biogas
Theoretical biogas composition is dependent on the stoichiometry of biogas production
reaction taking place in AD and elemental composition of substrate, according to Buswell
equation (Chernicharo, 2007). Elemental composition of sludge does not change
drastically after pretreatments. Therefore, biogas composition is not significantly affected
by pretreatment processes including TAP, which is also reflected in literature (Guo et al.,
2017; Kim et al., 2003; Park et al., 2014; Shehu et al., 2012; Toutian et al., 2020a).
Nonetheless, due to some changes in alkalinity and equilibrium of buffering reactions in
AD, some slight alterations in CH4/CO2 percentage or concentration of trace gases (NH3,
H2S, etc.) could take place (Campo et al., 2018; Chen et al., 2020).
3.2.3 Kinetics of biomethane production
The main purpose of AD is in fact maximum stabilization of sludge, meaning minimizing
degradation potential of its organic content before disposal (DWA, 2014). Single stage
AD is known to be a complex process of four hydrolytical and biochemical stages
occurring concomitantly, namely hydrolysis, acidogenesis, acetogenesis and
methanogenesis (Rosenwinkel et al., 2015). Pretreatment processes have proven to
accelerate the hydrolysis step, which is indeed the rate-limiting stage for process design
and retention times (Gonzalez et al., 2018). Nevertheless, it should be separately
considered, to what extent a pretreatment process affects kinetics of the whole AD process
and not only the hydrolysis step.
Overall kinetics of full AD after implementing TAP is improved, if two conditions are
fulfilled. First, same stabilization degree of sludge as that of without TAP in shorter time
is achieved. Second, sludge liquor contains same soluble chemical oxygen demand
(COD) as that of without TAP. In this case, digester volume reduces and overall retention
time decreases. For more clarification, Figure 3.7 schematically shows different BY
curves for WAS with and without pretreatment for a normal AD. Most studies of TAP
show BY curves similar to curve number 3 after pretreatments, leading to an increase of
final BY (Chi et al., 2011; Liu et al., 2019; Liu et al., 2020; Nagler et al., 2016). However,
curve number 3 does not lead to reduction of time required for same stabilization degree
of sludge as that of without TAP, since it needs as much time as needed by not pretreated
WAS (curve number 1). This might be due to inherent process limitations of AD.
Maximum biogas yield in complex biological system of AD is a result of healthy
syntrophy of all participating microorganisms and ongoing biochemical processes. This
necessitates that all affecting parameters (pH, VFAs, alkalinity, ammonia, microbial
populations, etc.) are in certain ranges. Any deviation from these suitable ranges results
in a slight inhibition in biomethane production (Appels et al., 2008; Chen et al., 2008).
Therefore, significant increase of solubilized organic matter accessible to microorganisms
after pretreatment does not necessarily mean that they convert it all faster to biogas.
In contrast to curve number 3 and ideally preferred, curve number 4 represents a
pretreatment which leads to increased BY and enhanced kinetics of AD, with reduction
of digestion volume (due to less time needed for maximum stabilization). Curve number
2 represents increase of kinetics of AD and reduction of digestion volume (with maximum
stabilization degree), but without increase in BY. Biomethane potential curves similar to
curves number 2 and 4 were not found in literature after TAP. This implies that increase
in kinetics of hydrolysis step (enhanced solubilization) after TAP, does not necessarily
lead to reduction of time (or digestion volume). This is also recognizable from multiple
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability
of waste activated sludge
26
peaks in curves of daily biomethane production rates of batch biomethane potential tests
(Guo et al., 2016; Guo et al., 2017; Zhang et al., 2015). These peaks represent temporary
inhibition of methanogenesis which could be due to concentration increase of metabolite
precursors of methanogenesis step (i.e. volatile fatty acids, acetate, H2). The reason for
this hypothesis is that as the inhibition factor is relieved (i.e. consumption of these
precursors), biomethane production rate increases again. However, conclusive statements
regarding authentic causing reasons need further systematic research.
Figure 3.7 Schematic illustration of kinetics of biomethane production of WAS with and without
pretreatment and its effect on maximum stabilization time reduction.
(Liu et al., 2020) calculated kinetic rates of four main stages of semi-continuous AD after
TAP for three hydraulic retention times (HRT) of 25, 20 and 15 days, recently. It was
shown that decreasing HRT led to significant increase of soluble COD in hydrolysis step
of AD (44.1-155.6% for TAP pretreated sludge relative to not pretreated sludge), while
slightly decreased the kinetics of other three steps of acidogenesis, acetogenesis and
methanogenesis (0.1-13.9%). This indicated that after pretreatments, reduction of HRT
of AD could lead to increase of soluble COD in sludge liquor. This was accompanied by
reduction of methane production kinetics to a slighter degree, although final BY was still
higher than that of sludge without pretreatment. In this regard, more systematic research
still enlightens the effects of TAP on reduction of HRT needed for AD with maximum
stabilization.
3.2.4 Other issues
Digester volume is mainly dependent on sludge retention time (i.e. HRT) and volumetric
flow of feed. Volumetric flow of feed is itself dependent on its water content (or TS). AD
is normally fed with maximum 4-7% TS after mixing mechanically thickened WAS and
primary sludge (PS) (ATV, 1996). This limit is due to high viscosity of sludge in higher
TS contents which hinders mass and heat transport in digester lowering its efficiency.
Viscosity of sludge has shown to decrease significantly during low temperature TAP (Tan
and Li, 2017; Wang et al., 2016b). This fact can be used to double the digestion capacity.
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability
of waste activated sludge
27
To actualize this, WAS should be highly dewatered (>15% TS) before TAP. This would
be similar to TH, in which AD of sludge with 10% TS is possible due to significant
viscosity reduction (Barber, 2016), above which AD is prone to inhibition risks.
Therefore, low temperature TAP of highly dewatered WAS with subsequent high solids
AD (~10% TS) should be investigated, as a promising alternative to TH in terms of
doubling digestion capacity.
One of the challenging issues regarding operation of AD and activated sludge systems is
combating foam production (Ganidi et al., 2009; Subramanian and Pagilla, 2015),
especially during winter months. Different operational issues in activated sludge systems
lead to enrichment of filamentous bacteria which are responsible for promotion of foam
production. After being transported from activated sludge tanks to digestion tanks as
WAS, they become more problematic. Foaming problem significantly hinders biogas
production and increases the operating costs (e.g. through antifoam addition). According
to plant operators in Germany (personal communications) TAP (Pondus®) has shown to
be effective in suppression of foam production. However, due to lack of information in
literature, systematic research is needed to address its efficiency and mechanisms on foam
reduction in AD or activated sludge tanks.
For countries, where digested sludge utilization in agriculture is legally allowed, effect of
TAP on odor suppression and hygienization of digestate is also an area, which lacks
information in literature and needs more research.
3.3 Dewaterability and sludge liquor quality
3.3.1 Dewatering potential
Improving dewaterability of sludge was the main motivation of implementing early
pretreatment techniques and still remains one of top costs saving incentives (Neyens and
Baeyens, 2003). Dewaterability of digested sludge plays a significant role in reduction of
costs related to final sludge disposal. Specifically, final wet mass of sludge and polymer
consumption are impactful with 59% and 17% share on disposal costs (centrifuges) for
WWTPs in Germany (DWA, 2011). Dewatering potential of sludge is evaluated by
measuring TS of dewatered cake and TSS (total suspended solids) of liquor on WWTPs
(DWA, 2008). It is desirable to maximize both of these parameters with optimal polymer
use to yield a higher dewatered cake and less loaded liquor. Therefore, it is advised that
researchers report on both dewatering potential parameters and polymer consumption, to
make it easier for economic comparison of different studies. Mimicking real dewatering
processes on WWTPs via lab-scale equipment is very challenging which makes
dewatering potential difficult to be quantified. In this regard, (Kopp and Dichtl, 2001)
developed a thermogravimetric method which predicts more precise results (parameter
known as DS(A) ±1.5% deviation from real achievable TS). This method needs special
equipment and procedure, calibration as well as personal experience. More simply
measured parameters such as capillary suction time (CST), specific resistance to filtration
(SRF) or TS of dewatered cake determined by lab centrifugation of a sludge sample have
also been used by researchers. These parameters are usually used to compare effects of
different process conditions on dewaterability potential. However, due to different
conditions used by researchers, it is not easy to deduce conclusive remarks. Accordingly,
it is highly recommended that experts develop and propose a unified standard method to
measure sludge dewaterability in lab scale similar to that of biochemical methane
potential by (Holliger et al., 2016). Such a standard method can be used by researchers
leading to more authentic comparisons between different studies.
Regarding effect of TAP on dewatering potential of digested sludge, there is not as much
information in literature as for solubilization of organic matter or anaerobic
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability
of waste activated sludge
28
biodegradability. A summary of results of dewatering performance after TAP or alkaline
pretreatment is presented in Table 3.4. It should be noted that data regarding CST (or
NCST) do not represent dewatering potential parameter on full scale WWTPs which is
cake solids percentage. Therefore, they should be viewed only as results of sludge
capability for water release. Results are categorized into two groups of dewaterability
before AD (BAD) or after AD (AAD). Dewaterability after pretreatment and before AD
is of interest for those WWTPs, where there is no AD in place and sludge is dewatered
and disposed directly or sent to a central AD. In these scenarios dewatering liquor can be
used as a carbon source for biological wastewater treatment process. According to BAD
results, dewaterability generally tends to worsen if NaOH or KOH are used as alkali
reagent. On the contrary, Ca(OH)2 or Mg(OH)2 improved dewaterability. This is probably
due to decrease in ratio of monovalent to divalent cations, when Ca(OH)2 or Mg(OH)2
are used instead of NaOH or KOH which promotes final dewaterability (Higgins and
Novak, 1997).
Dewaterability after AD is of more interest for WWTPs with AD in place. Results of
dewatering potential after TAP showed to be ambiguous. Some TAP studies in full- and
lab-scale showed improvements in dewaterability in terms of DS (A) parameter (DWA,
2016; Nagler et al., 2016). However, this was accompanied by increase in polymer usage
due to increase of soluble matter. On the contrary, there were studies, in which no increase
in dewaterability (Toutian et al., 2020a) or even deterioration (Huang et al., 2016) have
been reported. It should be noted that measurement methods of these two studies were
different, namely measurement of cake TS after lab centrifuge and measuring of CST,
respectively. To come to more rigorous conclusions, further systematic research with
standardized dewaterability test methods is needed. This is specifically a very important
issue when comparing low temperature TAP to TH, since one of main and widely reported
advantages of TH is improvement of dewaterability at high temperatures (T>160°C). To
shed light into mechanisms through which low temperature TAP affects dewaterability,
it would also be interesting to consider influencing parameters such as sludge age and
mixing ratio (WAS:PS), sludge chemical characteristics (pH, buffering system and
capacity, concentration of NH4
+− N , PO4
3− −P, Ca2+, Mg2+, CH4, etc.), P-removal
process type (biological or chemical), different EPS content and seasonal variations of
sludge.
3.3.2 Sludge liquor quality
Complex organic matter is broken down by microorganisms to simpler products during
AD. CH4 and CO2 are final products of carbonaceous organic matter degradation which
are transported to gaseous phase as biogas. Organic nitrogen and phosphorous are
degraded to ammonium and orthophosphate which remain solubilized in sludge liquor.
TAP targets rigid cell membranes in WAS which leads to release of intracellular organic
matter. This facilitates access of microorganisms to a higher fraction of organic matter.
As a result, both organic and inorganic (particulate and soluble) loads of sludge liquor
increase as reflected in increase of parameters total COD, soluble COD, total
phosphorous, PO4
3− − P , total nitrogen, NH4
+−N, and total suspended solids.
Part of load increase in sludge liquor is due to non-biodegradable organic matter such as
refractory dissolved organic carbon, dissolved organic nitrogen or dissolved organic
phosphorous. Since they cannot be removed by conventional process of wastewater
treatment, they can pose a risk to concentration increase of the effluent quality parameters
on WWTP (Dwyer et al., 2008). Knowing that discharge regulations are constantly
subject to stricter limits, it is important to take this issue into account before implementing
a pretreatment on WWTP.
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability of waste activated sludge
29
Table 3.4 Summary of literature dewaterability results after thermal alkaline or alkaline pretreatment of sludge before or after anaerobic digestion.
Before/After
anaerobic
digestion
(BAD/AAD)
Scale
Sludge type
(TS%)
Temperature
(°C)
Time
(h)
Alkali reagent
Alkali dosage
(mg per g TS)
pH
Dewaterability results
Reference
AADb
Full-
scale
WAS (6-7%)
65-70
2-2.5
NaOH
12-32
-
Improvement by 2.5% points with maximum cake solids of 31%.
Cake solids increased from 24.7 to 28.6% TS as polymer dosage
increased from 10.9 to 17.5 kg per t TS.
(Li et al.,
2017b)
AADb
Pilot
WAS (6.5%)
65-70
2-2.5
NaOH
12-32
-
No increase in TS of sludge cake after centrifugation with same
polymer dosage.
(Toutian
et al.,
2020a)
AADb
Lab
WAS (4-6%)
70
1
NaOH
32
-
Dewaterability parameter DS(A) increased from 27.0 to 32.6% with
polymer usage increase from 12.7 to 16 kg per t TS.
(Nagler et
al., 2016)
AADb
Lab
WAS (6-7%)
70
2
NaOH
12-32
-
Dewaterability parameter DS(A) increased from 26.7 to 31.3% with
polymer usage increase from 11.2 to 13.3 kg per t TS.
(DWA,
2016)
AADb
Lab
WAS (5.1%)
170
0.016
NaOH
50
-
Increase in flocculants need in dewatering test (centrifuge) from 13.1
kg per t TS for control to 23.4 kg per t TS for pretreated digested
sludge.
(Chi et al.,
2011)
AADa
Lab
Mixed
sludge (2%)
-
0.5, 1.0,
1.5, 2.0
NaOH
200
-
No change in TS of sludge cake (centrifuge with polymer) and
turbidity of sludge liquors.
(Li et al.,
2013)
AAD
Lab
WAS (2.8%)
-
-
NaOH and/or
Mg(OH)2
-
10
Deterioration of dewaterability in terms of CST with NaOH and
significant improvement after adding Mg(OH)2 (blank→ 348 s, 100:0
(NaOH:Mg(OH)2)→546 s, 75:25→77 s).
(Huang et
al., 2016)
AADa
Lab
WAS+PS
(2.1%)
35, 55
144
NaOH
-
8, 10,
12
At 35°C, NCST of all digested sludges were almost the same and at
55°C, NCST of untreated (no alkali) was higher than treated sludges
at different pHs.
(Wang et
al., 2018)
BAD and
AADa
Lab
WAS (1.1%)
-
0.5
NaOH
112, 200, 312,
488
10, 11,
12,
12.5
CST of disintegrated sludge increased with pH. CST decreased by
22% for digested sludge (microwave + pH 12)
(Dogan
and Sanin,
2009)
BAD
Lab
Mixed
sludge (6%)
20-120
1
NaOH,
KOH,
Ca(OH)2,
Mg(OH)2
36-68 (NaOH)
8-12
36 mg NaOH per g TS makes CST longer than blank. Best results
were obtained for 100°C, Ca(OH)2 at pH=10 and 60 min. CST
decreased from 34 to 22 s with Ca(OH)2. TS of sludge cake increased
from 28% to 46%. Monovalent ions such as Na+ and K+ give longer
CSTs.
(E.
Neyens, J.
Baeyens,
C.
Creemers,
2003)
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability of waste activated sludge
30
BAD
Lab
WAS
0-40
0-24
NaOH,
Ca(OH)2
0-1600 (NaOH),
0-2960
(Ca(OH)2)
-
NaOH treatment: CST increases significantly for doses of 320 mg per
g TS and then decreases to initial levels. Turbidity increases for dose
of 160 mg per g TS and then decreases. Sludge cake solids changed
very slightly (5-8%TS).
Ca(OH)2 treatment: CST and turbidity remain within initial levels.
Sludge cake solids significantly improved from 8% to 35% TS)
(Li et al.,
2008)
BAD
Lab
and
pilot
WAS
12.5%)
140-220
0.5-2.0
Ca(OH)2
51-105
9-11
Dewaterability was improved with increasing pretreatment
temperature but the impact of the pretreatment time was not
significant (cake TS up to 26-42.5% for centrifuge and 39.5-69% for
filter press, raw sludge 15%) for thermal pretreatments.
Addition of Ca(OH)2 gave better performance on the subsequent
mechanical dewatering of the pretreated sludge compared to pure
hydrothermal pretreatment, and the higher the pH value, the better the
dewaterability of the pretreated sludge.
(Li et al.,
2017a)
BAD
Lab
WAS (3%)
60, 80, 90
3, 6
NaOH
-
6-8,
10, 12
CST decreased from 2500 s to 1500 s after four successive steps of
TAP and dewatering.
(Liu et al.,
2017)
a With neutralization before AD
b Without neutralization before AD
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability
of waste activated sludge
31
(Toutian et al., 2020a) reported that Pondus® resulted in 0.8-1.1 mg/L increase in effluent
sCOD of WWTPs in Berlin due to formation of refractory COD in TAP, while with TH
(130-170°C ) this was 2-15 mg/L. This is expectable, considering lower temperature of
Pondus® in comparison to TH which leads to less intensive Maillard reactions. There is
not as much information for these parameters as for solubilization degrees and
biomethane potentials in literature. Therefore, further research is needed to clarify to what
extent different operating conditions of TAP affect WWTP effluent quality parameters.
To do such measurements, pretreated WAS should be digested in continuous AD. Then,
liquors from dewatering of digested sludge should be incubated in continuous aerobic
tests which mimic conditions of activated sludge systems on a WWTP. This procedure
should also be followed for a basis scenario in which not pretreated WAS is tested to
make an authentic comparison on the effects of TAP. This leads to measurement of
remaining not biodegradable nutrient fractions in wastewater which were introduced to
system vie pretreatment process.
Regarding inorganic load increase in sludge liquor, mainly NH4
+−N and PO4
3− −P
concentrations should be considered, as these exert extra treatment costs (through aeration
and chemicals) and additional required treatment capacity in the main-stream. Sludge
liquor is usually directed to head of WWTP to be treated in main-stream. Alternatively,
it can be first majorly treated in a separate stage through an added side-stream treatment
process and then be directed to main-stream for further treatment. It should be noted that
through innovative nutrient recovery processes, increase of NH4
+−N and PO4
3− −P in
sludge liquor can be utilized, positively. Especially for phosphorous as a critical nutrient,
this benefits numerous commercialized processes which aim to recover phosphorous from
digested sludge or its liquor. There is also a lack of sufficient information regarding
increase of these nutrient in sludge liquor after TAP and further research is needed to
show how TAP operating conditions increase sludge liquor quality parameters for more
rigorous conclusions on its economic aspects. As an example, (Toutian et al., 2020a)
showed that after Pondus NH4
+−N and PO4
3− −P increased 35% and 17%, respectively.
These increases should be taken into consideration in economics studies.
3.4 Important process design parameters
Economic and efficient low temperature TAP process involves various factors which are
closely related and should all be considered when investigating or designing this process
(Figure 3.8). Deviations from an optimal and correct design of TAP results in an
inefficient process or worsen its economics. In following some of the important process
design aspects of TAP are discussed in more details.
3.4.1 Total solids of WAS input to TAP
TS percentage of WAS plays a key role in economics and efficiency of TAP. Water as
the necessary reaction medium in WAS facilitates mixing leading to better mass and heat
transport. However, as TS of WAS increases, viscosity of WAS exponentially increases
and sludge adopts more of a non-Newtonian fluid (Pseudo-plastic) behavior (Cao et al.,
2016; Cheng and Li, 2015), which in turn worsens mixing and pumping efficiency.
Luckily, after/during pretreatment viscosity of sludge already reduces due to
disintegration of its microbial structure (Farno et al., 2016; Feng et al., 2017; Wang et al.,
2016b).
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability
of waste activated sludge
32
Figure 3.8 Different linked parameters influencing thermal alkaline pretreatment design in full-scale.
On the contrary, water unfavorably consumes energy (e.g. heat) and reactive reagents
(e.g. alkali) which are actually intended to affect the solids in WAS. The schematic
relationship between TS percentage in sludge, its viscosity and heat/alkali consumption
in TAP are shown in Figure 3.9.
Figure 3.9 Schematic relationship between viscosity and required heat and alkali reagent for thermal
alkaline pretreatment vs total solids percentage in sludge.
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability
of waste activated sludge
33
Regarding alkali consumption, pH plays the main role in terms of strength of reactive
driving force as it directly reflects the concentration of OH- in the liquid phase. Therefore,
it is important to achieve a certain pH at start of pretreatment to apply the needed reactive
ionic strength for attacking microbial biomass and to assure maximum disintegration
potential of alkali. For a solid weight unit of WAS (e.g. 1 kg solids as a basis), required
alkali reagent decreases, as its water content decreases (increase in TS). This is due to
alkalinity capacity of sludge, which increases alkali consumption.
Alkali consumption is not the only parameter dependent on TS of WAS. Due to following
neutral pH requirement (6.5-7.5) of sludge before AD, it is necessary to reduce pH of
WAS after TAP. Although this can be done through adding inorganic acids (e.g. HCl), it
is economically favorable to avoid this. Adding inorganic acids also increases inorganic
solids due to formation of salts. The goal is to exploit potential of released organic acids
during pretreatment to neutralize WAS after TAP (self-neutralization effect). Fraction of
organic solids in sludge and subsequently its organic acid release yield during TAP is
limited. Therefore, its solid content (or TS) should be high enough to allow for
neutralization of pretreated thickened sludge and not to consume released organic acids
for buffer capacity of extra liquor content in sludge.
Concerning heat consumption, amount of heat required to increase temperature of sludge
up to a certain point in TAP directly correlates with volumetric flow rate of sludge. Barber
(2016) showed crucial importance of reducing water content of sludge prior to TH on
steam consumption reduction. That is in fact, the main reason of required pre-dewatering
stage (up to 16-18% TS) prior to TH in full-scale.
As an example, Pondus® suggests increasing TS of WAS up to 6-8% before TAP. This
eliminates excessive use of alkali, extra neutralization before AD and heating the digester
through following procedure:
WAS has usually 0.6-0.8% TS after secondary clarifiers (DWA, 2014). In this range of
TS, it contains a considerable amount of mechanically separable water which contributes
to its significant large volumes. Therefore, it is normally thickened with dewatering
equipment up to 6-8% TS. This reduces nearly 90% of its original volume contributing
to significant subsequent digestion volume savings. Afterwards, WAS is pretreated
through Pondus® process leading to hydrolyzed WAS with a final pH of 6.8-7.0 (due to
its optimized operating conditions) and TWAS=70°C. Following, pretreated WAS is mixed
with same volume of PS (which normally has TS in range of 3-6% (DWA, 2014)) before
sending to digester (leading to TS~5% and neutral pH). This leads to a sludge mixture
with T2=40°C according to following equation:
∆𝑄
=𝜌𝑉𝐶(𝑇2−𝑇𝑊𝐴𝑆)+𝜌𝑉𝐶(𝑇2−𝑇𝑃𝑆)=0
3.1
Assumptions here were: average temperature of PS being 10°C (TPS), density and heat
capacity of WAS and PS being almost same due to >90% water content in both and fully
insulated mixing chamber and digester. In this way, TAP+AD would need as much
heating energy as an AD alone with optimized alkali dosage and no neutralization with
acids.
Accordingly, there is an optimal range of TS percentage in WAS for TAP above which
mass and energy transport efficiencies decline and below which energy and/or reactive
reagents are excessively consumed. Therefore, researchers should take TS of sludge into
account for a properly designed TAP and its techno-economic assessment.
3.4.2 Optimum alkali dosage
After thickening WAS to a certain TS, there is an optimum point (or range) for alkali
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability
of waste activated sludge
34
reagent dosage to perform a complete and economic TAP. When dosed below this point,
there would not be enough reactant (ionic strength) available to thoroughly disintegrate
sludge (pH below maximum pH required for a full alkaline disintegration). Consequently,
less disintegration products, i.e. organic acids, are released which in turn weakens self-
neutralization process. By dosing alkali reagent above optimal point, disintegration
process would be thorough. However, due to limited amount of released organic acids,
sludge would not be able to self-neutralize that surplus amount of alkali. This in turn,
necessitates extra step of neutralization with acids. On the contrary, in case of optimum
dosage of alkali reagent, not only thermal alkaline disintegration is complete but also
organic acids released are enough to self-neutralize sludge (Campo et al., 2018; Li et al.,
2017b; Nagler et al., 2016; Toutian et al., 2020a). Hence, no additional costs of extra
alkali or acid are necessary. This has also been graphically illustrated in Figure 3.10 for
further clarification. The progressions in Figure 3.10 are only for illustration of optimum
point of alkali dosage and real pathways (and above-mentioned mechanisms from
authors) need to be verified by precise lab research. Pondus® consumes 12-32 mg NaOH
per g TS of sludge, which is in the lowest range of alkali dosages reported for TAPs in
literature.
Figure 3.10 Schematic probable illustration of pH of WAS and concentration of released OH- and H+ after
thermal alkaline pretreatment versus alkali dosage
3.4.3 Alkali addition prior to thermal heating
Regarding cost effectiveness of TAP, it is very important to carry out alkali dosing step
before thermal heating. The reason lies again in release of organic acids from
disintegration of microbial biomass into liquid phase leading to self-neutralization of
sludge. When thermal heating is performed before adding alkali, a certain amount of
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability
of waste activated sludge
35
organic acids are released which leads to reduction of pH. This in turn leads to two
unfavorable issues. Firstly, there would be more alkali reagent needed for a complete
alkaline disintegration (e.g. to increase pH up to 12), as some of alkali would be consumed
for neutralization of released organic acids by foregoing thermal pretreatment. Secondly,
since part of organic acids are already released by thermal pretreatment, there would not
be sufficient acids released for self-neutralization of sludge after the following alkaline
pretreatment. Consequently, extra acid addition would be needed to neutralize sludge
before AD. This has been graphically illustrated in Figure 3.11 for more clarification.
Again, the progressions in Figure 3.11 are only for illustration of this issue and real
pathways (and above-mentioned mechanisms from authors) need to be determined by
precise lab research. In Pondus®, alkali addition is performed before thermal heating as
reported by (Heo et al., 2003; Li et al., 2017b; Nagler et al., 2016; Toutian et al., 2020a).
Figure 3.11 Schematic probable illustration of importance of carrying out alkaline step before thermal
heating step in thermal alkaline pretreatment to reduce extra alkali and acid costs.
Chapter 3: Impact of process parameters of thermal alkaline pretreatment on biogas yield and dewaterability
of waste activated sludge
36
3.5 Conclusions
Thermal alkaline pretreatment of WAS as a potential alternative to thermal hydrolysis
was reviewed. Following conclusions were drawn:
• Biogas yield increase of WAS after pretreatment lessens as initial biodegradability
of sludge increases. Depending on initial biodegradability, biogas yield increase
of 22-97% is expected.
• Low temperature TAP (T<100°C) increases biogas yield to a comparable extent
as of high temperature thermal hydrolysis (T=170°C).
• Alkali dosage and resulting initial pH have a significant effect on biogas yield. It
is recommended to keep alkali dosage below 80 mg NaOH per g TS of WAS for
maximum biomethane production and achieving self-neutralization after TAP.
Keeping pH high by constant dosing of alkali should be avoided. It is advisable
to dose alkali based on organic solid weight unit of sludge (e.g. mg NaOH per g
VS sludge) instead of only adjusting to a fixed pH. This makes calculation of
alkali consumption possible for comparison between studies.
• Treatment time between 1.5 and 5.0 h is sufficient for low temperature TAP. After
fixing temperature and alkali dosage, treatment time can be varied in this range to
achieve self-neutralization point after TAP.
• The potential benefit of TAP to reduce digestion time needed for maximum
stabilization of WAS (kinetics evaluation of biogas production) cannot be
confirmed and needs more research.
• Effect of low temperature TAP on dewatering potential of digestate seems to be
ambiguous and needs more systematic research. Especially, final TS and polymer
consumptions should be investigated for more rigorous conclusions. Therefore, a
standardized method for dewaterability measurement in lab-scale should be
developed for better comparability of results.
• Impact of TAP on sludge liquor quality parameters (dissolved nutrients as well as
refractory organic matter), suppression of foaming in digester and hygienization
of digestate needs more research.
• Optimized process conditions (temperature, alkali dosage and treatment time, TS
of WAS and performing alkali dosing step before heating step) with goal of self-
neutralization of WAS after TAP are key factors of an efficient and economic
TAP.
Finally, despite extensive studies in literature and existing full-scale installations, TAP
still needs more research for clarification of its underlying mechanisms, performance in
terms of biogas yield increase and downstream effects on sludge dewaterability and liquor
quality, and finally on its economic benefits for the WWTP operator.
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Chapter 4: Pilot study of thermal alkaline pretreatment of waste activated sludge: Seasonal effects on
anaerobic digestion and impact on dewaterability and refractory COD
43
4 Pilot study of thermal alkaline pretreatment of waste
activated sludge: Seasonal effects on anaerobic digestion
and impact on dewaterability and refractory COD
Highlights
• Biogas yield of mixed sludge follows a sinusoidal trend throughout a year.
• Thermal alkaline pretreatment (TAP) effect on biogas differs in winter and
summer.
• Dewaterability of digestate did not improve after TAP implementation.
• Effluent COD increase in WWTPs after TAP was predicted through modelling.
• TAP shows potential of biogas increase with small impact on effluent COD of
WWTP.
Contents of this chapter were published in Water Research journal
Volume 182, 1 September 2020, 115910
Received 20 February 2020
Revised 17 April 2020
Accepted 2 May 2020
Available online 22 May 2020
https://doi.org/10.1016/j.watres.2020.115910
Chapter 4: Pilot study of thermal alkaline pretreatment of waste activated sludge: Seasonal effects on
anaerobic digestion and impact on dewaterability and refractory COD
44
Abstract
Thermal alkaline pretreatment (TAP) of waste activate sludge (WAS) was carried out in
pilot-scale over a year to investigate its seasonal effects on anaerobic digestion and its
impact on dewaterability, sludge liquor quality and formation of soluble refractory COD
(sCODref). Temperature of TAP was set at 65-70°C and pH was increased by initial dosing
of sodium hydroxide [NaOH] (50% w/w, 1-2.5 mL/ L sludge) as alkali agent following
2-2.5 h reaction time. Pilot digesters were fed with primary sludge (PS) and hydrolyzed
WAS (HWAS) and compared to a reference digester fed with PS and untreated WAS.
Biogas yield increase due to TAP of WAS showed a sinusoidal trend throughout the year
with maximum in summer (+42%), minimum in winter (+3%) and average of +20%,
indicating a strong seasonal effect on TAP efficiency. Ammonium [ NH4
+−N],
orthophosphate [PO4
3− −P] and sulphate [SO4
2−] in sludge liquor increased by 34.6%,
17.0% and 21.6% with TAP, respectively. Centrifugation tests showed no significant
difference in dewaterability of both digestates with respect to total solids of sludge cake.
Normalized capillary suction time of digestate increased due to TAP, indicating a lower
capability for water release. Furthermore, detected sCODref after batch aerobic
biodegradation tests showed an increase of 30.3% with TAP. Hence, implementation of
TAP of WAS in full-scale will potentially lead to an increase of 0.8-1.1 mg/L of sCODref
in effluent of six wastewater treatment plants (WWTP) in Berlin. In conclusion, TAP of
WAS leads to increase in biogas production with a slighter negative impact on effluent
COD quality than high-temperature thermal hydrolysis.
Keywords
Thermal alkaline pretreatment; Biogas yield; Dewatering; Return load; Zahn-Wellens
test; Effluent COD
4.1 Introduction
Activated sludge process is the most widely used treatment process on WWTPs
throughout the world. Waste activated sludge (WAS) as the unfavorable byproduct of this
process puts WWTPs under significant financial burden for its disposal (Appels et al.,
2008). Therefore, it is economically beneficial to reduce WAS amounts as much as
possible. Anaerobic digestion (AD) as a well-established process leads to reduction of
organic matter of WAS, while producing biogas as a renewable energy source. However,
biodegradability of WAS in AD is relatively low (35-40% removal of volatile solids (VS))
due to its complex microbial structure (DWA, 2014). Besides, WAS shows a low
dewatering potential after AD (DWA, 2008). Accordingly, different pretreatment
techniques have been investigated in order to enhance biodegradation and dewaterability
of WAS (Carrere et al., 2016; Carrere et al., 2010; Neumann et al., 2016).
Biogas produced in AD is often utilized on-site in a combined heat and power (CHP)
plant to maximize energy recovery. Produced electrical energy can be used directly at the
WWTP or be injected to the grid. On the other hand, produced thermal energy cannot be
stored efficiently and thus must be used instantly, most often for digester heating and
auxiliary heat demand of the plant (e.g. buildings, hot water). For modern WWTPs with
efficient digestion and CHP, heat production from CHP exceeds actual heat demand at
the WWTP, leaving excess heat from the cooling cycle and exhaust gas that is wasted to
the atmosphere. By exploiting this readily available heat on site, thermal pretreatment
techniques have been more attractive to WWTP operators than alternative processes
which operate only on electrical energy. This is also reflected in the growing number of
full-scale installations of thermal pretreatment processes worldwide (Barber, 2016).
Chapter 4: Pilot study of thermal alkaline pretreatment of waste activated sludge: Seasonal effects on
anaerobic digestion and impact on dewaterability and refractory COD
45
Thermal hydrolysis (TH) and low temperature thermal alkaline pretreatment (TAP) are
two pretreatment techniques, which exploit thermal energy produced in a CHP. TH (130-
170°C) with specific thermal energy demand of 130-190 kWh/m3 is more energy
intensive than TAP (60-70°C) with 40-50 kWh/m3 (DWA, 2016). Moreover, TAP could
use even low temperature waste heat from a CHP cooling cycle due to lower operating
temperatures. Consequently, operation of steam generators as in TH with associated
operational risks is omitted. On the other hand, TAP needs a constant supply of alkali
agent to compensate for lower temperatures to boost its effect. Hence, it is of high interest
to know for WWTP operators, how TAP acts as a competitive alternative to TH on all
relevant aspects of WAS pretreatment before AD.
Along with benefits of TH and TAP in sludge handling, there comes also drawback of
increase in solubilized organic (carbon, nitrogen, phosphorus) and inorganic
(orthophosphate [PO4
3− −P], ammonium [NH4
+−N]fraction of sludge liquor after AD.
Beside additional costs of treating this increased return load, it could also lead to
impairment of effluent quality of WWTP e.g. in terms of increase in effluent COD
through soluble refractory COD (sCODref) (Toutian et al., 2020). With strict local and
national regulations on effluent quality parameters including COD, this is an important
deciding factor before implementing pretreatment processes (EC, 1991; Germany, 1996).
Regarding TAP, effects of alkali agent and dosage, treatment time and temperature on
disintegration of WAS has already been investigated extensively in batch-mode lab-scale
studies (Campo et al., 2018; Kim et al., 2013a; Shehu et al., 2012; Vigueras-Carmona et
al., 2011). Therefore, change of TAP operational parameters was not subject of current
study. Instead, other TAP knowledge gaps in literature were addressed. These studies
mainly focus on one or two aspects such as COD solubilization degree, biogas yield or
dewaterability and do not consider TAP effects on the whole process chain of sludge
treatment. Moreover, biochemical methane potential results from batch tests are subject
to inoculums which are not adapted to pretreated WAS and may not be fully
representative of continuous AD operation. Besides, since WAS is normally mixed with
primary sludge (PS) before AD on WWTPs, it is also important to investigate this
potential synergistic effect on biogas and dewaterability instead of WAS or pretreated
WAS alone. Furthermore, whether long term TAP poses a potential adverse impact on
stability of AD has not been investigated yet. Finally, due to seasonal changes of influent
properties of a WWTP and subsequent varying operational parameters, characteristics of
WAS also alters, resulting in different seasonal outcomes in biogas yield and dewatering
potential (Denkert and Reza-Tehrani, 2017; Hai et al., 2014). This necessitates yearlong
continuous pilot trials mimicking real conditions on WWTPs for more rigorous
conclusions on the effects of TAP.
Consequently, the aim of this pilot study was to investigate the effects of TAP of WAS
on characteristics change of WAS (pH, sCOD, PO4
3− −P,NH4
+−N, etc.), seasonal
biogas yield increase throughout one operational year, long term stability of digester,
dewaterability of digestate, return load increase of sludge liquor and sCODref after aerobic
biodegradation of sludge liquor. To estimate the impact of TAP implementation on
WWTP effluent quality, a ‘conversion factor’ was defined to correlate formation of
sCODref to the amount of pretreated VS in WAS. Using this factor, the effect of TAP on
effluent COD increase of six WWTPs in Berlin was predicted through modelling.
Whenever possible, results of this TAP study were compared with authors’ previous study
on TH (Toutian et al., 2020).
Chapter 4: Pilot study of thermal alkaline pretreatment of waste activated sludge: Seasonal effects on
anaerobic digestion and impact on dewaterability and refractory COD
46
4.2 Materials and methods
4.2.1 Characteristics of sludge
Thickened WAS was taken from a large municipal WWTP in Berlin operated with pre-
denitrification and enhanced biological phosphorus removal (EBPR). This WWTP is
referred to as reference WWTP (WWTPref) in this study. WAS is thickened with
centrifuges after polymer dosing (0.7 kg active substance per kg total solids (TS)). PS
was taken from the primary settling tanks. Activated sludge as inoculum for Zahn-
Wellens test was collected from the return activated sludge pipeline. Inoculum for
anaerobic digesters was digestate taken from digestion tanks on WWTPref. The
characteristics of WAS and PS are summarized in Table 4.1. The high concentrations of
sCOD, NH4
+−N and PO4
3− −P are due to a certain degree of pre-hydrolysis between
sampling and analysis time (one to two days), as explained in Figure S2 of supplementary
material.
Table 4.1 Characteristics of WAS and PS in this study
Parameter
Unit
WAS
PS
pH
-
6.2 (0.2)a
5.7 (0.3)
Total Solids (TS)
%
6.54 (1.17)
4.50 (0.52)
Volatile Solids (VS)
%TS
80.13 (0.90)
84.94 (1.75)
Total COD (TCOD)
mg/L
85455 (12400)
63775 (10804)
Soluble COD (sCOD)
mg/L
3081 (1308)
5010 (1799)
Total Nitrogen (TN)
mg/L
4589 (503)
1409 (224)
NH4
+−N
mg/L
158 (75)
103 (26)
Total Phosphorus (TP)
mg/L
2460 (382)
426 (73)
PO4
3− −P
mg/L
595 (175)
90 (18)
aMean value with standard deviations in parentheses
4.2.2 Pilot scale thermal alkaline pretreatment
TAP was carried out batch-wise in a pilot plant (Pondus® Verfahrenstechnik GmbH,
Germany) on-site at the WWTPref during one day of the week in a 24h continuous
operation. Trials took place in time period of July 2018 - June 2019 for one year (week 1
to week 52). Operational optimization and maintenance were carried out between week
26 to week 33. The process flow diagram of the TAP pilot plant is shown in Figure 4.1a
(see Figure S3 for a photo of pilot plant). The operational parameters were suggested by
process supplier (Pondus®) and are summarized in Table 4.2. Thickened WAS was first
sieved through a stainless steel sieve with mesh size of 5 mm (Figure S4) to protect the
influent pump against potential coarse material. Then, WAS was mixed with alkali dosed
at 1-2.5 mL sodium hydroxide [NaOH] (50% w/w)/L of WAS. Subsequently, WAS was
pumped through a spiral coil heat exchanger placed in a water bath which was heated up
to 65-70°C by an electrical heating element. After being heated, WAS entered the reactor
vessel through a nozzle, applying 2-2.5 hours hydraulic retention time (HRT) for the
reaction. Afterwards, WAS flowed into the second spiral coil heat exchanger which was
intended to cool down the hydrolyzed WAS (HWAS) after pretreatment. Cooling water
of the WWTPref was used to continuously cool the HWAS down to 22-24°C. In the end,
HWAS was collected in a 1 m3 container to be transported for AD. In each TAP run,
around 500 L WAS was hydrolyzed.
Chapter 4: Pilot study of thermal alkaline pretreatment of waste activated sludge: Seasonal effects on
anaerobic digestion and impact on dewaterability and refractory COD
47
4.2.3 Transportation and storage of sludge
As no parallel pilot digesters were available at the WWTPref, AD was operated at another
site (DBI Institute, Freiberg). Consequently, weekly transport of freshly taken WAS, PS
and HWAS was required by truck (3 hours drive). The sludges were then transferred to 3
containers (each with 1 m3 volume). Each container was equipped with a mechanical
stirrer to avoid settling. An ambient air cooling system was used to minimize the inherent
autolysis of sludge during one week storage time. Samples of digested sludge from two
digesters were sent back weekly to Berlin for further analyses.
Figure 4.1 a) Schematic process flow diagram of pilot plant of thermal alkaline pretreatment. b)
Configuration of anaerobic digesters and feedstock containers WAS, PS and HWAS.
4.2.4 Anaerobic digestion
The specifications of anaerobic digesters are listed in Table 4.3 and configuration of
digesters and feedstock containers are depicted in Figure 4.1b (see Figure S5 for a photo
a)
b)
Chapter 4: Pilot study of thermal alkaline pretreatment of waste activated sludge: Seasonal effects on
anaerobic digestion and impact on dewaterability and refractory COD
48
of digesters). Operational parameters similar to those of the digesters on the WWTPref
were maintained. Accordingly, mesophilic digestion at 37°C with 20 days HRT was
implemented. Digester A was fed daily with PS and WAS and served as a reference.
Digester B was fed with PS and HWAS after TAP. The mixing ratio between PS and
WAS (or HWAS) for both digesters was 1:1 (volume basis). Organic loading rate was
comparable for both digesters (1.5 kg VS/(m3.d)). Each digester was equipped with a
mechanical stirrer and online measurement systems of biogas volume, flow rate,
composition, temperature and pressure. Data of biogas production were normalized to the
standard temperature and pressure conditions (273.15 K, 101.33 kPa) (VDI, 2016).
Temperature, pH, redox potential and conductivity and in fermentate were also monitored
online. In the beginning, digesters were each fully inoculated with anaerobically digested
sludge from digesters of the WWTPref to boost the initiation phase of biogas production.
Table 4.2 Specifications of the pilot plant of thermal alkaline pretreatment
Parameter
Unit
Value
Feeding rate of WAS
L/h
25-30
Hydraulic retention time of reactor
h
2-2.5*
Temperature of reactor
°C
65-70
NaOH dosage (50% w/w)
mL/L sludge
1-2.5*
HWAS effluent temperature
°C
22-24
*Ranges of these parameters are due to operational challenges of pilot plant over one year of experiments
and diurnal and seasonal operational changes on the WWTP which affect properties of WAS (TS, viscosity,
etc).
4.2.5 Stability of digester
pH, volatile organic acids (VOA) and total alkalinity of carbonate (TAC) of fermentate
were measured to assess stability of fermentation process in digesters (Voß et al., 2009).
VOA corresponds to volatile fatty acids content and is measured as equivalent mg/L
acetic acid. TAC represents buffer capacity and is measured as equivalent mg/L calcium
carbonate [CaCO3].
Table 4.3 Characteristics of anaerobic digesters
Parameter
Digester A
Digester B
Volume [L]
1800
1800
Feed type and mixing ratio*
PS + WAS (1:1)
PS + HWAS (1:1)
Hydraulic retention time [d]
20
20
Organic loading rate [kg
VS/(m3.d)]
1.5
1.5
Temperature [°C]
37
37
*Volume basis
4.2.6 Dewaterability test
Capillary suction time (CST) tests: This test is typically used to determine the optimum
polymer type and dosage in sludge dewatering units. Moreover, it does not give direct
information regarding final achievable TS of dewatered sludge in full-scale dewatering
units (DWA, 2008). Nonetheless, it could still be used as a comparison parameter of water
release capability. CST was measured using a CST-meter (Type 304B, Triton
Electronics). For each measurement, circa 5 mL of sample was poured into the respective
Chapter 4: Pilot study of thermal alkaline pretreatment of waste activated sludge: Seasonal effects on
anaerobic digestion and impact on dewaterability and refractory COD
49
cylinder (diameter=1.5 cm, height=3 cm). Paper filter used was of type ‘Filtrak FN8’, 280
g/m2 and size of 70 x 90 mm (Hego Biotec GmbH). Before measurements, paper filters
were dried for 30 min in oven at 105°C and used after being cooled down to ambient
temperature in desiccator. To obtain normalized CST (NCST), CST value of each sample
was divided by its respective TS (APHA, 1998; DWA, 2008).
Centrifugation tests: Weekly samples of digestate were centrifuged at 25000 g for 30 min
(Avanti J-E, Beckman Coulter). The sludge cakes were separated and used for
determination of TS according to standard methods (APHA, 1998; DWA, 2008). For the
samples with polymer addition, the digestate was dosed with 12 g polymer as active
substance (0.5 % w/w solution, PolyChemie GmbH) per kg TS of sludge and mixed for
20 seconds at 600 rpm before centrifugation test using a mechanical stirrer (Hei-
TORQUE Value 100, Heidolph GmbH). The polymer used and the dosage were the same
as in the dewatering unit of the WWTPref. This test was performed over a period of 8
weeks (weeks 41-48).
4.2.7 Zahn-Wellens test
Zahn-Wellens test was used as a batch simulation of activated sludge process on a WWTP
to remove organic carbonaceous matter. For Zahn-Wellens Test, procedures of OECD
Guideline 302 B were followed (EMPA, 1992). Details of Zahn-Wellens test procedure
in this study were previously described (Toutian et al., 2020). The Zahn-Wellens test
period in current study was limited to 7 days, as this short-term biodegradation can be
used to mimic aerobic degradation of organic substances in activated sludge (Germany,
1996). The supernatant of the centrifugation test described in section 4.2.6 was taken as
test substance. Tests were run in duplicate and biodegradation was monitored by sCOD
measurement. Tests were carried out three times with samples from three successive
weeks (weeks 44-46).
4.2.8 Analytics
TS and VS were measured according to standard methods (APHA, 1998). The pH was
measured using a Hach Lange multimeter HQ40D model (IntelliCal probes). Total COD
(TCOD), total phosphorus (TP) and total nitrogen (TN) were determined after proper
dilution of the sample by photometric measurements using test cuvettes (Hach Lange, DR
5000). To measure sCOD, PO4
3− −P and NH4
+−N, samples were filtered through 0.45
µm membrane filters (CHROMAFIL, Macherey-Nagel) after proper dilutions. To
calculate soluble organic phosphorus (sPorg), soluble inorganic phosphorus (PO4
3− −P)
was measured and deduced from measured soluble total phosphorus (sTP). VOA and
TAC were measured using method by Nordmann (1977). Sulphate [SO4
2−] was measured
through the method suggested by Sheen et al. (1935).
4.2.9 Calculation methods
4.2.9.1 Solubilization degrees
To quantify the effect of TAP on release of COD, NH4
+−N and PO4
3− −P, solubilization
degrees (SD) after TAP and before AD were defined as following:
𝑆𝐷𝑥=[𝑥]𝑓 − [𝑥]0
𝑇𝑋−[𝑥]0×100
4.1
𝑆𝐷𝑥 (%): solubilization degree of COD, PO4
3− −P or NH4
+−N
TX (mg/L): TCOD, TP or TN
Chapter 4: Pilot study of thermal alkaline pretreatment of waste activated sludge: Seasonal effects on
anaerobic digestion and impact on dewaterability and refractory COD
50
[x]f (mg/L): sCOD, PO4
3− −P or NH4
+−N after TAP
[x]0 (mg/L): sCOD, PO4
3− −P or NH4
+−N before TAP
4.2.9.2 Specific biogas production and volatile solids reduction
To compare biogas yield of the two digesters, gliding specific biogas productions (SBP)
were defined and calculated over 8 weeks (3 HRTs of the digesters) through the following
equation:
𝐺𝑙𝑖𝑑𝑖𝑛𝑔 𝑆𝐵𝑃[ 𝑁𝐿
𝑘𝑔 𝑉𝑆𝑎𝑑𝑑𝑒𝑑]
=𝑏𝑖𝑜𝑔𝑎𝑠 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑 𝑖𝑛 8 𝑠𝑢𝑐𝑐𝑒𝑠𝑠𝑖𝑣𝑒 𝑤𝑒𝑒𝑘𝑠 [𝑁𝐿]
𝑎𝑑𝑑𝑒𝑑 𝑉𝑆 𝑖𝑛 8 𝑠𝑢𝑐𝑐𝑒𝑠𝑠𝑖𝑣𝑒 𝑤𝑒𝑒𝑘𝑠 [𝑘𝑔]
4.2
Gliding volatile solids reductions (VSR) were then calculated over 8 weeks (3 HRTs of
the digesters) through the following equation:
𝑉𝑆𝑅 [%]=𝑉𝑆 𝑟𝑒𝑚𝑜𝑣𝑒𝑑 𝑖𝑛 8 𝑠𝑢𝑐𝑐𝑒𝑠𝑠𝑖𝑣𝑒 𝑤𝑒𝑒𝑘𝑠 [𝑘𝑔]
𝑎𝑑𝑑𝑒𝑑 𝑉𝑆 𝑖𝑛 8 𝑠𝑢𝑐𝑐𝑒𝑠𝑠𝑖𝑣𝑒 𝑤𝑒𝑒𝑘𝑠 [𝑘𝑔]×100
4.3
Theoretically, 1 kg removed COD equals to exactly 350 L produced methane [CH4] in
standard conditions (VDI, 2016). Furthermore, 1 kg VS of mixed sludge was assumed to
correspond to 1.4 kg COD (Appels et al., 2008). Therefore, weekly removed volatile
solids were calculated through following equation:
𝑤𝑒𝑒𝑘𝑙𝑦 𝑉𝑆 𝑟𝑒𝑚𝑜𝑣𝑒𝑑 [𝑘𝑔]
= 𝑤𝑒𝑒𝑘𝑙𝑦 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑 𝐶𝐻4 [𝑁𝐿]×1 𝑘𝑔 𝐶𝑂𝐷 𝑟𝑒𝑚𝑜𝑣𝑒𝑑
350 𝐿 𝐶𝐻4 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑
×1 𝑘𝑔 𝑉𝑆 𝑟𝑒𝑚𝑜𝑣𝑒𝑑
1.4 𝑘𝑔 𝐶𝑂𝐷 𝑟𝑒𝑚𝑜𝑣𝑒𝑑
4.4
4.2.9.3 Refractory sCOD after Zahn-Wellens test
sCOD elimination degree at time t during Zahn-Wellens test period was calculated with
the following equation (EMPA, 1992):
𝐸𝐷𝑡=[1− 𝑠𝐶𝑂𝐷𝑠𝑡 −𝑠𝐶𝑂𝐷𝑠∗
𝑠𝐶𝑂𝐷𝑏𝑡 −𝑠𝐶𝑂𝐷𝑏∗]×100
4.5
EDt (%): sCOD elimination degree at time t
sCODst (mg/L): sCOD in test suspension at time t
sCODs* (mg/L): sCOD in test suspension after 3 hours (to compensate for adsorption
effect of inoculum)
sCODbt (mg/L): sCOD in blank at time t
sCODb* (mg/L): sCOD in blank after 3 hours
Final remaining sCOD after Zahn-Wellens tests of sludge liquors of digesters were
defined as sCODref.
4.2.9.4 Conversion factor to predict refractory soluble COD
sCODref was used to model effluent sCOD increase of WWTP after implementing TAP.
Chapter 4: Pilot study of thermal alkaline pretreatment of waste activated sludge: Seasonal effects on
anaerobic digestion and impact on dewaterability and refractory COD
51
To do this, a conversion factor (CF) was defined as following for each digester:
𝐶𝐹 =𝑠𝐶𝑂𝐷𝑟𝑒𝑓 ×𝑉
𝑉𝑆𝑖𝑛,20𝑑×106×100
4.6
CF (%): conversion factor of digester
sCODref (mg/L): refractory sCOD concentration of digester
V (L): volume of digester
VSin,20d (kg): sum of volatile solids fed to digester during 20 days HRT before taking
samples for Zahn-Wellens test
In fact, CF shows how much sCODref is produced per unit of VS fed to each digester.
4.2.9.5 Modelling effluent soluble COD increase of a WWTP
CF relates the sCODref to the volatile solids fed to the digesters. Therefore, CF, daily
PS+WAS production and daily flow rate of a WWTP could be used to predict the increase
in effluent COD of a WWTP as following:
𝑠𝐶𝑂𝐷𝐸𝑓𝑓,𝑖𝑛𝑐 =𝐶𝐹
100×𝑉𝑆𝑃𝑆+𝑊𝐴𝑆
𝑄×106
4.7
sCODEff,inc (mg/L): increase in effluent sCOD of WWTP
CF (%): conversion factor of digester A or B
VSPS+WAS (t/d): PS+WAS production of WWTP as VS
Q (m3/d): volume flow rate of WWTP
This is a simple modelling approach based on average Q and VS of a
4.3 Results and discussion
4.3.1 Characteristics of WAS after TAP
To investigate effect of TAP on WAS, characteristics of WAS and HWAS were
determined. As shown in Figure 4.2a, TS of WAS and HWAS were 6.75 and 6.67%
respectively, indicating no significant TS change during TAP. This is an advantage of
TAP over TH, since it removes necessity of pre-dewatering of WAS to 16% before TH
and diluting it again afterwards to be fit for the following AD process. VS/TS of WAS
and HWAS were 80.28 and 79.24%, respectively, showing no significant loss of organics
after TAP (Figure 4.2b).
The pH of WAS directly after dosing of NaOH was as high as 12-12.5. At extreme pH
values, extracellular polymeric substances (EPS) solubilizes and ionization of hydroxyl
radicals leads to swelling of particulate substances (Neyens et al., 2004). Subsequently,
several reactions such as saponification of lipids in cell wall leads to release of
intracellular organic matter (Neyens et al., 2004). Due to release of organic acids during
TAP through chemical and biological activities, the pH drops down again. In this study,
the applied NaOH dosage caused pH of HWAS to be not higher than 7.0 after TAP
(Figure 4.2c). Authors speculate that there also might be another reason for that which
relates to configuration of TAP in this study. In fact, in this pilot plant alkali is added to
sludge before it is heated up to 70°C. This leads to less alkali dosage needed to bring pH
up to 12.0. Afterwards, heating sludge causes other part of organic acids to be released
which bring the pH down to values lower than 7.0. But if sludge were first heated up to
70°C and then alkali were added, part of alkali would be used by organic acids released
by thermal effect. This would not only increase the amount of NaOH needed to bring pH
Chapter 4: Pilot study of thermal alkaline pretreatment of waste activated sludge: Seasonal effects on
anaerobic digestion and impact on dewaterability and refractory COD
52
up 12.0, but also would prevent pH of sludge to come down to 7.0 after TAP, since part
of organic acids were already released by thermal effect. Accordingly, process
configuration of current study avoids the use of mineral acids to neutralize the pH after
TAP and thus saves costs on chemicals. This is a very important techno-economical
aspect, since many studies in literature have reported on neutralization required after
TAP. In a study, Ruiz-Hernando et al. (2014) estimated acid costs needed for post-
neutralization of alkaline pretreated WAS (157 g NaOH/kg TS) to be almost half of the
costs of alkali consumption.
Figure 4.2 Monitored parameters of WAS and HWAS. Box-plots show minimum, 25% quartile, median,
75% quartile and maximum throughout the study. a) Total solids (n=35) b) Volatile solids (n=38) c) pH
(n=36) d) Soluble COD (n=27) e) PO4
3− −P (n=26) f) Soluble organic phosphorus measured as PO4
3− −P
(n=24) g) NH4
+−N (n=24).
The sCOD of WAS increased significantly from 2740 to 19840 mg/L after TAP (Figure
4.2d). This corresponded to SDCOD equal to 20.3%. Regarding SDCOD, there is a wide
range of values in literature. Demir (2018) reported 78% SDCOD through TAP of WAS
(conventional activated sludge process) with NaOH at 90°C for 25 minutes. The same
value was also reported by Kim et al. (2013a). Wang et al. (2016a) reached 30-50%
SDCOD after TAP of WAS with NaOH at 25-90°C for 1 hour. By increasing treatment
time up to 24 hours at 90°C SDCOD increased to 49-57% (Wang et al., 2016a). In another
study, Haigang et al. (2016) reported only 9-15% SDCOD even at higher temperatures of
105-135°C. This wide range of SDCOD is due to different pretreatment conditions and
WAS characteristics. Authors have previously reported 20.6 to 41.5% SDCOD after TH
(130-170°C) of WAS from same WWTP as in current study which is above SDCOD
a)
b)
c)
d)
e)
f)
g)
Chapter 4: Pilot study of thermal alkaline pretreatment of waste activated sludge: Seasonal effects on
anaerobic digestion and impact on dewaterability and refractory COD
53
achieved here after TAP (Toutian et al., 2020).
The PO4
3− −P increased significantly from 560 to 1044 mg/L which corresponded to
SDPO4
3−−P equal to 22.8% (Figure 4.2e). The high values of PO4
3− −P in WAS show a
pre-hydrolysis step between sampling time and analytical measurements. Since the WAS
in this study was from an EBPR plant, the increase in PO4
3− −P of HWAS could be
attributed to two different mechanisms or a combination of them. First mechanism could
be thermal and chemical destruction of orthophosphate bonds of phospholipids in cell
membranes. The SDPO4
3−−P up to 43.1% due to thermal destruction was previously
reported after TH of bio-P WAS at significantly higher temperatures (130-170°C)
(Toutian et al., 2020). The second mechanism could be due to biological release of
PO4
3− −P during TAP, where volatile fatty acids are available and redox potential is also
in required functional range of polyphosphate accumulating organisms due to addition of
NaOH (Cheng-Nan et al., 2001). The sPorg which is soluble organic phosphorus also
increased after TAP from 28 to 232 mg/L (Figure 4.2f).
The NH4
+−N increased from 141 to 350 mg/L after TAP (Figure 4.2g). This
corresponded to SDNH4
+−N equal to 3.9%. Considering usual high concentrations of
NH4
+−N in sludge liquor after AD, this shows that most part of ammonification process
still happens through microorganisms’ activity during AD. In a previous study, NH4
+−N
release was insignificant even after high temperatures of TH (130-170°C), indicating
different chemical and biological reaction pathways for NH4
+−N and PO4
3− −P (Toutian
et al., 2020).
4.3.2 Specific biogas production and volatile solids reduction
Gliding SBPs for digesters A and B are depicted in Figure 4.3a. The gliding SBP of
digester B was higher than in digester A. A noticeable fact is that SBPs of both digesters
increased as temperature of activated sludge tanks of WWTPref decreased (week 10 to
week 37, i.e. October to March). Subsequently, SBPs started to decrease again as
temperature of activated sludge tanks of WWTPref increased (week 4 to week 10 and week
37 to week 50, i.e. April to September). Minimum gliding SBP was 248 NL/kg VSadded,
while maximum gliding SBP raised to 464 NL/kg VSadded, showing 87% difference in
biogas yield by AD of PS+WAS at its lowest and highest values. As a result, effect of
TAP on SBP of WAS was also different in various seasons of the year. As shown in
Figure 4.3a, relative increase of gliding SBP varied widely from +3% (minimum value
during winter) to +42% (maximum value during summer) before the maintenance period.
With restart of the tests after maintenance period, relative increase of gliding SBP raised
from +3% to +38%. The average value of relative increase in gliding SBP was +20% for
the whole test period. This was quite in agreement with findings of Hai et al. (2014) who
reported the same sinusoidal trend in water temperature of WWTP and methane
production rate from AD of WAS throughout a year. According to their study, the
difference was as high as nearly 100% with methane production rate being at its maximum
during winter (minimum temperature of WWTP=14°C) and at its minimum during
summer (maximum temperature of WWTP=27°C). This corresponds well with the 87%
difference in current study. Hai et al. (2014) also showed that this sinusoidal trend in
temperature of water in WWTP throughout the year fitted the ratio of unbiodegradable
particulates (XU) to total WAS organics (Xorg). This ratio ranged from 40 to 65% between
minimum and maximum temperatures, showing more decay of WAS biomass in higher
water temperatures. This clearly implies that the significantly higher TAP effect on biogas
yield during summer in current study could be attributed to higher ratio of
unbiodegradable organics in WAS which is more susceptible to destruction by
Chapter 4: Pilot study of thermal alkaline pretreatment of waste activated sludge: Seasonal effects on
anaerobic digestion and impact on dewaterability and refractory COD
54
pretreatment. In another study, Wang et al. (1997) have reported lower VSR of WAS in
summer resulting in higher relative increase in BMP after thermal pretreatment at 60-
100°C. In their study, for three HRTs in AD and after thermal pretreatment at 60°C, ‘VSR
relative increase’ decreased from 18.1, 25.0 and 29.3% during summer to 11.8, 16.4 and
15.6% during winter, respectively. Sinusoidal trends in biomethane yield of microalgae
and biohydrogen yield of sludge throughout a year has also been reported by Adams et
al. (2011) and Xu et al. (2012), respectively.
Figure 4.3 a) Gliding specific biogas productions (SBP) with respective relative increase and temperature
in activated sludge tank of reference WWTP b) Gliding volatile solids reductions (VSR) with respective
relative increase.
Griffin and Wells (2017) reported that temperature is the principal factor which causes
significant change of microbial community in activated sludge throughout different
seasons. Furthermore, Kim et al. (2013b) investigated the effect of certain operational
parameters on population of general and rare bacterial taxa in an activated sludge system
over two years. They concluded that temperature was a significant parameter for rare
bacterial taxa. On the other hand, general bacterial taxa were affected more by influent
b)
a)
Chapter 4: Pilot study of thermal alkaline pretreatment of waste activated sludge: Seasonal effects on
anaerobic digestion and impact on dewaterability and refractory COD
55
and effluent biological oxygen demand and dissolved oxygen. Therefore, microbial
change of WAS throughout the year could be a reason for wide range of biomethane yield
increase after TAP or TH reported in literature.
As a comparison to TAP, authors reported up to +27% increase in biomethane yield of
only WAS after TH (170°C) in single batch biochemical methane tests (Toutian et al.,
2020). It was not investigated if this +27% increase changes throughout different seasons
of the year and whether co-digestion of PS has a synergistic positive effect on biogas
yield.
Gliding VSRs of digesters A and B are shown in Figure 4.3b. The gliding VSRs of
digester B (ranging from 48.2 to 60.3% with average of 52.8%) were higher than those of
digester A (ranging from 40.4 to 56.6% with average of 48.5%). Relative increase of
gliding VSR varied from 2% to 23% throughout whole test period. Characteristics of
produced biogas and fermentates from two digesters are presented in Table 4.4. CO2 and
CH4 contents of biogas from digesters A and B were 56.6 and 56.4%, showing no effect
of TAP on methane content of biogas.
Table 4.4 Characteristics of fermentate in digesters and produced biogas measured online (average values
with standard deviations in parentheses out of 423295 data points)
Parameter
[Unit]
Unit
Digester A
Digester B
Fermentate
Temperature
[°C]
37.3 (0.8)
37.3 (0.7)
pH
[-]
6.7 (1.5)
6.3 (2.5)
Redox potential
[mV]
-436.0 (177.8)
-441.0 (176.0)
Conductivity
[mS/cm]
5.7 (2.3)
8.5 (3.5)
Biogas
Pressure
[mbar]
3.6 (3.9)
5.2 (4.0)
Temperature
[°C]
18.6 (14.1)
18.0 (4.6)
Flow
[NL/h]
32.0 (21.3)
41.7 (24.5)
CH4
[Vol.-%]
56.6 (12.3)
56.4 (14.6)
CO2
[Vol.-%]
35.2 (7.4)
34.1 (8.8)
H2
[ppm]
47.1 (77.5)
35.0 (28.3)
H2S
[ppm]
1449.9 (825.1)
1229.1 (715.1)
4.3.3 Stability of digester
pH: As shown in Figure 4.4a, both digesters had same pH values (6.95) at the beginning
of test period. Subsequently, pH of both digesters increased steadily during first fourteen
weeks and then fluctuated slightly at these increased levels as digesters were stabilized
by feeding routine. pH of digester A ranged from 6.88 to 7.54 with median value equal to
7.16, while pH of digester B was between 7.02 and 7.59 with median value equal to 7.35
(Figure 4.4b). Higher pH range of digester B was due to pretreatment of WAS with NaOH
resulting in presence of more hydroxyl ions [OH-]. This slight pH increase did not cause
any operational problem for digester B and was regarded as no harm for anaerobic
digestion performance.
VOA: VOAs measured as equivalent acetic acid are shown in Figure 4.4c. A slight
increasing trend in VOAs concentration for both digesters during the test period could be
seen. Nevertheless, VOAs of digester A ranged from 918 to 2193 mg/L with median value
Chapter 4: Pilot study of thermal alkaline pretreatment of waste activated sludge: Seasonal effects on
anaerobic digestion and impact on dewaterability and refractory COD
56
equal to 1401 mg/L, while VOAs of digester B were between 1056 and 2286 mg/L with
median value equal to 1522 mg/L (Figure 4.4d). This accounted for 8.6% increase in
VOAs in digester B due to TAP, indicating positive effect of TAP on fermentation.
Figure 4.4 Monitored parameters in fermentates. Left side: Individual data, Right side: Box plots. a and b
for pH (n=117), c and d for VOA (n=116), e and f for TAC (n=116), and g and h for VOA/TAC (n=116).
TAC: TAC of digesters during test period is shown in Figure 4.4e. TAC of digester A
ranged from 2732 to 5769 mg/L with median value equal to 4021 mg/L, while TAC of
a)
b)
c)
d)
e)
f)
g)
h)
Chapter 4: Pilot study of thermal alkaline pretreatment of waste activated sludge: Seasonal effects on
anaerobic digestion and impact on dewaterability and refractory COD
57
digester B was between 3646 and 7218 mg/L with median value equal to 5451 mg/L
(Figure 4.4f). Therefore, TAP enhanced the buffer capacity of digester B by 35.6%. This
shows a potential for increasing organic loading rate of digester by implementing TAP of
WAS before AD, since OLR was in the order of 1.5 kg VS/(m3.d). Possibility of
increasing OLR combined with significant reduction of viscosity after TAP could
potentially reduce the size of digestion tanks even to half resulting in significant capital
cost savings (Wang et al., 2016b).
VOA/TAC: VOA/TAC ratios of digesters A and B during test period are shown in Figure
4.4g. This ratio ranged from 0.23 to 0.50 mg/L with median value equal to 0.35 for
digester A, while it was between 0.17 and 0.41 mg/L with median value equal to 0.28 for
digester B (Figure 4.4h). The normal range for this parameter differs from one digester to
another and is dependent of different parameters.
However, ratios between 0.15 and 0.45 have been reported as reference values (Voß et
al., 2009). Higher ratios indicate tendency towards over-acidification of digester and
lower ratios indicate under-loaded status of digester. In both cases, organic load should
be adjusted in order that digester gets back to normal working condition. Conclusively,
both digesters in this study were stable throughout test period. Again, lower ratio of
digester B offers the potential of increasing organic loading rate, as explained above.
4.3.4 Dewaterability of digestate
CST test: Results of NCST tests are shown in Figure 4.5a. NCST for both digestates
increased in winter months and decreased again in spring months, indicating worsening
of dewaterability during winter. The exacerbation of digestate dewaterability in winter is
an already known problem for WWTP operators (Denkert and Reza-Tehrani, 2017). The
digestate of digester B had a median NCST value of 22 s.kg/g, while digester A had a
median NCST value of 15 s.kg/g (Figure 4.5b). Therefore, TAP caused the digestate of
digester B to show less capability for releasing water. This could be attributed to addition
of Na+ cations in the sludge by TAP, which increases monovalent to divalent cation ratio
in digestate of digester B leading to aggravation of dewatering (Higgins and T. Novak,
1997). Liu et al. (2017) compared two TAP scenarios, a one-stage TAP (90°C, pH=12.0,
3 h) with a four-stage TAP (60-90°C, pH=6.0-12.0, 3-6 h) for dewaterability assessments.
In both scenarios, CST increased directly after TAP from below 100 s up to 1600 and
2500 s implying worsening water release capability. They also showed that increase in
loosely-bound EPS correlates well with increase in CST. Dogan and Sanin (2009) also
reported increase in CST of digested sludge pretreated with NaOH (pH=10 and 11) from
138 to 157 and 151 s.
Centrifugation test: Figure 4.5c shows individual weekly results of centrifugation tests of
the digestates. TS of sludge cakes for digesters A and B had median values of 14.3 and
14.4%, respectively (Figure 4.5d). Accordingly, there was no significant difference
between sludge cake TS of digesters A and B resulted from pretreatment. Insignificant
improvement in TS of dewatered digestate after TAP (Pondus®), was also reported in a
full-scale TAP plant (Dünnebeil, 2018). Li et al. (2013) has also reported no improvement
in dewaterability after alkaline post-treatment of pre-digested sludge (5% of digester
volume after 24h digestion) returned back to the digester. On the contrary to TAP, authors
have previously reported up to +4% increase in TS of digestate (14.3 to 18.6%) after TH
(170°C) (Toutian et al., 2020). Since dewaterability improvement leads to significant
disposal cost savings on WWTPs, this should be taken into consideration when deciding
between TH and TAP in techno-economical assessments.
Chapter 4: Pilot study of thermal alkaline pretreatment of waste activated sludge: Seasonal effects on
anaerobic digestion and impact on dewaterability and refractory COD
58
Figure 4.5 a) Normalized capillary suction time (NCST) of digestates. b) Box-plot of NCST of digestates
(n=22). c) TS of sludge cakes after centrifugation tests. d) Box-plot of TS of sludge cakes after
centrifugation tests with polymer (n=8).
4.3.5 Sludge liquor load increase
𝑁𝐻4
+−𝑁 : Figure 4.6a shows results of NH4
+−N measurements during test period.
Median NH4
+−N concentrations of digesters A and B were 1338 and 1801 mg/L,
respectively (Figure 4.6b). This shows 34.6% higher NH4
+−N concentration in digester
B than in digester A, which is attributed to higher degradation of organics resulted from
TAP. This increase puts WWTPs under extra burden of nitrogen removal in main- or side-
stream treatment which reflects in additional costs of aeration and carbon source plus
capacity shortages. It also reduces the content of nutrients in dewatered sludge, which
lessens its value for agricultural utilization. However, it could be regarded as a potential
positive effect of TAP for new WWTP concepts with nutrients recovery units on side
stream in the future.
𝑃𝑂4
3− −𝑃: Results of PO4
3− −P measurements during test period are shown in Figure
4.6c. PO4
3− −P increased constantly in the initial thirteen weeks in sludge liquors of both
digesters. This was due to the fact that in the beginning, both digesters were fully filled
with digested sludge from the WWTPref. This WWTP had a struvite precipitation unit
(AirPrex®) after digesters, which caused significant reduction of PO4
3− −P content in
digestate (Heinzmann and Engel, 2006). From thirteenth week on, the concentration of
PO4
3− −P in both digesters reached stabilized levels. Digester B had a median PO4
3− −P
concentration of 594 mg/L, while digester A had a median PO4
3− −P concentration of
695 mg/L (Figure 4.6d). This 17.0% increase in PO4
3− −P concentration is due to higher
degradation of organics in digester B. Again, this increase causes extra chemical costs for
phosphorus removal plus potential capacity lack for WWTPs and reduction of nutritional
value of dewatered sludge for agricultural utilization. However, with increasing demands
on phosphorus recovery as a vital non-renewable resource, this increase could lead to
a)
c)
b)
d)
Chapter 4: Pilot study of thermal alkaline pretreatment of waste activated sludge: Seasonal effects on
anaerobic digestion and impact on dewaterability and refractory COD
59
enhancing the recovery potential of renewable fertilizers such as struvite on WWTPs.
𝑆𝑂4
2−: Sulphate measurement results are shown in Figure 4.6e. Sulphate concentration
for both digesters increased constantly to the end of the test. This may be attributed to a
slight activity shift of sulphate reducing bacteria towards methanogenic bacteria
throughout the test period. Nevertheless, sludge liquor of digester B had 21.6% higher
sulphate concentration than digester A due to higher degradation of organics in digester
B, when considering the whole test period (Figure 4.6f).
Figure 4.6 Monitored parameters in sludge liquors. Left side: Individual data, Right side: Box plots. a and
b for NH4
+− N (n=44), c and d for PO4
3− −P (box-plot drawn for stabilized concentrations after week 13,
n=30) and e and f for SO4
2− (n=44).
4.3.6 Refractory soluble COD of sludge liquor
Results of Zahn-Wellens tests of liquors from digesters A and B are depicted in Figure
4.7. Figure 4.7a shows the sCOD elimination degrees during 7-day Zahn-Wellens test
period. Reference substance (ethylene glycol) was degraded more than 99% in less than
7 days. This demonstrated the validity of the Zahn-Wellens tests (EMPA, 1992). The
average elimination degrees for digesters A and B after 7 days were 25.8 and 30.2%,
a)
b)
c)
d)
e)
f)
Chapter 4: Pilot study of thermal alkaline pretreatment of waste activated sludge: Seasonal effects on
anaerobic digestion and impact on dewaterability and refractory COD
60
respectively. This higher elimination degree of digester B could be attributed to an
increase of aerobically biodegradable organics after implementing TAP.
Figure 4.7 Results of 7-day Zahn-Wellens tests of sludge liquors. Three identically-marked lines represent
results of three separate tests from three successive weeks (each data point represents average value of
duplicate measurements). a) Soluble COD elimination degrees. b) Absolute soluble COD concentrations
For predicting the increase in effluent COD of a WWTP after implementing TAP, the
absolute residual sCOD concentrations should be used in conversion factor. sCOD of
liquors from digester A decreased from 400 to 299 mg/L, while sCOD of liquors from
digester B decreased from 612 to 429 mg/L (average value out of three successive weeks)
(Figure 4.7b) Conclusively, TAP resulted in 30.3% increase in sCODref. This indicated
that in spite of higher sCOD elimination degree from sludge liquors of digester B, the
sCODref after aerobic treatment is still higher than that of digester A with lower
a)
b)
Chapter 4: Pilot study of thermal alkaline pretreatment of waste activated sludge: Seasonal effects on
anaerobic digestion and impact on dewaterability and refractory COD
61
elimination degree. It should be noted that aerobic inoculum in batch Zahn-Wellens tests
were not adapted to liquors after AD. Hence, long-term biodegradation of sludge liquor
could be better than batch-wise aeration tests. Moreover, results of this section may not
be representative of the whole year, as they were performed for three weeks. Nonetheless,
return load of refractory organic carbonaceous matter increased after implementing TAP.
How much this sCODref contributes to increase in effluent sCOD of a WWTP, is presented
and discussed in section 4.3.7.
4.3.7 Effluent soluble COD increase of WWTP
To predict the effect of TAP implementation on effluent sCOD increase of a WWTP, real
data from six WWTPs in Berlin were used. These data and modelling results of effluent
sCOD increase after TAP implementation are presented in Table 4.5. TAP of WAS before
AD is predicted to result in 0.8-1.1 mg/L increase in effluent sCOD of these WWTPs. A
higher PS+WAS production per inflow volume unit leads to a higher increase in effluent
sCOD after TAP implementation. In a previous study, authors predicted 2-15 mg/L
effluent sCOD increase for these six WWTPs after implementing TH (130-170°C)
(Toutian et al., 2020). Therefore, TAP has a less potential adverse impact on effluent
sCOD than TH. This is an important deciding factor for WWTPs, which their effluent
COD is not far below the regulated limits before implementation of pretreatments.
To the best of the authors' knowledge, there is no distinctive literature available regarding
increase of effluent sCOD after implementation of TAP for a comparison to results of this
study. According to the technical report of German association of water and wastewater
on sewage sludge disintegration, even thermal pretreatment of sludge at 70°C could lead
to measurable sCODref formation (DWA, 2016). However, it suggests that this amount of
sCODref would be roughly 50% of that of high temperature pretreatment processes such
as thermal hydrolysis, i.e. below 2.5 mg/L or 5% of effluent load. This is very well in
accordance with the results of the current study.
4.4 Conclusions
Thermal alkaline pretreatment of WAS was carried out in pilot scale over one year. The
following conclusions were drawn:
• Biogas yield of mixed sludge (PS+WAS) showed a sinusoidal trend throughout
seasons of a year, showing up to +87% variation between winter (highest) and
summer (lowest)
• TAP led to higher biogas yield increase in summer (+42%) and a lower increase
in winter (+3%), with an average value of +20% for the whole year
• Strong seasonal variations of WAS composition could be a reason for a wide range
of increase in biogas yield due to TAP or TH reported in literature
• Digester showed long-term stability with TAP during one year of operation and
potential for increase in organic loading rate
• Dewaterability of digestate showed no increase in TS of sludge cake by TAP in
contrast to positive effects on dewaterability reported for TH
• NH4
+−N and PO4
3− −P in sludge liquor increased by 34.6% and 17.0%
indicating increase in return load and related treatment costs
• Modeling results predicted 0.8-1.1 mg/L increase in effluent refractory sCOD of
six WWTPs in Berlin after implementing TAP which is lower than previously
reported for TH
In summary, TAP of sludge showed potential in reducing disposal costs through reduction
of volatile solids, with a slight impact on the effluent COD of WWTP.
Chapter 4: Pilot study of thermal alkaline pretreatment of waste activated sludge: Seasonal effects on anaerobic digestion and impact on dewaterability and refractory
COD
62
Table 4.5 Characteristics of six WWTPs in Berlin and expected effluent sCOD increase after implementing TAP
a WWTP3 has no anaerobic digestion now
*Only WAS was pretreated by TAP
**VS/TS of mixed sludge assumed to be 0.8 for all six WWTPs
***European Commission COD discharge limit=125 mg/L (EC, 1991)
****German national COD discharge limit=75 mg/L (Germany, 1996)
*****Regional COD discharge limit=65 mg/L
Parameter
Unit
WWTP1
WWTP2
WWTP3
WWTP4
WWTP5
WWTP6
Known data
Population equivalent
Million
0.423
0.245
1.402
1.275
0.266
0.677
Total flow (Q)
m3/d
63481
36803
210360
191252
39882
101536
Daily TS produced (PS+WAS)
t/d
33
19
89
86
20
59
Daily VS produced (PS+WAS)
t/d
26.4
15.2
71.2
68.8
16
47.2
Influent COD
mg/L
975
1379
819
954
1069
886
COD elimination
%
96
97
95
95
96
95
Effluent COD
mg/L
40
47
43a
52
42
41
Predictions
Daily sCODref produced (with AD)
g/d
143195
82445
386191
373174
86785
256015
Increase in effluent sCOD (with AD)
mg/L
2.3
2.2
1.8
2.0
2.2
2.5
Daily sCODref produced (with TAP+AD)
g/d
205453
118291
554101
535423
124517
367325
Increase in effluent sCOD (with TAP+AD)
mg/L
3.2
3.2
2.6
2.8
3.1
3.6
Increase in effluent sCOD caused by TAP
mg/L
1.0
1.0
0.8
0.8
0.9
1.1
Chapter 4: Pilot study of thermal alkaline pretreatment of waste activated sludge: Seasonal effects on
anaerobic digestion and impact on dewaterability and refractory COD
63
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Chapter 5: Effect of temperature on biogas yield increase and formation of refractory COD during thermal
hydrolysis of waste activated sludge
66
5 Effect of temperature on biogas yield increase and
formation of refractory COD during thermal hydrolysis
of waste activated sludge
Highlights
• Biomethane potential of WAS increased with temperature of thermal hydrolysis
(TH).
• Refractory soluble COD increased with temperature of TH.
• Slight decrease in TH temperature significantly reduced refractory sCOD.
• Impact of TH on effluent COD increase of a WWTP was predicted.
• Optimum TH temperature can be found for a WWTP to comply with COD
discharge limit.
Contents of this chapter were published in Water Research journal
Volume 171, 15 March 2020, 115383
Received 4 December 2019
Accepted 7 December 2019
Available online 11 December 2019
https://doi.org/10.1016/j.watres.2019.115383
Chapter 5: Effect of temperature on biogas yield increase and formation of refractory COD during thermal
hydrolysis of waste activated sludge
67
Abstract
Thermal hydrolysis (TH) increases the anaerobic biodegradability of waste activated
sludge (WAS), but also refractory organic and nutrient return load to a wastewater
treatment plant (WWTP). This could lead to an increase in effluent chemical oxygen
demand (COD) of the WWTP. The aim of this study was to investigate the trade-off
between increase in biogas production through TH and anaerobic digestion and increase
in refractory COD in dewatered sludge liquors at different temperatures of TH in lab-
scale. WAS was thermally hydrolyzed in temperature range of 130-170°C for 30 min to
determine its biomethane potential (BMP). After BMP test, sludge was dewatered and
sludge liquor was aerated in Zahn-Wellens test to determine its non-biodegradable soluble
COD known as refractory soluble COD (sCODref). With increasing temperature in the
range of 130-170°C, BMP of WAS increased by 17 to 27%, while sCODref increased by
3.9 to 8.4%. Dewaterability was also enhanced through relative increase in cake solids by
12-30%. A conversion factor was defined through mass balance to relate sCODref to
volatile solids of raw WAS. Based on the conversion factor, expected increase in effluent
CODs of six WWTPs in Berlin were predicted to be in the range of 2-15 mg/L after
implementation of TH at different temperatures. It was concluded that with a slight
decrease in temperature, formation of sCODref could be significantly reduced, while still
benefiting from a substantial increase in biogas production and dewaterability
improvement.
Keywords
Thermal Hydrolysis; Biomethane Potential; Zahn-Wellens Test; Anaerobic Digestion;
Sludge Liquor; Effluent COD
5.1 Introduction
The activated sludge process is the most widely used process for wastewater treatment
throughout the world for its convenience and efficiency. Waste activated sludge (WAS)
is the unfavorable byproduct of this process. Its handling and disposal accounts for up to
50% of operational costs on a wastewater treatment plant (WWTP) (Appels et al.
2008).Therefore, it is profitable to reduce the amount of WAS as much as possible before
disposal. Anaerobic digestion (AD) has established itself as an efficient way of reducing
sludge with benefit of producing biogas as a renewable energy source (Appels et al. 2008).
However, digestibility of WAS is limited due to its microbial structure, whereas
pretreatment processes are known to boost its AD (Barjenbruch and Kopplow 2003).
Different physical, chemical, thermal and biological pretreatment methods have been
investigated extensively for their potential effects on enhancing AD of WAS (Carlsson et
al. 2012, Carrere et al. 2016, Carrere et al. 2010, Joo et al. 2015, Neumann et al. 2016,
Tyagi and Lo 2011, Zhen et al. 2017). Amongst all, thermal pretreatments have shown
the highest potential when it comes to energy efficiency and integration on a WWTP
(Cano et al. 2015, Fernández-Polanco and Tatsumi 2016, Gonzalez et al. 2018). High
temperature thermal pretreatment, also known as thermal hydrolysis (TH), has been
commercialized and implemented in full-scale globally as an established WAS
pretreatment process (Barber 2016).
TH increases biogas production basically through solubilization of hardly biodegradable
organic matter, which in turn leads to increase in concentration of organic and inorganic
substances in sludge liquors. In practice, sludge liquors are usually returned back to
headworks of WWTP for retreatment (“return load”). Part of this increased return load
from WAS-TH (NH4
+−N, PO4
3− −P, etc.) can be eliminated in the main treatment line
Chapter 5: Effect of temperature on biogas yield increase and formation of refractory COD during thermal
hydrolysis of waste activated sludge
68
without modification of the process steps, albeit with increasing operational costs. This
part can also be recovered, although installation of recovery units is needed (Mehta et al.
2014). Another part of return load, referred to as dissolved organic part (organic nitrogen,
phosphorus and carbon) cannot be eliminated thoroughly in conventional treatment lines
and poses a risk to impair WWTP effluent quality control parameters (Dwyer et al. 2009,
Dwyer et al. 2008, Gupta et al. 2015).
There is abundant research concerning AD enhancement after TH over a wide range of
temperatures. Many researchers have reported the optimum temperature of TH to be in
the range of 160-180°C regarding maximum biogas yield, after which the anaerobic
biodegradability decreases (Bjerg-Nielsen et al. 2018, Choi et al. 2018, Higgins et al.
2017, Jeong et al. 2019, Svensson et al. 2018). Most commercialized processes also
operate TH in the temperature range of 165-170°C. Nevertheless, it has not yet been
reported to which extent lower temperatures during TH can mitigate the finally remaining
refractory soluble chemical oxygen demand (sCODref). sCODref is a part of the soluble
COD (sCOD), which is resistant to biological anaerobic and aerobic degradation and can
lead to potential deterioration of WWTP effluent quality.
The aim of this study was to assess the effect of decreasing usual temperature of TH on
biogas yield of WAS and on the following aerobic biodegradability of its centrates in a
series of batch tests. Consequently, a ‘conversion factor’ was defined which can be used
to predict the effect of TH temperature on effluent COD increase of a WWTP. Data of
six WWTPs in Berlin and conversion factor were used to show their potential effluent
COD increase after TH implementation.
5.2 Materials and Methods
5.2.1 Characteristics of sludge
Thickened WAS was obtained from a large WWTP in Berlin operated with pre-
denitrification and enhanced biological phosphorus removal (EBPR). WAS was stored at
4°C in refrigerator no longer than two days before the tests. The characteristics of WAS
are summarized in Table 5.1. The required aerobic inoculum (for Zahn-Wellens test) was
collected from the aerated activated sludge tanks and anaerobic inoculum (for biomethane
potential test) from the anaerobic sludge digestion tanks.
Table 5.1 Characteristics of WAS
Parameter
Unit
Value
pH
-
6.45 (0.05)a
Total Solids (TS)
%
7.2 (0.1)
Volatile Solids (VS)
%TS
79.8 (0.1)
Total COD (TCOD)
mg/L
95133 (9265)
Soluble COD (sCOD)
mg/L
1225 (601)
Total Nitrogen (TN)
mg/L
5418 (35)
NH4
+−N
mg/L
112 (8)
Total Phosphorous (TP)
mg/L
2612 (295)
PO4
3− −P
mg/L
380 (150)
aStandard deviations in parentheses
5.2.2 Procedure of the experiments
The procedure of the experiments carried out is shown in Figure 5.1. First, WAS was
thermally hydrolyzed and subsequently anaerobically digested to determine its biogas
yield. In the next step, the digestate was dewatered and the supernatant was used for the
Chapter 5: Effect of temperature on biogas yield increase and formation of refractory COD during thermal
hydrolysis of waste activated sludge
69
Zahn-Wellens test. Each step is described in detail in the following.
Figure 5.1 The schematic flow diagram of experiments. Numbers represent steps of experiments.
Solubilization degrees were calculated after thermal hydrolysis. BMPs were measured during batch
anaerobic digestion tests. Conversion factors were calculated after Zahn-Wellens tests.
5.2.2.1 High temperature thermal hydrolysis
TH was carried out with a lab-scale rig (Cambi Group AS, Norway, see Figure S6). The
rig included a steam generating unit, a reactor and a flash tank. Thickened WAS was
hydrolyzed at desired temperatures and related pressures by injecting vapor into the 1-
litre volume reactor. It was followed by an abrupt pressure relief through an orifice into
a flash chamber (“steam explosion”) to maximize the breakdown of the microbial cell
structures. TH time was fixed on 30 minutes and time was recorded as soon as the reactor
reached the desired temperature. TH temperature was varied from 130°C to 170°C within
10°C intervals (TH130, TH140, TH150, TH160 and TH170). For each temperature, the
whole test series shown in Figure 5.1 were run two times. After each test, the test rig was
cleaned with water for 30 min at 170°C to wash out the remainders of the previous TH of
WAS.
5.2.2.2 Biomethane potential test
An automatic test system (AMPTS II, Bioprocess Control, Sweden) was used to measure
biomethane potential (BMP). Inoculum to substrate ratio (ISR) of 2 based on volatile
solids (VS) content was chosen and kept constant throughout the tests to avoid volatile
fatty acids (VFA) inhibition and to check the kinetics. 500 ml bottles were filled with
inoculum and sludge and flushed with nitrogen gas for one minute to assure anaerobic
conditions. Intermittent automatic mixing (30 sec on/off) was applied to all bottles with
speed adjustment set on 30%. The tests were run under mesophilic temperature of 37°C
(±1) until daily methane production was below 1 percent of total volume of methane
produced (Holliger et al. 2016). The system included intermediate absorption of CO2 and
H2S using a solution of NaOH (3 M), in order that only methane volume is finally
measured and recorded. Activity of anaerobic inoculum was checked through parallel
incubating of microcrystalline cellulose (CAS Number: 9004-34-6). Anaerobic inoculum
was also incubated separately to determine its endogenous methane production. Tests
were always run in triplicate.
Chapter 5: Effect of temperature on biogas yield increase and formation of refractory COD during thermal
hydrolysis of waste activated sludge
70
5.2.2.3 Dewaterability test
Samples of digestate were centrifuged at 25000 g for 30 min (Avanti J-E, Beckman
Coulter). The sludge cakes were separated and used for determination of total solids (TS)
according to standard methods (APHA 1998, DWA 2008). In order to have same samples
for duplicate TS measurements, the cakes were cut in half vertically.
5.2.2.4 Zahn-Wellens test
Anaerobically digested sludge after the BMP tests was filtered with polyester filters and
then centrifuged at 6000 rpm for 10 min, and the supernatant was taken as test substance
(“centrate”). For Zahn-Wellenst test, procedures of OECD Guideline 302 B were
followed (EMPA 1992). Aerobic inoculum was washed three times with tap water to
reduce the amount of inherent sCOD before mixing with the samples. To check the
activity of the inoculum and the validity of the test, biodegradability of ethylene glycol
(CAS Number: 107-21-1) as a reference compound was always monitored and checked
in parallel. Moreover, the aerobic inoculum was aerated separately to determine its
inherent sCODref. The mineral medium was prepared before the start of the test and
included the nutrients described in standard procedure (EMPA 1992).
1-litre volume bottles were used for the test, in which 500 mL of mineral medium was
always mixed with a suitable amount of inoculum and test substance and filled up to 1
liter. The bottles were placed in an incubator set at 22(±1) °C and aerated for 28 days
using two sets of laboratory air pumps. Continuous mixing was provided by magnetic
stirrers. pH value was regularly adjusted to 7.0 (±0.1) using NaOH (3.0 M) and HCl (3.0
M). The loss of water in the bottles due to evaporation was always compensated with
deionized water before taking samples. Tests were run in duplicate and biodegradation
was monitored by sCOD measurement.
5.2.3 Analytics
Total solids (TS) and VS were determined using the guidelines of the standard methods
for the examination of water and wastewater (APHA 1998). pH was measured using a
Hach Lange multimeter HQ40D model (IntelliCal probes). Total COD (TCOD), total
phosphorous (TP) and total nitrogen (TN) were determined after proper dilution of the
sample by photometric measurements using test cuvettes (Hach Lange, DR 5000). To
measure soluble COD (sCOD), PO4
3− −P and NH4
+−N, samples were first centrifuged
at 6000 rpm for 10 min and the supernatant was then filtered through 0.45 µm membrane
filters (CHROMAFIL, Macherey-Nagel). All chemicals used in this study were of
analytical grade.
5.2.4 Calculation methods
5.2.4.1 Solubilization degree
To quantify the effect of TH on release of sCOD, NH4
+−N and PO4
3− −P, solubilization
degrees were defined as following:
𝑆𝐷𝐶𝑂𝐷 =[𝑠𝐶𝑂𝐷]𝑓 − [𝑠𝐶𝑂𝐷]0
𝑇𝐶𝑂𝐷−[𝑠𝐶𝑂𝐷]0×100
5.1
SDCOD (%): solubilization degree of COD
TCOD (mg): total COD
[sCOD]f (mg): fraction of sCOD after TH
[sCOD]0 (mg): fraction of sCOD before TH
Chapter 5: Effect of temperature on biogas yield increase and formation of refractory COD during thermal
hydrolysis of waste activated sludge
71
𝑆𝐷𝑃𝑂4−𝑃 =[𝑃𝑂4
3− −𝑃]𝑓 − [𝑃𝑂4
3− −𝑃]0
𝑇𝑃− [𝑃𝑂4
3− −𝑃]0×100
5.2
SDPO4-P (%): solubilization degree of PO4
3− −P
TP (mg): total phosphorous
[𝑃𝑂4
3− −𝑃]f (mg): fraction of PO4
3− −P after TH
[𝑃𝑂4
3− −𝑃]0 (mg): fraction of PO4
3− −P before TH
𝑆𝐷𝑁𝐻4−𝑁 =[NH4
+−N]𝑓 − [NH4
+−N]0
𝑇𝑁− [NH4
+−N]0×100
5.3
SDNH4-N (%): solubilization degree of 𝑁𝐻4
+−𝑁
TN (mg): total nitrogen
[𝑁𝐻4
+−𝑁]f (mg): fraction of NH4
+−N after TH
[𝑁𝐻4
+−𝑁]0 (mg): fraction of NH4
+−N before TH
5.2.4.2 Biomethane potential and volatile solids reduction
BMP of each sample was determined using the following equation:
𝐵𝑀𝑃𝑠= 𝑉𝑠−𝑚𝑖𝑠
𝑚𝑖𝑏 ×𝑉𝑏
𝑚𝑠𝑠
5.4
BMPs (NmL/g VSadded): biomethane potential of the sample
Vs (mL): volume of methane produced in sample bottle
Vb (mL): volume of methane produced in blank bottle
mis (g): VS of anaerobic inoculum in sample bottle
mib (g): VS of anaerobic inoculum in blank bottle
mss (g): VS of substrate in sample bottle
Volatile solids reduction (VSR) of each sample was calculated using the following
equation:
𝑉𝑆𝑅 =𝑚𝑠0 −𝑚𝑠𝑓
𝑚𝑠0 ×100
5.5
VSR (%): volatile solids reduction
ms0 (g): initial VS of substrate in sample bottle
msf (g): final VS of substrate in sample bottle
Final VS of substrate in sample bottle (msf) was measured using the gas measurement data
(with assumption of 1 g VS=1.42 g COD):
𝑚𝑠𝑓 =𝐵𝑀𝑃𝑠×𝑚𝑠𝑠 (𝑚𝐿 𝐶𝐻4 𝑝𝑟𝑜𝑑𝑢𝑐𝑒𝑑 𝑏𝑦 𝑡ℎ𝑒 𝑠𝑎𝑚𝑝𝑙𝑒)
350 𝑚𝐿 𝐶𝐻4
1 𝑔 𝐶𝑂𝐷𝑟𝑒𝑚𝑜𝑣𝑒𝑑 ×(1.42 𝑔 𝐶𝑂𝐷
1 𝑔 𝑉𝑆 )
5.6
5.2.4.3 Soluble COD elimination degree in Zahn-Wellens test
sCOD elimination degree at time t was calculated with the following equation (EMPA
1992):
Chapter 5: Effect of temperature on biogas yield increase and formation of refractory COD during thermal
hydrolysis of waste activated sludge
72
𝐸𝐷𝑡=[1− 𝑠𝐶𝑂𝐷𝑠𝑡 −𝑠𝐶𝑂𝐷𝑠∗
𝑠𝐶𝑂𝐷𝑏𝑡 −𝑠𝐶𝑂𝐷𝑏∗]×100
5.7
EDt : sCOD elimination degree at time t
sCODst : sCOD in test suspension at time t
sCODs* : sCOD in test suspension after 3 hours
sCODbt : sCOD in blank at time t
sCODb* : sCOD in blank after 3 hours
5.2.4.4 Conversion factor to predict refractory soluble COD
Different sources and sinks of sCODref during experiments in this study are shown in
Figure 5.2.
Figure 5.2 Different sinks and sources (formation or degradation points) of refractory sCOD calculation in
this study.
Ultimately remaining sCOD after Zahn-Wellens test was measured and defined as
sCODref. Afterwards, inherent sCODref fraction of aerobic inoculum (blank) was deduced
from it based on mixing fractions. Due to mixing of WAS with anaerobic inoculum during
BMP tests, anaerobic inoculum was also incubated separately in Zahn-Wellens test to
determine its inherent sCODref. Subsequently, a parameter named “conversion factor”
(CF) was defined which can be used to estimate the amount of remaining sCODref in
centrate after aerobic degradation (i.e. in effluent of an activated sludge tank) based on
the total amount of VS in WAS fed to the anaerobic digestors (see Figure 5.1). This factor
is also useful for comparison of the effect of different TH temperatures on formation of
sCODref and its potential impact on the WWTP effluent quality. First, CF for anaerobic
inoculum was defined as following:
Chapter 5: Effect of temperature on biogas yield increase and formation of refractory COD during thermal
hydrolysis of waste activated sludge
73
𝐶𝐹𝐴𝑛.,𝑖𝑛𝑜. =𝑠𝐶𝑂𝐷𝑟𝑒𝑓
𝑉𝑆𝑖𝑛 ×100
5.8
CFAn.,ino. (%): Conversion Factor for anaerobic inoculum
sCODref (g): fraction of sCOD remaining after Zahn-Wellens test of anaerobic inoculum
VSin (g): fraction of VS of anaerobic inoculum fed into BMP test bottles
After calculating CF for anaerobic inoculum, CF for WAS and thermally hydrolyzed
WAS at different temperatures were calculated using the following equations:
𝐶𝐹𝑖=𝑠𝐶𝑂𝐷𝑟𝑒𝑓,𝑖
𝑉𝑆𝑖𝑛,𝑖 ×100
5.9
𝑠𝐶𝑂𝐷𝑟𝑒𝑓,𝑖 =𝑠𝐶𝑂𝐷𝑟𝑒𝑓,𝑠 −(𝐶𝐹𝐴𝑛,𝑖𝑛𝑜.
100 ×𝑉𝑆𝐴𝑛,𝑖𝑛𝑜.,𝑠)
5.10
CFi (%): Conversion Factor for WAS or thermally hydrolyzed WAS at different
temperatures
sCODref,i (g): fraction of sCOD remaining after Zahn-Wellens test of WAS or thermally
hydrolyzed WAS at different temperatures without anaerobic inoculum
VSin,i (g): fraction of VS of WAS or thermally hydrolyzed WAS at different temperatures
fed into BMP test bottles
sCODref,s (g): fraction of sCOD remaining after Zahn-Wellens test of WAS or thermally
hydrolyzed WAS at different temperatures with anaerobic inoculum
VSAn,ino.,s (g): fraction of VS of anaerobic inoculum fed into BMP test bottles of WAS or
thermally hydrolyzed WAS at different temperatures
VS was chosen instead of COD here, because COD measurements of thickened WAS are
not as accurate and common as of VS in practice.
5.2.4.5 Modelling the effect of thermal hydrolysis on the effluent COD of WWTPs
Residual sCOD in effluent of a WWTP without AD of sludge originates from different
external sources, including also industrial wastewater. AD of sludge is an internal source
of sCODref, which increases COD concentration in the WWTP effluent through the return
load. By implementing TH of WAS before AD, another contributor of sCODref is added
in the process scheme. Influent volume rate of a WWTP equals its effluent volume rate.
Therefore, daily production of WAS in relation to daily wastewater flow of the WWTP
determines the potential increase of effluent sCOD by TH implementation. Using
conversion factors of TH from this study and knowing WAS productions and volume
flow rates of WWTPs, the potential increase in effluent sCOD at these WWTPs can be
predicted for different TH temperatures.
First, WAS production as VS per unit volume of influent is calculated by using the
following equation:
𝑆𝑊𝐴𝑆𝑃 =𝑉𝑆𝑊𝐴𝑆
𝑄×106
5.11
SWASP (g/m³): Specific WAS Production
VSWAS (t/d): daily WAS production as VS
Q (m3/d): daily volume flow rate of WWTP
Then, by having SWASP and CF, sCOD increase in effluent of the WWTPs after
implementing AD or AD preceded by TH of WAS at different temperatures are calculated
Chapter 5: Effect of temperature on biogas yield increase and formation of refractory COD during thermal
hydrolysis of waste activated sludge
74
as following:
𝑠𝐶𝑂𝐷𝐸𝑓𝑓,𝑖𝑛𝑐 =𝐶𝐹𝑖
100×𝑆𝑊𝐴𝑆𝑃
5.12
sCODEff,inc (mg/L): Increase in effluent sCOD
5.3 Results and discussion
5.3.1 Effect of temperature on solubilization of COD, PO4
3− −P and NH4
+−N
pH, TS and VS/TS ratio of WAS and hydrolyzed WAS are shown in Figure 5.3a. With
increasing temperature of TH between 130 and 170°C in 10°C intervals, pH decreases
from 6.10 to 5.98, 5.89, 5.79, 5.50 and 5.35 after TH, respectively. Reduction of pH is
mainly due to release of organic acids from microbial structure (Bougrier et al. 2008,
Jeong et al. 2019, Wilson and Novak 2009, Xue et al. 2015). With increasing temperature
of TH, more vapor needs to be injected into the reactor, which resulted in reduction of TS
from 7.20% to 4.62, 4.69, 3.88, 4.40 and 3.87% for respective temperatures. On the
contrary, VS/TS ratio did not change significantly ranging from 79.62 to 80.32%,
showing almost no loss of organic content during TH at the tested temperatures (Jo et al.
2018).
As shown in Figure 5.3b, SDCOD increased with increasing temperature from 130 to
170°C within 10°C intervals from 20.6 to 23.4, 29.3, 35.5 and 41.5%. This is also in
accordance with results of Jeong et al. (2019) which reported 15-40% increase in SDCOD.
Moreover, Wilson and Novak (2009) showed increase of SDCOD from 8.0% up to 21.4%
for WAS hydrolyzed at 130-200 °C. There is an agreement in literature that with
increasing temperature of TH, SDCOD constantly increases in this temperature range
(Barber 2016, Pilli et al. 2014).
With increase in TH temperature from 130 to 170°C, SDPO4-P similarly increased from
21.9% to 27.7, 31.9, 41.0 and 43.1%, respectively (Figure 5.3c). Laurent et al. (2011) also
reported an increase of orthophosphate from 29 to 50 mg/L with TH at 170°C. Due to the
fact that WAS was taken from a WWTP with EBPR in this study, it was concluded that
solubilized orthophosphate increased during TH through destruction of orthophosphate
bonds of phospholipids in cell membranes. Release of orthophosphate after TH of EBPR-
WAS could be regarded as the first step of a potential solution to the problem of struvite
precipitation in digesters, pipelines and subsequent equipment. When chemically
precipitated and separated after TH and before digestion, it could reduce maintenance
costs related to removing incrustations or lower consumption of the costly anti-scalant
chemicals which are currently applied to hinder precipitation processes in digesters
(Heinzmann and Engel 2006, Kuroda et al. 2002, Liao et al. 2003, Takiguchi et al. 2004).
SDNH4-N was negligible (<1%) for all TH temperatures from 130 to 170°C (Figure 5.3d).
This is in agreement with Xue et al. (2015) who reported no significant increase of NH4
+−
N after TH at temperatures of 120-180°C for 30 min treatment time. However, according
to their study, NH4
+−N drastically increased in higher temperatures (> 160°C), when
combined with longer treatment time (> 30 min). Similarly, Jeong et al. (2019) showed a
very slight increase of NH4
+−N for temperatures of 100 to 160°C followed by a sharp
increase for temperatures above 180°C up to 220°C. Wilson and Novak (2009) also
reported increase of total ammonia nitrogen (TAN) from 100 mg/L up to 250 mg/L for
TH of WAS at 170°C. Similarly, the release percentage was also intensified in their study
by increasing temperature to 190 and 220°C, resulting in increase of TAN to 650 and 900
mg/L, respectively. They clearly showed that in spite of significant solubilization of
Chapter 5: Effect of temperature on biogas yield increase and formation of refractory COD during thermal hydrolysis of waste activated sludge
75
Figure 5.3 Characteristics of WAS and thermally hydrolyzed WAS at different temperatures. Solubilization degrees are defined in section 5.2.4.1 a) TS, VS and VS/TS. b)
sCOD concentrations and solubilization degrees. c) 𝑃𝑂4
3− −𝑃 concentrations and solubilization degrees d) 𝑁𝐻4
+−𝑁 concentrations and solubilization degrees.
(a)
(b)
(c)
(d)
Chapter 5: Effect of temperature on biogas yield increase and formation of refractory COD during thermal
hydrolysis of waste activated sludge
76
proteins in temperature range of 130-170°C, TAN release intensified above 170°C. All
of these results show that ammonification of organic nitrogen in WAS could also occur
non-biologically, although at severe TH conditions (longer TH time or higher
temperature).
5.3.2 Effect of temperature on the biomethane potential
BMP of microcrystalline cellulose was 344±9 NmL/g VSadded, which was consistent with
the values reported in literature (Raposo et al. 2011). This proved the activity of anaerobic
inoculum and validity of the gas measurements.
Specific methane production of WAS and hydrolyzed WAS at temperatures of 130, 150
and 170°C are depicted in Figure 5.4a as illustrative examples of gas production curves.
Kinetically observed, despite a total increase in specific methane production through
pretreatment, the time needed for sludge to reach the ultimate specific methane production
did not change significantly. The percentage of daily specific methane production to the
ultimate specific methane production is presented in Figure 5.4b. As it could be seen, the
rate of methane production increases from the start until the day 2 and then decreases
continually until the end of the test. However, for the TH temperature of 170°C (and to a
lesser intensity for 150°C), there was another increase of methane production rate at the
day 5 until the day 7, after which it decreased again. This effect could be attributed to two
probable reasons: First, higher TH temperatures (>150°C) could cause an increase in
formation of more slowly biodegradable compounds, which need more time to be
hydrolyzed and broken down to favorable substrates. Another reason could be the
increase of VFAs caused by higher release of available substrates at higher TH
temperatures (Jeong et al. 2019, Xue et al. 2015). That could potentially lead to partial
and temporary inhibition of methanogens during the first methane production peak (Pratt
et al. 2012). Consequently, this partial inhibition is mitigated as soon as VFAs are
converted, which is followed by a second peak of methane production rate.
BMP absolute values as well as BMP percentage increases of thermally hydrolyzed WAS
in comparison to WAS without TH at different temperatures are shown in Figure 5.4c.
Overall, reproducibility of absolute BMP results was good between duplicate experiments
for each temperature (variation<10%). While BMP of WAS was 212 NmL/g VSadded,
BMP of thermally hydrolyzed WAS at temperatures of 130 to 170°C increased to 253,
247, 255, 250 and 274 NmL/g VSadded, respectively. Even at the lowest TH temperature
of 130°C, an increase of BMP up to 17.1% was achieved compared to the WAS without
TH. By further increasing the temperature of TH from 140 to 170°C, BMP increased by
17.4, 21.0, 18.9 and 27.1% respectively. Svensson et al. (2018) reported 18% BMP
increase of thermally hydrolyzed digested sludge in temperature range of 153-175°C with
negligible change in the range of 134 to 153°C. Jeong et al. (2019) studied thermal
hydrolysis of WAS with different TS values between 1 to 7% in temperature range of 100
to 220°C and reported maximum biomethane yield at temperature of 180°C. Higgins et
al. (2017) reported 6% increase in BMP of mixed sludge with increases of TH temperature
from 130 to 170°C. They also reviewed several studies which have reported increase of
BMP for WAS or mixed sludge until a threshold in temperature (160-180°C), after which
it declined. This has been reported to be due to reaction of simple sugars with amino acids
and production of Melanoidins through Maillard reactions at higher temperatures (Dwyer
et al. 2008).
VSR increased from 42.8% for WAS without TH to 50.8, 50.7, 51.2, 50.3, and 55.1% for
thermally hydrolyzed WAS at temperatures of 130 to 170°C (Figure 5.4d). Therefore, in
comparison to WAS without TH, VSR increased by 18.7, 18.4, 19.6, 17.5 and 28.6% for
thermally hydrolyzed WAS at respective temperatures.
Chapter 5: Effect of temperature on biogas yield increase and formation of refractory COD during thermal hydrolysis of waste activated sludge
77
Figure 5.4 BMP-tests results of WAS and thermally hydrolyzed WAS at different temperatures. For more clarity only the results of test number 4 are presented in a and b (refer
to Table 5.2). a) Specific methane production during BMP-test period. b) Percentage of daily specific methane production to the ultimate methane production during BMP-test
period. c) Absolute BMP values and BMP percentage increases of thermally hydrolyzed WAS in comparison to not pretreated WAS. d) Absolute VS reductions and VS
reduction percentage increases of thermally hydrolyzed WAS in comparison to not pretreated WAS.
(a)
(b)
(c)
(d)
Chapter 5: Effect of temperature on biogas yield increase and formation of refractory COD during thermal
hydrolysis of waste activated sludge
78
5.3.3 Effect of temperature on dewaterability of digested sludge
Results of dewaterability tests are shown in Figure 5.5. The final TS of WAS after AD
was 14.28%, while with increasing TH temperature from 130 to 170°C, TS in cake
increased constantly from 15.98 to 18.58%, making 12-30% relative increase. Higgins et
al. (2017) also reported constant increase in dewaterability of mixed sludge from 27 to
32% in cake solids, an increase of 18.5% within temperature range of 130-170°C.
Dewaterability plays a decisive role on disposal costs, as it directly deals with amount of
water which is bound to flocs of mechanically dewatered sludge. The most important cost
saving by implementation of TH comes from disposal costs reduction due to
dewaterability improvement.
Figure 5.5 Dewaterability test results of digested WAS and digested thermally hydrolyzed WAS in
temperature range 130-170°C.
5.3.4 Effect of temperature on refractory soluble COD
sCOD elimination percentage during the Zahn-Wellens test for reference substance
(ethylene glycol), centrates of digested WAS and thermally hydrolyzed WAS at
temperatures of 130, 150 and 170°C are shown as illustrative examples in Figure 5.6a.
The reference substance was always degraded (>99%) in less than 7 days which proved
good activity of aerobic inoculum and hence the validity of all tests. For other samples,
curves of sCOD elimination split into two separate phases. During the first 7 days of the
test, sCOD concentration increased as shown by “negative” elimination degrees, which
could probably be attributed to a higher rate of hydrolysis of particulate COD into sCOD
in comparison to the parallel elimination rate of sCOD. Afterwards, sCOD elimination
starts to increase continuously until the end of the test. In this period, sCOD elimination
outpaces hydrolysis of particulate COD (COD of organic substances greater than 0.45
µm) into sCOD. At the end of the aerobic biodegradability test period, sCOD elimination
rate diminished.
As described in section 5.2.4.4, a conversion factor (CF) was defined to relate mass of
sCODref after Zahn-Wellens test to VS input mass into BMP bottles. The mass balance
through BMP and Zahn-Wellens tests based on input and output concentrations, volumes
and fractions is presented in Table 5.2 for further clarification. Moreover, the final CF
average values are shown in Figure 5.6b. Reproducibility of absolute Zahn-Wellens
Chapter 5: Effect of temperature on biogas yield increase and formation of refractory COD during thermal hydrolysis of waste activated sludge
79
Figure 5.6 Zahn-Wellens tests results of WAS and thermally hydrolyzed WAS at different temperatures. For more clarity only the results of test number 4 are presented
in a (refer to Table 5.2). a) sCOD elimination during the test period b) Conversion factor and percentage increases in conversion factor of thermally hydrolyzed WAS in
comparison to not pretreated WAS. c) Conversion factor percentage increase and BMP percentage increase versus different TH temperatures. Data are fitted to
exponential models (refer to Table 5.3). d) Conversion factor percentage increase versus BMP percentage increase.
(a)
(b)
(c)
(d)
Chapter 5: Effect of temperature on biogas yield increase and formation of refractory COD during thermal hydrolysis of waste activated sludge
80
Table 5.2 Mass balance of BMP tests and the following Zahn-Wellens tests
aAnaerobic inoculum
bNot pretreated WAS
TH: Thermally hydrolyzed at designed temperature
BMP tests
Zahn Wellens tests
Test
No.
Substrate
TS
[%]
VS
[%]
Substrate
mass [g]
Substrate
VS [g]
An.
inoculum
mass [g]
An.
inoculum
VS [g]
Number
of
bottles
Mass
of
each
bottle
[g]
Total
mass
[g]
Water
content
[g or
mL]
Refractory
sCOD
concentration
[mg/L]
Fraction
of
refractory
sCOD
[mg]
Fraction of
refractory
sCOD from
An.
inoculum
[mg]
Fraction of
refractory
sCOD from
substrate[mg]
VS input of
substrate[g]
Conversion
factor [%]
1
An. inoculuma
3.53
2.50
0.00
0.00
300.00
7.50
2
300
600
579
1051
608
608
0
15.00
4.06
WASb
6.31
5.14
58.69
3.02
241.31
6.03
4
300
1200
1151
1201
1382
978
404
12.08
3.35
TH130
4.08
3.33
81.88
2.73
218.12
5.45
4
300
1200
1156
1326
1533
884
649
10.92
5.94
TH170
3.16
2.57
98.17
2.52
201.83
5.05
4
300
1200
1159
1751
2030
819
1210
10.08
12.01
2
An.inoculum
3.44
2.46
0.00
0.00
300.00
7.38
2
300
600
579
955
553
553
0
14.76
3.75
WAS
4.68
3.83
72.92
2.79
227.08
5.59
4
300
1200
1155
930
1074
838
236
11.16
2.12
TH140
2.77
2.28
105.13
2.40
194.87
4.79
4
300
1200
1162
995
1156
718
438
9.60
4.56
TH160
2.36
1.93
116.77
2.25
183.23
4.51
4
300
1200
1164
1030
1199
676
522
9.00
5.80
3
An. inoculum
3.68
2.55
0.00
0.00
400.00
10.20
2
400
800
771
920
709
709
0
20.40
3.48
WAS
5.70
4.60
86.81
3.99
313.19
7.99
3
400
1200
1134
1085
1231
833
398
11.97
3.32
TH140
3.65
2.97
120.14
3.57
279.86
7.14
3
400
1200
1141
1005
1147
744
403
10.71
3.76
TH150
3.50
2.84
123.94
3.52
276.06
7.04
3
400
1200
1142
1070
1222
734
488
10.56
4.62
TH160
3.29
2.67
129.28
3.45
270.72
6.90
3
400
1200
1143
1000
1143
719
424
10.35
4.09
4
An. inoculum
4.13
2.88
0.00
0.00
400.00
11.52
2
400
800
767
765
587
587
0
23.04
2.55
WAS
7.00
5.60
81.82
4.58
318.18
9.16
3
400
1200
1125
875
984
700
284
13.74
2.07
TH130
5.33
4.26
101.05
4.30
298.95
8.61
3
400
1200
1129
805
909
658
251
12.90
1.95
TH150
4.78
3.83
109.30
4.19
290.70
8.37
3
400
1200
1131
830
939
639
299
12.57
2.38
TH170
4.61
3.70
112.06
4.15
287.94
8.29
3
400
1200
1132
1075
1217
633
583
12.45
4.69
Chapter 5: Effect of temperature on biogas yield increase and formation of refractory COD during thermal
hydrolysis of waste activated sludge
81
results and related CF was not as good as that of BMP results. A reason for that might be
having filtered samples with remaining particles of different sizes after filtration process
of digested sludge before starting Zahn-Wellens test. In other words, having more
identical filtered samples after dewatering could lead to better reproducibility, since the
Zahn-Wellens test is a well-reproducible test per se (EMPA 1992). One issue to bear in
mind is that samples should at best represent the properties of centrate of dewatering
machines on the WWTP to have the best prediction results. As shown in Figure 5.6b, the
CF increased from 2.6% for WAS to 3.94, 4.26, 3.62, 5.08 and 8.42% for thermally
hydrolyzed WAS at temperatures of 130 to 170°C, respectively. This corresponded to a
51.2, 63.5, 39.1, 95.1 and 223.4% increase in CF for thermally hydrolyzed WAS at
respective temperatures in comparison to WAS without TH. The colors of the samples
before and after BMP tests for different temperatures are displayed in Figure S7.
Intensification of color with TH temperature for samples after BMP tests was obviously
noticeable (Figure S7b).
Moreover, the relative increase of CF and BMP for TH temperatures of 130 to 170°C is
shown in Figure 5.6c. Stepwise increase of BMP and CF with increase in temperature
clearly shows that there is a trade-off between biogas increase and refractory sCOD
increase. Interestingly, by lowering the TH temperature from 170 to 160°C, the CF
increase dropped from 223 to 95%, while BMP increase dropped only from 27 to 19%.
Both parameters were fitted to exponential models. The results of this fitting are presented
in Table 5.3. Percentage increase of CF versus percentage increase of BMP in temperature
range of 130-170°C is shown in Figure 5.6d.
Table 5.3. Curve fitting results of percentage increase in BMP and CF
Function (Y)
Variable (x)
Equation
R2
BMP percentage increase
[%]
Temperature of TH
[°C]
Y=0.148×exp(0.1001𝑥)
0.71
CF percentage increase
[%]
Temperature of TH
[°C]
Y=0.2814×exp(0.3352𝑥)
0.61
There are no quite similar studies, to which the results of this section can be compared.
However, Higgins et al. (2017) reported effluent sCOD of anaerobic digesters fed with
thermally hydrolyzed mixed sludge to be 12.5 to 9.0 g/L in temperature range of 130-
170°C. In another study by Dwyer et al. (2008), sCOD of thermally hydrolyzed mixed
sludge after digestion in TH temperature range of 140-165°C was 7-11 g/L. None of them
carried out an aerobic biodegradability test, similar to what was done in this study.
It should be noted that the inocula used in BMP and Zahn-Wellens tests of this study were
not adapted to thermally hydrolyzed sludge (batch tests). In other words, a potential
positive adaptation effect of the biomass over time could not be simulated in this study.
This aspect needs continuous biological tests over a longer period of time.
5.3.5 Modelling the effect of thermal hydrolysis on effluent COD of six WWTPs
in Berlin
The relationship between different values of SWASP (defined in section 5.2.4.5) and
increase in effluent COD of WWTPs using CFs (calculated in section 5.3.3) is shown in
Figure 5.7a. Equation 5.12 was used to depict these graphs. As shown, increase in effluent
COD of a WWTP is dependent of its SWASP, the implementation of AD or TH+AD, and
the TH temperature.
To make use of these results, actual data from six WWTPs in Berlin was obtained to
Chapter 5: Effect of temperature on biogas yield increase and formation of refractory COD during thermal
hydrolysis of waste activated sludge
82
predict the effect of TH temperature on their effluent COD increase (see Table 5.4).
SWASP for these WWTPs lies in the range of 141-251 g/m³. Effect of TH temperature
on effluent COD increase of these six WWTPs is shown in Figure 5.7b. A higher SWASP
(more WAS production per inflow volume unit) leads to a higher increase in effluent
COD after TH implementation.
Figure 5.7. Results of refractory sCOD increase based on conversion factors and mass balances from this
study. a) Increase in effluent COD of a WWTP based on implementation of AD and TH at different
temperatures depending on its specific WAS production (for definition of SWASP refer to section 5.2.4.5).
b) Predicted increase in effluent COD of six WWTPs in Berlin based on implementation of AD and TH at
different temperatures depending on their specific WAS productions.
However, SWASP and TH temperature are not the only determining factors yet, when
considering implementation of TH. Since each WWTP has its own unique effluent COD
concentration before TH determined mainly by its inflow composition and process,
different final effluent CODs are to be expected with TH. Looking again at Table 5.4 ,
although the order of SWASP follows
WWTP6>WWTP5>WWTP4>WWTP3>WWTP2>WWTP1, the order of predicted final
effluent COD with TH is WWTP4>WWTP3>WWTP2=WWTP6>WWTP5>WWTP1
due to the actual effluent COD before TH. Accordingly, effluent COD of a WWTP before
implementing TH, its SWASP, temperature of TH, and the regional COD discharge limit
should be taken into account when considering suitability of TH for a specific region or
(a)
(b)
Chapter 5: Effect of temperature on biogas yield increase and formation of refractory COD during thermal hydrolysis of waste activated sludge
83
Table 5.4. Characteristics of six WWTPs in Berlin and expected effluent CODs after implementing TH at different temperatures
Parameter
Unit
WWTP1
WWTP2
WWTP3
WWTP4
WWTP5
WWTP6
Known Data
Population
equivalent
Million
0.423
0.245
1.402
1.275
0.266
0.677
Qi,ave
m3/d
63481
36803
210360
191252
39882
101536
WAS as VSa
t/d
8.96
5.40
39.04
37.12
8.08
25.44
SWASP
g/m3
141
147
186
194
203
251
Influent COD
mg/L
975
1379
819
954
1069
886
COD elimination
%
96
97
95
95
96
95
Effluent COD
mg/L
40
47
43
52
42
41
Effluent COD
after AD
mg/L
40
47
48b
52
42
41
Predictions
Effluent COD
after AD + TH130
mg/L
42
49
50
55
45
44
Effluent COD
after AD + TH140
mg/L
42
49
51
55
45
45
Effluent COD
after AD + TH150
mg/L
41
48
50
54
44
44
Effluent COD
after AD + TH160
mg/L
43
51
52
57
47
47
Effluent COD
after AD + TH170
mg/L
48
56
59
63
54
56
Low High
a VS/TS of WAS assumed to be 0.8 for all six WWTPs
b WWTP3 has no anaerobic digestion now
* European Commission COD discharge limit=125 mg/L (EC 1991)
** German national COD discharge limit=75 mg/L (Germany 1996)
*** Regional COD discharge limit=65 mg/L
Chapter 5: Effect of temperature on biogas yield increase and formation of refractory COD during thermal
hydrolysis of waste activated sludge
84
plant. It should be noted that other controlled effluent quality parameters such as TN, TP,
etc. could also be affected by TH, but were not the focus of this study.
To the best of the authors' knowledge, there is no distinctive literature available regarding
the increase of effluent COD after TH implementation for a comparison to the results of
this study. However, a technical report of the German association of water and wastewater
(DWA) on sewage sludge disintegration estimates roughly 5 mg/L or 10% increase in
effluent COD load for TH implementation at usual temperatures (165°C) according to
full-scale experience (DWA 2016). This is within the range of the results of the current
study.
5.4 Conclusions
Lab-scale TH of WAS in temperature range of 130-170°C was carried out. The following
conclusions were drawn:
• Increase in biomethane potential of WAS (17-27%) was accompanied by increase
in refractory sCOD in return load (3.9 to 8.4%) and dewaterability enhancement
(12-30%)
• Reducing TH temperature from 170 to 160°C decreased refractory sCOD by half
while still benefiting from increase in biogas production by 19% and
dewaterability enhancement by 20%
• Reproducibility of Zahn-Wellens results was not as good as that of BMP results,
which may be attributed to filtered samples with different particle sizes
• A conversion factor can be defined und used to predict the impact of TH
temperature on refractory sCOD in sludge liquor
• Increase in effluent COD of a specific WWTP after TH is dependent of its WAS
production, total volume flow rate and TH temperature
• Conversion factor and WWTP data can be used to find an optimum TH
temperature regarding maximum biogas yield while still complying with local
COD discharge limits
References
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Wastewater, Washington DC, USA.
Appels, L., Baeyens, J., Degrève, J. and Dewil, R. (2008) Principles and potential of the
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Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of
waste activated sludge before anaerobic digestion
88
6 Comparative cost-benefit analysis of thermal hydrolysis
and thermal alkaline pretreatment of waste activated
sludge before anaerobic digestion
Highlights
• Digester capacity increases 25-28% by implementing TH while TAP has no effect
• TH of only WAS needs no extra fuel for heating sludge
• TAP and TH both enhance electricity and heat balance of WWTP
• TAP and TH both result in high revenues during plant lifetime
Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of
waste activated sludge before anaerobic digestion
89
Abstract
Through a mass and energy balance a cost-benefit analysis was developed for
implementation of thermal hydrolysis (TH) and thermal alkaline pretreatment (TAP) of
waste activated sludge (WAS) for two wastewater treatment plants (WWTPs) of
Waßmannsdorf in Berlin and Rostock. TAP and TH increase biogas production by 11-
22% and 15-30% for Waßmannsdorf, respectively. For Rostock, biogas increases 7-15%
and 10-20% after TAP and TH. TAP and TH enhance total electricity balance by 13-28%
and 10-30% for Waßmannsdorf and 13-23% and 5-20% for Rostock. TAP and TH
improve total heat balance of Waßmannsdorf by 11-22% and 5-29% and of Rostock by
55-81% and 62-97%, respectively. Polymer costs are decreased by 7-14% after TAP and
increased by 52-144% after TH for Waßmannsdorf. For Rostock, polymer costs decrease
by 5-10% after TAP and increase by 54-153% after TH. Disposal costs are decreased by
8-16% and 23-44% after TAP and TH for Waßmannsdorf, respectively. For Rostock,
disposal costs decrease by 5-11% and 21-37% after TAP and TH. Operating costs of
sludge treatment line decrease by 23-62% and 28-81% after TAP and TH for
Waßmannsdorf. For Rostock, TAP and TH result in 14-32% and 21-48% decrease in
operating costs of sludge treatment line. Due to significantly higher capital costs, TH had
longer return of investment time. Total revenue which indicates net profits of process
during its lifetime is 10.1-30.4 and 4.9-32.8 million euros after TAP and TH for
Waßmannsdorf. For Rostock, total revenue is 1.6-4.8 and 0.7-5.5 million euros for TAP
and TH. Digestion capacity increases 25% and 28% after implementing TH for
Waßmannsdorf and Rostock, while TAP has no effect on digester volume. TAP increases
effluent COD by 1 mg/L while TH increases effluent COD by 11 and 15 mg/L for
Waßmannsdorf and Rostock, respectively. Due to the current high effluent COD of
WWTP Waßmannsdorf, TAP is a safer process to prevent environmental fines. For
WWTP Rostock, TH can also be implemented due to lower current effluent COD and
higher local limits. Economically seen, TAP and TH are both profitable and other key
factors such as refractory COD formation, digestion capacity increase or simplicity of
process can be considered by decision making.
Keywords
Cost-benefit analysis; Pondus; Cambi; Berlin; Rostock; Waste activated sludge
6.1 Introduction
Thermal alkaline pretreatment (TAP) and thermal hydrolysis (TH) can both lead to
reduction of sludge disposal costs due to increase of organic matter reduction in anaerobic
digestion (AD) (Toutian et al., 2020b; Toutian et al., 2020a). Thermal hydrolysis has
shown to improve dewatering potential of digestate which also results in significant
disposal costs reduction (Barber, 2016). Utilization of biogas in a combined heat and
power (CHP) production unit leads to increase of electricity and heat production which
improves economics of wastewater treatment plant (WWTP). However, due to increase
of polymer consumption, return load treatment costs and pretreatment process costs
(energy, chemicals, etc.) profitability of pretreatment should be evaluated thoroughly
through a cost-benefit analysis. A comprehensive and thorough cost-benefit analysis
needs case specific and detailed local data. Moreover, refractory COD formation from
pretreatment which leads to increase of effluent COD can also be a prohibitive factor for
some WWTPs.
Aim of this chapter is to present results of a cost-benefit analysis for WWTP
Waßmannsdorf in Berlin and WWTP Rostock to compare economic viability of TH and
Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of
waste activated sludge before anaerobic digestion
90
TAP for each WWTP. This analysis shows through different scenarios which parts of
sludge treatment line are affected through pretreatment and if they lead to reduction or
increase of costs. Furthermore, profitability of each process in terms of return of
investment time and total net revenue for each of the two WWTPs are discussed. Finally,
refractory COD increase in effluent of WWTPs are presented and discussed.
6.2 Calculation method
A comprehensive mass and energy balance was developed for sludge treatment line of
the two WWTPs in this study. Following, based on this mass and energy balance a cost-
benefit analysis was performed to evaluate economic viability of each pretreatment
technique.
6.2.1 Characteristics of WWTPs
Characteristics of WWTP Waßmannsdorf and Rostock are presented in Table 6.1. Data
were provided by WWTPs operators.
Table 6.1. Characteristics of WWTP Waßmannsdorf and Rostock
Parameter
Unit
Waßmannsdorf
Rostock
WWTP
P.E.a
1840000
400000
Total Q
m3 per d
230000
45700
WAS TSb
t per d
69.6
9.3
WAS TSSc
g per L
6.0
3.7
WAS VS/TS
%
80%
81%
WAS VS
t per d
55.7
7.5
WAS flow
m3 per d
11600
2500
PS TS
t per d
50.0
14.8
PS TSS
g per L
50.0
40.0
PS VS/TS
%
85%
83%
PS VS
t per d
42.5
12.2
PS flow
m3 per d
1000
370
WAS or PS temperature
°C
13
13
aPopulation Equivalent, bTotal solids, cTotal suspended solids
The main difference between the two WWTPs is in volumetric flow rate of wastewater
(i.e. treatment capacity or plant size). Waßmannsdorf receives almost five times as much
daily wastewater flow as Rostock. The most important difference between these WWTPs
is in ratio of volatile solids of PS and WAS. Waßmannsdorf has a PS/WAS volatile solids
ratio of 0.76, while this ratio for Rostock is 1.63. In other words, Waßmannsdorf produces
more organic solids from WAS than PS, whereas in Rostock, organic solids from PS are
more than that of WAS. This affects cost-benefit analysis of pretreatments which was
considered in this study.
6.2.2 Definition of scenarios
Overall, eight scenarios with pretreatment and one basis scenario without pretreatment
were modelled for each WWTP. Characteristics of these scenarios are presented in Table
6.2. Explanations regarding selection of related values are given in following sections.
Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of
waste activated sludge before anaerobic digestion
91
Table 6.2. Characteristics of scenarios defined for cost-benefit analysis of each WWTP
No.
Scenario
Description
Biogas
increase
of WAS
Dewatering
potential
increase
Polymer
consumption
increase
1
AD
WWTP with AD
-
-
-
2
TAPmin+AD
WWTP with TAP and
AD
+20%
0%TS
0%
3
TAPmax+AD
WWTP with TAP and
AD
+40%
0%TS
0%
4
THmin+
AD
(TS+4%)
WWTP with TH and
AD
+27%
+4%TS
+66% Waß.
+73% Ros.
5
THmax+
AD
(TS+4%)
WWTP with TH and
AD
+54%
+4%TS
+66% Waß.
+73% Ros.
6
THmin+
AD
(TS+6%)
WWTP with TH and
AD
+27%
+6%TS
+109% Waß.
+121% Ros.
7
THmax+
AD
(TS+6%)
WWTP with TH and
AD
+54%
+6%TS
+109% Waß.
+121% Ros.
8
THmin+
AD
(TS+8%)
WWTP with TH and
AD
+27%
+8%TS
+158% Waß.
+178% Ros.
9
THmax+
AD
(TS+8%)
WWTP with TH and
AD
+54%
+8%TS
+158% Waß.
+178% Ros.
6.2.2.1 Biogas increase or volatile solids reduction
Digestion of TAP-processed WAS and PS in pilot-scale showed 20% increase in biogas
production (Toutian et al., 2020a). Assuming there is no synergistic effect from co-
digestion of PS, biogas increase from pretreatment of WAS should be 40% (due to same
organic solids share: 50% PS and 50% TAP-processed WAS). On the other hand, before
long-term pilot trials of TAP with fixed process parameters, lab-scale study of TAP was
also performed. In these trials, effects of process parameters of NaOH dosage and
temperature on biogas production and refractory COD in sludge liquor were investigated.
Extensive results of these trials are presented in bachelor thesis of (Gerundt, 2018)).
Concisely, biochemical methane potential (BMP) test of TAP-processed WAS in lab-
scale showed averagely 20% increase in biogas production (see Figure S8).
Conclusively, pilot trials showed 40% increase in biogas of WAS while lab trials showed
20%. This difference could be due to adaptation of microbiome via long-term continuous
process of AD in pilot-scale trials. Therefore, to account for lab-scale data (to be on the
safe side) and to present a range for biogas increase (min/max value) instead of only one
value, two scenarios of TAPmin+AD (20% increase in biogas of WAS) and TAPmax+AD
(40% increase in biogas of WAS) were defined.
On the other hand, BMP test of TH-processed WAS in lab-scale showed 27% increase in
biogas production (Toutian et al., 2020b). It was assumed that digestion of TH-processed
WAS and PS in pilot-scale shows double as much biogas production from WAS, as it
happened with TAP trials. Therefore, two scenarios of THmin (27% increase in biogas of
Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of
waste activated sludge before anaerobic digestion
92
WAS) and THmax (54% increase in biogas of WAS) were defined, as it was done for TAP.
To compare the above-mentioned scenarios, a basis scenario without pretreatment was
also defined (only AD).
6.2.2.2 Dewatering potential and polymer consumption
TAP had shown no increase in TS of centrifuged sludge (Toutian et al., 2020a). Therefore,
it was assumed that dewatering potential does not change after TAP in this analysis. On
the contrary, TH has shown to increase TS of centrifuged sludge up to 4 percent (Toutian
et al., 2020b). Thus, for TH increase of 4% in absolute TS value of dewatered sludge was
predicted in this model. Moreover, to account for other data from literature higher
increase of TS of dewatered sludge after TH (+6% and +8%) were also defined, for
comparison purposes.
Polymer increase in dewatering for TAP was assumed to be zero due to no improvement
in dewatering. On the contrary, polymer consumption for TH was extrapolated from
(Barber, 2020) and was determined using a function which was dependent of initial
dewatering potential of WAS without pretreatment and final dewatering potential after
pretreatment with TH (see Figure S9 and Figure S10).
6.2.2.3 Nitrification of sludge liquor
For liquor treatment, increase of nitrogen and its removal was considered. According to
data from WWTP Waßmannsdorf, released ammonium nitrogen in digested sludge liquor
forms nearly 5% of its organic solids content. Therefore, it was assumed that 50 g N per
1 kg organic solids removed in AD is released into sludge liquor. Another assumption
was that all nitrified N is denitrified without adding external carbon source.
6.2.3 Mass and energy balance parameters assumptions
Factors for mass and energy balance are presented in Table 6.3.
Table 6.3. Factors of mass and energy balance used in modeling
Thickening
Unit
TAP
TH
Polymer consumption
kg AS per t TS
1a
4
Electricity consumption
kWh per m3
0.7
1.5
WAS TS loss to centrate
%
4
4
TAP
NaOH (50%) dosage
L per m3 WAS
2
-
Electricity consumption
kWh per m3
1
-
WAS temperature
°C
65
-
TH
Steam needed
kg per t TS
-
889
Steam pressure
bar
-
10
Steam temp
°C
-
184
Enthalpy of saturated steam
MJ per t
-
2781
Enthalpy of feed water (at 15°C)
MJ per t
-
63
Electricity consumption
kWh per m3
-
2
WAS temperature
°C
-
95
AD
COD/VS
t per t
1.47
1.47
Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of
waste activated sludge before anaerobic digestion
93
Methane content
%
60
60
Pumping and mixing consumption
unit
kWh per m3
4
4
CHP
Electrical efficiency
%
36.0b
36.0b
Thermal efficiency
%
49.0c
49.0c
High grade heat share
%
63
63
Dewatering
Electricity consumption
kWh per m3
2.5
2.5
TS loss to centrate
%
1
1
Liquor nitrification
Nitrogen released in sludge liquor
kg N per kg VS reduced
0.05
0.05
O2 consumption for Nitrification
kg O2 per kg N
4.3
4.3
Electricity needed for O2 production
kWh per kg O2
0.45
0.45
afor Rostock 1.9, bfor Rostock 37.5%, cfor Rostock 47.5%
6.2.4 Cost-benefit analysis parameters assumptions
Cost factors used throughout this study are presented in Table 6.4. Plant installation cost
of TAP was obtained from Pondus Verfahrenstechnik GmbH and for TH installation,
costs were estimated via personal communications. All other required data for the models
were obtained either from literature or personal communications.
Table 6.4. Cost factors used in cost-benefit analysis
Parameter
Unit
Value
NaOH (50%)
€ per t
320
Polymer
€ per kg ASa
3.00 for Waß., 2.39 for Ros.
Sludge disposal
€ per t
80 for Waß., 100 for Ros.
Credit of electricity production
€ per kWh
0.2
Personnel
€ per h
35
Personnel needed
h per t TS per a
9.7 for TAP, 38.9 for TH
Interest rate
% per a
5.2
Plant lifetime
year
20
Capital recovery factor (in German:
KFAKR)
-
0.0816
Plant maintenance costs percentage
% of CCb per a
3
Inflation rate
% per a
2.5
Discount factor for progressive
annual increase of costs (in
German: DFAKRP)
-
15.3934
aActive substance, bCapital costs
Heat is produced in excess amount and has no effect on costs. This excess amount does
not contribute to revenue, as it cannot be sold and must be used in internal process units.
On the other hand, produced electricity can be substituted for the electricity bought from
grid for entire WWTPs consumption, thus reducing electricity bills.
Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of
waste activated sludge before anaerobic digestion
94
6.3 Results and discussion
6.3.1 Mass and energy balance of sludge treatment line in Waßmannsdorf
Mass balance of sludge treatment line with and without pretreatment is presented in Table
6.5. It should be noted that to avoid repetition, TH scenarios with higher TS (+6% and
+8%) are not presented and discussed here. Mass balance of these scenarios are like
scenario of TS +4%. The only differences between these scenarios are in cake total weight
and sludge liquor volume which will be discussed in following. Due to higher TS of WAS
required for TH in comparison to TAP (16.5% vs 6.5%), volume of WAS is 62% less
than for TAP or in basis scenario. This significantly reduces pretreatment plant size for
TH in comparison to TAP. However, TH and TAP have different process conditions
(retention time, temp, etc) and effect of this volume reduction cannot be directly
compared. Nevertheless, this volume reduction affects capital and operating costs of the
two pretreatment techniques, which has been discussed in following cost-benefit analysis
section.
After TH, volume of WAS is 56% less than in TAP or basis scenario since steam has been
added to WAS for heating. This steam reduces TS of sludge from 16.5% to 15.3% while
WAS in effluent of TAP has same TS of 6.5% due to heating via heat exchanger. After
mixing PS and WAS before AD, TS of sludge is 5.8% for TAP and basis scenario and
8.1% for TH. Usual digesters function well with TS of untreated mixed sludge around 5-
6%. As higher TS reduces viscosity of untreated sludge, TS more than this range cannot
be practiced in AD, due to impaired mixing and decreased mass and heat transfer
efficiency. On the contrary, TH significantly reduces viscosity of sludge (Barber, 2016).
This enhances mass and heat transfer efficiency in AD and makes AD of sludge with TS
up to 10% possible without inhibition. Therefore, AD of sludge with 8.1% TS is feasible
for TH-pretreated sludge in Waßmannsdorf.
Before AD, sludge in TH scenario has 28% less volume than TAP or basis scenario. This
means, TH results in 28% reduction of digester volume. This is an economical benefit for
WWTPs which have had increased capacity of wastewater in recent years and now must
treat more sludge. Moreover, this capacity can be allocated to other biomass or organic
waste sources, leading to profits and improvement of digestion through co-digestion.
Digesters occupy large spaces and many WWTPs around the world, such as in the
Netherlands, have limitations in this regard. Moreover, since costs of building a new
digester are significantly high, this increase of digester capacity due to TH is one of key
deciding factors or incentives for such WWTPs.
Table 6.5. Mass balance of sludge treatment line for WWTP Waßmannsdorf
Parameter
Unit
AD
TAPmin+AD
TAPmax+AD
THmin+AD
(TS+4%)
THmax+AD
(TS+4%)
Before thickening of WAS
WAS TS
%
0.6%
0.6%
0.6%
0.6%
0.6%
WAS TS
t per d
69.6
69.6
69.6
69.6
69.6
WAS VS
t per d
55.7
55.7
55.7
55.7
55.7
WAS total
volume
m3 per
d
11600
11600
11600
11600
11600
PS TS
%
5.0%
5.0%
5.0%
5.0%
5.0%
PS TS
t per d
50.0
50.0
50.0
50.0
50.0
PS VS
t per d
42.5
42.5
42.5
42.5
42.5
PS total
volume
m3 per
d
1000
1000
1000
1000
1000
Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of
waste activated sludge before anaerobic digestion
95
After thickening of
WAS
WAS TS
%
6.5%
6.5%
6.5%
16.5%
16.5%
WAS TS
loss to
centrate
%
4%
4%
4%
4%
4%
WAS TS
t per d
66.8
66.8
66.8
66.8
66.8
WAS VS
t per d
53.5
53.5
53.5
53.5
53.5
WAS total
volume
m3 per
d
1001
1001
1001
379
379
After TAP or
TH of WAS
WAS TS
%
-
6.5%
6.5%
15.3%
15.3%
WAS TS
t per d
-
66.8
66.8
66.8
66.8
WAS VS
t per d
-
53.5
53.5
53.5
53.5
WAS total
volume
m3 per
d
-
1001
1001
438
438
After mixing WAS
with PS
TS
%
5.8%
5.8%
5.8%
8.1%
8.1%
TS
t per d
116.8
116.8
116.8
116.8
116.8
VS
%TS
82.1%
82.1%
82.1%
82.1%
82.1%
VS
t per d
96.0
96.0
96.0
96.0
96.0
TS-VS
t per d
20.9
20.9
20.9
20.9
20.9
Sludge total
volume
m3 per
d
2001
2001
2001
1438
1438
AD
Increase in
VSR of
WAS due to
pretreatment
%
-
20.0%
40.0%
27.0%
54.0%
VS
reduction of
WAS
%
-
60.0%
70.0%
63.5%
77.0%
VS of WAS
removed
t per d
-
32.1
37.4
33.9
41.2
VS
reduction of
PS
%
-
50.0%
50.0%
50.0%
50.0%
VS of PS
removed
t per d
-
21.3
21.3
21.3
21.3
VS removed
total
t per d
48.0
53.3
58.7
55.2
62.4
VS
reduction
total
%
50.0%
55.6%
61.1%
57.5%
65.0%
After AD
TS
%
3.4%
3.2%
2.9%
4.3%
3.8%
TS
t per d
68.8
63.5
58.1
61.6
54.4
VS
%TS
69.7%
67.1%
64.1%
66.1%
61.7%
VS
t per d
48.0
42.6
37.3
40.8
33.5
TS-VS
t per d
20.9
20.9
20.9
20.9
20.9
Sludge total
volume
t per d
2001
2001
2001
1438
1438
After
dewatering
TS
%
26.5%
26.5%
26.5%
30.5%
30.5%
TS loss to
centrate
%
1%
1%
1%
1%
1%
TS
t per d
68.2
62.9
57.6
61.0
53.9
VS
%TS
69.7%
67.1%
64.1%
66.1%
61.7%
VS
t per d
47.5
42.2
36.9
40.4
33.2
Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of
waste activated sludge before anaerobic digestion
96
Cake total
weight
t per d
243.5
224.6
205.7
187.8
165.8
Liquor
nitrification
Liquor total
volume
m3 per
d
1771
1789
1807
1262
1282
Nitrogen
released in
sludge liquor
kg N
per d
2399
2666
2933
2760
3120
After AD, total VS reduction increases from 50% for basis scenario to 56-61% for TAP
and 58-65% for TH. TS of dewatered sludge was 26.5% for basis scenario and TAP and
30.5%, 32.5% and 34.5% for TH, as was set in model. This increase in dewatering
potential is a benefit of TH process. Cake weight and liquor volume of different scenarios
with and without pretreatment is shown in Figure 6.1. TAP and TH resulted in 8-16% and
23-40% reduction of cake weight, respectively. Higher reduction of sludge cake from TH
is due to enhancement of dewatering potential after TH which lacks in TAP. Sludge liquor
volume reduces 26-29% after TH and increases 1-2% after TAP. Reduction of liquor
volume is due to high pre-dewatering of WAS before TH.
Figure 6.1. Cake weight and liquor volume of different scenarios with and without pretreatment for
WWTP Waßmannsdorf
Biogas production and biogas percentage increase of different scenarios with and without
pretreatment is shown in Figure 6.2. TAP increases total biogas production of
Waßmannsdorf up to 11-22%, while TH results in 15-30% increase in total biogas. This
difference is due to higher efficiency of TH in increasing biogas yield of WAS. Biogas
production of TH scenarios with TS+6% and TS+8% is same as TH scenario with
TS+4%.
Electricity consumption or production in different process units of sludge treatment line
with and without pretreatment is presented in Figure 6.3. Due to higher pre-dewatering
of WAS before TH (16.5% TS), electricity consumption in thickening increases 114%
for TH. TAP has same electricity consumption in thickening as in basis scenario since
both need same TS of WAS (6.5%). For pretreatment plant, TAP consumes 1001 kWh
per d, while TH consumes 757 kWh per d. Lower electricity consumption by TH is due
to less volumetric flow of WAS to pretreatment plant (almost one third, see Table 6.5).
Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of
waste activated sludge before anaerobic digestion
97
Figure 6.2. Biogas production biogas percentage increase of different scenarios with and without
pretreatment for WWTP Waßmannsdorf (TH scenarios with TS+6% and TS+8% are same as TS+4%)
Figure 6.3. Electricity consumption or production in different process units of sludge treatment line with
and without pretreatment for WWTP Waßmannsdorf (TH scenarios with TS+6% and TS+8% are same as
TS+4%)
Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of
waste activated sludge before anaerobic digestion
98
Due to same reason, electricity consumption in AD and dewatering unit for TH is 28%
less than basis scenario and TAP has no difference to basis scenario. In CHP, electricity
production increases 11-22% for TAP and 15-30% for TH. Electricity consumption for
nitrification of liquor increases 11-22% for TAP and 15-30% for TH, which matches
increase in organic solids reduction of AD after pretreatments. Finally, TAP results in 13-
28% increase of electricity production in sludge treatment line, while TH increases
electricity production by 10-30%. Therefore, both TAP and TH have nearly same effect
in reduction of electricity costs via more production of biogas. Electricity consumption
or production of TH scenarios with TS+6% and TS+8% is same as TH scenario with
TS+4%.
Figure 6.4. Heat consumption or production in different process units of sludge treatment line with and
without pretreatment for WWTP Waßmannsdorf. LGH: Low grade heat, HGH: High grade heat (TH
scenarios with TS+6% and TS+8% are same as TS+4%)
Heat consumption or production in different process units of sludge treatment line with
and without pretreatment is shown in Figure 6.4. Heat needed for heat exchanger in basis
scenario and TAP were assumed to be provided from low grade heat of the CHP.
Conversely, heat needed for TH was assumed to be provided by high grade heat from
CHP. Regardless of heating type, TAP and TH need 8.4% and 6.7% more heat than basis
scenario, respectively. Slightly more theoretical heat consumption of TAP despite its
lower process temperature is due to two times more WAS volume that it must heat up in
comparison to TH. However, as mentioned source of heating for TAP and TH are
practically different. TAP and TH both result in 11-22% and 15-30% increase in high-
and low-grade heat production in CHP. High grade heat balance for TAP is 11-22%
higher than that of basis scenario. On the contrary, TH shows 42-67% decrease in high
grade heat balance. It is to be noted, that despite this decrease, high grade heat balance is
Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of
waste activated sludge before anaerobic digestion
99
still positive. This means even after implanting TH high grade heat is available in much
extra amount on WWTP. Low grade heat balance for basis scenario is negative which
shows part of high-grade heat is needed for heating sludge before AD. TAP has a better
low grade heat balance than basis scenario and needs 3-50% less low-grade heat.
However, TAP has also a negative low grade heat balance and needs part of high-grade
heat for heating up sludge. TH showed a positive low grade heat balance and needs 587-
650% less than basis scenario. Total heat balance of sludge treatment line for
Waßmannsdorf shows that TAP and TH increase heat production up to 11-22% and 5-
29%, respectively. In conclusion, TAP and TH need no extra fuel and improve total heat
balance of WWTP. Furthermore, TH reduces amount of high-grade heat and increases
amount of low-grade heat, while TAP increases both amounts of high- and low-grade heat
on Waßmannsdorf in comparison to basis scenario. Heat consumption or production of
TH scenarios with TS+6% and TS+8% is same as TH scenario with TS+4%.
6.3.2 Cost-benefit analysis of Waßmannsdorf
Costs and savings of sludge treatment line with and without pretreatment for WWTP
Waßmannsdorf are shown in Figure 6.5. Polymer costs decrease 7-14% for TAP and
increase 52-144% for TH. Decrease of polymer costs for TAP is due to enhanced
reduction of organic solids in AD after implementing TAP. Increase of polymer costs for
TH is due to increase of polymer consumption in thickening of WAS before TH and
dewatering after AD. TAP and TH result in 8-16% and 23-40% decrease in disposal costs,
respectively. Plant installation and maintenance costs, personnel and NaOH costs are also
shown in Figure 6.5. Total operating costs of sludge treatment line was reduced 23-62%
after TAP and 28-81% after TH. Savings due to pretreatment are 0.8-2.1 million euros
p.a. for TAP and 1.0-2.8 million euros p.a. for TH. Due to different capital costs of TH
and TAP, two parameters of return of investment time and total revenue must be
discussed, too. Comparing three scenarios of TH with different TS of dewatered cake
shows that savings due to reduction of disposal costs are offset with increase of polymer
costs. Therefore, improvement degree of dewatering potential (%TS) through TH plays
no role in decreasing operating costs in Waßmannsdorf and all scenarios show same final
operating costs.
Figure 6.6 shows return of investment duration and total revenue during plant lifetime
after implementing pretreatments. TAP has return of investment time of 1.0-2.7 years and
yields total revenue of 10.1-30.4 million euros. On the other hand, TH shows return of
investment time of 3.6-10.4 years and yields total revenue of 4.9-32.8 million euros.
Again, it is noteworthy to mention that TH scenarios with TS+6% and TS+8% dewatered
cake show nearly same return of investment time and total revenue results as those of TH
with TS+4%.
In conclusion, TAP and TH both show monetary profits during their plant lifetime. With
lower increase in biogas potential (min scenarios), profitability of TAP is significantly
more than TH, while with higher increase of biogas potential (max scenarios), TH slightly
excels. Besides, capital cost of TH is nearly 5 times as much as that of TAP and can play
a key role when initial budget is limited. Moreover, return of investment time is averagely
3.7 times that of TAP. On the other hand, for higher plant lifetimes (>20 years) TH leads
to more total revenue due to its higher annual savings. Overall, considering all factors in
this cost-benefit analysis, TAP and TH are both profitable for WWTP Waßmannsdorf.
Final decision can be made considering their different process conditions, side benefits
and most importantly formation of refractory COD which will be discussed in section
6.3.5.
Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of waste activated sludge before anaerobic digestion
100
Figure 6.5. Costs and savings of sludge treatment line with and without pretreatment for WWTP Waßmannsdorf
Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of
waste activated sludge before anaerobic digestion
101
Figure 6.6. Return of investment duration, total revenue during plant lifetime and pretreatment plant
installation costs for WWTP Waßmannsdorf
6.3.3 Mass and energy balance of sludge treatment line in Rostock
Mass balance of sludge treatment line with and without pretreatment for WWTP Rostock
is presented in Table 6.6. Like for Waßmannsdorf, TH scenarios with higher TS (+6%
and +8%) are not presented and discussed here. Mass balance of these scenarios are like
scenario of TS +4%. The only differences between these scenarios are in cake total weight
and sludge liquor volume which will be discussed in following.
As mentioned earlier, a significant difference of WWTP Rostock and Waßmannsdorf is
in VS ratio of PS/WAS. In Rostock VS from WAS is significantly lower than PS. WAS
in Rostock is thickened up to TS of 4.3% via floatation process, while in Waßmannsdorf
WAS is thickened up 6.5% via centrifuges. After thickening of WAS to required TS, TAP
and TH show 34% and 75% less WAS volume for pretreatment than basis scenario,
respectively. After mixing PS and WAS before AD, TS of sludge is 4.1%, 4,7 and 5.5%
for basis scenario, TAP and TH, respectively. In this range of TS digester functions
without any problems regarding mass or heat transfer. In comparison, Waßmannsdorf had
TS of 5.8% and 8.1% for TAP and TH, respectively.
Similar to reduction of digester volume after TH in Waßmannsdorf, Rostock also benefits
from 25% less digester volume after implementing TH. On the contrary, 12% increase in
digester capacity after TAP for Rostock in comparison to 0% for Waßmannsdorf is to be
expected. This is due to improvement of TS of thickened WAS after replacing floatation
system with centrifuges or improving TS of WAS up to 6.5% with current flotation. This
capacity increase can be economically beneficial by allocating AD capacity to external
co-substrates or avoiding building new digesters. After AD, total VS reduction increases
from 51% for basis scenario to 55-59% for TAP and 56-61% for TH. TS of dewatered
sludge increased from 24.5% for basis scenario and TAP up to 28.5%, 30.5% and 32.5%
for TH, as it was set in modeling data.
Cake weight and liquor volume of different scenarios with and without pretreatment is
shown in Figure 6.7. TAP and TH resulted in 5-11% and 21-37% reduction of cake
weight, respectively. Sludge liquor volume reduces 24-26% after TH and 12-13% after
TAP. For comparison, Waßmannsdorf showed no reduction of liquor volume after TAP.
This is due to improvement of TS of WAS before TAP in Rostock, as discussed above.
Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of
waste activated sludge before anaerobic digestion
102
Table 6.6. Mass balance of sludge treatment line for WWTP Rostock
Parameter
Unit
AD
TAPmin+AD
TAPmax+AD
THmin+AD
(TS+4%)
THmax+AD
(TS+4%)
Before thickening of WAS
WAS TS
%
0.4%
0.4%
0.4%
0.4%
0.4%
WAS TS
t per d
9.25
9.25
9.25
9.25
9.25
WAS VS
t per d
7.5
7.5
7.5
7.5
7.5
WAS total
volume
m3 per
d
2500
2500
2500
2500
2500
PS TS
%
4.0%
4.0%
4.0%
4.0%
4.0%
PS TS
t per d
14.8
14.8
14.8
14.8
14.8
PS VS
t per d
12.21
12.21
12.21
12.21
12.21
PS total
volume
m3 per
d
370
370
370
370
370
After thickening of
WAS
WAS TS
%
4.3%
6.5%
6.5%
16.5%
16.5%
WAS TS
loss to
centrate
%
4%
4%
4%
4%
4%
WAS TS
t per d
8.9
8.9
8.9
8.9
8.9
WAS VS
t per d
7.2
7.2
7.2
7.2
7.2
WAS total
volume
m3 per
d
203
133
133
50
50
After TAP or
TH of WAS
WAS TS
%
-
6.5%
6.5%
15.3%
15.3%
WAS TS
t per d
-
8.9
8.9
8.9
8.9
WAS VS
t per d
-
7.2
7.2
7.2
7.2
WAS total
volume
m3 per
d
-
133
133
58
58
After mixing WAS
with PS
TS
%
4.1%
4.7%
4.7%
5.5%
5.5%
TS
t per d
23.7
23.7
23.7
23.7
23.7
VS
%TS
82.0%
82.0%
82.0%
82.0%
82.0%
VS
t per d
19.4
19.4
19.4
19.4
19.4
TS-VS
t per d
4.3
4.3
4.3
4.3
4.3
Sludge total
volume
m3 per
d
573
503
503
428
428
AD
Increase in
VSR of
WAS due to
pretreatment
%
-
20.0%
40.0%
27.0%
54.0%
VS
reduction of
WAS
%
-
61.2%
71.4%
64.8%
78.5%
VS of WAS
removed
t per d
-
4.4
5.1
4.7
5.7
VS
reduction of
PS
%
-
51.0%
51.0%
51.0%
51.0%
VS of PS
removed
t per d
-
6.2
6.2
6.2
6.2
VS removed
total
t per d
9.9
10.6
11.4
10.9
11.9
VS
reduction
%
51.0%
54.8%
58.6%
56.1%
61.2%
Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of
waste activated sludge before anaerobic digestion
103
total
After AD
TS
%
2.4%
2.6%
2.4%
3.0%
2.8%
TS
t per d
13.8
13.0
12.3
12.8
11.8
VS
%TS
69.0%
67.3%
65.3%
66.6%
63.8%
VS
t per d
9.5
8.8
8.0
8.5
7.5
TS-VS
t per d
4.3
4.3
4.3
4.3
4.3
Sludge total
volume
t per d
573
503
503
428
428
After dewatering
TS
%
24.5%
24.5%
24.5%
28.5%
28.5%
TS loss to
centrate
%
1%
1%
1%
1%
1%
TS
t per d
13.6
12.9
12.2
12.7
11.7
VS
%TS
69.0%
67.3%
65.3%
66.6%
63.8%
VS
t per d
9.4
8.7
8.0
8.4
7.5
Cake total
weight
t per d
52.9
50.1
47.2
41.8
38.6
Liquor
nitrification
Liquor total
volume
m3 per
d
523
455
458
389
392
Nitrogen
released in
sludge liquor
kg N
per d
495
532
568
545
594
Biogas production and biogas percentage increase of different scenarios with and without
pretreatment for Rostock is shown in Figure 6.8. TAP increases total biogas production
up to 7-15%, while TH results in 10-20% increase. The lower total biogas increase of
Rostock in comparison to Waßmannsdorf is due to less VS production of WAS as feed
of pretreatment. Biogas production of TH scenarios with TS+6% and TS+8% is same as
TH scenario with TS+4%.
Figure 6.7. Cake weight and liquor volume of different scenarios with and without pretreatment for WWTP
Rostock
Electricity consumption or production in different process units of sludge treatment line
with and without pretreatment is presented in Figure 6.9. Due to higher pre-dewatering
of WAS before TH (16.5% TS), electricity consumption in thickening increases 114%
Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of
waste activated sludge before anaerobic digestion
104
for TH. For TAP, it was assumed that floatation system is replaced by centrifuges.
Centrifuges on Waßmannsdorf have 0.7 kWh per m3 sludge electricity consumption
which is equal to current consumption of floatation system in Rostock. Therefore, TAP
has same electricity consumption in thickening as in basis scenario. For pretreatment
plant, TAP consumes 133 kWh per d, while TH consumes 101 kWh per d. Lower
electricity consumption by TH is due to less volumetric flow of WAS to pretreatment
plant (see Table 6.6).
Figure 6.8. Biogas production biogas percentage increase of different scenarios with and without
pretreatment for WWTP Rostock (TH scenarios with TS+6% and TS+8% are same as TS+4%)
Electricity consumption in AD and dewatering unit for TH and TAP is 25% and 12% less
than basis scenario. Again, for comparison, Waßmannsdorf had no difference in
electricity consumption of AD and dewatering unit after TAP. This is due to TS of WAS
being already 6.5% with its current centrifuges. In CHP, electricity production increases
7-15% for TAP and 10-20% for TH. Electricity consumption for nitrification of liquor
increases 7-15% for TAP and 10-20% for TH. Finally, TAP results in 13-23% increase
of electricity production in sludge treatment line, while TH increases electricity
production by 5-20%. In conclusion, implementing TAP leads to higher electricity
balance than TH for WWTP Rostock, while for WWTP Waßmannsdorf both TAP and
TH showed nearly same total electricity balances. This is because replacing floatation
system of WAS thickening in Rostock with centrifuges and improving its TS from 4.5%
to 6.5% reduces electricity consumption of anaerobic digestion and dewatering units.
Electricity consumption and production of TH scenarios with TS+6% and TS+8% is same
as TH scenario with TS+4%.
Heat consumption or production in different process units of sludge treatment line with
and without pretreatment in Rostock is shown in Figure 6.10. Regardless of heating type,
TAP and TH both need 12% more heat than basis scenario. Same theoretical heat
consumption of TAP in comparison to TH despite its lower process temperature is due to
higher WAS volume that it must heat up. However, source of heating for TAP and TH
are practically different, as mentioned earlier. There is one important difference between
Waßmannsdorf and Rostock in heating up sludge. In Waßmannsdorf after mixing WAS
and PS in TAP and TH, temperature lies around 39°C and 38°C, respectively (AD
temp=37°C). Therefore, sludge can be directly fed to digester without warming or cooling
Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of
waste activated sludge before anaerobic digestion
105
(no heat exchanger needed). On the other hand, temperature of sludge after mixing WAS
and PS in TAP and TH in Rostock lies around 27 and 24°C, respectively (AD
temp=39°C). This is due to different volumetric flows of PS and WAS in Waßmannsdorf
and Rostock. Therefore, in Rostock a heat exchanger must be installed after TAP or TH
unit to heat up sludge to desired temperature of 39°C for AD. This heat can be provided
by low grade heat from CHP.
Figure 6.9. Electricity consumption or production in different process units of sludge treatment line with
and without pretreatment for WWTP Rostock (TH scenarios with TS+6% and TS+8% are same as
TS+4%)
TAP and TH result in 7-15% and 10-20% increase in high- and low-grade heat production
in CHP, respectively. High grade heat balance for TAP is 7-15% higher than that of basis
scenario. On the contrary, TH shows 31-41% decrease in high grade heat balance. Like
in Waßmannsdorf, despite this decrease, high grade heat balance is still positive. This
means even after implanting TH high grade heat is available in much extra amount. Low
grade heat balance for basis scenario is negative which shows part of high-grade heat
must be used for heating sludge before AD. TAP has a better low grade heat balance than
basis scenario and needs 34-42% less low-grade heat. However, TAP has also a negative
low grade heat balance and needs part of high-grade heat for heating up sludge.
Conversely, TH showed a positive low-grade heat balance and needs 131-142% less than
basis scenario. Total heat balance of sludge treatment line for Rostock shows that TAP
and TH increase heat production up to 55-81% and 63-97%, respectively. In conclusion,
TAP and TH need no extra fuel and improve total heat balance of Rostock WWTP.
Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of
waste activated sludge before anaerobic digestion
106
Furthermore, TH reduces amount of high-grade heat and improves amount of low-grade
heat available on Rostock in comparison to basis scenario. Meanwhile, TAP increases
amounts of both high- and low-grade heat. Heat consumption or production of TH
scenarios with TS+6% and TS+8% is same as TH scenario with TS+4%.
Figure 6.10. Heat consumption or production in different process units of sludge treatment line with and
without pretreatment for WWTP Rostock. LGH: Low grade heat, HGH: High grade heat (TH scenarios
with TS+6% and TS+8% are same as TS+4%)
6.3.4 Cost-benefit analysis of Rostock
Costs and savings of sludge treatment line with and without pretreatment for WWTP
Rostock are shown in Figure 6.11. Polymer costs decrease 5-10% for TAP and increase
54-153% for TH. Waßmannsdorf showed nearly comparable range of increase or
decrease in polymer consumption. TAP and TH resulted in 5-11% and 21-37% decrease
in disposal costs, respectively. Plant installation and maintenance costs, personnel and
NaOH costs are shown in Figure 6.11, too. Total operating costs of sludge treatment line
was reduced 14-32% after TAP and 21-48% after TH. In comparison to Waßmannsdorf,
decrease of total operating costs after implementing pretreatments is less in Rostock.
Savings due to pretreatments are 160-370 thousand euros p.a. for TAP and 240-550
thousand euros p.a. for TH. Comparing three scenarios of TH with different TS of
dewatered cake shows that savings due to reduction of disposal costs are slightly higher
than increase of polymer costs when TS of dewatered cake increases. In comparison, this
effect was negligible for Waßmannsdorf. Therefore, improvement degree of dewatering
potential (%TS) through TH slightly decreases final operating costs in Rostock.
Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of waste activated sludge before anaerobic digestion
107
Figure 6.11. Costs and savings of sludge treatment line with and without pretreatment for WWTP Rostock
Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of
waste activated sludge before anaerobic digestion
108
Figure 6.12 shows return of investment duration and total revenue during plant lifetime
after implementing pretreatment. TAP showed return of investment time of 2.3-5.3 years
and yielded total revenue of 1.6-4.8 million euros. On the other hand, TH showed return
of investment time of 5.4-12.6 years and yielded total revenue of 0.7-5.5 million euros.
For lower increase of biogas production (min scenarios), TAP has higher total revenue
than TH, regardless of %TS of dewatered cake. For higher increase of biogas (max
scenarios), total revenue of TAP is same as TH (TS+4%) which increase with TS of
dewatered cake up to nearly 10% more.
Figure 6.12. Return of investment duration, total revenue during plant lifetime and pretreatment plant
installation costs for WWTP Rostock
In conclusion, TAP and TH both lead to monetary profits during their plant lifetime for
Rostock. With lower increase in biogas potential (min scenarios), profitability of TAP is
significantly more than TH with TS+4%. With increase of TS of cake, TH scenarios
reduce their difference in revenue with TAP. However, with higher increase of biogas
potential (max scenarios), TH with TS+4% yields as much revenue as TAP. With increase
of TS of cake, TH scenarios yield more revenue than TAP.
Capital cost of TH is nearly 4 times as much as that of TAP and can play a key role when
initial budget is limited. Moreover, return of investment time is averagely 2.3 times that
of TAP. On the other hand, for higher plant lifetimes (>20 years) TH leads to more total
revenue due to its higher annual savings. Overall, considering all factors in this cost-
benefit analysis, TAP and TH are both profitable for WWTP Rostock. Final decision can
be made considering their different process conditions, side benefits and most importantly
formation of refractory COD which will be discussed in section 6.3.5.
6.3.5 Refractory COD increase in Effluent
Increase in refractory COD for WWTP Waßmannsdorf was modelled in previous
chapters. Effect of refractory COD formation due to pretreatment of WAS on increase of
effluent COD for WWTP Waßmannsdorf and Rostock is shown in Figure 6.13. TH
(170°C) can lead to increase of soluble refractory COD up to 11 mg/L in effluent (Toutian
et al., 2020b). This means current average effluent COD of Waßmannsdorf can increase
from 52 to 63 mg/L. German national discharge COD is 75 mg/L. However, local
environmental organization can lower this value according to local situations and
conditions. For example, Waßmannsdorf has an effluent COD limit of 65 mg/L.
Therefore, implementing TH in Waßmannsdorf can potentially lead to violation of
Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of
waste activated sludge before anaerobic digestion
109
discharge limits which result in financial fines. On the other hand, TAP results in less
than 1 mg/L increase in effluent COD of Waßmannsdorf (Toutian et al., 2020a), which is
far less than TH. It is to be noted, that a mono-incineration plant is planned to be built in
Waßmannsdorf. The condense wastewater from this plant will contain refractory COD
which results in a slight increase in effluent COD of WWTP. This leaves less space for
increase of refractory COD from a pretreatment process. Therefore, due to much higher
increase of refractory COD in effluent and possibility of imposed fines, TH is not
recommended for WWTP Waßmannsdorf and TAP can be implemented, more safely.
Current average effluent COD of Rostock is 32 mg/L and effluent COD limit for is 75
mg/L. According to modeling criteria in previous chapters, increase of effluent COD in
Rostock after implementing TH can be up to 15 mg/L. However, increase of COD in
effluent after TAP is limited to 1 mg/L. As shown in Figure 6.13, Rostock has more
capacity for increase of refractory COD in its effluent than Waßmannsdorf. Therefore,
although TAP increases effluent COD far less than TH, but TH can also be implemented,
due to higher difference between effluent COD and the COD limit. It is to be noted here,
that Rostock also is building a mono-incineration plant like Waßmannsdorf. Therefore, it
must be expected that refractory COD from condense wastewater of this plant will slightly
increase effluent COD of WWTP. Considering all discussions above, TAP is much safer
than TH in terms of avoiding volitation of effluent COD limits in Rostock, too.
Figure 6.13. Effect of refractory COD formation due to pretreatment of WAS on increase of effluent COD
for WWTP Waßmannsdorf and Rostock
6.4 Conclusions
Cost-benefit analyses of TH and TAP were performed for WWTPs Waßmannsdorf and
Rostock. Overview of effects of TAP and TH on parameters of mass and energy balance
and cost-benefit analysis is presented in Table 6.7. TAP and TH both increase biogas
production resulting in more electricity and heat production in CHP. Moreover, TAP and
TH enhance total electricity balance of WWTP. TAP works well with low-grade heat and
increases both low- and high-grade heat balances, resulting in a better total heat balance
Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of
waste activated sludge before anaerobic digestion
110
of WWTP. Meanwhile, TH needs high-grade heat and lowers high-grade heat balance.
On the other hand, TH improves low-grade heat balance, which finally results in a better
total heat balance of WWTP.
Table 6.7. Overall comparison of TAP and TH effects on parameters of mass/energy balance and cost-
benefit analysis of sludge treatment line of WWTPs Waßmannsdorf and Rostock
Waßmannsdorf
Rostock
Parameter
Unit
TAP
TH
TAP
TH
Mass
balance
Digester volume
%a
0
+28
+12
+25
Cake weight
%
-(8-16)
-(23-40)
-(5-11)
-(21-37)
Sludge liquor volume
%
+(1-2)
-(26-29)
-(12-13)
-(24-26)
Total biogas production
%
+(11-22)
+(15-30)
+(7-15)
+(10-20)
Energy
balance
Total electricity balance
%
+(13-28)
+(10-30)
+(13-23)
+(5-20)
High grade heat balance
%
+(11-22)
-(42-67)
+(7-15)
-(31-41)
Low grade heat balance
%
+(3-50)
+(587-650)
+(34-42)
+(131-142)
Total heat balance
%
+(11-22)
+(5-29)
+(55-81)
+(62-97)
Cost-benefit
analysis
Polymer costs
%
-(7-14)
+(52-144)
-(5-10)
+(54-153)
Disposal costs
%
-(8-16)
-(23-40)
-(5-11)
-(21-37)
Operating costs
%
-(23-62)
-(28-81)
-(14-32)
-(21-48)
Savings from pretreatment
M €/a
0.8-2.1
1.0-2.8
0.16-0.37
0.24-0.55
Capital costs
M €
2.2
10.0
0.8
3.0
Return of investment time
a
1.0-2.7
3.6-10.4
2.3-5.3
5.4-12-6
Total revenue
M €
10.1-30.4
4.9-32.8
1.6-4.8
0.7-5.5
Effluent COD increase
mg/L
1
11
1
15
aAll percentage values are relative to values of basis scenario without pretreatment
Economically seen, polymer costs increase with TH and decrease with TAP. Both TH
and TAP decrease disposal costs of sludge. Furthermore, TAP and TH both reduce
operating costs of sludge treatment line. Capital costs of TH are significantly higher than
TAP due to complexity of process. Therefore, return of investment time of TH is also
longer than TAP. Total revenues of TAP and TH are mainly dependent of biogas potential
increase. When effect of pretreatment on biogas increase is lower, TAP tends to be more
profitable than TH. On the contrary, when effect of pretreatment on biogas increase is
higher, TH yields slightly more revenue.
The difference between Rostock and Waßmannsdorf in production of WAS as feed of
pretreatment affects their cost-benefit analysis. Generally, relative decrease of operating
costs of Waßmannsdorf for both TAP and TH is more than Rostock due to production of
more WAS. Therefore, as ratio of VS of WAS/PS decreases, savings due to pretreatment
also decreases. Moreover, due to different volumetric flows of PS and WAS in
Waßmannsdorf and Rostock, sludge must be heated up to AD temperature in Rostock.
This requires installing a new heat exchanger before AD. Meanwhile this is not needed
in Waßmannsdorf, since mixing WAS and PS after pretreatment meets temperature
needed for AD. Accordingly, WWTPs should consider these parameters when referring
to or using results of this cost-benefit analysis.
Beside cost-benefit analysis, other key factors such as refractory COD formation leading
to increase of COD in WWTP effluent, simplicity of process, and increase of digester
capacity also influence the final decision. TH increases refractory COD of effluent
significantly more than TAP. For WWTP Waßmannsdorf, due to already near the limit
effluent COD without pretreatment, TAP is a safer process. For WWTP Rostock though,
TH can also be implemented since current effluent COD is much lower than limits.
However, due to construction of mono-incineration plant increase of effluent COD from
Chapter 6: Comparative cost-benefit analysis of thermal hydrolysis and thermal alkaline pretreatment of
waste activated sludge before anaerobic digestion
111
condense wastewater should be considered. Generally, TAP has a simpler process than
TH with milder temperature and ambient pressure. On the other hand, by TAP, handling
of concentrated NaOH (50% w/w) as a highly corrosive chemical must be taken into
consideration. Another advantage of TH over TAP is almost 25% increase of digester
capacity which can be economically very beneficial for WWTPs with overreached
digesters capacities and limited space or budget for building new digesters. Finally, TAP
and TH both have financial benefits and for each WWTP according to side effects such
as increase of effluent COD or digester capacity, the more suitable decision should be
made.
6.5 References
Barber, B., 2020. Sludge thermal hydrolysis: Application and potential / Bill Barber.
IWA Publishing, London.
Barber, W.P., 2016. Thermal hydrolysis for sewage treatment: A critical review. Water
research 104, 53–71.
Gerundt, K., 2018. Thermochemische Hydrolyse von Überschussschlamm –
Auswirkungen auf Faulgaserträge und die Bildung schwer abbaubarer organischer
Substanzen. Bachelor thesis, Berlin.
Toutian, V., Barjenbruch, M., Loderer, C., Remy, C., 2020a. Pilot study of thermal
alkaline pretreatment of waste activated sludge: Seasonal effects on anaerobic
digestion and impact on dewaterability and refractory COD. Water research 182,
115910.
Toutian, V., Barjenbruch, M., Unger, T., Loderer, C., Remy, C., 2020b. Effect of
temperature on biogas yield increase and formation of refractory COD during
thermal hydrolysis of waste activated sludge. Water research 171, 115383.
Chapter 7: Conclusions and recommendations for future research
112
7 Conclusions and recommendations for future research
7.1 Conclusions
Thermal hydrolysis (TH) and low temperature thermal alkaline pretreatment (TAP) of
waste activated sludge (WAS) were subjects of this study with aims of assessing biogas
yield, dewatering potential, refractory COD in effluent and cost-benefit analysis.
In chapter 3, impact of process parameters of thermal alkaline pretreatment on biogas
yield and dewaterability of waste activated sludge were investigated in a literature review.
Following conclusions were drawn:
• Higher initial biodegradability of WAS without pretreatment leads to decrease of
biogas or biodegradability enhancement in anaerobic digestion (AD) after
pretreatment.
• Low temperature TAP can be as effective as high temperature TH when it comes
to increase of biogas yield. This also indicates that chemicals (alkali) can
compensate for reduced effect of lower temperature.
• It is recommended to keep alkali dosage below 80 mg NaOH per g total solids of
WAS. It is also suggested that alkali is dosed per unit of solid matter in sludge.
• Kinetics of hydrolysis and acidification steps of AD are improved after TAP.
• More systematic studies are needed to clarify effect of TAP on dewatering
potential and polymer consumption. Moreover, it is needed to develop a
standardized dewatering potential test.
• Characterization and measurement of recalcitrant organic matter in sludge liquor
which finally lead to worsening quality parameters of WWTP need to be
investigated.
• A proper designed TAP process does not need acids for neutralization of
hydrolyzed WAS before AD. Therefore, it is needed to thicken sludge to a certain
solids content (around 6-7 percent total solids) before TAP. Moreover, it is
required to first add alkali to WAS and then heat it up to 70°C.
In chapter 4, pilot-scale study of low temperature TAP of WAS to investigate its effects
on anaerobic digestion and dewaterability and refractory COD was done. Following
conclusions were drawn:
• Biogas yield of WAS and PS mixture showed an alternating sinusoidal trend
through a year. Biogas yield increases after TAP up to +42% at its peak in summer
and to +3% at its lowest during winter. Therefore, TAP increases biogas yield of
WAS averagely +20% throughout the year.
• Digester maintains its smooth stable condition throughout the year.
• Centrifugation tests showed no significant difference between TS of digested
sludge with and without pretreatment.
• Ammonium and orthophosphate showed an increase of 35% and 17%,
respectively.
• Modeling the effect of TAP on effluent COD using data from six WWTPs in
Berlin and results of pilot tests showed that TAP leads to 0.8-1.1 mg/L increase
in effluent COD.
In chapter 5, lab-scale study on TH of WAS to investigate its effects on biogas yield
increase and formation of refractory COD over temperature range of 130-170°C was
Chapter 7: Conclusions and recommendations for future research
113
done. Following conclusions were drawn:
• Testing TH of only WAS in temperature range of 130-170°C showed that biogas
increase of 17-27% can be expected. Mass balance showed that refractory COD
in sludge liquor increases 50-200%. Modeling using data from six WWTPs in
Berlin and a newly introduced conversion factor showed that TH leads to 2-15
mg/L increase in effluent COD for temperature range of 130-170°C. Lab-scale
centrifugation tests of dewaterability showed 12-30% relative enhancement and
up to 4% absolute improvements (from 14.3% to 18.6%) in cake total solids.
• It was shown that reducing TH temperature from 170 to 160°C decreased
refractory COD by half while still benefiting from increase in biogas production
by 19% and relative dewaterability enhancement by 20%.
• Increase in effluent COD of a specific WWTP after TH depends on its WAS total
solids production, total volume flow rate and TH temperature. When performing
aerobic biodegradability tests, a conversion factor can be defined und used to
predict the impact of TH temperature on refractory soluble COD in sludge liquor.
After determining conversion factor, by using WWTP data, optimum TH
temperature regarding maximum biogas yield can be found, which does not lead
to surpassing COD limits in discharge.
In chapter 6, using data from experiments in previous chapters, literature, and operational
data from WWTPs Waßmannsdorf in Berlin and Rostock, a cost-benefit analysis was
done. Following conclusions were drawn:
• Digestion capacity increases 25-28% after implementing TH, while TAP has no
effect on digester volume.
• TAP and TH increase biogas production by 11-22% and 15-30% for
Waßmannsdorf, respectively. For Rostock, biogas increases 7-15% and 10-20%
after TAP and TH.
• TH of only WAS needs no extra fuel for heating sludge and high-grade heat from
CHP suffices.
• TAP and TH enhance total electricity balance by 13-28% and 10-30% for
Waßmannsdorf and 13-23% and 5-20% for Rostock.
• TAP and TH improve total heat balance of Waßmannsdorf by 11-22% and 5-29%
and of Rostock by 55-81% and 62-97%, respectively.
• Polymer costs are decreased by 7-14% after TAP and increased by 52-144% after
TH for Waßmannsdorf. For Rostock, polymer costs decrease by 5-10% after TAP
and increase by 54-153% after TH.
• Disposal costs are decreased by 8-16% and 23-44% after TAP and TH for
Waßmannsdorf, respectively. For Rostock, disposal costs decrease by 5-11% and
21-37% after TAP and TH.
• Operating costs of sludge treatment line decrease by 23-62% and 28-81% after
TAP and TH for Waßmannsdorf. For Rostock, TAP and TH result in 14-32% and
21-48% decrease in operating costs of sludge treatment line.
• Due to significantly higher capital costs (5x for Waßmannsdorf and 4x for
Rostock), TH had longer return of investment time (3.7x for Waßmannsdorf and
2.3x for Rostock) than TAP.
• Total revenue which indicates net profits of process during its lifetime is 10.1-
30.4 and 4.9-32.8 million euros after TAP and TH for Waßmannsdorf. For
Rostock, total revenue is 1.6-4.8 and 0.7-5.5 million euros for TAP and TH.
Chapter 7: Conclusions and recommendations for future research
114
• TAP increases effluent COD by 1 mg/L, while TH increases effluent COD by 11
and 15 mg/L for Waßmannsdorf and Rostock, respectively.
7.2 Final statement
Low temperature TAP of WAS increases biogas production which leads to improvement
of heat and electricity balance of WWTP. Moreover, disposal costs are reduced. TAP also
reduces viscosity of sludge which eases pumping and mixing capabilities. TAP did not
improve dewatering potential of digested sludge. An advantage of TAP is that it does not
increase effluent COD of WWTP significantly and can be safely implemented in WWTPs
with strict effluent limits. TAP has simpler process conditions and significantly less
capital costs than TH.
TH of WAS increases biogas production slightly better than TAP, which results in
improvement of heat and electricity balance of WWTP. It also reduces disposal costs
more than TAP. TH reduces viscosity of sludge significantly, which makes its pumping
and mixing easier through treatment line. TH leads to increase of capacity of digester and
makes digestion of sludge with higher organic loading rates possible. Dewatering
potential is improved after TH, although it increases polymer consumption. Formation of
refractory COD and increase of effluent COD of WWTP are higher after TH than TAP.
This must be considered before implementing this process on WWTP with effluent CODs
near limits. Nevertheless, it was shown in this study that lowering temperature of TH can
reduce formation of refractory COD. However, decreasing TH temperature lowers its
benefits of biogas increase and dewatering potential enhancement.
TAP and TH both reduce operating costs of sludge treatment line and yield high revenues
during their plant lifetime. If increase in biogas potential after pretreatment is low, TAP
yields more revenue than TH. On the contrary, if increase in biogas potential after
pretreatment is high, TH yields slightly more revenue than TAP.
Although TAP and TH have both shown significant financial potentials, incentives of
implementation are different among various WWTPs and countries. For example, due to
high temperatures in TH and only when both WAS and PS are pretreated, class A
biosolids is produced. Class A biosolid is allowed to be used as fertilizer on agricultural
farms according to sludge disposal standards in the US. However, in some countries such
as Switzerland, use of digested sludge in agricultural farming is not permitted. In
Germany, utilization of digested sludge in agriculture has declined over the last years,
while thermal utilization (incineration) has grown. This is mainly due to presence of
resistant pollutants which can eventually enter human beings’ water and food chain and
cause diseases. Therefore, production of class A biosolids is not an advantage of TH
process in Germany.
Although results of this study (in relative forms) can be used as a guide, local pilot
experimenting and cost-benefit analysis using data from full-scale plants are still
recommended for WWTPs which plan to implement TAP or TH.
Finally, TAP and TH both are profitable and beside cost-benefit analysis, simplicity of
process, digestion capacity increase and refractory organic matter formation are factors
which should be considered when selecting pretreatment type for each specific WWTP.
7.3 Recommendations for future research
Thermal alkaline pretreatment and thermal hydrolysis have been investigated in this study
for their effects in sludge treatment line. Nevertheless, following viewpoints are
recommended to be considered in future research:
• Zahn-Wellens test used in this dissertation for determining aerobic
biodegradability of sludge liquor is a batch test. For more solid conclusions on
Chapter 7: Conclusions and recommendations for future research
115
aerobic biodegradability of sludge liquor after pretreatment, it is recommended to
run long-tern continuous or semi-continuous tests. This also allows the biomass
to adapt to characteristics of sludge liquor and possibly improve its treating
efficiency over time. In the end, results of these tests series can be compared to
real increase in COD of WWTP after implementing a pretreatment technique to
validate the test.
• Dewatering test procedures used in literature suffer from being too simple and not
very practicable. These methods do not represent the real dewatering parameters
and their values from full-scale dewatering equipment. Moreover, polymer
consumption which is a key cost factor is not included in conventional dewatering
tests in literature. Since effect of pretreatments on dewatering potential of digested
sludge is a significant factor on operation costs in sludge treatment line, it is
crucial to have authentic information and data in this regard. It is recommended
that experts in sludge dewatering area design, develop and propose a lab-scale
standard test which is applicable by researchers and leads to more genuine unified
results.
• TAP needs more pilot or full-scale investigations with focus on its effect on
dewatering potential of digested sludge. It is also required to determine polymer
consumption after implementing TAP.
• TAP is practiced with WAS with TS in the range of 6-8%. It is recommended to
test if TAP can also be run with highly thickened sludge up to 16-18%. If feasible,
this reduces digester volume needed as TH also does. This is especially
meaningful for WWTPs with near-limit capacity of digester. Moreover, due to
sludge volume reduction before pretreatment, heat demand also decreases.
However, it is not clear if this affects self-neutralization of sludge after
pretreatment.
• Due to sinusoidal trend in biogas yield of pilot experiments through the year seen
in this study, it is recommended to perform batchwise biochemical methane
potential tests and dewaterability tests on WAS throughout a year to determine if
seasons affect its behavior.
• The pretreatment techniques in this study, focused on implementation of these
processes before AD. It would also be interesting to perform same experiments
with pretreatment as an intermediate step (between two digesters) or as post-
digestion process. Mass and energy balance as well as final cost-benefit analysis
of these configurations is also different than configuration with pretreatment
before AD.
• Scenarios with sludge drying step and thermal utilization of sludge via
incineration or with modern technologies such as nutrients recovery
(orthophosphate and ammonium) can be added to mass and energy balance and
cost-benefit analysis in this dissertation to further investigate economic viability
of these two pretreatment techniques for WWTPs which already possess these
operational units.
• With increase of concerns regarding climate change and effects of greenhouse
gases, it is also interesting to perform environmental impact assessments of these
two pretreatment techniques.
• It would be interesting to perform a comparative cost-benefit analysis of TH or
TAP with other pretreatment technique such as microwave or ultrasonic or
combination of these with thermal and chemical pretreatments.
Supplementary Material
116
Supplementary Material
Measurement method description of pH in Figure 3.4:
Characteristics and source of waste activated sludge (WAS) were same as the sludge
described by (Toutian et al., 2020b). Aim of this test was to measure pH of WAS directly
after adding alkali and mixing. To do so, 200 mL of WAS (1.7 or 7.7 % TS) was poured
into a 500 mL beaker for each measurement. Different amounts of alkali (with unit of mg
NaOH per g total solids of sludge) were added to sludge. Sludge was mixed swiftly for
30 s per hand and pH of sludge was measured thereafter.
Figure S1. Increase of interest in thermal alkaline pretreatment of sludge showed by returned number of
published peer-reviewed articles by searching terms of ‘thermal alkaline’ and ‘sludge’ on Web of Science®.
Supplementary Material
117
Figure S2. Characteristics of waste activated sludge (WAS) and primary sludge (PS) over 10 days storage
time at room temperature. As shown, pH decreases due to acidification, while PO4
3− −P and sCOD increase
due to hydrolytic and biochemical processes.
Supplementary Material
118
Figure S3. The pilot plant of thermal alkaline pretreatment; 1) WAS influent pump 2) NaOH dosing pump
3) Electrical heater 4) Heat exchanger for heating 5) Thermostat 6) Reactor 7) Heat exchanger for cooling
Figure S4. Sieve used to protect the influent pump of WAS against course material a) before use b) after
use
a
b
Supplementary Material
119
Figure S5. The two parallel-in-work anaerobic digesters (DBI gGmbH, Freiberg, Germany)
Digester A
Digester B
Supplementary Material
120
Figure S6. The lab scale thermal hydrolysis test rig. 1) steam generator 2) Sludge input hopper 3) reactor
4) pressure regulator (to set the temperature) 5) sludge flash tank
Supplementary Material
121
Figure S7. Centrate samples of a) thermally hydrolyzed and b) digested thermally hydrolyzed WAS at
different temperatures filtered with 0.45 µm filter
Supplementary Material
122
Figure S8. Results of biomethane yield from lab-scale thermal alkaline pretreatment of WAS at 70°C and
different dosages of NaOH. Increase in biomethane is between 15 and 26%.
Supplementary Material
123
Figure S9. Polymer consumption versus total solids of dewatered cake after TH for WWTP Waßmannsdorf
(TS of cake without pretreatment 26.5%) correlated with data from (Barber, 2020)
Figure S10. Polymer consumption versus total solids of dewatered cake after TH for WWTP Rostock (TS
of cake without pretreatment 24.5%) correlated with data from (Barber, 2020)
Supplementary Material
124
Table S1. Details of data references for Figure 3.2, Figure 3.3a, Figure 3.5a and Figure 3.6.
No.
Reference
Scale
Sludge
type (%TS)
Pretreatment
temp. (°C)
Pretreatment
time (h)
Pretreatment
alkali agent
Pretreatment
alkali dosage
Pretreatment
pH
Anaerobic digestion
1
(Nagler et al.,
2016)
Lab
WAS (4-6%)
39, 50, 70, 85,
100, 120, 150,
180 and 200
1
NaOH
16, 32 ,64
mg/g TS
-
Batch mesophilic (37°C) AD.
2
(Toutian et al.,
2020a)
Pilot
WAS (6.5%)
65-70
2-2.5
NaOH
1-2.5 mL
NaOH
(50%)/kg
sludge
-
Semi-continuous mesophilic AD (37°C).
3
(Ruffino et al.,
2016)
Lab
WAS (5-6%)
20, 70, 90
1.5-3.0
NaOH,
Ca(OH)2
(Neutralization*
with HCl)
0.04-0.2 g
alkali/g TS
-
Batch mesophilic (35°C) AD.
4
(Campo et al.,
2018)
Lab
WAS (5%) and
digested sludge
20, 70, 90
1.5
NaOH,
Ca(OH)2
(Neutralization
with HCL)
40, 80 mg/g
TS
-
Batch mesophilic (35°C) AD.
5
(Heo et al., 2003)
Lab
WAS
25, 35, 55
4
NaOH
-
-
-
6
(Zhang et al.,
2019)
Lab
WAS (2.6%)
80, 170
0.5, 1.3, 12
NaOH (0.1M)
0.1 mol/L
-
BMP experiment at 35°C.
7
(Ferrer et al.,
2009)
Lab
Mixed
Sludge (3.9%)
70
9, 24, 48, 72
-
-
-
Batch and semi-continuous (10 d SRT)
thermophilic AD (55°C).
8
(Demir, 2018)
Lab
WAS (5%)
70, 80, 90
0.17, 0.33,
0.50
NaOH
8, 16, 24 mg/g
TS
-
Semi-continuous (SRT 10 d) thermophilic
(55°C) AD
9
(Abudi et al.,
2016)
Lab
WAS (14.2%)
+rice straw
(92.6%)
70, 90
9, 10
NaOH
(With
neutralization)
-
11
Batch mesophilic (37°C) AD.
10
(Xu et al., 2014)
Lab
WAS (2.9-
3.2%)
70, 90
9, 10, 184
NaOH (5N)
(With
neutralization)
-
10, 11
Batch mesophilic (35°C) AD.
11
(Kim et al.,
2013)
Lab
WAS (1.2%)
60, 75, 90
6
NaOH
(Neutralization
with HCL)
0, 0.1, 0.2
mol/L
-
Batch mesophilic (35°C) AD.
Supplementary Material
125
12
(Zhang et al.,
2015)
Lab
WAS (22.9%)
80, 100, 120
1
NaOH
(Neutralization
with HCl)
20, 40, 60, 80
mg/g TS
-
Batch mesophilic (37°C) AD.
13
(Wang et al.,
2016c)
Lab
WAS (5.9-
8.7%)
120
0.5
NaOH
(Neutralization
with HCL)
-
10
Semi-continuous mesophilic (35°C) AD.
14
(Kim et al.,
2003)
Lab
WAS (3.8%)
121
0.5
NaOH, KOH,
Mg(OH)2,
Ca(OH)2
(Neutralization
with HCL)
0-21000 mg/L
-
Batch mesophilic (37°C) AD.
15
(Guo et al.,
2017)
Lab
Dewatered
WAS (19%)
134.95
1
NaOH
(Neutralization
with HCL (6M))
23.77 mg/g
TS
-
Batch mesophilic (37°C) AD.
16
(Guo et al.,
2016)
Lab
Dewatered
WAS (19%)
105, 120, 135
1
NaOH
(Neutralization
with 6M HCL)
5, 20, 35 mg/g
TS
-
Batch mesophilic (37°C) AD.
17
(Valo et al.,
2004)
Lab
WAS (1.7%)
130, 150, 170
0.25, 0.50,
1.00
KOH
(With and
without
neutralization)
1680, 3650
mg/L
10, 12
Batch and continuous mesophilic AD (35°C).
18
(Chi et al., 2011)
Lab
WAS (4.1%,
5.1%)
110, 130, 150,
170, 190, 210
0.016-0.18,
0.37, 0.52,
0.73, 0.85
NaOH (Without
neutralization)
0, 0.05, 0.10,
0.15, 0.20,
0.25 g/g SS
-
Batch and semi-continuous (30 d HRT)
thermophilic AD (55°C).
19
(Kavitha et al.,
2017)
Lab
WAS (1%)
50–70
0-48
NaOH (1N)
-
6-10
Batch mesophilic (35°C) AD.
20
(Dong et al.,
2016)
Lab
WAS (2.5%)
60, 80, 100,
120 for
thermal and
100 for
alkaline
0.25, 0.50,
0.75, 1 and
1.25 for
thermal
Ca(OH)2 (5M)
-
4-12
Semi-continuous mesophilic(35°C)
fermentation and AD.
21
(Vlyssides,
2004)
Lab
WAS (10%)
50, 60, 70, 80,
90
10
Lime
-
8, 9, 10, 11
Batch thermophilic AD (55°C).
22
(Liu et al., 2019)
Lab
WAS (3.1%)
(with and
without
polyacrylamide)
50, 70, 90
6, 12, 18
NaOH
-
8, 10, 12
Batch mesophilic (35°C) AD.
Supplementary Material
126
23
(Chen et al.,
2020)
Lab
WAS+PS
(2.7%)
134
0.5
NaOH (10M)
(Neutralization
with 4M HCl or
4M NaOH)
-
12.0 ± 0.1
Semi-continuous mesophilic AD (37 ± 1 °C).
24
(Park et al.,
2005)
Lab
WAS (2%)
121
0.5
NaOH
(Neutralization
with HCl)
7 g/L sludge
-
Batch mesophilic (41°C) AD.
*Neutralization refers to neutralizing pretreated sludge before anaerobic digestion
Supplementary Material
127
Table S2. Kruskal-Wallis Test for different initial biomethane yield ranges.
Factor
Statistic
df
p
Initial biomethane yield (L CH4 per kg VSadded)
24.030
5
< .001
Table S3. Dunn's Post Hoc Comparisons for different initial biomethane yield ranges (L CH4 per kg VSadded).
Comparison
z
W i
W j
p
p bonf
p holm
0-50 - 100-150
0.501
69.500
64.125
0.308
1.000
0.727
0-50 - 150-200
2.227
69.500
46.536
0.013
0.194
0.130
0-50 - 200-250
4.111
69.500
24.318
< .001
< .001
< .001
0-50 - 250-300
3.242
69.500
37.588
< .001
0.009
0.008
0-50 - 50-100
2.733
69.500
44.788
0.003
0.047
0.038
100-150 - 150-200
1.639
64.125
46.536
0.051
0.759
0.304
100-150 - 200-250
3.496
64.125
24.318
< .001
0.004
0.003
100-150 - 250-300
2.580
64.125
37.588
0.005
0.074
0.054
100-150 - 50-100
2.031
64.125
44.788
0.021
0.317
0.169
150-200 - 200-250
2.022
46.536
24.318
0.022
0.324
0.169
150-200 - 250-300
0.909
46.536
37.588
0.182
1.000
0.727
150-200 - 50-100
0.193
46.536
44.788
0.423
1.000
0.727
200-250 - 250-300
-
1.257
24.318
37.588
0.104
1.000
0.522
200-250 - 50-100
-
2.087
24.318
44.788
0.018
0.277
0.166
250-300 - 50-100
-
0.846
37.588
44.788
0.199
1.000
0.727
Supplementary Material
128
Table S4. Kruskal-Wallis Test for different temperature ranges.
Factor
Statistic
df
p
Temperature (°C)
2.833
6
0.830
Table S5. Dunn's Post Hoc Comparisons for temperature ranges (°C).
Comparison
z
W i
W j
p
p bonf
p holm
0-50 - 100-125
0.506
43.500
36.833
0.306
1.000
1.000
0-50 - 125-150
-0.182
43.500
46.000
0.428
1.000
1.000
0-50 - 150-175
-0.538
43.500
52.100
0.295
1.000
1.000
0-50 - 175-200
-0.509
43.500
54.000
0.305
1.000
1.000
0-50 - 50-75
0.238
43.500
40.435
0.406
1.000
1.000
0-50 - 75-100
0.285
43.500
39.750
0.388
1.000
1.000
100-125 - 125-150
-1.033
36.833
46.000
0.151
1.000
1.000
100-125 - 150-175
-1.268
36.833
52.100
0.102
1.000
1.000
100-125 - 175-200
-0.967
36.833
54.000
0.167
1.000
1.000
100-125 - 50-75
-0.481
36.833
40.435
0.315
1.000
1.000
100-125 - 75-100
-0.367
36.833
39.750
0.357
1.000
1.000
125-150 - 150-175
-0.481
46.000
52.100
0.315
1.000
1.000
125-150 - 175-200
-0.440
46.000
54.000
0.330
1.000
1.000
125-150 - 50-75
0.656
46.000
40.435
0.256
1.000
1.000
125-150 - 75-100
0.704
46.000
39.750
0.241
1.000
1.000
150-175 - 175-200
-0.095
52.100
54.000
0.462
1.000
1.000
150-175 - 50-75
0.993
52.100
40.435
0.160
1.000
1.000
150-175 - 75-100
1.026
52.100
39.750
0.152
1.000
1.000
175-200 - 50-75
0.773
54.000
40.435
0.220
1.000
1.000
175-200 - 75-100
0.803
54.000
39.750
0.211
1.000
1.000
50-75 - 75-100
0.091
40.435
39.750
0.464
1.000
1.000
Supplementary Material
129
Table S6. Kruskal-Wallis Test for different alkali dosage ranges.
Factor
Statistic
df
p
Alkali dosage (mg NaOH per g TS)
24.959
7
< .001
Table S7. Dunn's Post Hoc Comparisons for different alkali dosage ranges (mg NaOH per g TS).
Comparison
z
W i
W j
p
p bonf
p holm
0-20 - 100-200
-2.315
19.050
39.583
0.010
0.288
0.214
0-20 - 20-40
-0.679
19.050
24.042
0.249
1.000
1.000
0-20 - 200-300
-2.345
19.050
42.875
0.010
0.267
0.214
0-20 - 300-400
-0.093
19.050
20.000
0.463
1.000
1.000
0-20 - 40-60
-3.506
19.050
43.633
< .001
0.006
0.006
0-20 - 400-700
0.712
19.050
11.000
0.238
1.000
1.000
0-20 - 60-100
-0.409
19.050
22.900
0.341
1.000
1.000
100-200 - 20-40
1.810
39.583
24.042
0.035
0.985
0.598
100-200 - 200-300
-0.297
39.583
42.875
0.383
1.000
1.000
100-200 - 300-400
1.766
39.583
20.000
0.039
1.000
0.619
100-200 - 40-60
-0.488
39.583
43.633
0.313
1.000
1.000
100-200 - 400-700
2.354
39.583
11.000
0.009
0.260
0.214
100-200 - 60-100
1.604
39.583
22.900
0.054
1.000
0.761
20-40 - 200-300
-1.899
24.042
42.875
0.029
0.805
0.546
20-40 - 300-400
0.408
24.042
20.000
0.342
1.000
1.000
20-40 - 40-60
-2.945
24.042
43.633
0.002
0.045
0.042
20-40 - 400-700
1.176
24.042
11.000
0.120
1.000
1.000
20-40 - 60-100
0.125
24.042
22.900
0.450
1.000
1.000
200-300 - 300-400
1.884
42.875
20.000
0.030
0.835
0.546
200-300 - 40-60
-0.078
42.875
43.633
0.469
1.000
1.000
200-300 - 400-700
2.430
42.875
11.000
0.008
0.211
0.181
200-300 - 60-100
1.734
42.875
22.900
0.041
1.000
0.622
300-400 - 40-60
-2.445
20.000
43.633
0.007
0.203
0.181
300-400 - 400-700
0.686
20.000
11.000
0.246
1.000
1.000
300-400 - 60-100
-0.252
20.000
22.900
0.401
1.000
1.000
40-60 - 400-700
3.004
43.633
11.000
0.001
0.037
0.036
40-60 - 60-100
2.338
43.633
22.900
0.010
0.272
0.214
400-700 - 60-100
-0.949
11.000
22.900
0.171
1.000
1.000
Supplementary Material
130
Table S8. Statistical analysis for data in Figure 5b.
Parameter
Value
Pearson r
0.676
N
15
T Statistic
3.307
DF
13
P-value
0.005
Table S9. Kruskal-Wallis Test for different treatment time ranges (h).
Factor
Statistic
df
p
Treatment time (h)
10.779
6
0.095
Table S10. Dunn's Post Hoc Comparisons for different treatment time ranges (h).
Comparison
z
W i
W j
p
p bonf
p holm
0.0-0.5 - 0.5-1.0
1.869
48.720
34.684
0.031
0.648
0.586
0.0-0.5 - 1.0-1.5
0.222
48.720
46.846
0.412
1.000
1.000
0.0-0.5 - 1.5-5.0
-1.313
48.720
64.600
0.095
1.000
1.000
0.0-0.5 - 10.0-20.0
-0.162
48.720
50.875
0.436
1.000
1.000
0.0-0.5 - 20.0-25.0
1.101
48.720
28.750
0.135
1.000
1.000
0.0-0.5 - 5.0-10.0
1.844
48.720
34.412
0.033
0.684
0.586
0.5-1.0 - 1.0-1.5
-1.369
34.684
46.846
0.085
1.000
1.000
0.5-1.0 - 1.5-5.0
-2.412
34.684
64.600
0.008
0.167
0.167
0.5-1.0 - 10.0-20.0
-1.193
34.684
50.875
0.117
1.000
1.000
0.5-1.0 - 20.0-25.0
0.323
34.684
28.750
0.373
1.000
1.000
0.5-1.0 - 5.0-10.0
0.033
34.684
34.412
0.487
1.000
1.000
1.0-1.5 - 1.5-5.0
-1.367
46.846
64.600
0.086
1.000
1.000
1.0-1.5 - 10.0-20.0
-0.286
46.846
50.875
0.388
1.000
1.000
1.0-1.5 - 20.0-25.0
0.965
46.846
28.750
0.167
1.000
1.000
1.0-1.5 - 5.0-10.0
1.368
46.846
34.412
0.086
1.000
1.000
1.5-5.0 - 10.0-20.0
0.829
64.600
50.875
0.204
1.000
1.000
1.5-5.0 - 20.0-25.0
1.736
64.600
28.750
0.041
0.866
0.701
1.5-5.0 - 5.0-10.0
2.404
64.600
34.412
0.008
0.170
0.167
10.0-20.0 - 20.0-25.0
1.035
50.875
28.750
0.150
1.000
1.000
10.0-20.0 - 5.0-10.0
1.200
50.875
34.412
0.115
1.000
1.000
20.0-25.0 - 5.0-10.0
-0.307
28.750
34.412
0.379
1.000
1.000
Supplementary Material
131
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Publication bibliography and contribution of authors
133
Publication bibliography and contribution of authors
1. Toutian, V., Barjenbruch, M., Unger, T., Loderer, C. and Remy, C. 2020. Effect of
temperature on biogas yield increase and formation of refractory COD during
thermal hydrolysis of waste activated sludge. Water research 171, 115383.
Published and accessible online: https://doi.org/10.1016/j.watres.2019.115383
Contribution of authors:
Vahid Toutian: Conceptualization, Methodology, Experimentation, Validation, Formal
analysis, Writing Original Draft, Review & Editing, Visualization, Supervision
Matthias Barjenbruch: Review & Editing, Supervision
Tina Unger: Experimentation, Validation, Formal analysis
Christian Loderer: Review & Editing, Supervision, Funding acquisition, Project
administration
Christian Remy: Conceptualization, Validation, Review & Editing, Supervision,
Funding acquisition, Project administration
2. Toutian, V., Barjenbruch, M., Loderer, C. and Remy, C. 2020. Pilot study of
thermal alkaline pretreatment of waste activated sludge: seasonal effects on
anaerobic digestion and impact on dewaterability and refractory COD. Water
research 182, 115910.
Published and accessible online: https://doi.org/10.1016/j.watres.2020.115910
Contribution of authors:
Vahid Toutian: Conceptualization, Methodology, Experimentation, Validation, Formal
analysis, Writing Original Draft, Review & Editing, Visualization, Supervision
Matthias Barjenbruch: Review & Editing, Supervision
Christian Loderer: Review & Editing, Supervision, Funding acquisition, Project
administration
Christian Remy: Conceptualization, Validation, Review & Editing, Supervision,
Funding acquisition, Project administration
3. Toutian, V., Barjenbruch, M., Loderer, C. and Remy, C. 2020. Impact of process
parameters of thermal alkaline pretreatment on biogas yield and dewaterability of
waste activated sludge. Water research 202, 117465.
Published and accessible online: https://doi.org/10.1016/j.watres.2021.117465
Publication bibliography and contribution of authors
134
Contribution of authors:
Vahid Toutian: Conceptualization, Methodology, Investigation, Validation, Formal
analysis, Writing Original Draft, Review & Editing, Visualization, Supervision
Matthias Barjenbruch: Review & Editing, Supervision
Christian Loderer: Review & Editing, Supervision, Funding acquisition, Project
administration
Christian Remy: Conceptualization, Validation, Review & Editing, Supervision,
Funding acquisition, Project administration
