applied
sciences
A icle
Compos Quali y and Sani a ion on Indus ial Scale
Compos ing o Municipal Solid Was e and Sewage Sludge
Ana B. Siles-Cas ellano, Juan A. López-González * , Maca ena M. Ju ado, Ma ía J. Es ella-González,
F ancisca Suá ez-Es ella and Ma ía J. López


Ci a ion: Siles-Cas ellano, A.B.;
López-González, J.A.; Ju ado, M.M.;
Es ella-González, M.J.;
Suá ez-Es ella, F.; López, M.J.
Compos Quali y and Sani a ion on
Indus ial Scale Compos ing o
Municipal Solid Was e and Sewage
Sludge. Appl. Sci. 2021,11, 7525.
h ps://doi.o g/10.3390/app11167525
Academic Edi o : Elza Bon empi
Recei ed: 30 June 2021
Accep ed: 12 Augus 2021
Published: 17 Augus 2021
Publishe ’s No e: MDPI s ays neu al
wi h ega d o ju isdic ional claims in
published maps and ins i u ional a il-
ia ions.
Copy igh : © 2021 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
This a icle is an open access a icle
dis ibu ed unde he e ms and
condi ions o he C ea i e Commons
A ibu ion (CC BY) license (h ps://
c ea i ecommons.o g/licenses/by/
4.0/).
Uni o Mic obiology, Depa men o Biology and Geology, CITE II-B, Ag i ood Campus o In e na ional
Excellence ceiA3, CIAIMBITAL, Uni e si y o Alme ia, 04120 Alme ia, Spain; [email p o ec ed] (A.B.S.-C.);
[email p o ec ed] (M.M.J.); [email p o ec ed] (M.J.E.-G.); [email p o ec ed] (F.S.-E.); [email p o ec ed] (M.J.L.)
*Co espondence: [email p o ec ed]; Tel.: +34-950-015-891
Abs ac :
Municipal solid was e and sewage sludge a e p oduced in la ge quan i ies ha a e o en
managed h ough indus ial compos ing ea men . Because o hei o igin, composi ion, and
complexi y, ensu ing adequa e s abiliza ion o he o ganic ma e , and sani a ion o ecal con aminan s
du ing compos ing is o he u mos signi icance, and di icul o achie e on an indus ial scale. In
his s udy, he ope a ions o six indus ial compos ing acili ies ha p ocess municipal solid was e
and sewage sludge we e e alua ed om he poin o iew o he sani a ion achie ed and he quali y
o he compos p oduced. In addi ion, he esul s we e compa ed using he model o indus ial
compos om g een was e. Di e ences be ween he plan s we e asc ibable o ope a ions o he han
compos ing sys ems. High phy o oxici y and ecal con amina ion abo e legisla ion h esholds we e
ound in compos p oduced om municipal solid was e. In con as , compos om sewage sludge
we e mo e s able and ma u e han hose p oduced om g een was e, and also had an adequa e
le el o sani a ion. The aw ma e ial and ope a ional ac o s a e o g ea ele ance o ob ain a s able,
ma u e, and pa hogen- ee compos .
Keywo ds:
ecal con amina ion; Salmonella; ecal coli o ms; ma u a ion; s abili y; indus ial compos
1. In oduc ion
A p esen , he amoun o was es gene a ed by he wel a e socie y has inc eased
conside ably, and he numbe o indus ial acili ies dedica ed o o ganic was e ea men
has enla ged no ably [
1
]. Indus ial compos ing has become he mos widesp ead o ganic
was e ea men p ocedu e. The e is a need o a ool ha uni es economic ac i i y and
en i onmen al well-being in a sus ainable way ha also in eg a es o ganic was e in o he
ci cula economy model implemen ed in Eu ope [
2
]. This p ocess esul s in he s abiliza ion
o o ganic ma e , gene a ing a inal p oduc ha can be used as a soil condi ione and a
supply o nu ien s [
3
,
4
]. The e is a need o ake in o accoun he pa icula i ies o indus ial
compos ing wi h espec o small-scale compos ing in o de o a oid he isk o gene a ing
p oduc s ha a e unsui able o ag onomic applica ion [5]. I no , he isk is high.
In a compos ing p ocess, he s a ing aw ma e ials can be e y di e se in composi ion
and o igin [
6
]. The knowledge o he cha ac e is ics o he aw ma e ials is essen ial o
p o ide he op imal compos ing condi ions o ob ain good quali y compos [
7
]. The e
a e many s udies in li e a u e dealing wi h speci ic bio-ma e ials and co-compos ing mix-
u es [
7
–
9
], among which u ban was e, i.e., municipal solid was e (MSW) and sewage
sludge (SS), a e o special in e es because o i s p oblema ic managemen [
9
]. Mos MSW
ypically includes o ganic esidues om households, ma ke s and ood p ocessing ha
con ibu e o i s high o ganic con en and he e o e biodeg adable ac ion. This ac ion
usually consis s o mo e han 50% o he o al was e gene a ed and can be as high as 80% o
sou ce sepa a ed MSW [
10
,
11
]. Hence, MSW is a highly compos able was e [
12
]. The use o
compos p oduced om MSW has inc eased in ecen yea s. I s use exe s a posi i e e ec
Appl. Sci. 2021,11, 7525. h ps://doi.o g/10.3390/app11167525 h ps://www.mdpi.com/jou nal/applsci
Appl. Sci. 2021,11, 7525 2 o 13
on he ac i i ies o enzymes in ol ed in he ca bon, ni ogen and phospho us cycles [
13
]
and p o ides a high amoun o o ganic ma e [
14
]. The e o e, i is use ul as an ag icul u al
soil condi ione and e ilize [
15
]. In his sense, he o he main was e ma e ial o u ban
o igin, sewage sludge, has managemen al e na i es ha include e en he mochemical
decomposi ion [
16
]. Rega ding he compos ing o SS, some key aspec s mus be aken in o
accoun . SS possess ce ain cha ac e is ics ha a e common o MSW, which makes i also
sui able o compos ing. Howe e , due o i s low C/N a io, equi es co-compos ing wi h,
e.g., lignocellulosic was e [
17
]. Among he mos nega i e aspec s o MSW and SS a e he
con en o hea y me als and he p esence o ecalci an xenobio ic subs ances, which a e
capable o mig a ing and dis ibu ing h ough en i onmen s [
18
,
19
]. Besides, esea ch is
inc easingly ocused on he s udy o echniques o imp o e he quali y o he inal p oduc
ob ained, wi hou o ge ing he conse a ion o na u al esou ces [
20
] and he educ ion
in en i onmen al impac [
21
]. Fo he compos o be sui able o ag icul u e, ma u i y
and s abili y should be ensu ed. The s abili y is o en e alua ed using seed ge mina ion
es s [
7
]. Howe e , espi ome y is one o he bes ools o moni o he s abili y o he
ma e ial du ing he compos ing p ocess [
22
]. The ma u i y iden i ies he g ade o o ganic
ma e ans o ma ion du ing he p ocess and he alida ion o he elimina ion o phy o ox-
ici y in he compos gene a ed is p e e ed [
23
]. On he o he hand, a co ec ly p epa ed
compos mus no con ain pa hogens ha pose isks o heal h and he en i onmen . Fecal
con amina ion indica o s a e used o de e mine ha he compos is sa e [24] and o e i y
ha he sani a ion condi ions we e eached du ing he p ocess. This is especially impo an
o la ge pile sizes used a indus ial scale compos ing in which i is di icul o es ablish
ha he desi ed condi ions a e eached in he whole mass.
Due o he abo e conside a ions, i is impo an o conduc a s udy o e alua e whe he
he u ban was e bio-ma e ials and he condi ions o he compos ing p ocess in luence he
quali y o he compos ob ained. In his s udy, a compa a i e analysis is ca ied ou o he
compos p oduced om indus ial compos ing acili ies ha p ocess municipal solid was e
(MSW) and sewage sludge (SS) and compa e hem wi h hose p oduced om g een was e
(VR). To achie e his goal, he ollowing speci ic objec i es we e ealized: (1) o e alua e he
physico-chemical cha ac e is ics o aw ma e ials in di e en indus ial acili ies p ocessing
u ban and g een was e; (2) o de e mine he e olu ion o he o ganic ac ions o hese
bio-ma e ials du ing compos ing; (3) o es ima e he p esence o ecal con amina ion in
he inal compos as an indica o o sani iza ion in indus ial compos ing p ocesses; (4) o
compa e he compos s gene a ed in he p ocesses, aking in o accoun physico-chemical
p ope ies, biological s abili y, ma u i y and ecal con amina ion o he subs a es.
2. Ma e ials and Me hods
2.1. Sampling S a egy and Raw Ma e ials
This s udy was ca ied ou in di e en indus ial compos ing companies ha p ocess
di e en aw ma e ials, loca ed in he sou heas o Spain (Alme ia, G anada, Mu cia
and Alican e). Th ee acili ies dedica ed o each o he ollowing o ganic was e we e
selec ed: ege al esidue (VR), municipal solid was e (MSW) and sewage sludge (SS).
Table 1
shows he main ea men cha ac e is ics o each compos ing p ocess in each acili y.
The compos ing piles we e p epa ed ou side, in a ow o 7–15 m in leng h and 3–5 m. The
samples we e aken du ing he aw ma e ial (RM) and inal p oduc phases (FP). Du ing
RM and FP phases, samples co esponding o nine di e en poin s o he compos ing pile
we e aken. Th ee samples we e aken supe icially (0.5 m), h ee we e aken a a dep h o
1.5 m and he las h ee as close as possible o he bo om o he piles. Fo his, a p obe ha
eached 1–2 m deep was used. Each o he samples ob ained om he di e en poin s was
mixed o ob ain a ep esen a i e sample, ob aining a inal mass o 3 kg, which was di ided
in o h ee sub-samples o abou 1 kg. A e , he samples we e s o ed in acuum bags and
ozen a −20 ◦C, o u he analysis.
Appl. Sci. 2021,11, 7525 3 o 13
Table 1. Cha ac e is ics o he indus ial compos ing p ocesses.
Facili y aWas e Mix u e Me hod o Compos ing Time o Compos ing P ocess
(Mon hs)
VR1 Cucumbe and zucchini c op esidues:
s alks, lea es Open ai , u ned wind- ows 4
VR2 Cucumbe and zucchini c op esidues:
s alks, lea es Open ai , u ned wind- ows 4
VR3 Peppe c op esidues: s alks, lea es Open ai , u ned wind- ows 3
MSW1 Municipal solid was e bIn- essel u ned wind- ows in bays 3.5
MSW2 Municipal solid was e bIn- essel u ned wind- ows in bays 4.5
MSW3 Municipal solid was e bIn- essel unnel compos ing
( u ning by auge s) 3
SS1 Sewage sludge + s aw
1:1 / Open ai , u ned wind- ows 3.5
SS2 Sewage sludge + p uning was es
1:1 / Open ai , u ned wind- ows 3
SS3 D ied sewage sludge + p uning was es
1:2 /
In- essel unnel compos ing
( u ning by auge s) 3
a
Vege al esidue (VR): p uning and ga dening emains, ag icul u al (ho icul u al) o om ag icul u e; sewage sludge (biosolids) (SS):
c ude, ac i e, sludge; municipal solid was e (MSW): o ganic ac ion o municipal solid was e.
b
All acili ies p ocessed mixed municipal
solid was e.
2.2. Analy ical Me hods
2.2.1. Physico-Chemical Pa ame e s
Di e en physico-chemical analyses we e ca ied ou on he bio-ma e ials and in-
dus ial compos s p oduced. Mois u e was de e mined a 105
◦
C o 24 h. The elec ical
conduc i i y (EC) and pH we e measu ed by p epa ing a 1/10 (w/ ) ex ac in wa e . The
o ganic ma e con en (OM) was de e mined by in oducing a mu le u nace a 550
◦
C o
3.5 h. The de e mina ion o o al ca bon (C) and o al ni ogen (N) was ca ied ou using an
Elemen a Va io Mic o CHNS (Elemen a Analysensys eme GmbH, Hanau, Ge many).
Soluble o ganic ca bon (SOC), educing suga s (RS) and o al p o eins (TP) we e ana-
lyzed using he desc ibed me hods [
25
], a b ie desc ip ion o hese me hods is p o ided
nex . Fo SOC a 1/10 dilu ion o he samples was pe o med in 0.5 M K
2
SO
4
and shaken
o 30 min a 200 pm. Subsequen ly, il e ed ex ac s we e ob ained om his dilu ion and
measu ed using a TOC-VCSN analyze (Shimadzu, Co., Kyo o, Japan). Reducing suga s
(RS) we e analyzed by he DNS me hod [
26
]. To al p o eins (TP) we e spec opho ome -
ically analyzed acco ding o he me hod desc ibed by Low y and modi ied by [
27
]. Fo
N-NH
4+
and N-NO
3
−
, a 1/10 dilu ion was made and shaken o 30 min a 200 pm. Fo
he de e mina ion o N-NH
4+
, a Hach 9663 p obe (Hach, Lo eland, CO, USA) was used.
Fo N-NO3
−Ni achek 404 p obe (KPG P oduc s Ld ., Ho e, UK) was used.
The cellulose (CEL), hemicellulose (HC) and lignin (LIG) ac ions we e de e mined
using he ANKOM 200/220 analyze (Ankom Technology, Macedonia, NY, USA). Fi s ,
samples we e subjec ed o diges ion by using a neu al de e gen (NDF), ob aining hemi-
cellulose, cellulose and lignin ac ions. A e diges ion using an acid de e gen (ADF),
he sum ac ion o cellulose and lignin was ob ained. Finally, his sum ac ion was sub-
jec ed o a ea men wi h concen a ed sul u ic acid (ADL), he ac ion ha was ob ained
con ained lignin (h p://www.ankom.com/p ocedu es.aspx (accessed on 10 June 2021)).
2.2.2. Indica o Pa ame e s o Biological S abili y and Ma u i y
The phy o oxici y o he samples was e alua ed, ollowing he me hod o [
28
], and
sligh ly modi ied and acco ding o [
7
]. The biodeg adabili y o he samples was e alua ed
using he dynamic espi a ion index (DRI) [
29
] and dynamic accumula ed espi a ion
ac i i y (AT
4
) [
5
]. Fo his pu pose, 100 g o he sample, wi h 60% humidi y, we e placed
in a 500 mL eac o and placed in a wa e ba h a 37
◦
C. An oxygen senso (Alphasense
L d., Essex, UK) was used o measu e he exhaus ai . The alues o he oxygen le els
we e con olled by a da a acquisi ion sys em. Fo he DRI calcula ion, he a e age alue
Appl. Sci. 2021,11, 7525 4 o 13
o oxygen consumed in he mos ac i e 24 h o biological ac i i y was used, ha is, as g
o oxygen consumed pe kg o o ganic ma e (OM) and pe hou (g O2 kg
−1
OM h
−1
).
AT
4
was exp essed as g o oxygen pe kg o OM ma e (g O2 kg
−1
OM) a e ou days o
cumula i e espi ome ic ac i i y.
2.2.3. Fecal Con amina ion and Pa hogens
Fo he de e mina ion o each o he indica o g oups o ecal con amina ion, 10 g o
he di e en compos s samples we e suspended in 90 mL o s e ile saline (NaCl, 0.9%) and
s i ed o 30 min a 120 pm. Se ial decimal dilu ions o he samples we e pe o med in
an app op ia e medium o each mic obial g oup. The de e mina ion o o al and ecal
coli o ms, Esche ichia coli and ecal en e ococci was ca ied ou using he mos p obable
numbe (MPN) echnique by inocula ing 1 mL o he dilu ions made in each speci ic
medium. Fo he quan i ica ion o o al coli o ms, 10 mL ubes o lac ose b o h (Sha lab,
S.L., Ba celona, Spain) wi h 0.2 g L
−1
b omoc esol pu ple and Du ham bell, we e used as
a cul u e medium and i we e incuba ed a 37
◦
C o 24–48 h in a wa e ba h. A e he
incuba ion pe iod, he o al coli o m coun was ca ied ou , as indica ed by he change
in he colo o he medium om pu ple o yellow and he accumula ion o gas in he
Du ham bell. The de e mina ion o ecal coli o ms was made om posi i e o al coli o ms.
Fo his, 0.1 mL o he posi i e ubes we e aken and ans e ed o a ube wi h 10 mL o
lac ose b o h wi h b omoc esol pu ple and Du ham bell which was incuba ed a 44.5
◦
C o
24–48 h
. Again, as in he case o o al coli o ms, posi i es we e sea ched o by he same
me hod. F om he posi i e ecal coli o ms, he posi i e ubes we e quan i ied by s eaking
wi h a pla inum loop in eosin me hylene blue aga (Pan eac, ITW, Glen iew, IL, USA) and
incuba ing a 37
◦
C o 24–48 h. The appea ance o colonies wi h a me allic lus e indica ed
he p esence o E. coli. The ecal en e ococci we e de e mined by ubes wi h 10 mL o Ro he
b o h (Oxoid Ld ., Han , UK) and incuba ing a 37
◦
C o 24 h in a wa e ba h. The p esence
o ecal en e ococci was con i med by he p esence o u bidi y and/o sedimen a ion in
he ubes and con i ma ion by G am s aining and mic oscopy.
The quan i ica ion o he le el o sul a e- educing clos idia was pe o med by coun -
ing colonies pe g am o sample (c u g
−1
) in SPS aga . Fo his, 1 mL o he di e en se ial
dilu ion was aken and ans e ed o a ube con aining 15 mL o SPS aga (Sha lab, S.L.,
Ba celona, Spain), sealed wi h pa a in c ea ing anae obic condi ions and incuba ed a 37
◦
C
o 24–48 h. Colonies o sul i e- educing clos idia showed a cha ac e is ic black colo .
The de ec ion o Salmonella spp. was ca ied ou by weighing 25 g o he sample
ha was placed in a lask wi h 0.1% bu e ed pep one wa e (Pan eac, ITW, Glen iew,
IL, USA) and incuba ing a 37
◦
C o 24 h. Subsequen ly, 1 mL was aken which was
placed in a ube wi h 10 mL o seleni e and cys ine b o h (Oxoid Ld ., Han , UK). A e he
incuba ion ime o he b o h media, i was inocula ed on Hek oen En e ic Aga (Pan eac,
ITW, Glen iew, IL, USA) and incuba ed a 37
◦
C o 24 h. A e he incuba ion ime, he
p esence o Salmonella was de ec ed by he appea ance o g een colonies wi h o wi hou a
blackened a ea. Suspec ed colonies we e biochemically con i med by inocula ion in Kligle
i on aga (Pan eac, ITW, Glen iew, IL, USA) and incuba ed a 37 ◦C o 24 h.
2.3. Da a Analysis
The pa ame e s analyzed we e pe o med in iplica e, using he mean o he p esen-
a ion o he da a. The no mali y and homogenei y o he a iances we e e i ied using he
Shapi o–Wilk and Le ene es s. S a is ical analysis was pe o med a a signi icance le el
o p< 0.05. By means o he analysis o a iance (ANOVA) and he Fische compa ison
es o leas signi ican di e ence (LSD), he mean alues o each sampling acili y we e
compa ed. The p esence o ca ego ies wi hin inal compos samples collec ed om acili ies
compos ing di e en aw ma e ials was in es iga ed using s epwise linea disc iminan
analysis (DA), in o de o ind simple equa ions o es ima ion o he composi ion o hese
was es om easily analyzable pa ame e s. Spea man co ela ions be ween di e en pa am-
e e s and a mul iple eg ession analysis wi h s epwise selec ion o a iables we e calcula ed.
Appl. Sci. 2021,11, 7525 5 o 13
S a g aphics Cen u ion (Ve sion 18.1.8) (S a -Poin Inc., The Plains, VA, USA) was used o
he analysis.
3. Resul s and Discussion
3.1. Bio-Ma e ials Cha ac e iza ion
The physical-chemical s a e o he bio-ma e ials ha a e compos ed is essen ial o
gua an ee an e icien p ocess. Especially in indus ial acili ies, whe e he olume o was e
ea ed can cause addi ional di icul ies [
5
]. Table 2shows he alues eco ded in aw
ma e ials in he physical-chemical pa ame e s o he nine indus ial acili ies analyzed
( h ee acili ies o each ype o bio-ma e ial). Mois u e pe cen ages we e be ween 60 and
85%, excep in a ege able was e acili y (VR3). The h ee aw ma e ials analyzed we e
ound, in a leas one si ua ion, in alues highe han 80% humidi y. This can lead o
anae obic condi ions in he piles a he s a o he compos ing p ocess [
30
]. The e o e,
condi ioning ope a ions should y o coun e ac hese p oblems. Rega ding he pH
si ua ion in aw ma e ials, he beha io was es ic ed o he na u e o he esidue. Wi hin
he esidues cha ac e ized in he s udy, he MSW had a ypical sligh acid cha ac e [
31
].
On he o he hand, SS and g een was e egis e ed alues abo e pH neu ali y. On he
con a y, in he conduc i i y o he bio-ma e ials, wi h a clea di e ence, SS and MSW we e
mo e sui able as s a ing subs a es han g een was es. The p esence o o ganic ma e
was be ween 65 and 80% in all cases. Thus, all he esidues con ained a high o ganic load
suscep ible o compos ing. Despi e he abo e, he o m in which o ganic ma e is p esen is
highly a iable and la gely dependen on he o igin o he was e [
32
]. Mo eo e , his is jus
wha occu ed in he p esen s udy. The C/N a ios did no each he alue o 10 in SS, no
he alue o 15 in VR. In con as , he a io was op imal (20–30) o MSW. The espi ome y
o he bio-ma e ials e ealed he highe consump ion o oxygen associa ed wi h MSW wi h
espec o SS. In u n, i was ound ha he aw ma e ials ha each he indus ial acili ies
we e subs a es wi h a high con en o esh, non-s abilized o ganic ma e . This ac was
ully demons a ed h ough he esul o he phy o oxici y o bio-ma e ials. The ull o ali y
o he analyzed esidues, p esen ed alues lowe han 50% in he ge mina ion index, which
is he h eshold alue o oxici y o his pa ame e [33].
Table 2. P ope ies o aw ma e ials om indus ial compos ing acili ies *.
Facili y ** M (%) pH EC
(mS cm−1)OM (%) C/N AT4
(gO2kg−1OM)
DRI
(gO2kg−1OM h−1)GI (%)
VR1 85.94 d 8.10 e 16.78 c 74.06 de 12.63 d 52.38 a 1.07 ab 7.70 ab
VR2 66.62 b 8.04 e 14.73 cd 73.48 d 13.30 d 67.03 a 1.70 b 0.00 a
VR3 29.27 a 6.78 c 13.53 d 77.13 10.61 c 38.98 a 0.46 a 1.91 b
MSW1 78.19 c 5.16 b 3.98 ab 74.73 e 28.47 196.56 b 3.01 c 0.00 a
MSW2 84.83 cd 3.94 a 2.65 a 75.15 e 28.05 160.95 b 2.73 c 0.00 a
MSW3 61.06 b 7.22 d 5.37 b 71.73 c 20.10 e 49.65 a 0.74 a 0.00 a
SS1 81.87 cd 7.95 e 3.02 a 71.97 c 9.29 b 23.27 a 0.62 a 40.42 c
SS2 66.37 b 8.63 g 2.56 a 64.91 a 9.50 ab 57.83 a 0.95 a 18.70 b
SS3 67.67 b 8.40 g 3.06 a 66.14 b 6.44 a 31.86 a 0.61 a 38.34 c
* Da a a e mean alues (n = 3), hose wi h he same le e in he same column a e no signi ican ly di e en om each o he (LSD, p< 0.05).
Abb e ia ions: M: mois u e; EC: elec ical conduc i i y; OM: o ganic ma e ; C/N: ca bon-ni ogen a io; AT
4
: dynamic accumula ed
espi a ion ac i i y; DIR: dynamic espi ome ic index GI: ge mina ion index. All da a a e on a d y weigh basis. ** Vege al esidue (VR);
municipal solid was e (MSW); sewage sludge (SS).
3.2. E olu ion o Soluble and Polyme ic F ac ions
In a compos ing p ocess, he decomposi ion o polyme ic esidual ma e occu s
h ough mic obial ac ion. The mic obio a consume nu ien s in chemical o ms solubilized
and simple. I his is no he case, he nu ien s mus be bio ans o med in o me abolizable
compounds. The exoenzymes sec e ed by he mic oo ganisms p oduce he elease o he
basic sou ces o nu ien s. This ac ion allows he g ow h o he compos ing mic obiome.
The e o e, du ing compos ing he e mus be conside able consump ion o he soluble

Appl. Sci. 2021,11, 7525 6 o 13
and polyme ic ac ions o he was e ma e ial. In he p esen s udy, bo h ac ions we e
e alua ed a he beginning and a he end o he p ocess (Figu e 1). This quan i ica ion
allowed he obse a ion o he deg ada ion ha hese ac ions su e ed du ing compos ing
(Figu e 2). Acco ding o he esul s ob ained in he polyme ic ac ion, he ibe s con en o
he aw ma e ials anged be ween 30 and 40% o he d y weigh (Figu e 1a). As expec ed,
and also epo ed ea lie by o he s [
34
], he main ac ion he eo , consis ed o cellulose
(10–20%). The ibe con en was ela i ely cons an among all he samples, ega dless o
he aw ma e ial. In con as , he soluble ac ion was highly dependen on i (Figu e 1c).
The SOC ep esen ed a ound 2.5% o he d y weigh in MSW, bu ba ely eached 0.2% in
SS. On he o he hand, in SS he majo i y o he soluble ac ion was clea ly he soluble
p o ein. This esul can be explained by he high con en o ni ogen ac ion con ained in
hese esidues due o hei o igin [
35
]. Ni ic and ammonia ac ions did no ep esen a
conside able soluble ac ion ei he in he SS o in MSW s udied.
Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 o 14
SS2 66.37 b 8.63 g 2.56 a 64.91 a 9.50 ab 57.83 a 0.95 a 18.70 b
SS3 67.67 b 8.40 g 3.06 a 66.14 b 6.44 a 31.86 a 0.61 a 38.34 c
* Da a a e mean alues (n = 3), hose wi h he same le e in he same column a e no signi ican ly di e en om each o he
(LSD, p < 0.05). Abb e ia ions: M: mois u e; EC: elec ical conduc i i y; OM: o ganic ma e ; C/N: ca bon-ni ogen a io;
AT4: dynamic accumula ed espi a ion ac i i y; DIR: dynamic espi ome ic index GI: ge mina ion index. All da a a e on
a d y weigh basis. ** Vege al esidue (VR); municipal solid was e (MSW); sewage sludge (SS).
3.2. E olu ion o Soluble and Polyme ic F ac ions
In a compos ing p ocess, he decomposi ion o polyme ic esidual ma e occu s
h ough mic obial ac ion. The mic obio a consume nu ien s in chemical o ms solubilized
and simple. I his is no he case, he nu ien s mus be bio ans o med in o me abolizable
compounds. The exoenzymes sec e ed by he mic oo ganisms p oduce he elease o he
basic sou ces o nu ien s. This ac ion allows he g ow h o he compos ing mic obiome.
The e o e, du ing compos ing he e mus be conside able consump ion o he soluble and
polyme ic ac ions o he was e ma e ial. In he p esen s udy, bo h ac ions we e e alu-
a ed a he beginning and a he end o he p ocess (Figu e 1). This quan i ica ion allowed
he obse a ion o he deg ada ion ha hese ac ions su e ed du ing compos ing (Fig-
u e 2). Acco ding o he esul s ob ained in he polyme ic ac ion, he ibe s con en o
he aw ma e ials anged be ween 30 and 40% o he d y weigh (Figu e 1a). As expec ed,
and also epo ed ea lie by o he s [34], he main ac ion he eo , consis ed o cellulose
(10–20%). The ibe con en was ela i ely cons an among all he samples, ega dless o
he aw ma e ial. In con as , he soluble ac ion was highly dependen on i (Figu e 1c).
The SOC ep esen ed a ound 2.5% o he d y weigh in MSW, bu ba ely eached 0.2% in
SS. On he o he hand, in SS he majo i y o he soluble ac ion was clea ly he soluble
p o ein. This esul can be explained by he high con en o ni ogen ac ion con ained in
hese esidues due o hei o igin [35]. Ni ic and ammonia ac ions did no ep esen a
conside able soluble ac ion ei he in he SS o in MSW s udied.
Figu e 1. (a) Fibe ac ions in aw ma e ials; (b) soluble ac ions in aw ma e ials; (c) ibe ac ions in compos ; (d)
soluble ac ions in compos . Abb e ia ions: cellulose (Cel), hemicellulose (Hemicel), lignin (Lig), educing suga s (RS),
o al p o eins (TP), soluble o ganic ca bon (SOC), N-NH4+ (NH4), N-NO3- (NO3).
Figu e 1.
(
a
) Fibe ac ions in aw ma e ials; (
b
) soluble ac ions in aw ma e ials; (
c
) ibe ac ions in compos ; (
d
) soluble
ac ions in compos . Abb e ia ions: cellulose (Cel), hemicellulose (Hemicel), lignin (Lig), educing suga s (RS), o al
p o eins (TP), soluble o ganic ca bon (SOC), N-NH4+(NH4), N-NO3
−(NO3).
Th oughou he compos ing p ocess, he bio-ma e ials unde go a se ies o bio ans-
o ma ions. These changes a e de ec able by analyzing he a ia ion in he soluble and
polyme ic ac ions [
25
]. The e o e, in his s udy bo h ac ions we e also quan i ied in he
compos p oduced (Figu e 1b,d) despi e he ac ha in indus ial scaling p ocesses, was e
can o en no be bio ans o med as co ec ly as in ideal compos ing. The a iable con en
o hese ac ions in compos s e ealed si ua ions in which biodeg ada ions we e mo e
in ense. Figu e 1b,d showed ha he deg ada ion o o ganic ma e was clea ly de ec ed
h ough he e olu ion o he soluble ac ion. Zhang e al. [
36
] also p e iously desc ibed
his beha io . The esul s o he ibe ac ions equi e long pe iods o ac i e biodeg ada-
ion o be e ec i ely mine alized. The ecalci ance o hese polyme ic s uc u es causes a
challenge o mic obial deg ada ion [37].
Despi e he abo e, du ing compos ing i is common o ind quan i iable deg ada i e
p ocesses in all ac ions o o ganic ma e . Tha is p ecisely wha was in ended o be
de e mined in his s udy. Fo his, he deg ada ion pe cen ages eached by he di e en
Appl. Sci. 2021,11, 7525 7 o 13
ac ions we e e alua ed and a e shown in Figu e 2. The da a we e highly e ealing
o analyze he pe o mance o he acili ies. Deg ada ion o he holocellulose ac ion
(hemicellulose + cellulose) was de ec ed in up o se en o he nine acili ies s udied, wi h
si ua ions whe e his ac ion was consumed by up o 50% (SS1 and SS2). In ac , hese
esul s di e ed g ea ly depending on he aw ma e ial o be compos ed. Thus, in MSW
he e was ha dly a signi ican deg ada ion o holocellulose while in SS i was in ense.
On he o he hand, in nei he o hese wo esidual ma e ials a consump ion o he mos
ecalci an ac ion o he ibe s (lignin) was epo ed. Rega ding he soluble ac ions, a
conside able deple ion o nu ien s was de ec ed in bo h esidues. In ac , i was highe han
ha de ec ed in g een was e acili ies. Al hough he use o his ac ion by he mic obio a
p oduced a iable esul s depending on he compos ed bio-ma e ials. In his way, i was
possible o co obo a e a g ea e dec ease in SOC le els in MSW, compa ed wi h ha
egis e ed in SS. E en, his dec ease a exceeded 70% in wo MSW acili ies (MSW1 and
MSW2). On he con a y, acco ding o he esul s o he SS acili ies, in his bio-ma e ial, he
consump ion o he soluble ac ion was mainly a ibu ed o he educ ion in he ni ogen
ac ion o he o ganic ma e . The educ ion in his ni ogen ac ion, as a whole, is widely
desc ibed in compos ing [38].
Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 o 14
Th oughou he compos ing p ocess, he bio-ma e ials unde go a se ies o bio ans-
o ma ions. These changes a e de ec able by analyzing he a ia ion in he soluble and
polyme ic ac ions [25]. The e o e, in his s udy bo h ac ions we e also quan i ied in he
compos p oduced (Figu e 1b,d) despi e he ac ha in indus ial scaling p ocesses, was e
can o en no be bio ans o med as co ec ly as in ideal compos ing. The a iable con en
o hese ac ions in compos s e ealed si ua ions in which biodeg ada ions we e mo e
in ense. Figu e 1b,d showed ha he deg ada ion o o ganic ma e was clea ly de ec ed
h ough he e olu ion o he soluble ac ion. Zhang e al. [36] also p e iously desc ibed
his beha io . The esul s o he ibe ac ions equi e long pe iods o ac i e biodeg ada-
ion o be e ec i ely mine alized. The ecalci ance o hese polyme ic s uc u es causes a
challenge o mic obial deg ada ion [37].
Despi e he abo e, du ing compos ing i is common o ind quan i iable deg ada i e
p ocesses in all ac ions o o ganic ma e . Tha is p ecisely wha was in ended o be de-
e mined in his s udy. Fo his, he deg ada ion pe cen ages eached by he di e en ac-
ions we e e alua ed and a e shown in Figu e 2. The da a we e highly e ealing o analyze
he pe o mance o he acili ies. Deg ada ion o he holocellulose ac ion (hemicellulose
+ cellulose) was de ec ed in up o se en o he nine acili ies s udied, wi h si ua ions whe e
his ac ion was consumed by up o 50% (SS1 and SS2). In ac , hese esul s di e ed
g ea ly depending on he aw ma e ial o be compos ed. Thus, in MSW he e was ha dly
a signi ican deg ada ion o holocellulose while in SS i was in ense. On he o he hand, in
nei he o hese wo esidual ma e ials a consump ion o he mos ecalci an ac ion o
he ibe s (lignin) was epo ed. Rega ding he soluble ac ions, a conside able deple ion
o nu ien s was de ec ed in bo h esidues. In ac , i was highe han ha de ec ed in g een
was e acili ies. Al hough he use o his ac ion by he mic obio a p oduced a iable e-
sul s depending on he compos ed bio-ma e ials. In his way, i was possible o co obo-
a e a g ea e dec ease in SOC le els in MSW, compa ed wi h ha egis e ed in SS. E en,
his dec ease a exceeded 70% in wo MSW acili ies (MSW1 and MSW2). On he con a y,
acco ding o he esul s o he SS acili ies, in his bio-ma e ial, he consump ion o he
soluble ac ion was mainly a ibu ed o he educ ion in he ni ogen ac ion o he o -
ganic ma e . The educ ion in his ni ogen ac ion, as a whole, is widely desc ibed in
compos ing [38].
Figu e 2. Pe cen age o deg ada ion in o ganic ac ions du ing indus ial compos ing p ocesses. Abb e ia ions:
ege al esidue (VR); municipal solid was e (MSW); sewage sludge (SS); holocellulose (cellulose + hemicellulose);
soluble ac ion (include educing suga s, o al p o eins, N-NO3- and N-NH4+); Soluble O ganic Ca bon (SOC).
Figu e 2.
Pe cen age o deg ada ion in o ganic ac ions du ing indus ial compos ing p ocesses.
Abb e ia ions: ege al esidue (VR); municipal solid was e (MSW); sewage sludge (SS); holocellulose
(cellulose + hemicellulose); soluble ac ion (include educing suga s, o al p o eins, N-NO
3
−
and
N-NH4+); Soluble O ganic Ca bon (SOC).
3.3. Fecal Con amina ion in Compos s
The s udy o ecal con amina ion in he p esen s udy acked ou ep esen a i e
g oups: coli o ms ( o al, ecal and Esche ichia coli), ecal en e ococci, sul i e- educing
clos idia and Salmonella spp. The i s h ee indica o g oups we e quan i ied in hei le els
in compos s (Figu e 3a–d), while o Salmonella spp., only he p esence was sea ched. In ad-
di ion, he eco ded le els o en e ococci and E. coli we e compa ed wi h hose es ablished
by cu en Eu opean legisla ion [
39
]. This legisla ion indica es ha compos s o be used as
e ilize s canno con ain le els highe han 1000 c u g
−1
in E. coli o en e ococci (bu no
necessa y in bo h g oups) and mus ha e an absence o Salmonella spp. in 25 g o compos .
Acco ding o he esul s ob ained, only one acili y exceeded hese mic obial limi s and
con ained Salmonella spp. in he compos , MSW2. The es o he acili ies ul illed he
legisla ion. E en so, ou acili ies exceeded he en e ococci con en (VR2, MSW2, MSW3
and SS3). Howe e , hey did no exceed he E. coli and Salmonella spp. limi s, so hey
complied wi h he equi emen s. These esul s showed ha he ecal en e ococci g oup
was mo e esis an o he condi ions p e ailing du ing compos ing han he E. coli and
Appl. Sci. 2021,11, 7525 8 o 13
Salmonella spp. g oups. This beha io was also epo ed in compos ing by [
40
]. The jus i i-
ca ion o his esis ance lies in he e y na u e o he g oup o en e ococci. This mic obial
g oup, unlike he o he wo, is G am-posi i e and has a sligh ly highe he mo ole ance.
Sul i e- educing clos idia a e no delimi ed in he Eu opean legisla ion. In spi e o his,
he quan i ica ion o his g oup is o g ea in e es because i is a good indica o o ecal
con amina ion, and complemen a y o hose commonly used [
41
]. The coun s ob ained in
his s ic anae obic g oup we e ound o be closely linked o he na u e o he compos ed
was e. Thus, hose bio-ma e ials whe e anae obic si ua ions a e c ea ed, con ained a highe
amoun o sul i e- educing clos idia (SS > MSW > VR). In sho , acco ding o ou esul s,
he compos ing p ocesses ca ied ou in indus ial acili ies we e able o gene a e adequa e
sani iza ion condi ions o p oduce e ilize s ha sa is y he cu en legisla ion; wi h he
excep ion o he MSW2 ea men acili y, whe e minimum he mal sani iza ion condi ions
we e no main ained. The compos gene a ed in he la e acili y can only be disposed o
in a con olled land ill.
Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 o 14
3.3. Fecal Con amina ion in Compos s
The s udy o ecal con amina ion in he p esen s udy acked ou ep esen a i e
g oups: coli o ms ( o al, ecal and Esche ichia coli), ecal en e ococci, sul i e- educing clos-
idia and Salmonella spp. The i s h ee indica o g oups we e quan i ied in hei le els
in compos s (Figu e 3a–d), while o Salmonella spp., only he p esence was sea ched. In
addi ion, he eco ded le els o en e ococci and E. coli we e compa ed wi h hose es ab-
lished by cu en Eu opean legisla ion [39]. This legisla ion indica es ha compos s o be
used as e ilize s canno con ain le els highe han 1000 c u g−1 in E. coli o en e ococci
(bu no necessa y in bo h g oups) and mus ha e an absence o Salmonella spp. in 25 g o
compos . Acco ding o he esul s ob ained, only one acili y exceeded hese mic obial
limi s and con ained Salmonella spp. in he compos , MSW2. The es o he acili ies ul-
illed he legisla ion. E en so, ou acili ies exceeded he en e ococci con en (VR2, MSW2,
MSW3 and SS3). Howe e , hey did no exceed he E. coli and Salmonella spp. limi s, so
hey complied wi h he equi emen s. These esul s showed ha he ecal en e ococci
g oup was mo e esis an o he condi ions p e ailing du ing compos ing han he E. coli
and Salmonella spp. g oups. This beha io was also epo ed in compos ing by [40]. The
jus i ica ion o his esis ance lies in he e y na u e o he g oup o en e ococci. This mi-
c obial g oup, unlike he o he wo, is G am-posi i e and has a sligh ly highe he mo ol-
e ance. Sul i e- educing clos idia a e no delimi ed in he Eu opean legisla ion. In spi e
o his, he quan i ica ion o his g oup is o g ea in e es because i is a good indica o o
ecal con amina ion, and complemen a y o hose commonly used [41]. The coun s ob-
ained in his s ic anae obic g oup we e ound o be closely linked o he na u e o he
compos ed was e. Thus, hose bio-ma e ials whe e anae obic si ua ions a e c ea ed, con-
ained a highe amoun o sul i e- educing clos idia (SS > MSW > VR). In sho , acco ding
o ou esul s, he compos ing p ocesses ca ied ou in indus ial acili ies we e able o
gene a e adequa e sani iza ion condi ions o p oduce e ilize s ha sa is y he cu en
legisla ion; wi h he excep ion o he MSW2 ea men acili y, whe e minimum he mal
sani iza ion condi ions we e no main ained. The compos gene a ed in he la e acili y
can only be disposed o in a con olled land ill.
Figu e 3. Fecal con amina ion le els in indus ial acili ies (Log (c u g-1 compos )). (a) Vege al esidue (VR) acili ies, (b)
municipal solid was e (MSW) acili ies, (c) sewage sludge (SS) acili ies, (d) all he indus ial acili ies analyzed pe aw
ma e ials. Abb e ia ions: SR Clos idia: sul i e- educing clos idia; R.L: egula ion law acco ding o [39].
Figu e 3.
Fecal con amina ion le els in indus ial acili ies (Log (c u g
−1
compos )). (
a
) Vege al esidue (VR) acili ies,
(
b
) municipal solid was e (MSW) acili ies, (
c
) sewage sludge (SS) acili ies, (
d
) all he indus ial acili ies analyzed pe aw
ma e ials. Abb e ia ions: SR Clos idia: sul i e- educing clos idia; R.L: egula ion law acco ding o [39].
3.4. Compos s Cha ac e iza ion and Disc iminan and Co ela ions Analysis
The cha ac e iza ion o he inal compos s p oduced was decisi e in o de o know
in de ail which p ocesses esul ed in adequa ely ma u ed p oduc s. Resul s o he con ol
pa ame e s, oge he wi h he ma u i y indexes, a e shown in Table 3. The educ ion in
he mois u e con en o he was e gene a es a se e e educ ion in mic obial su i al, wi h
he consequen he mal dec ease in he piles. Thus, on occasions o was e sa u a ion,
he dehyd a ion o he ma e ial can cause a alse sensa ion o was e s abiliza ion [
42
].
MSW1 and MSW2 we e placed unde his p emise. Rega ding he inal pH alues, nei he
he sewage sludge no he MSW con ained a pH ha could hinde plan g ow h. In
ac , he mos un a o able alues o his pa ame e we e ob ained in he g een was es
(VR1 and VR3). In ac , his esul is usual in he case o g een was es o ho icul u al
o igin [
43
]. In he same way, he elec ical conduc i i y alues ound do no ep esen
a hind ance o c ops, excep o MSW3 and he VR acili ies. These con ol pa ame e s
signi ican ly a ec he de elopmen o compos ing. The e o e, hey ha e an impac on
he deg ee o mine aliza ion eached du ing he p ocess [
44
]. Acco ding o he esul s
ob ained, he deg ada ion o o ganic ma e in indus ial compos ing was in ense. The
Appl. Sci. 2021,11, 7525 9 o 13
alues we e be ween 26% (SS2) and 63% (VR2 and MSW3). P ecisely, his in e al ma ked
he di e en ia ion be ween he lowes and highes phy o oxici y de ec ed among he
samples. The e o e, he moni o ing o o ganic ma e was ele an o know he deg ee
o ma u i y o he bio-ma e ials. Simila ly, he C/N a io in o ms abou he a ailabili y
o unconsumed nu ien s in he piles [
45
]. In he p esen s udy, his pa ame e ob ained
i s maximum alue in he mos phy o oxic compos (MSW3) and he minimum in he
mos ma u e compos s (SS1 and SS2). Howe e , o know in de ail he s a e o ma u i y
and s abiliza ion, i is bes o use speci ic pa ame e s o his pu pose. Acco ding o he
specialized li e a u e on his subjec , he mos accu a e measu emen s a e hose ha include
a espi ome y and biological es [
46
]. This is p ecisely wha was chosen in his s udy. The
espi ome ic indices AT
4
and DRI p esen ed minimum alues in compos s o he indus ial
p ocesses o SS. In addi ion, he GI esul s es ablished a sepa a ion be ween he p ocesses
ha gene a ed pho o oxic (<50%), mode a ely phy o oxic (50–80%) and non-phy o oxic
(80–100%) ma e ials. As an o e all esul , only wo indus ial ea men acili ies, bo h SS,
we e able o gene a e phy o oxic- ee compos s. This esul highligh s he ha d wo k ahead
o he imp o emen o was e ea men acili ies a he indus ial le el.
Table 3. P ope ies o inal compos om indus ial compos ing acili ies *.
Facili y ** M (%) pH EC
(mS cm−1)OM (%) C/N AT4
(gO2kg−1OM)
DRI
(gO2kg−1OM h−1)GI (%)
VR1 41.05 g 9.18 8.48 c 48.43 c 11.73 bc 23.20 b 0.47 cd 45.79 bc
VR2 20.46 c 8.08 d 17.36 e 63.30 14.09 d 30.05 b 0.44 cb 2.66 a
VR3 24.72 d 9.68 g 9.97 d 39.64 b 10.91 b 25.72 b 0.38 bc 46.43 bc
MSW1 11.31 b 8.66 e 4.97 b 53.91 e 11.79 c 34.23 c 0.47 cd 32.73 b
MSW2 5.66 a 7.50 b 5.58 b 38.05 b 15.63 e 75.78 d 0.73 d 45.31 bc
MSW3 50.95 h 6.00 a 10.29 d 63.65 22.44 30.23 b 0.50 cd 0.00 a
SS1 30.80 e 7.72 c 4.67 b 47.19 c 8.92 a 1.32 a 0.04 a 91.08 d
SS2 32.33 8.26 d 2.72 a 26.09 a 9.63 a 9.04 a 0.20 acb 99.80 d
SS3 32.39 8.52 e 5.52 b 50.18 d 14.21 d 7.99 a 0.09 ab 52.68 c
* Da a a e mean alues (n = 3), hose wi h he same le e in he same column a e no signi ican ly di e en om each o he (LSD, p< 0.05).
Abb e ia ions: M: mois u e; EC: elec ical conduc i i y; OM: o ganic ma e ; C/N: ca bon-ni ogen a io; AT
4
: dynamic accumula ed
espi a ion ac i i y; DIR: dynamic espi ome ic index GI: ge mina ion index. All da a a e on a d y weigh basis. ** Vege al esidue (VR);
municipal solid was e (MSW); sewage sludge (SS).
In his s udy, ma u i y indexes, physico-chemical pa ame e s and mic obiological
con amina ion we e e alua ed in he compos s p oduced. This allowed he pe o mance o
a s a is ical s udy ha included a disc iminan and co ela ion analysis. The disc iminan
analysis loading plo o con ol pa ame e s, ecal con amina ion indica o s and deg ada ion
pe cen ages o he o ganic ac ions om municipal solid was e (MSW), sewage sludge (SS)
and ege al esidue (VR) is ep esen ed in Figu e 4. Da a a e g ouped in h ee classes: I,
MSW; II, VR; III, SS. This analysis was pe o med including he pa ame e s ga he ed in
Table 3and Figu es 2and 3. Two disc iminan unc ions we e ob ained ha explained 100%
o he a ia ion. The i s disc iminan unc ion accoun s o 80.33% o he a ia ion and
sepa a ed he samples in o h ee g oups in unc ion o he aw ma e ial. The second unc-
ion explained 19.67% o he a ia ion and sepa a ed he g een was es o bo h u ban was es
e alua ed in he wo k. In bo h unc ions, he sepa a ion was mainly based on espi ome y
indexes, he ecal coli o m g oup and holocellulose deg ada ion. This indica es ha bo h
he o igin o he was e o be ea ed and he sani a ion and s abiliza ion ope a ions du ing
compos ing we e undamen al o gene a e compos s o ag onomic in e es .