Energetic and economic assessment of sludge thermal hydrolysis in novel wastewater treatment plant configurations

Energetic and economic assessment of sludge thermal hydrolysis in novel wastewater treatment plant configurations Anton Taboada-Santos, Juan M. Lema and Marta Carballa Accepted Manuscript How to cite: Taboada-Santos, A., Lema, J., & Carballa, M. (2019). Energetic and economic assessment of sludge thermal hydrolysis in novel wastewater treatment plant configurations Waste Management, 92 30- 38. doi: https://doi.org/10.1016/j.wasman.2019.05.003 Copyright information: © Elsevier 2019 Energetic and economic assessment of sludge thermal hydrolysis in 1 novel wastewater treatment plant configurations 2 Anton Taboada-Santos*, Juan M. Lema and Marta Carballa 3 Department of Chemical Engineering, School of Engineering, Universidade de Santiago 4 de Compostela, E- 15782, Santiago de Compostela, Spain. 5 *Anton Taboada-Santos: corresponding author 6 E-mail: [email protected] 7 Tel: +34 881 816021; Fax: +34 881 816702 8 9 E-mail addresses: [email protected] (A. Taboada-Santos), [email protected] (J.M. 10 Lema), marta.carba[email protected] (M. Carballa). 11 Abstract 12 Novel wastewater treatment plants (WWTPs) are aimed to be more energetically 13 efficient than conventional ones. Their first step is a chemical oxygen demand (COD) 14 preconcentration stage with different alternatives, such as rotating belt filters (RBF), 15 chemically enhanced primary treatment (CEPT), high-rate activated sludge (HRAS), or 16 combinations thereof, in which energy requirements are substantially reduced. The 17 COD recovered as sludge allows a noticeable increase of biogas production in anaerobic 18 digestion (AD). In conventional WWTPs, sludge anaerobic biodegradability can be 19 significantly enhanced by applying sludge pretreatment methods, such as thermal 20 hydrolysis (TH), before AD. However, considering that novel-sludges are more 21 anaerobically biodegradable than conventional ones, the impact of TH on their methane 22 production is expected to result significantly lower. In this study, an energetic and 23 economic assessment of applying TH in novel WWTPs was performed. We found that 24 TH is only justified to reduce operational costs as long as sludge TS concentration in the 25 feeding to the TH unit is higher than 1-2%. The HRAS is the scenario that leads to the 26 lowest treatment costs (below 1 c€/m3 wastewater if sludge is thickened over 10% of 27 TS). However, the WWTP based on CEPT for COD preconcentration leads to the 28 lowest electricity consumption (below 0.01 kWh/m3 of wastewater), but even in the 29 most favourable conditions the energy autarky was not achievable. Results show that 30 the main impact of TH is mainly due to sludge disposal savings (270,000-430,000 31 €/year for a 500,000 inhabitants WWTP) rather than the increase of energy production 32 (achieves maximum savings of 35,000-60,000 €/year). Payback time is very dependent 33 on the WWTP size, ranging from 15 to 30 years for a 100,000 inhabitants WWTP and 34 from 2 to 4 years for a 1,000,000 inhabitants WWTP. 35 Keywords: anaerobic digestion, economy of scale, energy autarky, high-rate activated 36 sludge, payback time, sludge disposal. 37 1. Introduction 38 Traditional wastewater treatment plants (WWTPs) have traditionally applied the 39 conventional nitrification-denitrification process, which consumes high amounts of 40 electrical energy and chemical oxygen demand (COD) for aeration and conversion of 41 nitrate to nitrogen, respectively (Siegrist et al., 2008). They are electrical consumers 42 with an usual demand in the range of 0.3 to 0.6 kWh/m3 wastewater treated (Gikas, 43 2017; Wan et al., 2016). Novel WWTPs are expected to be less energetically demanding 44 since the aeration requirements are lower than in conventional ones and a higher 45 methane production can be achieved (Gu et al., 2017; Wan et al., 2016). Actually, some 46 researchers consider that WWTPs can reach the energy autarky or even become net 47 producers (Garrido et al., 2013; Siegrist et al., 2008). In novel WWTPs, COD is 48 recovered in a first stage followed by a partial nitritation-Anammox (PN-Anammox) 49 unit. The COD recovered as sludge is subsequently used to produce biogas in anaerobic 50 digestion (AD). Several preconcentration alternatives can be applied, such as rotating 51 belt filters (RBF), chemically enhanced primary treatment (CEPT), high-rate activated 52 sludge (HRAS) or combinations thereof (Lotti et al., 2014). 53 In recent years, the application of different pretreatment techniques for sewage sludge 54 before AD, such as ultrasounds, high pressure homogenizer, pulse electric fields or 55 thermal hydrolysis (TH), have gained importance in order to increase biogas yield and 56 reduce the final volume of sludge (Carrère et al., 2010; Zhen et al., 2017). Among them, 57 TH is the most attractive since it leads to a more efficient energy integration in the 58 WWTP (Cano et al., 2015). Besides, this technology increases dewaterability, reduces 59 odour emissions and viscosity and removes pathogens, obtaining a sterilized sludge that 60 meets EPA Class A biosolids standards (Barber, 2016; Higgins et al., 2017; Wang et al., 61 2018). 62 European Directive 86/278/CEE promotes the use of sewage sludge in agriculture and 63 up to 4-4.5 million ton of TS of sewage sludge were used in Europe in years 2010-2012 64 as a fertilizer (http://epp. eurostat.ec.europa.eu). Although this Directive does not 65 consider the presence of pathogens, there is an agreement between policy makers, 66 scientists and population that thresholds should be implemented. For this reason, the 67 European Commission developed the 3rd draft working document on sludge (EC, 68 2000), that was not finally implemented mainly to the associated costs to the proposed 69 more restrictive thresholds. The European Commission (EC, 2008) evaluated these 70 limits in order to implement a more restrictive legislation, but the lack of consensus 71 among the Member States of the European Commission made that the Directive 72 86/278/CEE could not be modified, confirming that it is not easy to establish more strict 73 limits at European level. However,11 out of 27 EU countries have adopted more 74 restrictive legislation (Kacprzak et al., 2017; Kelessidis and Stasinakis, 2012), 75 establishing thresholds for pathogens not achievable during mesophilic AD (Astals et 76 al., 2012). Therefore, mesophilic digested sludge usually needs further treatment before 77 its agricultural use, this representing up to 50% of the WWTP costs (Vázquez-Padín et 78 al., 2011), which can be avoided when TH is included before AD. 79 The methane production increase is a second advantage. There is a consensus in the 80 literature that, whereas biomethane potential (BMP) of primary sludge is barely affected 81 by TH (<20%), BMP of waste activated sludge (WAS) can be increased up to 76% 82 (Bougrier et al., 2006a, 2006b; Carrère et al., 2010; Fdz-Polanco et al., 2008; Perez- 83 Elvira et al., 2008). However, considering the noticeable higher BMP for sludges from 84 RBF, CEPT and HRAS in comparison with traditional ones (Ge et al., 2017; Ju et al., 85 2016; Paulsrud et al., 2014), the impact of TH on increasing methane yield is expected 86 to be lower. 87 The goal of this work is to study how the energetic and economic balance in a novel 88 WWTP can be affected by the installation of a thermal hydrolysis unit. 89 2. Materials and methods 90 In this section the materials and methods used to generate the data needed to perform 91 the energetic and economic assessment are presented. 92 2.1. Wastewater treatment and sludge production 93 RBF sludge was taken from a RBF placed in Blaricum WWTP (The Netherlands), with 94 a typical mesh size of 350 µm (Behera et al., 2018) treating around 1,600 m3/h of 95 wastewater. CEPT sludge was generated in a pilot plant located in a WWTP in the 96 north-west of Spain. The pilot plant, described by Suarez et al. (2009), was fed with 100 97 L/h of wastewater and operated at a hydraulic retention time (HRT) of 30 minutes with 98 the addition of 125-150 mg/L of ferric chloride. Two types of HRAS sludge were 99 considered: HRAS I, from a WWTP in the centre of Spain, which treats an average flow 100 of 2,200 m3/h of wastewater. The plant consists on a heterotrophic HRAS reactor 101 working with a HRT of 6-7 hours and a solid retention time (SRT) of 2.5-3 days 102 followed by a secondary settling tank with an HRT of 30 minutes. HRAS II was 103 generated in a pilot plant of 50 L located in the same WWTP, working at the same 104 conditions as the full-scale HRAS reactor, but with previous primary settling. 105 COD, TSS, VSS, TS, VS and pH were characterised according to Standard Methods 106 (APHA, 2005). VFAs were measured by gas chromatography with flame ionization 107 detection (FIC, HP 5890A). 108 2.2. Thermal hydrolysis pilot plant 109 The experiments were carried out in an automatic pilot-scale thermal system described 110 by Sapkaite et al. (2017). The pilot plant consists on a feeding tank, a progressive cavity 111 pump (Pmax = 12 bar), a steam boiler, a 20 L total volume hydrolysis reactor (Vworking = 112 10 L) connected to a flash tank (V = 100 L) with outlet pipes for steam and hydrolysed 113 sludge. It is equipped with automatic valves to control the steam entrance from the 114 boiler and the sludge exit from the reactor to the flash tank. A data acquisition and 115 control system is used to measure pressure and temperature and to control the operation. 116 The pump introduces 10 L of sludge into the reactor and then the steam valve is opened 117 until pressure and temperature reach the set-point values. TH was carried out at 170 ºC 118 during 20 minutes, since some authors reported that non-biodegradable compounds 119 begin to form at higher temperatures (Dwyer et al., 2008). At the end of the reaction 120 time, the decompression valve is automatically opened and the hydrolysed sludge flows 121 to the flash tank. 122 2.3. Biomethane potential tests 123 Biomethane potential (BMP) tests of the different sludges (RBF, CEPT, HRAS I and 124 HRAS II sludges) before and after TH were carried out in an AMPTS II equipment 125 (Bioprocess Control), following the protocol described by Holliger et al. (2016). The 126 tests were conducted in 2 L bottles (1.9 L of working volume) in triplicate and with an 127 ISR (inoculum to substrate ratio in terms of VS) of 2. The inoculum was anaerobic 128 flocculant biomass (15-20 g VS/L) from a mesophilic sewage sludge anaerobic digester. 129 The reactors were dosed with macro- and micro-nutrients, and pH was adjusted to 7.2- 130 7.5 with NaOH or HCl when necessary. After flushing the head space with nitrogen, 131 they were incubated at 37ºC. Accumulated methane production was monitored over 132 time to determine the COD fraction converted into methane. The assays lasted till 133 methane production during three consecutive days was less than 1% of the total 134 production. Methane production by each sludge was calculated as the difference 135 between the average production in the bottles with substrate minus the average 136 production in the blank (residual production of the inoculums). BMP was calculated as 137 the experimental ultimate methane production, expressed in L(N)/kg VS fed, where N 138 means normal conditions (1 atm, 0ºC). Anaerobic biodegradability (AB) was expressed 139 as the percentage of the initial COD of the substrate converted to methane. At the end of 140 the test, bottles were opened and pH and VFAs concentration were measured to confirm 141 that no acidification occurred. 142 3. Energetic and economic assessment: case study 143 3.1. Novel WWTPs configurations and energy demand inventory 144 The influence of TH on the energetic and economic balance in four different WWTPs 145 configurations, depending on the mainstream COD recovery technology, was evaluated; 146 three of them referring to novel scenarios based on CEPT (Figure 1A), HRAS (Figure 147 1B) and a combination of RBF and HRAS (Figure 1C) and one conventional activated 148 sludge (CAS) process (Figure 1D). A 500,000 inhabitants equivalent WWTP, with a 149 flow rate of 125 L/inhabitant·d and a COD of 500 mg/L (Wan et al., 2016) was 150 considered for all the scenarios. The energy consumption of the different units is 151 gathered in Table 1. 152 3.2. Thermal hydrolysis unit and sludge anaerobic digestion 153 A combined heat and power (CHP) full integration plant was considered for all the 154 WWTP schemes. Therefore, heat requirements of the TH unit and digester are satisfied 155 by the exhaust gases and hot water from CHP, respectively, and electrical requirements 156 are satisfied by the CHP electricity co-generation. For other scenarios (no heat 157 integration, heat recovery from flash, etc.), it is known that the increase in energy 158 production is clearly insufficient to cover the operational energy demand of the 159 pretreatment process (Cano et al., 2015; Carrère et al., 2010). Total energy production 160 (ET, kWh/m3 sludge) in an anaerobic process depends on the volatile solids load (VSL, 161 kg VS/m3 sludge) fed into the digester and on its biomethane production (BMP, m3(N) 162 mix RBF+HRAS sludges was 12%), being this value around 2-fold higher than in 313 conventional scenarios (4%). 314 The impact of sludge TS concentration in the feeding to the TH unit on the energy 315 demand of the WWTP is represented in Figure 4A. The optimised scenario consists on 316 thickening sludges up to 20% of TS. The three novel scenarios lead to a much lower 317 energy consumption in comparison with the conventional one when TH is not applied. 318 However, in the conventional scenario TH has a more relevant effect since it can reduce 319 almost 0.05 kWh/m3 wastewater, whereas for novel scenarios a maximum energy 320 reduction of 0.02-0.03 kWh/m3 wastewater can be achieved. Even so, in any of the 321 evaluated scenarios the WWTP energy autarky can be reached. Among them, CEPT is 322 the one that allow to obtain the lowest energy demand regardless sludge TS 323 concentration. However, this does not necessary mean that this technology achieves the 324 lowest treatment costs, since other treatment costs need to be considered. 325 4.5. Impact of thermal hydrolysis on operational costs 326 . The influence of TH on these costs is displayed in Figure 4B. TH has a beneficial 327 impact on WWTP operational costs even when sludge TS concentration is 1-2% for 328 novel and also conventional scenarios. These minimum values are lower than those 329 found in the previous section for the TH unit to be energetically profitable since sludge 330 management costs are greatly reduced. Therefore, even if the TH unit becomes 331 electricity demanding it can result economically favourable. Detailed information about 332 the contribution of each factor (electricity, sludge management and coagulant) on 333 WWTP operational costs without TH is shown in Section S1 in the Supporting 334 Information. 335 Although the alternative based on CEPT was the one with the lowest energy 336 requirements, it achieves the highest operational costs (Figure 4B) (mainly due to the 337 addition of ferric chloride), which are very comparable with the conventional scenario 338 (Figure 4B). WWTP based on HRAS and on the combination of RBF+HRAS resulted 339 on 2- to 3-fold lower operational costs. The former drives to the lowest operational costs 340 which can result even below 1 c€/m3 of wastewater (Figure 4B) when sludge is 341 thickened till more than 10% of TS. Moreover, the operational costs in these novel 342 scenarios would be very low affected by a fluctuation of the electricity cost since they 343 present a considerably lower energy demand. 344 The economic impact of additional self-produced electricity and sludge disposal savings 345 are specifically shown in Figure 5. In novel scenarios, for a TS concentration of 10%, 346 almost no benefit from electricity production in novel schemes is obtained, being sludge 347 disposal savings 270,000 €/year for RBF+HRAS scenario, 320,000 €/year for HRAS 348 alternative and 430,000 €/year for CEPT scenario (Figure 5). In conventional WWTPs, 349 the economic benefits of TH due to extra self-produced electricity are much higher 350 (110,000-145,000 €/year, Figure 5), being sludge disposal comparable to those of the 351 HRAS scenario (320,000 €/year, Figure 5). In the optimised scenario, TH in novel 352 configurations drives to an additional economic benefit of 35,000-58,000 €/year due to 353 extra self-produced electricity. Moreover, the sensitivity analysis shows that a potential 354 variation in electricity cost would have a negligible impact on the economic profits of 355 TH plants. As a conclusion, it appears that the impact of TH on reducing wastewater 356 treatment costs is mainly due to sludge disposal savings rather than other energetic 357 factors. 358 4.6. Economy of scale: influence of the WWTP size on the payback time of TH unit 359 Figure 6 shows the influence of sludge TS concentration on the payback times for the 360 TH unit in the different WWTP sizes considered in this work. The comparison of the 361 different WWTP configurations for a specific size shows that the conventional scenario 362 is the one that achieves the lowest payback times, whereas the alternative based on 363 RBF+HRAS is the one achieving the highest ones. The effect of TS concentration on 364 the payback time is much more relevant in the 100,000 inhabitants WWTP (Figure 6A), 365 and more specifically in the range 5-11% of TS. The minimum payback times for this 366 WWTP size are 23 years for the scenario based on RBF+HRAS and 15-16 for the 367 others, including the conventional alternative, what might result too high. For the 368 250,000 inhabitants WWTP (Figure 6B) payback times are 2-fold decreased compared 369 to the 100,000 inhabitants WWTP, achieving minimum values of 9 years for the 370 RBF+HRAS alternative and around 6 for the others. 371 For the 500,000 and 1,000,000 inhabitants WWTPs (Figure 7C and 7D, respectively), 372 considerably lower payback periods were determined. Sludge concentration has a much 373 lower influence on payback time than in smaller WWTPs. Minimum values of 5 years 374 for RBF+HRAS configuration and of 3-4 years for the other alternatives were 375 calculated for the 500,000 inhabitants WWTP. Very similar payback periods were 376 achieved for the 1,000,000 inhabitants WWTP, which range from 2 to 4 years, being the 377 influence of TS concentration almost negligible. Specific information regarding the 378 sensitivity analysis is gathered in Section S2 in the Supporting Information. 379 5. Conclusions 380 Sludge thermal hydrolysis approaches novel WWTPs to the energy self-sufficiency, 381 which is not reachable in any of the analysed configurations. In novel WWTP schemes, 382 the impact of thermal hydrolysis on the WWTP economy is mainly due to sludge 383 disposal savings rather than other energetic factors and a minimum total solids 384 concentration of approximately 1-2% to achieve a reduction in operational costs was 385 found. Payback times for a new thermal hydrolysis unit are greatly dependent on the 386 WWTP size, showing that their profitability is considerably higher in huge WWTPs. 387 Acknowledgements 388 The authors would like to thank the EU (ID199) and AEI (PCIN2015-22) for funding, in 389 the frame of the collaborative international Consortium Pioneer_STP financed under 390 Water Joint Programming Initiative. The authors belong to the Galician Competitive 391 Research Group ED431C 2017/029 and the CRETUS Strategic Partnership (ED431E 392 2018/01). These programmers are co-funded by FEDER (EU). Authors also want to 393 thank Professor Fernando Fdz-Polanco for his support, providing the full-scale data. 394 References 395 APHA, 2005. Standard Methods for the Examination of Water and Wastewater, 21st ed. American 396 Public Health Association/American Water Works Association/Water Environment 397 Federation, Washington DC, USA. 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Energy Rev. 69, 569 559–577. doi:10.1016/j.rser.2016.11.187 570 Web sites 571 http://appsso.eurostat.ec.europa.eu/nui/show.do?dataset=env_ww_spd&lang=en (last access 572 29.10.2018). 573 Table legends 574 Table 1. Data considered for the WWTP energetic evaluation. 575 Table 2. Novel sludges physico-chemical characterization. RBF: rotating belt filters 576 sludge, CEPT: chemically enhanced primary treatment sludge, HRAS: high-rate 577 activated sludge. 578 Table 1 579 Technology Energy demand (kWh/m3 wastewater) Wastewater pumping 0.03 (Longo et al., 2016) Rotating belt filters 0.04 (Salsnes, 2016) Chemically enhanced primary treatment 0.03 (Longo et al., 2017) High-rate activated sludge reactor 0.05* (Ge et al., 2015) Partial nitritation-anammox reactor 0.25** (Schaubroeck et al., 2015) Conventional primary treatment 0.03 (Greenfield and Batstone, 2005) Activated sludge reactor 0.45*** (Gikas, 2017; Siegrist et al., 2008) * calculated from the value of 0.66 kWh/kgCODoxidized ,** calculated as 60% of those of a conventional activated sludge reactor, 580 ***calculated from the value of 0. 37 kWh/m3 which was increased 20% to consider the organic load increase due to sludge 581 supernatant recycling. 582 Figure 5 617 618 0 100,000 200,000 300,000 400,000 500,000 600,000 10% 20% 10% 20% 10% 20% 10% 20% CEPT HRAS RBF+HRAS Conventional Economic savings (€/year) Scenario Figure 6 619 620 621 10 14 18 22 26 30 5 8 11 14 17 20 Payback (years) Sludge TS concentration (%) 4 6 8 10 12 14 5 8 11 14 17 20 Payback (years) Sludge TS concentration (%) 3 4 5 6 7 8 5 8 11 14 17 20 Payback (years) Sludge TS concentration (%) 1 2 3 4 5 6 5 8 11 14 17 20 Payback (years) Sludge TS concentration (%) A B C D