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A comparative life cycle assessment on mono- and co-digestion of food waste and sewage sludge
Available online at www.sciencedirect.com Available online at www.sciencedirect.com
ScienceDirect ScienceDirect
Energyonline Procedia 00 (2018) 000–000 Available onlineatat www.sciencedirect.com Available www.sciencedirect.com Energy Procedia 00 (2018) 000–000
ScienceDirect ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Energy Procedia 158 Energy Procedia 00(2019) (2017)4166–4171 000–000 www.elsevier.com/locate/procedia
10th th
International Conference on Applied Energy (ICAE2018), 22-25 August 2018, Hong Kong, 10 International Conference on Applied Energy China(ICAE2018), 22-25 August 2018, Hong Kong, China
A comparative life cycle assessment on mono- and co-digestion of The 15th International Symposium onon District Heating Cooling A comparative life cycle assessment monoandandco-digestion of food waste and sewage sludge food waste and sewage sludge Assessing the feasibility of using the heat demand-outdoor b Huanhuan Tongaa, Yen-Wah Tonga,* a,*, Ying Hong Pengb Huanhuan Tongdistrict , Ying Hong temperature functionTong for, aYen-Wah long-term heatPeng demand forecast Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585, a a
Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585, Singapore a,b,c a a b c c Singapore Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai Institute of Knowledge Based Engineering, School of Mechanical Engineering, b Institute of Knowledge Based Engineering, School of Mechanical200240, Engineering, China Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai a 200240, China IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Abstract Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France b
I. Andrić
*, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre
Abstract It is common practice for Singapore wastewater treatment plant (WWTP) to treat primary sludge and secondary active sludge by It is common practice for to Singapore wastewater treatment (WWTP) to treat primary and secondary active sludge by anaerobic digestion (AD) recover energy through biogas.plant However, sewage sludge (SS) sludge is low in biodegradable organic matter Abstract anaerobic digestion (AD) to recover energy through biogas. However, sludgeand (SS) low in biodegradable organic matter and biomethane potential, which result in low AD efficiency both fromsewage a processing aniseconomic standpoint. Co-digestion of and biomethane potential, which(FW) resultnot in low efficiency both fromalternative a processing an management, economic standpoint. of SS together with food waste onlyADprovides a practical forand FW but alsoCo-digestion improves the District heating are (FW) commonly addressed in the literature as digester one of for the most effective for improves decreasing the SS together with food waste only provides a practical alternative FW management, but also the performance and networks energy efficiency ofnot often under-performed anaerobic in WWTP, as the solutions potential synergistic effect greenhouse gas emissions from thethe building sector. These systems require high investments which returned through heat performance energy of often under-performed anaerobic digester intoWWTP, asco-digestion the are potential effect between FW and SS couldefficiency increase biogas production and system stability. Prior applying insynergistic reality, lifethe cycle sales. Due to climate and building renovation policies, demand in the future couldlife between FW(LCA) andthe SSischanged could increase theconditions biogasthe production and system stability. Prior toheat applying instrategy reality, cycle assessment required to understand environmental advantages and drawbacks of theco-digestion co-digestion sodecrease, that it prolonging the investment return period. It was assessment (LCA) is required to understand the environmental advantages drawbacks of the co-digestion strategy so that in it can be implemented more sustainability. found that everyday 4400and tonne of SS and 1000 t of FW were generated Thebemain scope of this paper is topossible assess the feasibility of the heat demand – of outdoor temperature function forgenerated heat demand can implemented more sustainability. Itoutcomes was found thatusing 4400 SS and 1000and t ofantagonistic, FW were in Singapore. For co-digestion, the may beeveryday categorized astonne neutral, synergistic, if methane forecast. The of Alvalade, located in Lisbon (Portugal), wasofused as a case study. The is consisted of the 665 Singapore. For district co-digestion, possible outcomes may be categorized as neutral, synergistic, anddistrict antagonistic, if when methane production from the mixture isthe equivalent, higher or lower than the sum mono-digestion. The LCA results show that buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district production from the mixture is equivalent, higher or lower than the sum of mono-digestion. The LCA results show that when the antagonistic situation happens, the co-digestion system becomes much less favorable, although it requires less water consumption renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, values antagonistic happens, the key co-digestion becomesare much less favorable, although itmaximization requiresheat lessdemand water consumption and land footsituation print. Therefore, the efforts of system plant manager suggested to be spent on theobtained of biogas output.were compared with results from a the dynamic heat demand previously developed and validated by the authors. and land foot print. Therefore, key efforts of plantmodel, manager are suggested to be spent on the maximization of biogas output. The results that when weather change is considered, the margin of error could be acceptable for some applications Copyright © showed 2018 Elsevier Ltd. only All rights reserved. ©(the 2019 The Published by Elsevier Ltd.20% for all weather scenarios considered). However, after introducing renovation error annual demand lower Copyright ©inAuthors. 2018 Elsevier Ltd.was All rights than reserved. Selection and peer-review under responsibility of the scientific committee of Applied Energy Symposium and Forum 2018: Low This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) scenarios, the error value increased upCUE2018. to 59.5% (depending the weather and renovation scenarios combination Selection and peer-review under responsibility of the scientific on committee of Applied Energy Symposium and Forum considered). 2018: Low carbon cities and urban energy systems, Peer-review under responsibility of the scientific committee of ICAE2018 – The 10th International Conference on Applied Energy. The value slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the carbon citiesof and urban energy systems, CUE2018. decrease Type in the number of heating hours by ofsemicolons 22-139h during the heating season (depending on the combination of weather and Keywords: your keywords here, separated ; Keywords: Type your keywords here, separated semicolons renovation scenarios considered). On thebyother hand, ;function intercept increased for 7.8-12.7% per decade (depending on the coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and the accuracy of heat demand estimations. 1.improve Introduction
1. Introduction © Food 2017 The Authors. by Elsevier waste (FW) Published is considered as an Ltd. attractive feedstock for anaerobic digestion (AD), as carbohydrate, protein, Peer-review under responsibility of the Scientific Committee of Thefor 15th International Symposium onasDistrict Heating and is considered an attractive feedstock anaerobic digestion (AD), carbohydrate, protein, andFood lipidwaste in the(FW) FW can be readilyas converted to biogas under anaerobic condition [1]. However, process inhibition Cooling. and lipid in the FW can be readily converted to biogas under anaerobic condition [1]. However, process inhibition Keywords: Heat demand; Forecast; Climate change 1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. 1876-6102 Copyright © 2018 Elsevier Ltd. All of rights reserved. committee of the Applied Energy Symposium and Forum 2018: Low carbon cities Selection and peer-review under responsibility the scientific Selection peer-review responsibility of the scientific committee of the Applied Energy Symposium and Forum 2018: Low carbon cities and urbanand energy systems, under CUE2018. and urban energy systems, CUE2018. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 – The 10th International Conference on Applied Energy. 10.1016/j.egypro.2019.01.814
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may occur in AD of FW alone due to the imbalance in nutrient content, accumulation of volatile fatty acids (VFAs) and process inhibition by high ammonia or salt content (mainly NaCl) [2]. Co-digesting FW with sewage sludge (SS) has been shown consistently in laboratory and full-scale studies to have a synergistic effect, which could overcome imbalance in nutrients, utilize the bacterial diversities in both substrate, and dilute the potential inhibitory [3-8]. All these effects lead to increase in the organic loading rate (OLR), biogas production and system stability. To assess the environmental advantages and disadvantages of mono- and co-digestion from a holistic view, life cycle assessment (LCA) was performed in this study. On one hand, the result of this study could provide information to governmental-level organizations on technology selection. On the other hand, it helps to identify key elements that contribute large environmental impact in the treatment chain. This finding suggests future direction of system improvement. 2. Life cycle assessment 2.1 Goal and scope definition of the LCA study The goal of this LCA was to evaluate the environmental performances of mono-digestion and co-digestion of FW & sludge. In this study, the functional unit (FU) was defined as the treatment of 1,000 tonnes of Singapore eatery FW [9] and 4,400 tonnes of wet sludge from wastewater treatment plant daily. There are currently four wastewater treatment plants (WTTPs) in Singapore to handle 100% of the used water from both the domestic and non-domestic sectors. In 2015, 574.8 Million m3 of wastewater were treated to standards fit for discharge into the sea. Based on the interviews with an official from PUB, it was found that 0.078 kg of dry primary sludge and 0.032 kg of dry activated sludge were generated per cubic meter of treated wastewater [10]. Assuming that the sludge entered the digester with the solid content of 4% [11], a total of 4,400 tonne of wet sludge needs to be digested daily. 2.2 Life cycle inventory Fig.1 drafts the system boundary of the mono- and co-digestion AD scenarios. The detailed description of each scenario is presented under respective subsections. (a)
Reject
Biogas
Wastewater
FW
Wet separation
Digestion
Dewater
Digestate
Incineration
Water Sludge
(b) Sludge
FW
Digestate
Digestion
Dewater
Biogas
Wastewater
Screw press separation
Reject
Digestion
Dewater
Biogas
Wastewater
Digestate
Incineration
Incineration
Fig.1 System boundaries for mono-digestion scenario (a) and co-digestion scenario (b)
Huanhuan Tong et al. / Energy Procedia 158 (2019) 4166–4171 Author name / Energy Procedia 00 (2018) 000–000
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2.2.1 Mono-digestion scenario (mono-AD) In this scenario, the eatery FW was digested in the dedicated FW-AD plant. After shredding, the FW was mixed with water to form aqueous suspension, and the natural buoyancy and sedimentation forces helped to separate the suspension into three layers. The floating layer (mainly containing plastics, foils, textiles, and wood) was skimmed off and the bottom layer (including glass, metal and heavy bones) was collected from base. The middle layer contained the putrescible biomass and was pumped to digester for biogas generation. Researchers showed that the added water could also help to transport the volatile fatty acid (VFAs) produced by acidogenic bacteria to methanogens in the digester [12]. This not only avoids the pH drop and the potential inhibition on the methanogenic microbes caused by VFAs accumulation, but also accelerates the hydrolysis process, leading to a better methane yield and VS reduction rate [13]. The detailed description was included our previous study [9]. Meanwhile the sewage sludge was digested in the WWTP. According to the site data from Singapore WTTPs, 43% of the sludge VS could be destroyed in the digester with the methane yield of 218 m3/tonne of incoming VS [11]. The element composition of the local sludge was extracted from work of Chan et al. [14]. 2.2.2 Co-digestion scenario (Co-AD) In co-digestion scenario, the eatery FW was collected and transported to WWTP. The screw press was employed to separate the inorganic impurity from the biomass. Screw press was selected instead of wet separation in codigestion scenario, as screw press required no water addition during treatment. In comparison, wet density separation requires a water addition of 0.6 m3 per tonne of incoming FW [9], which added burden to the already stressed freshwater supply in Singapore. Moreover, the sewage sludge already had a low total solid (TS) content of 2-4%. Water addition in FW should be avoided as this could further lower the OLR during co-digestion, and result in the inefficient utilization of digester capacity. Screw press separation applied high pressure to squeeze the incoming FW in extrusion chambers with perforation. The slurry, which was extruded through perforations, was purified organic fraction. The impurities (such as plastic, wooden substances, and metal) and certain portion of organic matter sticking to impurity materials were routed through extrusion chamber into the reject fraction. Based on the literature, the rejection rate for impurity and biomass were assumed as 95% and 30% by wet weight (Table 1). The biomass in rejected fraction was found to have the water content around 60% [15-17]. The mass flow of the screw press pretreatment is presented in Fig. 2. The resultant biomass slurry was co-digested with the primary sludge and secondary sludge in the digester. For codigestion, the possible outcomes may be categorized as neutral, synergistic, and antagonistic, if methane production from the mixture was equivalent, higher or lower than the sum of mono-digestion. Synergistic results may occur, if co-digestion overcome imbalance in nutrients, utilize the bacterial diversities in both substrate, improve buffer capacity and dilute the potential inhibitory. Neutral outcome happens, when the different substrate do not interfere each other during co-digestion. However, wastes containing potential toxicants, such as ammonia and sanitizers, or toxicant precursors, such as organic nitrogen, may inhibit biogas production during co-digestion, causing an antagonistic effect.
Fig.2 The mass flow of the screw press pretreatment for treating FW
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Table 1 Comparison of two pretreatment methods: screw press and wet density separation
In order to take into account all the possible outcomes, four co-digestion scenarios were modelled, which were denoted as Co-AD-30%, Co-AD-10%, Co-AD-neutral, Co-AD+10%. Table 2 lists the detailed explanation of these scenarios. Table 2 Four co-digestion scenarios Scenario
Comments
Co-AD-30%
Antagonism occurs, and methane yield of co-digestion is 30% less than the sum of mono-digestion.
Co-AD-10%
Antagonism occurs, and methane yield of co-digestion is 10% less than the sum of mono-digestion.
Co-AD-neutral
Neutral outcome happens, where methane yield of co-digestion is equal to the sum of mono-digestion.
Co-AD+10%
Synergistic effects occur, and methane yield of co-digestion is 10% more than the sum of mono-digestion.
3. Results and discussion Table 3 lists the environmental impact scores for the studied scenarios. The negative net score means that the scenario brings savings to environment, while the positive score means that the scenario generates burdens to the environment. The value in the bracket represents the percentage change compared with the Mono-AD scenario (positive value: increased environmental burden; negative value: decreased environmental burden). The results in Table 3 clearly show that the environmental burdens decrease with the increase of the biogas output. The CoAD+10% has the best environmental profile in all the impact categories among the Co-AD scenarios, ascribing to the raised biogas yield by synergistic effect. The power generation from biogas brings great environmental benefits, as the electricity from AD plant could replace the grid mix and avoid the consumption of fossil fuel as well as the pollution emission associated with the excavation and consumption of these fossil fuels. Except Odeplet, Co-AD-neutral performs worse than Mono-AD in all the other impact categories (Table 3). The improvement of Odeplet in Co-AD scenarios is due to the adoption of screw press pretreatment. The wet separation in Mono-AD scenario required 0.6 m3 of water per tonne FW, which has a high contribution to ODeplet impacts. In Singapore, where freshwater is scarce, 42% of the water in AD plant is supplied by sea water desalination. The
Huanhuan Tong et al. / Energy Procedia 158 (2019) 4166–4171 Author name / Energy Procedia 00 (2018) 000–000
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ODeplet burden of water consumption in wet separation is associated with the emission of CFC-113 (1,1,2Trichloro-1,2,2-trifluoroethane), which serves as an organic solvent during membrane fabrication and escapes into atmosphere to create ozone depletion problem [9]. The screw press in Co-AD scenario is assumed to have no water demanding (Table 3). However, as a compromise, the screw press has a higher loss of biomass within the range of 20-43%, which is the double of that of wet density separation (3-22%). The lost biomass ends in the reject faction, and is sent together with other inorganic impurity for incineration disposal. Given that these biomass still contains 60% of moisture, combustion is far less energy-efficient that digestion. Fig. 3 shows that the incineration of reject in Co-AD-neutral brings 132 MJ more electricity per FU than that in Mono-AD. Meanwhile 256 MJ less electricity per FU is harvested from FW-AD process in Co-AD-neutral. To sum up, Co-AD-neutral achieves 124 MJ less electricity output than Mono-AD. As electricity output is a great environmental advantage, the less power generation in CoAD-neutral scenario causes worse performance in comparison to Mono-AD. Even when synergistic effect occurs and the biogas yield is further increases by 10%, Co-AD scenario still cannot match up to Mono-AD. Table 3 Total environmental impacts of the considered scenarios (impact per functional unit) and the percentage change (%) of the specific CoAD scenario compared with Mono-AD scenario.
* Value in the bracket is calculated based on the following equations: Change% = (Co-ADX)–(Mono-AD)/Abs(Mono-AD), where X represent different Co-AD scenarios, i.e. -30%, -10%, -neutral and +10%.
4. Discussion Energy plays a dominant role in LCA, and hence the scenario with higher energy output earns more environmental credits. The results of this study suggest that significant efforts should be spent on maximizing biogas production during AD process. For example, better sorting method can be developed to effectively remove the impurity and at the same time retain as much biomass as possible in the following digestion process. Plant operating parameters should be adjusted and optimized to make sure synergy occur during co-digestion. AD
Reject Incineration
Digestate Incineration
100 0
Electricity (GJ)
-100 -200 -300 -400 -500 -600 -700 -800
FW
Sludge
Mono-AD -960 GJ
FW
Sludge
Co-AD-30% -599 GJ
FW
Sludge
Co-AD-10% -776 GJ
FW
Sludge
Co-AD-neutral -839 GJ
FW
Sludge
Co-AD+10% -915 GJ
Fig.3 Energy balance for the investigated five scenarios
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In this study, it is assumed that the screw press results in 30% loss of wet biomass into the reject fraction, which seems too pessimistic. Further endeavours are needed to collect more data from the co-digestion site to improve the accuracy and the representativeness of this data. Acknowledgements This research/project is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme. References [1] Chiu, S.L.H. and I.M.C. Lo, Reviewing the anaerobic digestion and co-digestion process of food waste from the perspectives on biogas production performance and environmental impacts. Environ Sci Pollut Res Int, 2016. 23(24): p. 24435-24450. [2] Dai, X., et al., High-solids anaerobic co-digestion of sewage sludge and food waste in comparison with mono digestions: stability and performance. Waste Manag, 2013. 33(2): p. 308-16. [3] Zhang, C., et al., Reviewing the anaerobic digestion of food waste for biogas production. Renewable and Sustainable Energy Reviews, 2014. 38: p. 383-392. [4] Zupančič, G.D., N. Uranjek-Ževart, and M. Roš, Full-scale anaerobic co-digestion of organic waste and municipal sludge. Biomass and Bioenergy, 2008. 32(2): p. 162-167. [5] Bolzonella, D., et al., Anaerobic codigestion of waste activated sludge and OFMSW: the experiences of Viareggio and Treviso plants (Italy). Water Science and Technology, 2006. 53(8): p. 203-211. [6] Edwards, J., et al., Anaerobic co-digestion of municipal food waste and sewage sludge: A comparative life cycle assessment in the context of a waste service provision. Bioresour Technol, 2017. 223: p. 237-249. [7] Lijo, L., et al., Decentralised schemes for integrated management of wastewater and domestic organic waste: the case of a small community. J Environ Manage, 2016. [8] Prabhu, M.S. and S. Mutnuri, Anaerobic co-digestion of sewage sludge and food waste. Waste Management & Research, 2016. 34(4): p. 307315. [9] Tong, H., et al., A comparative life cycle assessment on four waste-to-energy scenarios for food waste generated in eateries. Applied Energy, 2018((Accepted)). [10] Ng, B.J.H., et al., Municipal food waste management in Singapore: practices, challenges and recommendations. Journal of Material Cycles and Waste Management, 2015. 19(1): p. 560-569. [11] Yeshi, C., et al., Mass flow and energy efficiency in a large water reclamation plant in Singapore. Journal of Water Reuse and Desalination, 2013. 3(4): p. 402. [12] Nagao, N., et al., Maximum organic loading rate for the single-stage wet anaerobic digestion of food waste. Bioresource Technology, 2012. 118: p. 210-218. [13] Liotta, F., et al., Effect of total solids content on methane and volatile fatty acid production in anaerobic digestion of food waste. Waste Management and Research, 2014. 32(10): p. 947-953. [14] Chan, W.P. and J.Y. Wang, Comparison study on thermal degradation behaviours and product distributions for various types of sludge by using TG-FTIR and fixed bed pyrolysis. Journal of Analytical and Applied Pyrolysis, 2016. 121: p. 177-189. [15] Anaergia, Organic digestion from MSW. http://dpw.lacounty.gov/epd/ConversionTechnology/DownLoad/Anaergia_Organics_Solutions.pdf. 2014. [16] Bernstad, A., et al., Need for improvements in physical pretreatment of source-separated household food waste. Waste Manag, 2013. 33(3): p. 746-54. [17] Hansen, T.L., et al., Effects of pre-treatment technologies on quantity and quality of source-sorted municipal organic waste for biogas recovery. Waste Manag, 2007. 27(3): p. 398-405. [18] Cavinato, C., et al., Mesophilic and thermophilic anaerobic co-digestion of waste activated sludge and source sorted biowaste in pilot- and full-scale reactors. Renewable Energy, 2013. 55: p. 260-265. [19] Sanscartier, D., H.L. Maclean, and B. Saville, Electricity production from anaerobic digestion of household organic waste in Ontario: techno-economic and GHG emission analyses. Environ Sci Technol, 2012. 46(2): p. 1233-42. [20] Gandolfi, P.B., V. Nosiglia, and G. Vitali, Anaerobic digestion of municipal solid waste, biowaste & commercial wastes-examples of 1) successful revamping of existing plants 2) co-digestion of biowaste anad commercial waste with argicultural residues. http://www.biotecsistemi.it/media/cms/EU_BC20122DV.3.31.pdf. 2012. [21] Shuros, W.A. and C.L. Hartog, Source separated organic materials anaerobic digestion feasibility study. http://static1.squarespace.com/static/55118948e4b06b1b4f71b1f4/t/5613f95fe4b0b5fb6110529b/1444149599527/anaerobic_digestion_feasibility _study.pdf. 2009. [22] Cavinato, C., et al., Mesophilic and thermophilic anaerobic co-digestion of waste activated sludge and source sorted biowaste in pilot- and full-scale reactors. Renewable Energy, 2013. 55: p. 260-265.
ScienceDirect ScienceDirect
Energyonline Procedia 00 (2018) 000–000 Available onlineatat www.sciencedirect.com Available www.sciencedirect.com Energy Procedia 00 (2018) 000–000
ScienceDirect ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Energy Procedia 158 Energy Procedia 00(2019) (2017)4166–4171 000–000 www.elsevier.com/locate/procedia
10th th
International Conference on Applied Energy (ICAE2018), 22-25 August 2018, Hong Kong, 10 International Conference on Applied Energy China(ICAE2018), 22-25 August 2018, Hong Kong, China
A comparative life cycle assessment on mono- and co-digestion of The 15th International Symposium onon District Heating Cooling A comparative life cycle assessment monoandandco-digestion of food waste and sewage sludge food waste and sewage sludge Assessing the feasibility of using the heat demand-outdoor b Huanhuan Tongaa, Yen-Wah Tonga,* a,*, Ying Hong Pengb Huanhuan Tongdistrict , Ying Hong temperature functionTong for, aYen-Wah long-term heatPeng demand forecast Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585, a a
Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585, Singapore a,b,c a a b c c Singapore Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai Institute of Knowledge Based Engineering, School of Mechanical Engineering, b Institute of Knowledge Based Engineering, School of Mechanical200240, Engineering, China Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai a 200240, China IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France c Abstract Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France b
I. Andrić
*, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre
Abstract It is common practice for Singapore wastewater treatment plant (WWTP) to treat primary sludge and secondary active sludge by It is common practice for to Singapore wastewater treatment (WWTP) to treat primary and secondary active sludge by anaerobic digestion (AD) recover energy through biogas.plant However, sewage sludge (SS) sludge is low in biodegradable organic matter Abstract anaerobic digestion (AD) to recover energy through biogas. However, sludgeand (SS) low in biodegradable organic matter and biomethane potential, which result in low AD efficiency both fromsewage a processing aniseconomic standpoint. Co-digestion of and biomethane potential, which(FW) resultnot in low efficiency both fromalternative a processing an management, economic standpoint. of SS together with food waste onlyADprovides a practical forand FW but alsoCo-digestion improves the District heating are (FW) commonly addressed in the literature as digester one of for the most effective for improves decreasing the SS together with food waste only provides a practical alternative FW management, but also the performance and networks energy efficiency ofnot often under-performed anaerobic in WWTP, as the solutions potential synergistic effect greenhouse gas emissions from thethe building sector. These systems require high investments which returned through heat performance energy of often under-performed anaerobic digester intoWWTP, asco-digestion the are potential effect between FW and SS couldefficiency increase biogas production and system stability. Prior applying insynergistic reality, lifethe cycle sales. Due to climate and building renovation policies, demand in the future couldlife between FW(LCA) andthe SSischanged could increase theconditions biogasthe production and system stability. Prior toheat applying instrategy reality, cycle assessment required to understand environmental advantages and drawbacks of theco-digestion co-digestion sodecrease, that it prolonging the investment return period. It was assessment (LCA) is required to understand the environmental advantages drawbacks of the co-digestion strategy so that in it can be implemented more sustainability. found that everyday 4400and tonne of SS and 1000 t of FW were generated Thebemain scope of this paper is topossible assess the feasibility of the heat demand – of outdoor temperature function forgenerated heat demand can implemented more sustainability. Itoutcomes was found thatusing 4400 SS and 1000and t ofantagonistic, FW were in Singapore. For co-digestion, the may beeveryday categorized astonne neutral, synergistic, if methane forecast. The of Alvalade, located in Lisbon (Portugal), wasofused as a case study. The is consisted of the 665 Singapore. For district co-digestion, possible outcomes may be categorized as neutral, synergistic, anddistrict antagonistic, if when methane production from the mixture isthe equivalent, higher or lower than the sum mono-digestion. The LCA results show that buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district production from the mixture is equivalent, higher or lower than the sum of mono-digestion. The LCA results show that when the antagonistic situation happens, the co-digestion system becomes much less favorable, although it requires less water consumption renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, values antagonistic happens, the key co-digestion becomesare much less favorable, although itmaximization requiresheat lessdemand water consumption and land footsituation print. Therefore, the efforts of system plant manager suggested to be spent on theobtained of biogas output.were compared with results from a the dynamic heat demand previously developed and validated by the authors. and land foot print. Therefore, key efforts of plantmodel, manager are suggested to be spent on the maximization of biogas output. The results that when weather change is considered, the margin of error could be acceptable for some applications Copyright © showed 2018 Elsevier Ltd. only All rights reserved. ©(the 2019 The Published by Elsevier Ltd.20% for all weather scenarios considered). However, after introducing renovation error annual demand lower Copyright ©inAuthors. 2018 Elsevier Ltd.was All rights than reserved. Selection and peer-review under responsibility of the scientific committee of Applied Energy Symposium and Forum 2018: Low This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) scenarios, the error value increased upCUE2018. to 59.5% (depending the weather and renovation scenarios combination Selection and peer-review under responsibility of the scientific on committee of Applied Energy Symposium and Forum considered). 2018: Low carbon cities and urban energy systems, Peer-review under responsibility of the scientific committee of ICAE2018 – The 10th International Conference on Applied Energy. The value slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the carbon citiesof and urban energy systems, CUE2018. decrease Type in the number of heating hours by ofsemicolons 22-139h during the heating season (depending on the combination of weather and Keywords: your keywords here, separated ; Keywords: Type your keywords here, separated semicolons renovation scenarios considered). On thebyother hand, ;function intercept increased for 7.8-12.7% per decade (depending on the coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and the accuracy of heat demand estimations. 1.improve Introduction
1. Introduction © Food 2017 The Authors. by Elsevier waste (FW) Published is considered as an Ltd. attractive feedstock for anaerobic digestion (AD), as carbohydrate, protein, Peer-review under responsibility of the Scientific Committee of Thefor 15th International Symposium onasDistrict Heating and is considered an attractive feedstock anaerobic digestion (AD), carbohydrate, protein, andFood lipidwaste in the(FW) FW can be readilyas converted to biogas under anaerobic condition [1]. However, process inhibition Cooling. and lipid in the FW can be readily converted to biogas under anaerobic condition [1]. However, process inhibition Keywords: Heat demand; Forecast; Climate change 1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. 1876-6102 Copyright © 2018 Elsevier Ltd. All of rights reserved. committee of the Applied Energy Symposium and Forum 2018: Low carbon cities Selection and peer-review under responsibility the scientific Selection peer-review responsibility of the scientific committee of the Applied Energy Symposium and Forum 2018: Low carbon cities and urbanand energy systems, under CUE2018. and urban energy systems, CUE2018. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the scientific committee of ICAE2018 – The 10th International Conference on Applied Energy. 10.1016/j.egypro.2019.01.814
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may occur in AD of FW alone due to the imbalance in nutrient content, accumulation of volatile fatty acids (VFAs) and process inhibition by high ammonia or salt content (mainly NaCl) [2]. Co-digesting FW with sewage sludge (SS) has been shown consistently in laboratory and full-scale studies to have a synergistic effect, which could overcome imbalance in nutrients, utilize the bacterial diversities in both substrate, and dilute the potential inhibitory [3-8]. All these effects lead to increase in the organic loading rate (OLR), biogas production and system stability. To assess the environmental advantages and disadvantages of mono- and co-digestion from a holistic view, life cycle assessment (LCA) was performed in this study. On one hand, the result of this study could provide information to governmental-level organizations on technology selection. On the other hand, it helps to identify key elements that contribute large environmental impact in the treatment chain. This finding suggests future direction of system improvement. 2. Life cycle assessment 2.1 Goal and scope definition of the LCA study The goal of this LCA was to evaluate the environmental performances of mono-digestion and co-digestion of FW & sludge. In this study, the functional unit (FU) was defined as the treatment of 1,000 tonnes of Singapore eatery FW [9] and 4,400 tonnes of wet sludge from wastewater treatment plant daily. There are currently four wastewater treatment plants (WTTPs) in Singapore to handle 100% of the used water from both the domestic and non-domestic sectors. In 2015, 574.8 Million m3 of wastewater were treated to standards fit for discharge into the sea. Based on the interviews with an official from PUB, it was found that 0.078 kg of dry primary sludge and 0.032 kg of dry activated sludge were generated per cubic meter of treated wastewater [10]. Assuming that the sludge entered the digester with the solid content of 4% [11], a total of 4,400 tonne of wet sludge needs to be digested daily. 2.2 Life cycle inventory Fig.1 drafts the system boundary of the mono- and co-digestion AD scenarios. The detailed description of each scenario is presented under respective subsections. (a)
Reject
Biogas
Wastewater
FW
Wet separation
Digestion
Dewater
Digestate
Incineration
Water Sludge
(b) Sludge
FW
Digestate
Digestion
Dewater
Biogas
Wastewater
Screw press separation
Reject
Digestion
Dewater
Biogas
Wastewater
Digestate
Incineration
Incineration
Fig.1 System boundaries for mono-digestion scenario (a) and co-digestion scenario (b)
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2.2.1 Mono-digestion scenario (mono-AD) In this scenario, the eatery FW was digested in the dedicated FW-AD plant. After shredding, the FW was mixed with water to form aqueous suspension, and the natural buoyancy and sedimentation forces helped to separate the suspension into three layers. The floating layer (mainly containing plastics, foils, textiles, and wood) was skimmed off and the bottom layer (including glass, metal and heavy bones) was collected from base. The middle layer contained the putrescible biomass and was pumped to digester for biogas generation. Researchers showed that the added water could also help to transport the volatile fatty acid (VFAs) produced by acidogenic bacteria to methanogens in the digester [12]. This not only avoids the pH drop and the potential inhibition on the methanogenic microbes caused by VFAs accumulation, but also accelerates the hydrolysis process, leading to a better methane yield and VS reduction rate [13]. The detailed description was included our previous study [9]. Meanwhile the sewage sludge was digested in the WWTP. According to the site data from Singapore WTTPs, 43% of the sludge VS could be destroyed in the digester with the methane yield of 218 m3/tonne of incoming VS [11]. The element composition of the local sludge was extracted from work of Chan et al. [14]. 2.2.2 Co-digestion scenario (Co-AD) In co-digestion scenario, the eatery FW was collected and transported to WWTP. The screw press was employed to separate the inorganic impurity from the biomass. Screw press was selected instead of wet separation in codigestion scenario, as screw press required no water addition during treatment. In comparison, wet density separation requires a water addition of 0.6 m3 per tonne of incoming FW [9], which added burden to the already stressed freshwater supply in Singapore. Moreover, the sewage sludge already had a low total solid (TS) content of 2-4%. Water addition in FW should be avoided as this could further lower the OLR during co-digestion, and result in the inefficient utilization of digester capacity. Screw press separation applied high pressure to squeeze the incoming FW in extrusion chambers with perforation. The slurry, which was extruded through perforations, was purified organic fraction. The impurities (such as plastic, wooden substances, and metal) and certain portion of organic matter sticking to impurity materials were routed through extrusion chamber into the reject fraction. Based on the literature, the rejection rate for impurity and biomass were assumed as 95% and 30% by wet weight (Table 1). The biomass in rejected fraction was found to have the water content around 60% [15-17]. The mass flow of the screw press pretreatment is presented in Fig. 2. The resultant biomass slurry was co-digested with the primary sludge and secondary sludge in the digester. For codigestion, the possible outcomes may be categorized as neutral, synergistic, and antagonistic, if methane production from the mixture was equivalent, higher or lower than the sum of mono-digestion. Synergistic results may occur, if co-digestion overcome imbalance in nutrients, utilize the bacterial diversities in both substrate, improve buffer capacity and dilute the potential inhibitory. Neutral outcome happens, when the different substrate do not interfere each other during co-digestion. However, wastes containing potential toxicants, such as ammonia and sanitizers, or toxicant precursors, such as organic nitrogen, may inhibit biogas production during co-digestion, causing an antagonistic effect.
Fig.2 The mass flow of the screw press pretreatment for treating FW
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Table 1 Comparison of two pretreatment methods: screw press and wet density separation
In order to take into account all the possible outcomes, four co-digestion scenarios were modelled, which were denoted as Co-AD-30%, Co-AD-10%, Co-AD-neutral, Co-AD+10%. Table 2 lists the detailed explanation of these scenarios. Table 2 Four co-digestion scenarios Scenario
Comments
Co-AD-30%
Antagonism occurs, and methane yield of co-digestion is 30% less than the sum of mono-digestion.
Co-AD-10%
Antagonism occurs, and methane yield of co-digestion is 10% less than the sum of mono-digestion.
Co-AD-neutral
Neutral outcome happens, where methane yield of co-digestion is equal to the sum of mono-digestion.
Co-AD+10%
Synergistic effects occur, and methane yield of co-digestion is 10% more than the sum of mono-digestion.
3. Results and discussion Table 3 lists the environmental impact scores for the studied scenarios. The negative net score means that the scenario brings savings to environment, while the positive score means that the scenario generates burdens to the environment. The value in the bracket represents the percentage change compared with the Mono-AD scenario (positive value: increased environmental burden; negative value: decreased environmental burden). The results in Table 3 clearly show that the environmental burdens decrease with the increase of the biogas output. The CoAD+10% has the best environmental profile in all the impact categories among the Co-AD scenarios, ascribing to the raised biogas yield by synergistic effect. The power generation from biogas brings great environmental benefits, as the electricity from AD plant could replace the grid mix and avoid the consumption of fossil fuel as well as the pollution emission associated with the excavation and consumption of these fossil fuels. Except Odeplet, Co-AD-neutral performs worse than Mono-AD in all the other impact categories (Table 3). The improvement of Odeplet in Co-AD scenarios is due to the adoption of screw press pretreatment. The wet separation in Mono-AD scenario required 0.6 m3 of water per tonne FW, which has a high contribution to ODeplet impacts. In Singapore, where freshwater is scarce, 42% of the water in AD plant is supplied by sea water desalination. The
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ODeplet burden of water consumption in wet separation is associated with the emission of CFC-113 (1,1,2Trichloro-1,2,2-trifluoroethane), which serves as an organic solvent during membrane fabrication and escapes into atmosphere to create ozone depletion problem [9]. The screw press in Co-AD scenario is assumed to have no water demanding (Table 3). However, as a compromise, the screw press has a higher loss of biomass within the range of 20-43%, which is the double of that of wet density separation (3-22%). The lost biomass ends in the reject faction, and is sent together with other inorganic impurity for incineration disposal. Given that these biomass still contains 60% of moisture, combustion is far less energy-efficient that digestion. Fig. 3 shows that the incineration of reject in Co-AD-neutral brings 132 MJ more electricity per FU than that in Mono-AD. Meanwhile 256 MJ less electricity per FU is harvested from FW-AD process in Co-AD-neutral. To sum up, Co-AD-neutral achieves 124 MJ less electricity output than Mono-AD. As electricity output is a great environmental advantage, the less power generation in CoAD-neutral scenario causes worse performance in comparison to Mono-AD. Even when synergistic effect occurs and the biogas yield is further increases by 10%, Co-AD scenario still cannot match up to Mono-AD. Table 3 Total environmental impacts of the considered scenarios (impact per functional unit) and the percentage change (%) of the specific CoAD scenario compared with Mono-AD scenario.
* Value in the bracket is calculated based on the following equations: Change% = (Co-ADX)–(Mono-AD)/Abs(Mono-AD), where X represent different Co-AD scenarios, i.e. -30%, -10%, -neutral and +10%.
4. Discussion Energy plays a dominant role in LCA, and hence the scenario with higher energy output earns more environmental credits. The results of this study suggest that significant efforts should be spent on maximizing biogas production during AD process. For example, better sorting method can be developed to effectively remove the impurity and at the same time retain as much biomass as possible in the following digestion process. Plant operating parameters should be adjusted and optimized to make sure synergy occur during co-digestion. AD
Reject Incineration
Digestate Incineration
100 0
Electricity (GJ)
-100 -200 -300 -400 -500 -600 -700 -800
FW
Sludge
Mono-AD -960 GJ
FW
Sludge
Co-AD-30% -599 GJ
FW
Sludge
Co-AD-10% -776 GJ
FW
Sludge
Co-AD-neutral -839 GJ
FW
Sludge
Co-AD+10% -915 GJ
Fig.3 Energy balance for the investigated five scenarios
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In this study, it is assumed that the screw press results in 30% loss of wet biomass into the reject fraction, which seems too pessimistic. Further endeavours are needed to collect more data from the co-digestion site to improve the accuracy and the representativeness of this data. Acknowledgements This research/project is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme. References [1] Chiu, S.L.H. and I.M.C. Lo, Reviewing the anaerobic digestion and co-digestion process of food waste from the perspectives on biogas production performance and environmental impacts. Environ Sci Pollut Res Int, 2016. 23(24): p. 24435-24450. [2] Dai, X., et al., High-solids anaerobic co-digestion of sewage sludge and food waste in comparison with mono digestions: stability and performance. Waste Manag, 2013. 33(2): p. 308-16. [3] Zhang, C., et al., Reviewing the anaerobic digestion of food waste for biogas production. Renewable and Sustainable Energy Reviews, 2014. 38: p. 383-392. [4] Zupančič, G.D., N. Uranjek-Ževart, and M. Roš, Full-scale anaerobic co-digestion of organic waste and municipal sludge. Biomass and Bioenergy, 2008. 32(2): p. 162-167. [5] Bolzonella, D., et al., Anaerobic codigestion of waste activated sludge and OFMSW: the experiences of Viareggio and Treviso plants (Italy). Water Science and Technology, 2006. 53(8): p. 203-211. [6] Edwards, J., et al., Anaerobic co-digestion of municipal food waste and sewage sludge: A comparative life cycle assessment in the context of a waste service provision. Bioresour Technol, 2017. 223: p. 237-249. [7] Lijo, L., et al., Decentralised schemes for integrated management of wastewater and domestic organic waste: the case of a small community. J Environ Manage, 2016. [8] Prabhu, M.S. and S. Mutnuri, Anaerobic co-digestion of sewage sludge and food waste. Waste Management & Research, 2016. 34(4): p. 307315. [9] Tong, H., et al., A comparative life cycle assessment on four waste-to-energy scenarios for food waste generated in eateries. Applied Energy, 2018((Accepted)). [10] Ng, B.J.H., et al., Municipal food waste management in Singapore: practices, challenges and recommendations. Journal of Material Cycles and Waste Management, 2015. 19(1): p. 560-569. [11] Yeshi, C., et al., Mass flow and energy efficiency in a large water reclamation plant in Singapore. Journal of Water Reuse and Desalination, 2013. 3(4): p. 402. [12] Nagao, N., et al., Maximum organic loading rate for the single-stage wet anaerobic digestion of food waste. Bioresource Technology, 2012. 118: p. 210-218. [13] Liotta, F., et al., Effect of total solids content on methane and volatile fatty acid production in anaerobic digestion of food waste. Waste Management and Research, 2014. 32(10): p. 947-953. [14] Chan, W.P. and J.Y. Wang, Comparison study on thermal degradation behaviours and product distributions for various types of sludge by using TG-FTIR and fixed bed pyrolysis. Journal of Analytical and Applied Pyrolysis, 2016. 121: p. 177-189. [15] Anaergia, Organic digestion from MSW. http://dpw.lacounty.gov/epd/ConversionTechnology/DownLoad/Anaergia_Organics_Solutions.pdf. 2014. [16] Bernstad, A., et al., Need for improvements in physical pretreatment of source-separated household food waste. Waste Manag, 2013. 33(3): p. 746-54. [17] Hansen, T.L., et al., Effects of pre-treatment technologies on quantity and quality of source-sorted municipal organic waste for biogas recovery. Waste Manag, 2007. 27(3): p. 398-405. [18] Cavinato, C., et al., Mesophilic and thermophilic anaerobic co-digestion of waste activated sludge and source sorted biowaste in pilot- and full-scale reactors. Renewable Energy, 2013. 55: p. 260-265. [19] Sanscartier, D., H.L. Maclean, and B. Saville, Electricity production from anaerobic digestion of household organic waste in Ontario: techno-economic and GHG emission analyses. Environ Sci Technol, 2012. 46(2): p. 1233-42. [20] Gandolfi, P.B., V. Nosiglia, and G. Vitali, Anaerobic digestion of municipal solid waste, biowaste & commercial wastes-examples of 1) successful revamping of existing plants 2) co-digestion of biowaste anad commercial waste with argicultural residues. http://www.biotecsistemi.it/media/cms/EU_BC20122DV.3.31.pdf. 2012. [21] Shuros, W.A. and C.L. Hartog, Source separated organic materials anaerobic digestion feasibility study. http://static1.squarespace.com/static/55118948e4b06b1b4f71b1f4/t/5613f95fe4b0b5fb6110529b/1444149599527/anaerobic_digestion_feasibility _study.pdf. 2009. [22] Cavinato, C., et al., Mesophilic and thermophilic anaerobic co-digestion of waste activated sludge and source sorted biowaste in pilot- and full-scale reactors. Renewable Energy, 2013. 55: p. 260-265.