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Energy demand and carbon footprint of treating household food waste compared to its prevention
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Energy Procedia Procedia 00 161(2017) (2019)000–000 17–23 Energy
2nd International Conference on Sustainable Energy and Resource Use in Food Chains, www.elsevier.com/locate/procedia ICSEF 2018, 17-19 October 2018, Paphos, Cyrpus 2nd International Conference on Sustainable Energy and Resource Use in Food Chains, 2nd International Conference on Sustainable Energy and Resource Use in Food Chains, ICSEF ICSEF 2018, 17-19 October 2018, Paphos, Cyrpus 2018,carbon 17-19 October 2018, Paphos, Cyprus household food Energy demand and footprint of treating
waste compared to on itsDistrict prevention Energy demand and carbon footprint of treating household The 15th International Symposium Heating and Cooling food wasteK.compared toCuéllar-Franca its prevention Peter C. Slorach, Harish Jeswani, Rosa and Adisa Azapagic* Assessing the feasibility of using the heat demand-outdoor Centre for Energy use in Food (CSEF), Sustainable Industrial Systems, School Chemical Azapagic* Engineering and Peter C.Sustainable Slorach, Harish K.chains Jeswani, Rosa Cuéllar-Franca andof Adisa temperature function for long-term heat demand forecast Analytical Science, Thea University of Manchester,district M13 9PL, United Kingdom Centre for Sustainable Energy use in Food chains (CSEF), Sustainable Industrial Systems, School of Chemical Engineering and a,b,c Analytical Science, a a of Manchester, M13 b 9PL, United Kingdom c c The University
Abstract a
I. Andrić
*, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre
IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal
The majority of household food waste in the EU is sent to landfill or incinerated; a slowly-increasing fraction is collected separately b Abstract Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France and utilised for c anaerobic digestion (AD) or in-vessel composting (IVC). This study evaluates life cycle environmental impacts of Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France thesemajority four options to identify thewaste most in sustainable The are compared to waste prevention, upstream The of household food the EU is alternatives. sent to landfill orresults incinerated; a slowly-increasing fraction isinclusive collectedofseparately supply-chain impacts. Thedigestion results suggest AD has the lowest,(IVC). net-negative carbon footprint –40environmental kg CO2 eq. per tonne of and utilised for anaerobic (AD) orthat in-vessel composting This study evaluates life of cycle impacts wastefour treated and the highest the lifemost cyclesustainable energy recovery efficiency 12% are withcompared respect totothe totalprevention, primary energy recovered. If all these options to identify alternatives. Theof results waste inclusive of upstream ofAbstract the heat can be utilised, then both AD that and AD incineration can achieve maximum energy recovery efficiencies of around 25%. supply-chain impacts. The results suggest has the lowest, net-negative carbon footprint of –40 kg CO2 eq. per tonne of and with a recovery 3% through therecovered. combustion of Wastetreated landfilling has highest the highest carbonenergy footprint at 193efficiency kg CO2 eq./t waste and the life cycle recovery of 12% respectefficiency to the totalofprimary energy If all landfill gas.can IVC, credited theboth production ofincineration fertiliser, hascan a carbon of 80 kg CO it has the of lowest recovery 2 eq./t and ofDistrict the heat utilised,for then AD and achievefootprint energy efficiencies around 25%. heatingbenetworks are commonly addressed in the literature asmaximum one of the most recovery effective solutions for decreasing the eq. 2savings area achieved by the incineration of foodthe waste, with a netefficiency at 1%. Under the best conditions, the greatest CO 2 CO eq./t and recovery efficiency of 3% through combustion of Waste landfilling has the highest carbon footprint at 193 kg greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat However,has this is eclipsed by the t CO thatthe canlowest be avoided by negativegas. carbon kg CO2 eq./t landfill IVC,footprint credited of for–221 the production of .fertiliser, a carbon footprint of 2,800–3,100 80 kg CO2 eq./t and it has recovery 2 eq. sales. Due to the changed climate conditions and building renovation policies, heat demand in the future could decrease, preventing avoidable andbest potentially avoidable food CO waste. while waste may of be food best waste, treated with via AD or savings are unavoidable achieved by food the incineration a netefficiency atthe 1%. Under the conditions, the greatest 2 eq.Thus, prolonging the investment return period. incineration, the footprint savings are compared to the benefits ofeclipsed waste prevention. Therefore,tfood may bebe used withinbya this is by the 2,800–3,100 CO2waste eq. that can avoided negative carbon of negligible –221 kg CO 2 eq./t. However, The main scope of paper is to assess the of feasibility of but using the heatnot demand – outdoor for heat demand circular economy to this reclaim limited amount resources, it should be considered antemperature alternative tofunction prevention. preventing the avoidable andapotentially avoidable food waste. Thus, while unavoidable food waste may be best treated via AD or forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 incineration, the savings are negligible compared to the benefits of waste prevention. Therefore, food waste may be used within a buildings that vary in botha construction period and typology. Three weather scenarios an (low, medium, and three district circular economy to reclaim limited amount of resources, but it should not be considered alternative to high) prevention. renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were © 2018 The Authors. Published by Elsevier Ltd. © 2018 The Authors. Published by Elsevier Ltd. compared with results from a dynamic heat demand model, previously developed and validated by the authors. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) This isresults an open accessthat article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) The showed when only weather change considered, the margin of error could be acceptable for some applications © 2018 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of theis2nd International Conference on Sustainable Energy and Resource Use in Selection and peer-review under responsibility of the 2nd International Conference on Sustainable and Resource Use in (the error in annual demand was lower than 20% for all weather scenarios considered). However,Energy after introducing renovation This isChains, an openICSEF2018. access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Food Food Chains, scenarios, theICSEF2018 error value under increased up to 59.5% (depending on the weather and renovation scenarios combination considered). Selection and peer-review responsibility of the 2nd International Conference on Sustainable Energy and Resource Use in The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the Keywords: Anaerobic digestion; Circular economy; Incineration; In-vessel composting; Landfilling; Life cycle assessment. Food Chains, ICSEF2018. decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and renovation scenarios considered). the other hand, function increased for 7.8-12.7% per decade (depending on the Keywords: Anaerobic digestion; CircularOn economy; Incineration; In-vesselintercept composting; Landfilling; Life cycle assessment. coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations. * Corresponding author. Tel.: +44 (0)161 306 4363 E-mail address: [email protected]
© 2017 The Authors. by Elsevier Ltd. * Corresponding author.Published Tel.: +44 (0)161 306 4363 Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and E-mail address: [email protected] 1876-6102 © 2018 The Authors. Published by Elsevier Ltd. Cooling. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/)
Selection peer-review under responsibility of the 2nd International Conference on Sustainable Energy and Resource Use in Food Chains, 1876-6102and © 2018 The Authors. Published by Elsevier Ltd. Keywords: Heat demand; Forecast; Climate change license (https://creativecommons.org/licenses/by-nc-nd/4.0/) ICSEF2018. This is an open access article under the CC BY-NC-ND Selection and peer-review under responsibility of the 2nd International Conference on Sustainable Energy and Resource Use in Food Chains, ICSEF2018. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. 1876-6102 © 2019 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the 2nd International Conference on Sustainable Energy and Resource Use in Food Chains, ICSEF2018. 10.1016/j.egypro.2019.02.053
Peter C. Slorach et al. / Energy Procedia 161 (2019) 17–23 Slorach P. C. et al./ Energy Procedia 00 (2018) 000–000
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Nomenclature AD FW IVC LCA LFG MSW PED
Anaerobic digestion Food waste In-vessel composting Life cycle assessment Landfill gas Municipal solid waste Primary energy demand
1. Introduction It has been estimated that a third of all global food produced for human consumption is wasted along the supply chains [1], contributing 6.8% to global greenhouse gas emissions [2, 3]. Within developed nations, the main cause of food waste (FW) is the consumer. In the European Union (EU), of the 454 Mt of waste consumed annually [4], 46.5 Mt is wasted by households, accounting for 53% of all EU food waste [5]. In the FW management hierarchy, prevention is considered to be the best solution, followed by redistribution for human or animal consumption, energy recovery and finally landfilling [6-8]. In accordance with this, the EU is committed to the UN target of halving food waste at the consumer and retail levels by 2030 [9]. At the same time, the EU is pushing forward with its proposed Circular Economy Package which includes a target to recycle 65% of all municipal waste by 2030 [10]. Thus, adopting circular economy perspective, waste streams could instead be considered a valuable resource. Currently, the two leading options for the extraction of valuable resources from FW are anaerobic digestion, which produces biogas and a nutrient-rich digestate, and in-vessel composting, which produces a natural fertiliser. However, taking the UK as an example, only 10% of FW is collected separately for treatment via these routes [11]; the majority is embedded in general municipal waste where it is incinerated with energy recovery or disposed of in landfill. In the UK, 7.3 Mt of food waste are generated annually by households, of which only 1.6 Mt are classed as unavoidable (e.g. banana peel, egg shells and bones) [12]. Two previous life cycle assessment (LCA) studies [13, 14] have already highlighted the high relative carbon footprint savings of FW prevention in comparison to a range of treatment options in Ireland and Sweden. This paper focuses on UK conditions to compare the carbon footprint of waste prevention and the four treatment routes: anaerobic digestion; in-vessel composting; incineration; and landfilling. The FW is not considered to be burden-free but instead is treated like a valuable resource stream and thus includes the upstream burdens in the food supply chain. Primary energy demand (PED) of food waste treatment is also estimated. 2. Methodology The LCA study has been carried out following the methodology in the ISO 14040/14044 standards [15, 16]. GaBi V8.6 [17] has been used to model the systems. The hierarchist ReCiPe methodology [18] has been applied to estimate the carbon footprint and the Thinkstep method [17] to determine PED. 2.1
Goal and scope
The main goal of the study is to estimate PED and the carbon footprint of the current food waste treatment options. A further goal is to compare the carbon footprint of the treatment to the prevention of food waste to identify more sustainable options. For these purposes, the FW is considered in a circular economy model and it, therefore, includes the burdens of the food-production supply chain. This is in contrast to conventional LCA studies of waste which typically apply a zero-burden approach. The same analysis is not possible for PED due to a lack of data for the PED of food waste on a life cycle basis. The treatment routes included are anaerobic digestion (AD) with biogas utilisation, in-vessel composting (IVC), incineration with energy recovery and landfilling with landfill gas (LFG) utilisation. Credits are applied for the generation of electricity, heat and fertilisers where applicable. Figure 1 shows the system boundaries.
Peter C. Slorach et al. / Energy Procedia 161 (2019) 17–23 Slorach P. C. et al./ Energy Procedia 00 (2018) 000–000
Facility construction
Waste treatment mix
Operation resources
Electricity export credits
Landfill
Processing
Retail
19 3
Incineration
Household food waste
Heat export credits
Anaerobic digestion
Agriculture
In-vessel composting
Fertiliser credits
Fig. 1. System boundary for the management of household food waste
To enable comparison between the four treatment routes and with the prevention of the same quantity of waste, the functional unit is defined as the treatment or prevention of 1 tonne of household food waste. The scope of the study is from the agricultural production of the food to the end of life of any products or resources produced via the waste treatment routes. This includes the agriculture, food processing, transportation, packaging, food use, waste collection, waste treatment process and the environmental credits for any waste-derived products. LCA models have been created for the waste treatment routes, while existing studies (see Table 1) have been used to determine the carbon footprint of the production, supply and use of food. The results for the waste treatments are presented for two cases: i) current situation, representing average facilities currently operating in the UK; and ii) the best case, in which energy recovery is maximised by assuming the most favourable process parameters and by utilising the heat from AD and incineration. Energy recovery efficiencies for each treatment route are estimated as a percentage of the total PED in the supply chain that can be reclaimed, inclusive of PED credits associated with displaced products. 2.2
Inventory data and assumptions
Inventory data have been obtained from a number of plants and averaged to represent a typical waste facility operating in the UK. Annual UK household food waste composition has been used to consider the prevention option, with 78% of the waste classed as avoidable or potentially avoidable and the rest as unavoidable [12]. The carbon footprint of food waste generated across the food life cycle is given in Table 1 [12, 19]. The end-of-life disposal of waste is not shown as it is assessed within this study. An overview of the four treatment methods considered in the study is provided below. Table 1. Carbon footprint of UK household food waste along the life cycle [12, 19]
Life cycle stage Agriculture Processing Logistics Packaging Use Total
Carbon footprint [t CO2 eq./t] 2.00 0.36 0.86 0.18 0.35 3.74
Contribution [%] 53.4% 9.6% 22.9% 4.7% 9.4% 100%
Anaerobic digestion: The AD facility uses a single stage continuous mesophilic digester to treat 25,000 t of FW per year. The average biogas production rate is 137 Nm3/t FW. This requires 23 kWh/t FW of electricity and 82 kWh/t FW of heat, both of which are provided by the combustion of the biogas in a combined heat and power engine [2022]. The remaining 254 kWh of electricity are exported to the grid; the heat is not currently utilised in the UK market. For the best case, the excess heat is utilised to displace natural gas heating and the parameters are set to the most favourable limits of their ranges as follows: 187 Nm3/t FW of biogas produced, and 10 kWh/t of electricity and 36
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kWh/t of heat used by the plant. In the best case, fugitive methane emissions are also reduced from the current value of 2% of biogas production to 1% [20, 23, 24]. The digestate displaces mineral fertiliser based on nutrient composition and emissions from its application, while the electricity displaces the UK grid mix [25]. In-vessel composting: The IVC is carried out in a rotating drum as described by Eades et al. [26]. The capacity is 50,000 t/yr and the electricity use has been averaged over several plants at 93 kWh/t FW for the current situation and at 30 kWh/t FW in the best case [26-30]. The compost displaces mineral fertiliser based on nutrient composition and application emissions. Incineration: The incineration plant uses a moving-grate furnace as described by Doka [31] and has been modelled with an Ecoinvent tool [32]. Based on UK operating plants [33], the plant has a capacity of treating 300,000 t of municipal solid waste (MSW) per year. The energy outputs are scaled based on the lower heating value of FW of 3.8 MJ/kg [34, 35], compared to 8.9 MJ/kg for MSW [33]. In the current case, 255 kWh/t FW of electricity is generated, of which 81 kWh/t is used by the plant [33]. The generation rises to 338 kWh/t in the best case, with 48 kWh/t used by the plant, and up to 1,316 kWh of heat available, which displaces natural gas heating. Landfilling: The sanitary landfill has been modelled with the same Ecoinvent tool as the incineration process [32]. The leachate is sent to wastewater treatment and the LFG is collected and used to generate electricity in a gas engine. Currently, 68% of LFG is collected; in the best case, the collection rate is 85% [36]. Exported electricity displaces the UK grid mix. 3. Results and discussion The PED and carbon footprint of the waste treatment routes are presented in Table 2 for the current and best cases; these impacts do not include the upstream impacts related to the food supply chain. For the current case, AD performs best for both impacts which are net-negative for this treatment option (–1,993 MJ and –40 kg CO2 eq. per tonne FW). This is due to the highest amount of electricity exported from this system and the displacement of mineral fertilisers. IVC has the highest energy demand and, as it can only displace a small quantity of fertilisers, both its impacts are netpositive (1,332 MJ/t FW and 80 kg CO2 eq./t FW). Incineration also leads to savings in both impacts due to the exported electricity, but these are lower than for the AD (–941 MJ and –10 kg CO2 eq.). In the case of landfilling, the electricity generated from LFG is not sufficient to counteract the carbon footprint primarily related to the methane emissions; hence, this option has the highest carbon footprint (193 kg CO2 eq./t FW) and second highest PED after IVC (122 MJ/t FW). In the best case, incineration becomes the most favourable option for both impacts (–4,913 MJ and –221 kg CO2 eq./t FW). Although the utilisation of all the heat generated by the plant may be unlikely, these results shows that a highly-efficient incineration plant could potentially represent a better option than an AD plant for these two criteria. For IVC, the value of the compost is not able to counteract the processing burdens, but reducing electricity use can improve the performance. For landfilling, improving the LFG capture rates allows for an overall saving in PED and a halving of the carbon footprint on the current case. The PED of the whole food supply chain from production to consumer is estimated at 20.2 GJ/t [4]. Figure 2 presents the percentage of this value recovered when 1 t of FW is treated via the four management routes; this value also takes into account the PED of each treatment. For the current situation, credits for the PED of the displaced grid electricity and fertiliser manufacture result in AD having the greatest energy recovery efficiency of 12%. If in the best case all excess heat and electricity generated is used to displace heat from natural gas and the grid mix, respectively, then incineration has the best energy recovery efficiency of 26.5%. Table 2. Primary energy demand and carbon footprint of treating 1 t of household food waste via anaerobic digestion, in-vessel composting, incineration and landfill. Primary energy demand (MJ/t FW) Current situation Best case Anaerobic digestion -1,993 -4,376 In-vessel composting 1,332 757 Incineration -941 -4,913 Landfill 122 -33 a Excluding the upstream impacts of food waste; these are shown in Table 1.
Carbon footprint (kg CO2 eq./t FW) a Current situation Best case -40 -169 80 58 -10 -221 193 95
Peter C. Slorach et al. / Energy Procedia 161 (2019) 17–23 Slorach P. C. et al./ Energy Procedia 00 (2018) 000–000
Anaerobic digestion
In-vessel composting
Incineration
Landfill
30%
26.5%
23.5%
25%
21 5
20% 15%
12.0%
10%
7.4%
5%
0.75%
0%
2.7%
3.4%
0.77%
Current situation
Best case
Fig 2. The energy recovery efficiency for the treatment of 1 t of household food waste via the four treatment routes for the current and the best cases. Efficiency expressed as the percentage of the primary energy demand recovered relative to the primary energy used in the whole food supply chain.
The overall carbon footprint for FW managed via each treatment route and inclusive of the upstream supply chain burdens (3,740 kg CO2 eq./t FW) is displayed in Figure 3. Put within the context of the whole supply chain, the carbon footprint of AD is similar to that of incineration and only 6% lower than that of landfilling for both the current and best cases. IVC has a higher impact than incineration in both cases. It is also only 3% better than landfilling for the current situation and very close to landfilling in the best case. If the total avoidable or potentially avoidable fraction (78%) of 1 t of household FW can be prevented, then the carbon footprint of the remaining unavoidable waste, when treated via the current AD, would be 810 kg CO2 eq. This means that prevention can potentially save up to 2,890 kg CO2 eq. per tonne of FW currently generated [3,740 + (– 40) – 810]. Applying the same calculation to all the treatment routes for both the current situation and best case results in potential savings from prevention of 2,800–3,100 kg CO2 eq. per tonne of food waste currently generated. Thus, although AD and incineration offer carbon footprint savings and should be prioritised for waste that does arise, it is clear that the benefits compared to waste prevention are small. Furthermore, while IVC complies with a biological circular economy, the value of compost derived from FW represents neither a saving in terms of PED nor carbon footprint. Therefore, these findings highlight the importance of a full system analysis before assuming that a circular economy solution represents the most sustainable option. Current situation
Carbon footprint (t CO₂ eq./t FW)
4
3.70
3.57
3.82
3.80
Best case 3.73
3.93 3.52
3.83
3 2 1 0
Anaerobic digestion
In-vessel composting
Incineration
Landfill
Fig 3. Total carbon footprint of the treatment of 1 t of household food waste for the current and best cases, including the carbon footprint of food waste on a life cycle basis (3.74 t CO2 eq./t FW; see Table 1).
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4. Conclusions At present, anaerobic digestion is the best option for reducing the energy demand and carbon footprint of treating household food waste. Once credited for electricity generation and fertiliser displacement, it can reclaim 12% of the embedded energy in the food chain and has a net-negative carbon footprint, saving up to 40 kg CO2 eq. per tonne of waste. AD improves to a 24% energy recovery efficiency and a carbon footprint of –169 kg CO2 eq./t for the best operating conditions and if all heat can be utilised. However, under the best conditions, the greatest savings are achieved at a highly-efficient incineration plant that can export all the excess heat and electricity. This achieves a maximum energy recovery efficiency of 27% and a net-negative carbon footprint of –221 kg CO2 eq./t. However, this is eclipsed by the 2,800–3,100 kg CO2 eq./t that can be avoided by preventing the avoidable and potentially avoidable fractions of food waste. Although energy and resources can and should be reclaimed from unavoidable food waste by applying circular economy principles, care should be taken not to make food waste a commodity as prevention offers far greater savings. Acknowledgements This work was carried out as part of the UK Centre for Sustainable Energy Use in Food Chains (CSEF), funded by the UK Research Councils (EP/K011820/1). The authors acknowledge gratefully this funding. References [1] Gustavsson, J., et al., Global food losses and food waste. 2011, The Food and Agriculture Organization of the United Nations http://www.fao.org/docrep/014/mb060e/mb060e00.pdf. [2] FAO, Food wastage footprint & Climate Change. 2015, Food and Agriculture Organisation of the United Nations: http://www.fao.org/3/abb144e.pdf. [3] The World Bank. Total greenhouse gas emissions (kt of CO2 equivalent). 2016; Available from: http://data.worldbank.org/indicator/EN.ATM.GHGT.KT.CE. 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Energy Procedia Procedia 00 161(2017) (2019)000–000 17–23 Energy
2nd International Conference on Sustainable Energy and Resource Use in Food Chains, www.elsevier.com/locate/procedia ICSEF 2018, 17-19 October 2018, Paphos, Cyrpus 2nd International Conference on Sustainable Energy and Resource Use in Food Chains, 2nd International Conference on Sustainable Energy and Resource Use in Food Chains, ICSEF ICSEF 2018, 17-19 October 2018, Paphos, Cyrpus 2018,carbon 17-19 October 2018, Paphos, Cyprus household food Energy demand and footprint of treating
waste compared to on itsDistrict prevention Energy demand and carbon footprint of treating household The 15th International Symposium Heating and Cooling food wasteK.compared toCuéllar-Franca its prevention Peter C. Slorach, Harish Jeswani, Rosa and Adisa Azapagic* Assessing the feasibility of using the heat demand-outdoor Centre for Energy use in Food (CSEF), Sustainable Industrial Systems, School Chemical Azapagic* Engineering and Peter C.Sustainable Slorach, Harish K.chains Jeswani, Rosa Cuéllar-Franca andof Adisa temperature function for long-term heat demand forecast Analytical Science, Thea University of Manchester,district M13 9PL, United Kingdom Centre for Sustainable Energy use in Food chains (CSEF), Sustainable Industrial Systems, School of Chemical Engineering and a,b,c Analytical Science, a a of Manchester, M13 b 9PL, United Kingdom c c The University
Abstract a
I. Andrić
*, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre
IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal
The majority of household food waste in the EU is sent to landfill or incinerated; a slowly-increasing fraction is collected separately b Abstract Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France and utilised for c anaerobic digestion (AD) or in-vessel composting (IVC). This study evaluates life cycle environmental impacts of Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France thesemajority four options to identify thewaste most in sustainable The are compared to waste prevention, upstream The of household food the EU is alternatives. sent to landfill orresults incinerated; a slowly-increasing fraction isinclusive collectedofseparately supply-chain impacts. Thedigestion results suggest AD has the lowest,(IVC). net-negative carbon footprint –40environmental kg CO2 eq. per tonne of and utilised for anaerobic (AD) orthat in-vessel composting This study evaluates life of cycle impacts wastefour treated and the highest the lifemost cyclesustainable energy recovery efficiency 12% are withcompared respect totothe totalprevention, primary energy recovered. If all these options to identify alternatives. Theof results waste inclusive of upstream ofAbstract the heat can be utilised, then both AD that and AD incineration can achieve maximum energy recovery efficiencies of around 25%. supply-chain impacts. The results suggest has the lowest, net-negative carbon footprint of –40 kg CO2 eq. per tonne of and with a recovery 3% through therecovered. combustion of Wastetreated landfilling has highest the highest carbonenergy footprint at 193efficiency kg CO2 eq./t waste and the life cycle recovery of 12% respectefficiency to the totalofprimary energy If all landfill gas.can IVC, credited theboth production ofincineration fertiliser, hascan a carbon of 80 kg CO it has the of lowest recovery 2 eq./t and ofDistrict the heat utilised,for then AD and achievefootprint energy efficiencies around 25%. heatingbenetworks are commonly addressed in the literature asmaximum one of the most recovery effective solutions for decreasing the eq. 2savings area achieved by the incineration of foodthe waste, with a netefficiency at 1%. Under the best conditions, the greatest CO 2 CO eq./t and recovery efficiency of 3% through combustion of Waste landfilling has the highest carbon footprint at 193 kg greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat However,has this is eclipsed by the t CO thatthe canlowest be avoided by negativegas. carbon kg CO2 eq./t landfill IVC,footprint credited of for–221 the production of .fertiliser, a carbon footprint of 2,800–3,100 80 kg CO2 eq./t and it has recovery 2 eq. sales. Due to the changed climate conditions and building renovation policies, heat demand in the future could decrease, preventing avoidable andbest potentially avoidable food CO waste. while waste may of be food best waste, treated with via AD or savings are unavoidable achieved by food the incineration a netefficiency atthe 1%. Under the conditions, the greatest 2 eq.Thus, prolonging the investment return period. incineration, the footprint savings are compared to the benefits ofeclipsed waste prevention. Therefore,tfood may bebe used withinbya this is by the 2,800–3,100 CO2waste eq. that can avoided negative carbon of negligible –221 kg CO 2 eq./t. However, The main scope of paper is to assess the of feasibility of but using the heatnot demand – outdoor for heat demand circular economy to this reclaim limited amount resources, it should be considered antemperature alternative tofunction prevention. preventing the avoidable andapotentially avoidable food waste. Thus, while unavoidable food waste may be best treated via AD or forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 incineration, the savings are negligible compared to the benefits of waste prevention. Therefore, food waste may be used within a buildings that vary in botha construction period and typology. Three weather scenarios an (low, medium, and three district circular economy to reclaim limited amount of resources, but it should not be considered alternative to high) prevention. renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were © 2018 The Authors. Published by Elsevier Ltd. © 2018 The Authors. Published by Elsevier Ltd. compared with results from a dynamic heat demand model, previously developed and validated by the authors. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) This isresults an open accessthat article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) The showed when only weather change considered, the margin of error could be acceptable for some applications © 2018 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of theis2nd International Conference on Sustainable Energy and Resource Use in Selection and peer-review under responsibility of the 2nd International Conference on Sustainable and Resource Use in (the error in annual demand was lower than 20% for all weather scenarios considered). However,Energy after introducing renovation This isChains, an openICSEF2018. access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Food Food Chains, scenarios, theICSEF2018 error value under increased up to 59.5% (depending on the weather and renovation scenarios combination considered). Selection and peer-review responsibility of the 2nd International Conference on Sustainable Energy and Resource Use in The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the Keywords: Anaerobic digestion; Circular economy; Incineration; In-vessel composting; Landfilling; Life cycle assessment. Food Chains, ICSEF2018. decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and renovation scenarios considered). the other hand, function increased for 7.8-12.7% per decade (depending on the Keywords: Anaerobic digestion; CircularOn economy; Incineration; In-vesselintercept composting; Landfilling; Life cycle assessment. coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations. * Corresponding author. Tel.: +44 (0)161 306 4363 E-mail address: [email protected]
© 2017 The Authors. by Elsevier Ltd. * Corresponding author.Published Tel.: +44 (0)161 306 4363 Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and E-mail address: [email protected] 1876-6102 © 2018 The Authors. Published by Elsevier Ltd. Cooling. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/)
Selection peer-review under responsibility of the 2nd International Conference on Sustainable Energy and Resource Use in Food Chains, 1876-6102and © 2018 The Authors. Published by Elsevier Ltd. Keywords: Heat demand; Forecast; Climate change license (https://creativecommons.org/licenses/by-nc-nd/4.0/) ICSEF2018. This is an open access article under the CC BY-NC-ND Selection and peer-review under responsibility of the 2nd International Conference on Sustainable Energy and Resource Use in Food Chains, ICSEF2018. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. 1876-6102 © 2019 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the 2nd International Conference on Sustainable Energy and Resource Use in Food Chains, ICSEF2018. 10.1016/j.egypro.2019.02.053
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Nomenclature AD FW IVC LCA LFG MSW PED
Anaerobic digestion Food waste In-vessel composting Life cycle assessment Landfill gas Municipal solid waste Primary energy demand
1. Introduction It has been estimated that a third of all global food produced for human consumption is wasted along the supply chains [1], contributing 6.8% to global greenhouse gas emissions [2, 3]. Within developed nations, the main cause of food waste (FW) is the consumer. In the European Union (EU), of the 454 Mt of waste consumed annually [4], 46.5 Mt is wasted by households, accounting for 53% of all EU food waste [5]. In the FW management hierarchy, prevention is considered to be the best solution, followed by redistribution for human or animal consumption, energy recovery and finally landfilling [6-8]. In accordance with this, the EU is committed to the UN target of halving food waste at the consumer and retail levels by 2030 [9]. At the same time, the EU is pushing forward with its proposed Circular Economy Package which includes a target to recycle 65% of all municipal waste by 2030 [10]. Thus, adopting circular economy perspective, waste streams could instead be considered a valuable resource. Currently, the two leading options for the extraction of valuable resources from FW are anaerobic digestion, which produces biogas and a nutrient-rich digestate, and in-vessel composting, which produces a natural fertiliser. However, taking the UK as an example, only 10% of FW is collected separately for treatment via these routes [11]; the majority is embedded in general municipal waste where it is incinerated with energy recovery or disposed of in landfill. In the UK, 7.3 Mt of food waste are generated annually by households, of which only 1.6 Mt are classed as unavoidable (e.g. banana peel, egg shells and bones) [12]. Two previous life cycle assessment (LCA) studies [13, 14] have already highlighted the high relative carbon footprint savings of FW prevention in comparison to a range of treatment options in Ireland and Sweden. This paper focuses on UK conditions to compare the carbon footprint of waste prevention and the four treatment routes: anaerobic digestion; in-vessel composting; incineration; and landfilling. The FW is not considered to be burden-free but instead is treated like a valuable resource stream and thus includes the upstream burdens in the food supply chain. Primary energy demand (PED) of food waste treatment is also estimated. 2. Methodology The LCA study has been carried out following the methodology in the ISO 14040/14044 standards [15, 16]. GaBi V8.6 [17] has been used to model the systems. The hierarchist ReCiPe methodology [18] has been applied to estimate the carbon footprint and the Thinkstep method [17] to determine PED. 2.1
Goal and scope
The main goal of the study is to estimate PED and the carbon footprint of the current food waste treatment options. A further goal is to compare the carbon footprint of the treatment to the prevention of food waste to identify more sustainable options. For these purposes, the FW is considered in a circular economy model and it, therefore, includes the burdens of the food-production supply chain. This is in contrast to conventional LCA studies of waste which typically apply a zero-burden approach. The same analysis is not possible for PED due to a lack of data for the PED of food waste on a life cycle basis. The treatment routes included are anaerobic digestion (AD) with biogas utilisation, in-vessel composting (IVC), incineration with energy recovery and landfilling with landfill gas (LFG) utilisation. Credits are applied for the generation of electricity, heat and fertilisers where applicable. Figure 1 shows the system boundaries.
Peter C. Slorach et al. / Energy Procedia 161 (2019) 17–23 Slorach P. C. et al./ Energy Procedia 00 (2018) 000–000
Facility construction
Waste treatment mix
Operation resources
Electricity export credits
Landfill
Processing
Retail
19 3
Incineration
Household food waste
Heat export credits
Anaerobic digestion
Agriculture
In-vessel composting
Fertiliser credits
Fig. 1. System boundary for the management of household food waste
To enable comparison between the four treatment routes and with the prevention of the same quantity of waste, the functional unit is defined as the treatment or prevention of 1 tonne of household food waste. The scope of the study is from the agricultural production of the food to the end of life of any products or resources produced via the waste treatment routes. This includes the agriculture, food processing, transportation, packaging, food use, waste collection, waste treatment process and the environmental credits for any waste-derived products. LCA models have been created for the waste treatment routes, while existing studies (see Table 1) have been used to determine the carbon footprint of the production, supply and use of food. The results for the waste treatments are presented for two cases: i) current situation, representing average facilities currently operating in the UK; and ii) the best case, in which energy recovery is maximised by assuming the most favourable process parameters and by utilising the heat from AD and incineration. Energy recovery efficiencies for each treatment route are estimated as a percentage of the total PED in the supply chain that can be reclaimed, inclusive of PED credits associated with displaced products. 2.2
Inventory data and assumptions
Inventory data have been obtained from a number of plants and averaged to represent a typical waste facility operating in the UK. Annual UK household food waste composition has been used to consider the prevention option, with 78% of the waste classed as avoidable or potentially avoidable and the rest as unavoidable [12]. The carbon footprint of food waste generated across the food life cycle is given in Table 1 [12, 19]. The end-of-life disposal of waste is not shown as it is assessed within this study. An overview of the four treatment methods considered in the study is provided below. Table 1. Carbon footprint of UK household food waste along the life cycle [12, 19]
Life cycle stage Agriculture Processing Logistics Packaging Use Total
Carbon footprint [t CO2 eq./t] 2.00 0.36 0.86 0.18 0.35 3.74
Contribution [%] 53.4% 9.6% 22.9% 4.7% 9.4% 100%
Anaerobic digestion: The AD facility uses a single stage continuous mesophilic digester to treat 25,000 t of FW per year. The average biogas production rate is 137 Nm3/t FW. This requires 23 kWh/t FW of electricity and 82 kWh/t FW of heat, both of which are provided by the combustion of the biogas in a combined heat and power engine [2022]. The remaining 254 kWh of electricity are exported to the grid; the heat is not currently utilised in the UK market. For the best case, the excess heat is utilised to displace natural gas heating and the parameters are set to the most favourable limits of their ranges as follows: 187 Nm3/t FW of biogas produced, and 10 kWh/t of electricity and 36
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kWh/t of heat used by the plant. In the best case, fugitive methane emissions are also reduced from the current value of 2% of biogas production to 1% [20, 23, 24]. The digestate displaces mineral fertiliser based on nutrient composition and emissions from its application, while the electricity displaces the UK grid mix [25]. In-vessel composting: The IVC is carried out in a rotating drum as described by Eades et al. [26]. The capacity is 50,000 t/yr and the electricity use has been averaged over several plants at 93 kWh/t FW for the current situation and at 30 kWh/t FW in the best case [26-30]. The compost displaces mineral fertiliser based on nutrient composition and application emissions. Incineration: The incineration plant uses a moving-grate furnace as described by Doka [31] and has been modelled with an Ecoinvent tool [32]. Based on UK operating plants [33], the plant has a capacity of treating 300,000 t of municipal solid waste (MSW) per year. The energy outputs are scaled based on the lower heating value of FW of 3.8 MJ/kg [34, 35], compared to 8.9 MJ/kg for MSW [33]. In the current case, 255 kWh/t FW of electricity is generated, of which 81 kWh/t is used by the plant [33]. The generation rises to 338 kWh/t in the best case, with 48 kWh/t used by the plant, and up to 1,316 kWh of heat available, which displaces natural gas heating. Landfilling: The sanitary landfill has been modelled with the same Ecoinvent tool as the incineration process [32]. The leachate is sent to wastewater treatment and the LFG is collected and used to generate electricity in a gas engine. Currently, 68% of LFG is collected; in the best case, the collection rate is 85% [36]. Exported electricity displaces the UK grid mix. 3. Results and discussion The PED and carbon footprint of the waste treatment routes are presented in Table 2 for the current and best cases; these impacts do not include the upstream impacts related to the food supply chain. For the current case, AD performs best for both impacts which are net-negative for this treatment option (–1,993 MJ and –40 kg CO2 eq. per tonne FW). This is due to the highest amount of electricity exported from this system and the displacement of mineral fertilisers. IVC has the highest energy demand and, as it can only displace a small quantity of fertilisers, both its impacts are netpositive (1,332 MJ/t FW and 80 kg CO2 eq./t FW). Incineration also leads to savings in both impacts due to the exported electricity, but these are lower than for the AD (–941 MJ and –10 kg CO2 eq.). In the case of landfilling, the electricity generated from LFG is not sufficient to counteract the carbon footprint primarily related to the methane emissions; hence, this option has the highest carbon footprint (193 kg CO2 eq./t FW) and second highest PED after IVC (122 MJ/t FW). In the best case, incineration becomes the most favourable option for both impacts (–4,913 MJ and –221 kg CO2 eq./t FW). Although the utilisation of all the heat generated by the plant may be unlikely, these results shows that a highly-efficient incineration plant could potentially represent a better option than an AD plant for these two criteria. For IVC, the value of the compost is not able to counteract the processing burdens, but reducing electricity use can improve the performance. For landfilling, improving the LFG capture rates allows for an overall saving in PED and a halving of the carbon footprint on the current case. The PED of the whole food supply chain from production to consumer is estimated at 20.2 GJ/t [4]. Figure 2 presents the percentage of this value recovered when 1 t of FW is treated via the four management routes; this value also takes into account the PED of each treatment. For the current situation, credits for the PED of the displaced grid electricity and fertiliser manufacture result in AD having the greatest energy recovery efficiency of 12%. If in the best case all excess heat and electricity generated is used to displace heat from natural gas and the grid mix, respectively, then incineration has the best energy recovery efficiency of 26.5%. Table 2. Primary energy demand and carbon footprint of treating 1 t of household food waste via anaerobic digestion, in-vessel composting, incineration and landfill. Primary energy demand (MJ/t FW) Current situation Best case Anaerobic digestion -1,993 -4,376 In-vessel composting 1,332 757 Incineration -941 -4,913 Landfill 122 -33 a Excluding the upstream impacts of food waste; these are shown in Table 1.
Carbon footprint (kg CO2 eq./t FW) a Current situation Best case -40 -169 80 58 -10 -221 193 95
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Anaerobic digestion
In-vessel composting
Incineration
Landfill
30%
26.5%
23.5%
25%
21 5
20% 15%
12.0%
10%
7.4%
5%
0.75%
0%
2.7%
3.4%
0.77%
Current situation
Best case
Fig 2. The energy recovery efficiency for the treatment of 1 t of household food waste via the four treatment routes for the current and the best cases. Efficiency expressed as the percentage of the primary energy demand recovered relative to the primary energy used in the whole food supply chain.
The overall carbon footprint for FW managed via each treatment route and inclusive of the upstream supply chain burdens (3,740 kg CO2 eq./t FW) is displayed in Figure 3. Put within the context of the whole supply chain, the carbon footprint of AD is similar to that of incineration and only 6% lower than that of landfilling for both the current and best cases. IVC has a higher impact than incineration in both cases. It is also only 3% better than landfilling for the current situation and very close to landfilling in the best case. If the total avoidable or potentially avoidable fraction (78%) of 1 t of household FW can be prevented, then the carbon footprint of the remaining unavoidable waste, when treated via the current AD, would be 810 kg CO2 eq. This means that prevention can potentially save up to 2,890 kg CO2 eq. per tonne of FW currently generated [3,740 + (– 40) – 810]. Applying the same calculation to all the treatment routes for both the current situation and best case results in potential savings from prevention of 2,800–3,100 kg CO2 eq. per tonne of food waste currently generated. Thus, although AD and incineration offer carbon footprint savings and should be prioritised for waste that does arise, it is clear that the benefits compared to waste prevention are small. Furthermore, while IVC complies with a biological circular economy, the value of compost derived from FW represents neither a saving in terms of PED nor carbon footprint. Therefore, these findings highlight the importance of a full system analysis before assuming that a circular economy solution represents the most sustainable option. Current situation
Carbon footprint (t CO₂ eq./t FW)
4
3.70
3.57
3.82
3.80
Best case 3.73
3.93 3.52
3.83
3 2 1 0
Anaerobic digestion
In-vessel composting
Incineration
Landfill
Fig 3. Total carbon footprint of the treatment of 1 t of household food waste for the current and best cases, including the carbon footprint of food waste on a life cycle basis (3.74 t CO2 eq./t FW; see Table 1).
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4. Conclusions At present, anaerobic digestion is the best option for reducing the energy demand and carbon footprint of treating household food waste. Once credited for electricity generation and fertiliser displacement, it can reclaim 12% of the embedded energy in the food chain and has a net-negative carbon footprint, saving up to 40 kg CO2 eq. per tonne of waste. AD improves to a 24% energy recovery efficiency and a carbon footprint of –169 kg CO2 eq./t for the best operating conditions and if all heat can be utilised. However, under the best conditions, the greatest savings are achieved at a highly-efficient incineration plant that can export all the excess heat and electricity. This achieves a maximum energy recovery efficiency of 27% and a net-negative carbon footprint of –221 kg CO2 eq./t. However, this is eclipsed by the 2,800–3,100 kg CO2 eq./t that can be avoided by preventing the avoidable and potentially avoidable fractions of food waste. Although energy and resources can and should be reclaimed from unavoidable food waste by applying circular economy principles, care should be taken not to make food waste a commodity as prevention offers far greater savings. Acknowledgements This work was carried out as part of the UK Centre for Sustainable Energy Use in Food Chains (CSEF), funded by the UK Research Councils (EP/K011820/1). The authors acknowledge gratefully this funding. References [1] Gustavsson, J., et al., Global food losses and food waste. 2011, The Food and Agriculture Organization of the United Nations http://www.fao.org/docrep/014/mb060e/mb060e00.pdf. [2] FAO, Food wastage footprint & Climate Change. 2015, Food and Agriculture Organisation of the United Nations: http://www.fao.org/3/abb144e.pdf. [3] The World Bank. Total greenhouse gas emissions (kt of CO2 equivalent). 2016; Available from: http://data.worldbank.org/indicator/EN.ATM.GHGT.KT.CE. 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