Bioresource Technology 102 (2011)612-620 Contents lists available at ScienceDirect BIORESOURCE TECHNǒOGY Bioresource Technology ELSEVIER journal homepage:www.elsevier.com/locate/biortech Anaerobic digestion of source-segregated domestic food waste:Performance assessment by mass and energy balance Charles J.Banks2,Michael Chesshireb,Sonia Heaven,Rebecca Arnoldb School of Civil Engineering and the Environment,University of Southampton.Southampton S017 1BJ.UK BiogenGreenfinch,The Business Park,Coder Road,Ludlow SY8 1XE,UK ARTICLE INFO ABSTRACT Article history: An anaerobic digester receiving food waste collected mainly from domestic kitchens was monitored over Received 14 July 2010 a period of 426 days.During this time information was gathered on the waste input material.the biogas Received in revised form 1 August 2010 production,and the digestate characteristics.A mass balance accounted for over 90%of the material Accepted 2 August 2010 Available online 6 August 2010 entering the plant leaving as gaseous or digestate products.A comprehensive energy balance for the same period showed that for each tonne of input material the potential recoverable energy was 405 kWh.Bio- gas production in the digester was stable at 642 m3tonne-1VS added with a methane content of around Keywords: 62%.The nitrogen in the food waste input was on average 8.9 kg tonne-1.This led to a high ammonia con- Anaerobic digestion Food waste centration in the digester which may have been responsible for the accumulation of volatile fatty acids Energy that was also observed. Biogas 2010 Elsevier Ltd.All rights reserved. Mass balance 1.Introduction and industry in methods of processing source segregated house- hold food waste by the anaerobic digestion route. Many examples exist on the use of anaerobic digestion (AD)to There are,however.reasons why food waste has not been pop- treat the mechanically separated biodegradable fraction of munici- ular in the past as a single substrate,since digestion of this energy- pal waste.Both 'wet'and 'dry'anaerobic technologies have been rich material can lead to operational problems.The protein content used as part of mechanical-biological treatment (MBT)(Mata of food waste typically gives a high nitrogen content on hydrolysis, Alvarez,2003).There are also examples of the processing of mixed which leads to elevated concentrations of ammonia or ammonium source segregated biodegradable wastes such as kitchen and gar- ion in the digester.The distribution of the two species and their den wastes(Archer et al.,2005):but there are few reports of AD relative toxicity is pH dependent,with the more toxic form domi- plants operating entirely on source segregated household food nating at higher pH(Mata-Alvarez,2003).There is still uncertainty waste.Interest in this approach is growing within Europe due to concerning the concentration at which ammonia becomes inhibi- rising energy costs associated with the processing of wet waste. tory to methanogenesis,and this is reflected in the various limit the requirement to meet the diversion targets of the EU Landfill values given in recent literature.According to Mata-Alvarez directive(99/31/EC),and the need to comply with regulations for (2003),inhibition occurs at total ammonia concentrations of the disposal of animal by-products (EC 1774/2002).When AD is 1200 mgl-1 and above.Hartmann and Ahring (2005)showed used to process source segregated waste it not only produces bio- ammonia inhibition begins at free ammonia concentrations above gas,but also presents an opportunity to recover additional value 650 mg I'NH3-N,whereas Angelidaki et al.(2005)in a study of 18 from the waste material,in the form of a quality assured nutri- full-scale biogas plants in Denmark co-digesting manure and or- ent-rich fertiliser product that can applied to agricultural land used ganic waste only found decreases in efficiency when total ammo- in food production.If the waste is not source segregated and the nia was above 4000 mg NH3-NI-1.El Hadj et al.(2009)found organic fraction is recovered through a MBT plant,regulations in that methane generation in batch tests with a high-protein syn- many European countries do not permit the digestate product to thetic biowaste under mesophilic conditions fell by 50%at ammo- be used on land in this way (Stretton-Maycock and Merrington, nium ion concentrations of 3860 mg NH-N 1-1.Although 2009).Consequently,there is strong interest from government ammonia has been shown to create operational difficulties in anaerobic digesters,it is also recognised that populations can accli- mate,making it difficult to predict the exact concentration at Corresponding author.Tel.:+44(0)2380 594650:fax:+44 (0)2380 677519 which process instability or failure may occur (Fricke et al.. E-mail address:cjb@soton.ac.uk (C.J.Banks). 2007).It has been reported on a number of occasions that digestion 0960-8524/$-see front matter2010 Elsevier Ltd.All rights reserved. doi:10.1016j.biortech.2010.08.005
Anaerobic digestion of source-segregated domestic food waste: Performance assessment by mass and energy balance Charles J. Banks a, *, Michael Chesshire b , Sonia Heaven a , Rebecca Arnold b a School of Civil Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, UK b BiogenGreenfinch, The Business Park, Coder Road, Ludlow SY8 1XE, UK article info Article history: Received 14 July 2010 Received in revised form 1 August 2010 Accepted 2 August 2010 Available online 6 August 2010 Keywords: Anaerobic digestion Food waste Energy Biogas Mass balance abstract An anaerobic digester receiving food waste collected mainly from domestic kitchens was monitored over a period of 426 days. During this time information was gathered on the waste input material, the biogas production, and the digestate characteristics. A mass balance accounted for over 90% of the material entering the plant leaving as gaseous or digestate products. A comprehensive energy balance for the same period showed that for each tonne of input material the potential recoverable energy was 405 kWh. Biogas production in the digester was stable at 642 m3 tonne1 VS added with a methane content of around 62%. The nitrogen in the food waste input was on average 8.9 kg tonne1 . This led to a high ammonia concentration in the digester which may have been responsible for the accumulation of volatile fatty acids that was also observed. 2010 Elsevier Ltd. All rights reserved. 1. Introduction Many examples exist on the use of anaerobic digestion (AD) to treat the mechanically separated biodegradable fraction of municipal waste. Both ‘wet’ and ‘dry’ anaerobic technologies have been used as part of mechanical–biological treatment (MBT) (MataAlvarez, 2003). There are also examples of the processing of mixed source segregated biodegradable wastes such as kitchen and garden wastes (Archer et al., 2005); but there are few reports of AD plants operating entirely on source segregated household food waste. Interest in this approach is growing within Europe due to rising energy costs associated with the processing of wet waste, the requirement to meet the diversion targets of the EU Landfill directive (99/31/EC), and the need to comply with regulations for the disposal of animal by-products (EC 1774/2002). When AD is used to process source segregated waste it not only produces biogas, but also presents an opportunity to recover additional value from the waste material, in the form of a quality assured nutrient-rich fertiliser product that can applied to agricultural land used in food production. If the waste is not source segregated and the organic fraction is recovered through a MBT plant, regulations in many European countries do not permit the digestate product to be used on land in this way (Stretton-Maycock and Merrington, 2009). Consequently, there is strong interest from government and industry in methods of processing source segregated household food waste by the anaerobic digestion route. There are, however, reasons why food waste has not been popular in the past as a single substrate, since digestion of this energyrich material can lead to operational problems. The protein content of food waste typically gives a high nitrogen content on hydrolysis, which leads to elevated concentrations of ammonia or ammonium ion in the digester. The distribution of the two species and their relative toxicity is pH dependent, with the more toxic form dominating at higher pH (Mata-Alvarez, 2003). There is still uncertainty concerning the concentration at which ammonia becomes inhibitory to methanogenesis, and this is reflected in the various limit values given in recent literature. According to Mata-Alvarez (2003), inhibition occurs at total ammonia concentrations of 1200 mg l1 and above. Hartmann and Ahring (2005) showed ammonia inhibition begins at free ammonia concentrations above 650 mg l1 NH3-N, whereas Angelidaki et al. (2005) in a study of 18 full-scale biogas plants in Denmark co-digesting manure and organic waste only found decreases in efficiency when total ammonia was above 4000 mg NH3-N l1 . El Hadj et al. (2009) found that methane generation in batch tests with a high-protein synthetic biowaste under mesophilic conditions fell by 50% at ammonium ion concentrations of 3860 mg NHþ 4 -N l1 . Although ammonia has been shown to create operational difficulties in anaerobic digesters, it is also recognised that populations can acclimate, making it difficult to predict the exact concentration at which process instability or failure may occur (Fricke et al., 2007). It has been reported on a number of occasions that digestion 0960-8524/$ - see front matter 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.08.005 * Corresponding author. Tel.: +44 (0)2380 594650; fax: +44 (0)2380 677519. E-mail address: cjb@soton.ac.uk (C.J. Banks). Bioresource Technology 102 (2011) 612–620 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
CJ.Banks et al/Bioresource Technology 102 (2011)612-620 613 at high ammonia concentrations can give stable biogas production and type of waste was recorded.Water usage was monitored by at alkaline pH over extended periods of time under continuous separate meters,one for the industrial process water and another loading conditions.In digesters treating food waste these condi- for staff facilities (e.g.toilets and washrooms). tions can also lead to operation at elevated levels of volatile fatty acids in the digestate (Banks et al.,2008:Neiva Correia et al., 2.2.2.Biogas sampling analysis and quantification 2008).Similar conditions have been reported in thermophilic cattle A biogas sample was taken daily from the gas holder feeding the slurry digesters(Neilsen and Angelidaki,2008),and in other nitro- CHP and analysed for methane and carbon dioxide content using a gen rich substrates such as slaughterhouse waste (Banks and GA2000 portable infrared gas analyser(Geotechnical instruments. Wang.1999;Wang and Banks 2003). Leamington Spa,UK).Biogas volumes were recorded on an indus- The current work presents the results of a mass and energy bal- trial gas flow meter,and readings were manually adjusted for ance over a 14-month period for a full-scale food waste digester water vapour content and expressed at standard temperature operating at high ammonia and VFA concentrations.During the and pressure(STP)of 273.15 K and 101.325 kPa. study period the digester was fed mainly on food waste collected from domestic properties mixed with small amounts of commer- 2.2.3.Waste input sampling and analysis cial food waste and municipal green waste.Since the study was Daily composite samples of the shredded feedstock were taken completed the plant has continued to operate successfully as a for analysis of the total solids (TS)and volatile solids(VS)content commercial facility processing food waste. according to Standard Method 2540 G (APHA,2005).Further com- posites were prepared from the daily composites over two-week 2.Methods periods for determination of Total Kjeldahl Nitrogen(N),phospho- rus(P),and potassium(K).Total Kjeldahl N was determined using a 2.1.Digestion plant Kjeltech block digestion and steam distillation unit according to the manufacturer's instructions (Foss Ltd.,Warrington,UK).Sam- The plant was commissioned in March 2006 and for the first ples for Potassium and Phosphorus were extracted using concen- 9 months of operation was fed on mixed kitchen and garden waste trated HNOs in a CEM Microwave Accelerated Reaction System collected from domestic properties.From January 2007 the feed for Extraction (MARSX)(CEM Corporation,North Carolina,USA). was gradually switched to source segregated food waste only. Potassium was quantified using a Varian Spectra AA-200 atomic The study period began on 1 June,2007(day 0).and data for the absorption spectrophotometer (Varian,Australia)according to mass and energy balances was collected for 426 days.During the the manufacturer's instructions.Phosphorus was measured spec- study 3936 tonnes of waste were processed of which 95.5%was trophotometrically by the ammonium molybdate method (ISO source-segregated domestic food waste,with the remainder con- 6878:2004) sisting of commercial food waste from restaurants and local busi- nesses (2.9%)including a small amount of whey,and grass 2.2.4.Digester and digestate sampling and analysis cuttings (1.6%).The food waste received at the plant was first Samples of digestate were taken on a regular basis for analysis. shredded in a rotary counter-shear shredder to reduce the particle Total and volatile solids were measured as above.Ammonia was size,then passed to a feed preparation vessel where it was mixed determined using a Kjeltech steam distillation unit according to with recirculated whole digestate and macerated to give a particle the manufacturer's instructions (Foss Ltd.,Warrington,UK).VFA size less than 12 mm.The feed to the digester was via a buffer stor- were quantified in a Shimazdu GC-2010 gas chromatograph,using age tank providing 3 days storage,to allow continuous feeding over a flame ionization detector and a capillary column type SGE BP-21 weekends and public holidays.The digester itself was a 900 m with helium as the carrier gas at a flow of 190.8 ml min-,with a tank that was completely mixed by continuous gas recirculation split ratio of 100 giving a flow rate of 1.86 ml min-in the column and maintained at 42C by external heat exchangers:the choice and a 3.0 ml min-purge.The GC oven temperature was pro- of temperature was based on the previous experience and prefer- grammed to increase from 60 to 210C in 15 min,with a final hold ence of the plant operator.The digestate was passed batch-wise time of 3 min.The temperatures of injector and detector were 200 to a pasteurisation tank(60 m)where it was heated to 70C for and 250 C,respectively.Samples were prepared by acidification in a minimum of 1 h.Pasteurised digestate was transferred to the dig- 2%formic acid.A standard solution containing acetic,propionic, estate storage tank(900 m3),where it was kept until being ex- iso-butyric,n-butyric,iso-valeric,valeric,hexanoic and heptanoic ported to local farms for use on agricultural land as either acids,at three dilutions giving individual acid concentrations of separated fibre,liquor or whole digestate.The biogas generated 50.250 and 500 mgI-1,respectively,was used for calibration. was used to produce electricity using a 195 kW MAN Combined Alkalinity was measured by titration using 0.25 N H2SO4 to end- Heat and Power(CHP)unit with an assumed electrical conversion points of 5.7 and 4.3(Ripley et al.,1986).Digestate pH was mea- efficiency of 32%at full load and a potential for 53%recovery of heat sured using a combination glass electrode and meter calibrated via the jacket and exhaust cooling water streams.Electricity pro- in buffers at pH 4,7 and 9. duced by the CHP and imports and exports to the grid were all me- tered.The power requirements of the plant were calculated from 3.Results and discussion (CHP generator meter grid import meter-grid export meter). Some of the heat produced by the CHP was fed back into the pro- cess.Temperatures in all tanks were recorded continuously using 3.1.Feedstock characteristics,organic loading rate and retention time a SCADA.More detailed descriptions of individual components of Fig.1 shows values for TS and VS throughout the study period the plant are given in Chesshire(2007)and Arnold et al.(2010). for the domestic food waste and the commercial food waste (not including whey)components of the feedstock.The average solids 2.2.Sampling,measurement and analysis content was similar for domestic food waste(TS 27.7%,VS 24.4%) and commercial food waste (TS 27.8%,VS 24.3%).As can be seen 2.2.1.Quantification of input waste and other materials in Fig.1a and b,there was some variation in the TS and VS content All vehicles delivering waste to the plant were weighed on a of individual samples of domestic food waste but no strong evi- weighbridge before and after discharging their load.The origin dence of seasonal variation and the VS:TS ratio remained fairly
at high ammonia concentrations can give stable biogas production at alkaline pH over extended periods of time under continuous loading conditions. In digesters treating food waste these conditions can also lead to operation at elevated levels of volatile fatty acids in the digestate (Banks et al., 2008; Neiva Correia et al., 2008). Similar conditions have been reported in thermophilic cattle slurry digesters (Neilsen and Angelidaki, 2008), and in other nitrogen rich substrates such as slaughterhouse waste (Banks and Wang, 1999; Wang and Banks 2003). The current work presents the results of a mass and energy balance over a 14-month period for a full-scale food waste digester operating at high ammonia and VFA concentrations. During the study period the digester was fed mainly on food waste collected from domestic properties mixed with small amounts of commercial food waste and municipal green waste. Since the study was completed the plant has continued to operate successfully as a commercial facility processing food waste. 2. Methods 2.1. Digestion plant The plant was commissioned in March 2006 and for the first 9 months of operation was fed on mixed kitchen and garden waste collected from domestic properties. From January 2007 the feed was gradually switched to source segregated food waste only. The study period began on 1 June, 2007 (day 0), and data for the mass and energy balances was collected for 426 days. During the study 3936 tonnes of waste were processed of which 95.5% was source-segregated domestic food waste, with the remainder consisting of commercial food waste from restaurants and local businesses (2.9%) including a small amount of whey, and grass cuttings (1.6%). The food waste received at the plant was first shredded in a rotary counter-shear shredder to reduce the particle size, then passed to a feed preparation vessel where it was mixed with recirculated whole digestate and macerated to give a particle size less than 12 mm. The feed to the digester was via a buffer storage tank providing 3 days storage, to allow continuous feeding over weekends and public holidays. The digester itself was a 900 m3 tank that was completely mixed by continuous gas recirculation and maintained at 42 C by external heat exchangers: the choice of temperature was based on the previous experience and preference of the plant operator. The digestate was passed batch-wise to a pasteurisation tank (60 m3 ) where it was heated to 70 C for a minimum of 1 h. Pasteurised digestate was transferred to the digestate storage tank (900 m3 ), where it was kept until being exported to local farms for use on agricultural land as either separated fibre, liquor or whole digestate. The biogas generated was used to produce electricity using a 195 kW MAN Combined Heat and Power (CHP) unit with an assumed electrical conversion efficiency of 32% at full load and a potential for 53% recovery of heat via the jacket and exhaust cooling water streams. Electricity produced by the CHP and imports and exports to the grid were all metered. The power requirements of the plant were calculated from (CHP generator meter + grid import meter grid export meter). Some of the heat produced by the CHP was fed back into the process. Temperatures in all tanks were recorded continuously using a SCADA. More detailed descriptions of individual components of the plant are given in Chesshire (2007) and Arnold et al. (2010). 2.2. Sampling, measurement and analysis 2.2.1. Quantification of input waste and other materials All vehicles delivering waste to the plant were weighed on a weighbridge before and after discharging their load. The origin and type of waste was recorded. Water usage was monitored by separate meters, one for the industrial process water and another for staff facilities (e.g. toilets and washrooms). 2.2.2. Biogas sampling analysis and quantification A biogas sample was taken daily from the gas holder feeding the CHP and analysed for methane and carbon dioxide content using a GA2000 portable infrared gas analyser (Geotechnical instruments, Leamington Spa, UK). Biogas volumes were recorded on an industrial gas flow meter, and readings were manually adjusted for water vapour content and expressed at standard temperature and pressure (STP) of 273.15 K and 101.325 kPa. 2.2.3. Waste input sampling and analysis Daily composite samples of the shredded feedstock were taken for analysis of the total solids (TS) and volatile solids (VS) content according to Standard Method 2540 G (APHA, 2005). Further composites were prepared from the daily composites over two-week periods for determination of Total Kjeldahl Nitrogen (N), phosphorus (P), and potassium (K). Total Kjeldahl N was determined using a Kjeltech block digestion and steam distillation unit according to the manufacturer’s instructions (Foss Ltd., Warrington, UK). Samples for Potassium and Phosphorus were extracted using concentrated HNO3 in a CEM Microwave Accelerated Reaction System for Extraction (MARSX) (CEM Corporation, North Carolina, USA). Potassium was quantified using a Varian Spectra AA-200 atomic absorption spectrophotometer (Varian, Australia) according to the manufacturer’s instructions. Phosphorus was measured spectrophotometrically by the ammonium molybdate method (ISO 6878: 2004). 2.2.4. Digester and digestate sampling and analysis Samples of digestate were taken on a regular basis for analysis. Total and volatile solids were measured as above. Ammonia was determined using a Kjeltech steam distillation unit according to the manufacturer’s instructions (Foss Ltd., Warrington, UK). VFA were quantified in a Shimazdu GC-2010 gas chromatograph, using a flame ionization detector and a capillary column type SGE BP-21 with helium as the carrier gas at a flow of 190.8 ml min1 , with a split ratio of 100 giving a flow rate of 1.86 ml min1 in the column and a 3.0 ml min1 purge. The GC oven temperature was programmed to increase from 60 to 210 C in 15 min, with a final hold time of 3 min. The temperatures of injector and detector were 200 and 250 C, respectively. Samples were prepared by acidification in 2% formic acid. A standard solution containing acetic, propionic, iso-butyric, n-butyric, iso-valeric, valeric, hexanoic and heptanoic acids, at three dilutions giving individual acid concentrations of 50, 250 and 500 mg l1 , respectively, was used for calibration. Alkalinity was measured by titration using 0.25 N H2SO4 to endpoints of 5.7 and 4.3 (Ripley et al., 1986). Digestate pH was measured using a combination glass electrode and meter calibrated in buffers at pH 4, 7 and 9. 3. Results and discussion 3.1. Feedstock characteristics, organic loading rate and retention time Fig. 1 shows values for TS and VS throughout the study period for the domestic food waste and the commercial food waste (not including whey) components of the feedstock. The average solids content was similar for domestic food waste (TS 27.7%, VS 24.4%) and commercial food waste (TS 27.8%, VS 24.3%). As can be seen in Fig. 1a and b, there was some variation in the TS and VS content of individual samples of domestic food waste but no strong evidence of seasonal variation and the VS:TS ratio remained fairly C.J. Banks et al. / Bioresource Technology 102 (2011) 612–620 613
614 CJ.Banks et aL/Bioresource Technology 102 (2011)612-620 0.5 a 0.4 0.3 ·TS VS -21-day TS 。 21-day VS 0.0 0 100 200 300 400 Day 0.5 0.5 b C 0.4 0.4 03 0.3 ⊙0.2 0.2 y=0.796x+0.023 y=0.884x-0.002 0.1 R2=0.881 0.1 R2=0.970 0.0 0.0 0.0 0.1 0.2 0.3 0.4 0.5 .0 0.1 0.2 0.3 0.4 0.5 Domestic food waste TS.gg Commercial food waste TS.gg Fig.1.TS and VS content of domestic (a and b)and commercial(c)food waste during the study period (points show average value for triplicate determinations:lines show rolling 21-day averages). constant.Fig.1c shows the TS and VS values for commercial food The average nutrient content of the domestic food waste during waste:these spanned a greater range than for the domestic food the study period was 8.9,1.9 and 3.3 kg tonne-on a wet weight waste,reflecting greater differences in moisture content,but again (WW)basis for Total Kjeldahl Nitrogen (TKN),Phosphorus (P) the ratio of VS:TS was consistent. and Potassium (K).respectively,while the equivalent values for 12 10 a 8 64 2 0 10 b y6IeM JeM 9Uuo]6 86 A 2 0 122138 171182 245258 259/272 2744288 289303 305318 319350 366381 Days ▣Nitrogen(TKN)■Phosphorus(P)▣Potassium(K Fig.2.Variability in nutrient content of domestic food waste composite samples(a)and of digestate samples(b)during the study period
constant. Fig. 1c shows the TS and VS values for commercial food waste: these spanned a greater range than for the domestic food waste, reflecting greater differences in moisture content, but again the ratio of VS:TS was consistent. The average nutrient content of the domestic food waste during the study period was 8.9, 1.9 and 3.3 kg tonne1 on a wet weight (WW) basis for Total Kjeldahl Nitrogen (TKN), Phosphorus (P) and Potassium (K), respectively, while the equivalent values for Fig. 1. TS and VS content of domestic (a and b) and commercial (c) food waste during the study period (points show average value for triplicate determinations; lines show rolling 21-day averages). Fig. 2. Variability in nutrient content of domestic food waste composite samples (a) and of digestate samples (b) during the study period. 614 C.J. Banks et al. / Bioresource Technology 102 (2011) 612–620
CJ.Banks et al./Bioresource Technology 102 (2011)612-620 615 0.10 a 0.08 pue 0.06 o 以。 00 ◆TS 0.04 ▣VS 4品 0.02 0.00+ 0 100 200 300 400 Days 0.30 ◆ 0.06 b ) 0.20 9 0.04 0.02 ejql 0.10 y=0.630x+0.001 y=0.615x+0.033 R2=0.925 R2=0.620 0.00+ 0.00+ 0.00 0.02 0.04 0.06 0.08 0.10 0.00 0.10 0.20 0.30 0.40 Digestate TS.gg Fibre TS.gg Fig.3.TS and VS content of digestate (a and b)and fibre(c)during the study period(points show average value for triplicate determinations:note different scales for digestate and fibre). commercial food waste were 8.7,1.8 and 3.4 kg tonne-1 WW. than those for feedstock,apart from one high value for nitrogen Fig.2a shows the variability in fortnightly composite samples of in day 215-227.A nutrient mass balance taking into account water domestic food waste.The variations between consecutive samples additions showed outputs equal to 86.1%,32.8%and 96.4%of the may reflect the fact that only a small amount of material is ulti- input values of TKN,P and K,respectively.The lower recovery of mately used in laboratory analysis,and however much effort is TKN and particularly of P may indicate losses by precipitation made to prepare representative composites,subsamples may show e.g.of struvite(NH4MgPO6H2O)within the digester system. slight non-homogeneity due to unavoidable scale factors. The average organic loading rate during the study period was 2.5 kg Vs m-3 day-1 based on the nominal digester volume of 3.3.Biogas output and variability 900 m',or 2.7 kg VS m day-'based on the average volume of di- gester contents.The maximum and minimum weekly average Table 1 shows the total biogas production and the proportion of loadings based on actual volume were 3.46 and 0.91 kg VS m methane and carbon dioxide based on daily measurements.The day-,respectively,with the minimum corresponding to a Christ- specific biogas and methane yields are given on both a wet weight mas closure period.The average hydraulic retention time was and a VS basis,and show the food waste has a high methane poten- 80 days,based on the nominal digester volume divided by the tial in comparison to typical municipal residual waste streams.The mass input on a wet weight basis.More detailed information on high moisture content means,however,that the biogas production day-to-day variations in feedstock quantities is given in Arnold etal.(2010). Table 1 Gas production parameters during mass and energy balance period. 3.2.Digestate characteristics Item Unit Value 黑 Methane m3STP 385,488 62.6 Values for TS and VS content of digestate and fibre throughout Carbon dioxide mSTP 229984 37.4 the study period are shown in Fig.3.The average solids content Biogas m3 STP 615,472 100.0 was TS 4.5%,VS 2.9%for the digestate and TS 23.8%,VS 17.9%for Food waste input kg ww 3936.504 the fibre.As can be seen in Fig.3.there was some variation in kg VS 959.209 m3tonne-1wW 156 the TS and VS content of individual samples of digestate but the Specific biogas yield m3 tonne-1 VS 642 VS:TS ratio remained fairly constant. Specific methane yield m3tonne-1ww 98 The average nutrient content of the digestate during the study mtonne-1Vs 402 period was 5.6.0.4 and 2.3 kg tonne-1 WW for TKN.P and K. Volumetric biogas yield m m-3 reactor 1.59 respectively.Fig.2b shows the variability between fortnightly Volumetric methane yielda m3m-3 reactor 1.00 composite samples.As expected the values were more consistent a Based on volume of digester only
commercial food waste were 8.7, 1.8 and 3.4 kg tonne1 WW. Fig. 2a shows the variability in fortnightly composite samples of domestic food waste. The variations between consecutive samples may reflect the fact that only a small amount of material is ultimately used in laboratory analysis, and however much effort is made to prepare representative composites, subsamples may show slight non-homogeneity due to unavoidable scale factors. The average organic loading rate during the study period was 2.5 kg VS m3 day1 based on the nominal digester volume of 900 m3 , or 2.7 kg VS m3 day1 based on the average volume of digester contents. The maximum and minimum weekly average loadings based on actual volume were 3.46 and 0.91 kg VS m3 day1 , respectively, with the minimum corresponding to a Christmas closure period. The average hydraulic retention time was 80 days, based on the nominal digester volume divided by the mass input on a wet weight basis. More detailed information on day-to-day variations in feedstock quantities is given in Arnold et al. (2010). 3.2. Digestate characteristics Values for TS and VS content of digestate and fibre throughout the study period are shown in Fig. 3. The average solids content was TS 4.5%, VS 2.9% for the digestate and TS 23.8%, VS 17.9% for the fibre. As can be seen in Fig. 3, there was some variation in the TS and VS content of individual samples of digestate but the VS:TS ratio remained fairly constant. The average nutrient content of the digestate during the study period was 5.6, 0.4 and 2.3 kg tonne1 WW for TKN, P and K, respectively. Fig. 2b shows the variability between fortnightly composite samples. As expected the values were more consistent than those for feedstock, apart from one high value for nitrogen in day 215–227. A nutrient mass balance taking into account water additions showed outputs equal to 86.1%, 32.8% and 96.4% of the input values of TKN, P and K, respectively. The lower recovery of TKN and particularly of P may indicate losses by precipitation e.g. of struvite (NH4MgPO46H2O) within the digester system. 3.3. Biogas output and variability Table 1 shows the total biogas production and the proportion of methane and carbon dioxide based on daily measurements. The specific biogas and methane yields are given on both a wet weight and a VS basis, and show the food waste has a high methane potential in comparison to typical municipal residual waste streams. The high moisture content means, however, that the biogas production Fig. 3. TS and VS content of digestate (a and b) and fibre (c) during the study period (points show average value for triplicate determinations; note different scales for digestate and fibre). Table 1 Gas production parameters during mass and energy balance period. Item Unit Value % Methane m3 STP 385,488 62.6 Carbon dioxide m3 STP 229,984 37.4 Biogas m3 STP 615,472 100.0 Food waste input kg WW 3936,504 – kg VS 959,209 – Specific biogas yield m3 tonne1 WW 156 – m3 tonne1 VS 642 – Specific methane yield m3 tonne1 WW 98 – m3 tonne1 VS 402 – Volumetric biogas yielda m3 m3 reactor 1.59 – Volumetric methane yielda m3 m3 reactor 1.00 – a Based on volume of digester only. C.J. Banks et al. / Bioresource Technology 102 (2011) 612–620 615
616 CJ.Banks et al /Bioresource Technology 102(2011)612-620 15000 10000 -biogas ◆-CH4 ◆-C02 5000 100 200 300 400 Day 6 b 75 CM 65 -Average CH4 55 45 100 200 300 400 Day Fig.4.Weekly gas production(a)and daily methane percentage in biogas(b)during the study period. per tonne of imported material is similar to typical values reported The reasons for this change are not clear but the ammonia concen- for municipal solid waste(MSW).Volumetric gas production is cal- tration in the digesters had been increasing steadily and reached culated based on the volume of the digester only. around 5000 mg I-1 at this time.Subsequent work in laboratory- Variability in the biogas production and composition is shown scale digesters has suggested that high ammonia concentrations by reference to the weekly values for methane,carbon dioxide may cause a shift in the biochemical pathways leading to methane and biogas in Fig.4a.Total biogas production over a one-week per- formation (Banks and Zhang,2010).A slight decrease in biogas iod varied from a minimum 6364 m3 to a maximum of 13,438 m3 methane concentration can be seen before day 342 followed by although some of the peaks and troughs can be explained by differ- recovery.Total VFA concentrations continued to increase and ap- ences in the incoming load (e.g.suspension of some deliveries dur- proached 15.000 mgl-1 by the end of the monitoring period.of ing the Christmas-New Year period in 2007).Fig.4b shows the which propionic acid made up 11,500 mgI-.Despite the high variability in methane concentration based on daily readings,com- VFA values the specific and volumetric biogas yields remained pared to the calculated average for the whole study period. unaffected (Fig.6). 3.4.Digestion parameters 3.5.Overall mass balance Digestion parameters are reported from day 0 corresponding to The mass balance around the plant was calculated in two ways: the start of the mass and energy balance study,although measure- by wet weight (Table 2)and on a VS basis (Table 3).In the wet ment of VFA and ammonia only began some time after this. weight balance water additions from both the process and facilities The average digester pH in the study period was 8.13 with val- supplies were included as inputs.Methane and carbon dioxide vol- ues remaining mainly between 8.0 and 8.25(Fig.5a).From day 252 umes were corrected to STP and it was assumed that the spot val- to day 304,however,the pH rose to 8.64,then fell sharply to a min- ues for methane concentration are representative of a 24-h period. imum value of 7.24 by day 342.This fall appears to have been a re- Weights of digestate,fibre and rejects were taken from weigh- sult of a shift in alkalinity,with an increase in intermediate bridge data for materials leaving site.Stored materials are based alkalinity(IA).a fall in partial alkalinity (PA)and a rise in the IA/ on tank volumes and estimated quantities of fibre in the digestate PA ratio to 2.74(Fig.5b).Prior to this the IA/PA ratio was around hall.Weight data on all wastes generated by the operation (includ- 0.4 indicating stable operation (Ripley et al.,1986). ing canteen wastes and litter as well as feedstock contamination) The major factor affecting the intermediate alkalinity is the con was only collected from April 2008,and therefore underestimates centration of undissociated VFA.This fell between day 250-300. the total weight of material leaving the plant by this route.Con- with a decrease in the propionic acid concentration,followed by tamination of the feedstock itself,assessed by hand sorting of sam- a rapid increase after day 300 in both acetic and propionic acid ples(not reported here).was minimal.Evaporative water losses and a slower rise in the concentration of butyric etc.(Fig.5c). from the gas mixing system due to supersaturation followed by
per tonne of imported material is similar to typical values reported for municipal solid waste (MSW). Volumetric gas production is calculated based on the volume of the digester only. Variability in the biogas production and composition is shown by reference to the weekly values for methane, carbon dioxide and biogas in Fig. 4a. Total biogas production over a one-week period varied from a minimum 6364 m3 to a maximum of 13,438 m3 , although some of the peaks and troughs can be explained by differences in the incoming load (e.g. suspension of some deliveries during the Christmas–New Year period in 2007). Fig. 4b shows the variability in methane concentration based on daily readings, compared to the calculated average for the whole study period. 3.4. Digestion parameters Digestion parameters are reported from day 0 corresponding to the start of the mass and energy balance study, although measurement of VFA and ammonia only began some time after this. The average digester pH in the study period was 8.13 with values remaining mainly between 8.0 and 8.25 (Fig. 5a). From day 252 to day 304, however, the pH rose to 8.64, then fell sharply to a minimum value of 7.24 by day 342. This fall appears to have been a result of a shift in alkalinity, with an increase in intermediate alkalinity (IA), a fall in partial alkalinity (PA) and a rise in the IA/ PA ratio to 2.74 (Fig. 5b). Prior to this the IA/PA ratio was around 0.4 indicating stable operation (Ripley et al., 1986). The major factor affecting the intermediate alkalinity is the concentration of undissociated VFA. This fell between day 250–300, with a decrease in the propionic acid concentration, followed by a rapid increase after day 300 in both acetic and propionic acid and a slower rise in the concentration of butyric etc. (Fig. 5c). The reasons for this change are not clear but the ammonia concentration in the digesters had been increasing steadily and reached around 5000 mg l1 at this time. Subsequent work in laboratoryscale digesters has suggested that high ammonia concentrations may cause a shift in the biochemical pathways leading to methane formation (Banks and Zhang, 2010). A slight decrease in biogas methane concentration can be seen before day 342 followed by recovery. Total VFA concentrations continued to increase and approached 15,000 mg l1 by the end of the monitoring period, of which propionic acid made up 11,500 mg l1 . Despite the high VFA values the specific and volumetric biogas yields remained unaffected (Fig. 6). 3.5. Overall mass balance The mass balance around the plant was calculated in two ways: by wet weight (Table 2) and on a VS basis (Table 3). In the wet weight balance water additions from both the process and facilities supplies were included as inputs. Methane and carbon dioxide volumes were corrected to STP and it was assumed that the spot values for methane concentration are representative of a 24-h period. Weights of digestate, fibre and rejects were taken from weighbridge data for materials leaving site. Stored materials are based on tank volumes and estimated quantities of fibre in the digestate hall. Weight data on all wastes generated by the operation (including canteen wastes and litter as well as feedstock contamination) was only collected from April 2008, and therefore underestimates the total weight of material leaving the plant by this route. Contamination of the feedstock itself, assessed by hand sorting of samples (not reported here), was minimal. Evaporative water losses from the gas mixing system due to supersaturation followed by Fig. 4. Weekly gas production (a) and daily methane percentage in biogas (b) during the study period. 616 C.J. Banks et al. / Bioresource Technology 102 (2011) 612–620
CJ.Banks et al./Bioresource Technology 102(2011)612-620 617 9.0 8.5 a 舌 8.0 7.5 7.0- 0 100 200 300 400 Day 20000 3.0 b 2.5 15000 2.0 是 10000 1.5 pue vd 1.0 5000 0.5 0.0 0 100 200 300 400 Day ---0--PartialIntermediate- -IA/PA 20000 Q 15000 是 10000 5000 0 0 100 200 300 400 Day ◆-Acetic……Propionic lso-Butyric -n-Butyric…w…lso-Valeric -Valeric Total Fig.5.Digestion parameters during the study period:pH(a).alkalinity(b)and VFA(c).(Hexanoic and Heptanoic acids included in total VFA but individual values not shown. Error bars on total VFA show range of duplicate determinations.) condensation in the gas holder were not taken into account.Calcu- with only a slight change in the overall mass balance from 90.3%to lations also did not consider fugitive emissions of gas or liquid 90.6%(Table3). from the site. The method for determination of VS leads to volatilisation and For the VS mass balance,the VS of input food waste,digestate loss of intermediate soluble metabolites such as VFA and ammonia. and fibre was based on the average of all laboratory determinations If concentrations of these in the liquid digestate are taken into ac- for each parameter.The VS of the reject stream was taken as equal count and assumed to be vaporised during the standard analytical to that of the incoming food waste.The amount of material stored procedure,the VS mass balance increases to 95.7%. in tanks was based on tank volumes and an assumed VS equal to Considering the difficulties of obtaining representative samples the digestate storage.Calculations using a 21-day rolling average from very heterogeneous materials,the results of the mass balance of VS values for input food waste and digestate gave similar results, are considered to be acceptable for a full-scale plant
condensation in the gas holder were not taken into account. Calculations also did not consider fugitive emissions of gas or liquid from the site. For the VS mass balance, the VS of input food waste, digestate and fibre was based on the average of all laboratory determinations for each parameter. The VS of the reject stream was taken as equal to that of the incoming food waste. The amount of material stored in tanks was based on tank volumes and an assumed VS equal to the digestate storage. Calculations using a 21-day rolling average of VS values for input food waste and digestate gave similar results, with only a slight change in the overall mass balance from 90.3% to 90.6% (Table 3). The method for determination of VS leads to volatilisation and loss of intermediate soluble metabolites such as VFA and ammonia. If concentrations of these in the liquid digestate are taken into account and assumed to be vaporised during the standard analytical procedure, the VS mass balance increases to 95.7%. Considering the difficulties of obtaining representative samples from very heterogeneous materials, the results of the mass balance are considered to be acceptable for a full-scale plant. Fig. 5. Digestion parameters during the study period: pH (a), alkalinity (b) and VFA (c). (Hexanoic and Heptanoic acids included in total VFA but individual values not shown. Error bars on total VFA show range of duplicate determinations.) C.J. Banks et al. / Bioresource Technology 102 (2011) 612–620 617
618 CJ.Banks et al /Bioresource Technology 102 (2011)612-620 500 15000 400 a 12000 300 ◆input 200 9000 balance 100 6000 output 3000 stored in 100 tanks -200 100 200 300 400 Day 6000 5000 b 一input 4000 output 3000 2000 output change 1000 in storage -balance -1000 100 200 300 400 Day Fig.6.Mass balance (wet weight)during the study period:weekly(a)and cumulative(b). Fig.6a and b show the mass balance plotted on a weekly and of to measure directly the heat output associated with the CHP cumulative weekly basis.On a weekly basis (Fig.6a)there is some or the amount of this heat that used to maintain the temperature variability,a proportion of which can be attributed to the problem of the digestion plant.The calculated gross energy output of the of estimating the quantities of digestate and fibre stored on site. CHP plant was 2781 MWh which at a recovery value of 53%would From Fig.6b it can be seen that at the point when materials were provide a further 1474 MWh of energy in the form of extractable last taken off site and accurate weights recorded,the difference be- heat,in addition to the electrical energy output. tween input and output weights without consideration of storage When starting the CHP unit,a small amount of natural gas was was only 190 tonnes out of a total input of 4823 tonnes(4%). used before switching to biogas.During the study period this to- talled 1534 m(0.4%of the total methane production of the plant). No electricity was generated as a result and this component is 3.6.Gross electricity and heat outputs from the CHP unit therefore not included in Table 5,but is taken into account in the overall energy balance. Values for electricity and heat outputs during the study period At the time when the CHP unit is not generating electricity,due from the CHP unit only are shown in Table 4.There was no way to scheduled maintenance,breakdown,or gas quality below the threshold limit,the biogas is burnt in a separate boiler unit to pro- Table 2 duce hot water. Mass balance for study period (wet weight). Parameter Unit Value 3.7.Electricity and heat requirements of the process plant Food waste input kg 3936.504 Water input(washwater) kg 1490.000 Electrically-powered equipment involved in operation of the Total input kg 5426,504 plant included the raw waste shredder,macerators,feed pumps Methane kg 275,177 biogas compressor pumps,CHP and boiler water feed pumps,belt Carbon dioxide 451,473 Water vapour kg 12,526 press,air filtration and minor ancillary equipment such as convey- Digestate 3969.080 ors.The primary consumers of electricity included the heat dump- Fibrea g 39,240 ing fans,gas mixing compressors,air filtration unit for reception All waste leaving site kg 35.820 hall,air filtration biofilter for digestate hall,raw waste shredder, Total output 经 4783,315 Wet tanks 92.433 and pasteurisation heating pump.Intermediate consumers were Stored material Total storage 煙 30.000 the feed and discharge pumps.gas holder inflation fans and the 122,433 CHP water pump.There were no individual electricity meters on Balance accounted for 是 520,756 these and the power taken depends upon the equipment load,so 90.4% cannot be calculated directly from hours run and plate capacity. Any liquid digestate produc d recirculated through the cess and leaves Parasitic energy is therefore given as an overall figure representing the site as whole digestate. the total number of kWh consumed on site (Table 5)
Fig. 6a and b show the mass balance plotted on a weekly and cumulative weekly basis. On a weekly basis (Fig. 6a) there is some variability, a proportion of which can be attributed to the problem of estimating the quantities of digestate and fibre stored on site. From Fig. 6b it can be seen that at the point when materials were last taken off site and accurate weights recorded, the difference between input and output weights without consideration of storage was only 190 tonnes out of a total input of 4823 tonnes (4%). 3.6. Gross electricity and heat outputs from the CHP unit Values for electricity and heat outputs during the study period from the CHP unit only are shown in Table 4. There was no way of to measure directly the heat output associated with the CHP or the amount of this heat that used to maintain the temperature of the digestion plant. The calculated gross energy output of the CHP plant was 2781 MWh which at a recovery value of 53% would provide a further 1474 MWh of energy in the form of extractable heat, in addition to the electrical energy output. When starting the CHP unit, a small amount of natural gas was used before switching to biogas. During the study period this totalled 1534 m3 (0.4% of the total methane production of the plant). No electricity was generated as a result and this component is therefore not included in Table 5, but is taken into account in the overall energy balance. At the time when the CHP unit is not generating electricity, due to scheduled maintenance, breakdown, or gas quality below the threshold limit, the biogas is burnt in a separate boiler unit to produce hot water. 3.7. Electricity and heat requirements of the process plant Electrically-powered equipment involved in operation of the plant included the raw waste shredder, macerators, feed pumps, biogas compressor pumps, CHP and boiler water feed pumps, belt press, air filtration and minor ancillary equipment such as conveyors. The primary consumers of electricity included the heat dumping fans, gas mixing compressors, air filtration unit for reception hall, air filtration biofilter for digestate hall, raw waste shredder, and pasteurisation heating pump. Intermediate consumers were the feed and discharge pumps, gas holder inflation fans and the CHP water pump. There were no individual electricity meters on these and the power taken depends upon the equipment load, so cannot be calculated directly from hours run and plate capacity. Parasitic energy is therefore given as an overall figure representing the total number of kWh consumed on site (Table 5). Fig. 6. Mass balance (wet weight) during the study period: weekly (a) and cumulative (b). Table 2 Mass balance for study period (wet weight). Parameter Unit Value Food waste input kg 3936,504 Water input (washwater) kg 1490,000 Total input kg 5426,504 Methane kg 275,177 Carbon dioxide kg 451,473 Water vapour kg 12,526 Digestate a kg 3969,080 Fibre a kg 39,240 All waste leaving site kg 35,820 Total output kg 4783,315 Wet tanks kg 92,433 Stored material kg 30,000 Total storage kg 122,433 Balance accounted for kg 520,756 % 90.4% a Any liquid digestate produced is recirculated through the process and leaves the site as whole digestate. 618 C.J. Banks et al. / Bioresource Technology 102 (2011) 612–620
CJ.Banks et al./Bioresource Technology 102(2011)612-620 619 Table 3 Table 6 Mass balance for study period(VS). Overall energy balance. Parameter Unit Value Parameter Unit Value Food waste input kg WW 3936,504 CHP net electrical output kWh 852,346 Food waste VS kg VS kg-1 ww 0.244 Parasitic electrical requirement of kWh 232,694 Total input kg VS 959.209 process plant Methane kg VS 275.177 Net energy output as electricity kWh 619.652 Carbon dioxide kg VS 451.473 Recoverable heat output from CHP kwh 14741R5 Digestate (includes separated kg ww 3969.080 Parasitic heat requirement of plant kWh 446,334 and whole digestate) Net energy output as heat kWh 1027,851 kg VS kg-wW 0.029 CHP natural gas used kWh 18.413 kg VS 115.521 Energy required for digestate use kWh 34350 Fibre kg ww 39240 Total potentially recoverable energy kWh 1594,740 kg VS kg-1 ww 0.179 (heat and electricity) kg VS 7040 Total potentially recoverable energy kWh 405 Reject kg WW 35,820 per wet tonne of food waste kg vs kg-1 ww 0.244 kg VS 8728 a Includes heat energy generated but not used at the time of the study. Total output kg VS 857,938 Wet tanks kg Ww 92.433 kg Vs kg-1 ww 0.029 3.8.Energy used in digestate transport and application kg Vs 2690 Stored material kg WW 30.000 kg VS kg-1 ww 0.179 The energy required for transport and application of the dige kg VS 5382 state to land was 34,350 kWh and was calculated from actual vehi- Total storage kg VS 8072 cle trips made,taking into account the vehicle types used and Balance kg VS 93,198 mileages covered.Vehicle fuel efficiencies were based on emis- 903% sions factors from the EU Environment Agency's Corinair database (EEA,2002).Energy used in application of the digestate to land was calculated using tractor power efficiency conversions and esti- mated hours run and gave a value of 17 MJ tonne-1,which agrees Table 4 with that quoted in Berglund and Borjesson(2006).More detail on Electrical and heat outputs from the CHP plant during the study period. the land application calculations is given in Banks et al.(in review). Parameter Unit Value CHP gross energy kWh 2781.481 100.0 3.9.Overall energy balance CHP gross electrical output kWh 890.074 32.0 CHP parasitic electrical requirement kWh 37.728 1.4 Table 6 summarises the overall energy balance for the process- CHP net electrical output kWh 852346 30.6 CHP gross heat output kWh 1891,407 68.0 ing of feedstock,including delivery to the recipient sites and appli- CHP recoverable heat output kWh 1474185 53.0 cation to land during the study period based on the above data. CHP waste heat kWh 417222 15.0 Calculated as electrical output divided by conversion efficiency taken as 32 4.Conclusions The specific methane yield of food waste was 98 m3 tonne-1 wet weight or 402 m'tonne-VS,and productivity remained high Table 5 throughout the study period.The nitrogen content led to high Breakdown of process energy requirements. ammonia concentrations that buffered VFA accumulation.Net Parameter Unit Value recoverable energy was 405 kWh tonne-1 wet weight,including CHP electrical parasitic kWh digestate transport and utilisation.The mass balance was 90.4% 37.728 Rest of plant parasitic kWh 232,694 (wet weight).and 95.7%(VS basis)allowing for loss of volatile com- Total electrical parasitic kWh 270.422 ponents.Since study ended the plant has continued in successful of gross electrical output 30.4% commercial operation and provides a sustainable route for recov- Heat requirement to raise feedstock temperature kWh 202.674 ery of products from domestic food waste. Heat requirement for pasteurisation kWh 150.709 Heat requirement to maintain tank temperatures kWh 92,951 Total parasitic heat requirement kWh 446,334 Acknowledgements of recoverable heat 30.3% Total electrical parasitic,divided by CHP gross electrical output from Table 4. Funding for this project was provided by Advantage West Mid- lands and by Defra from the New Technologies Demonstrator Pro- gramme.The authors also gratefully acknowledge the support and assistance of Biocycle South Shropshire Ltd. The energy required to raise the temperature of the feedstock. pasteurise the digestate and maintain the temperature of the References heated tanks was calculated based on input volumes,tank dimen- sions and insulation values,and is given in Table 5.Temperature APHA,2005.Standard Methods for the Examination of Water and Wastewater.21st differentials between tanks and ambient were taken from average ed.American Public Health Association,American Water Works Association, monthly values for the UK Meteorological Office station at Water Environment Federation,Washington.USA. Angelidaki,L.Boe,K..Ellegaard,L.2005.Effect of operating conditions and reactor Lyonshall.Feedstock materials were considered to be at ambient configuration on efficiency of full-scale biogas plants.Water Sci.Technol.52(1- temperatures.It was assumed that the buffer tank,digester and 2).189-194. pasteuriser were always full and maintained at operating Archer.E,Baddeley.A,Klein.A..Schwager,J..Whiting.K..2005.MBT:A Guide for Decision Makers-Processes.Policies and Markets.Juniper Consulting Ltd..Uley. temperatures. Gloucestershire.UK
The energy required to raise the temperature of the feedstock, pasteurise the digestate and maintain the temperature of the heated tanks was calculated based on input volumes, tank dimensions and insulation values, and is given in Table 5. Temperature differentials between tanks and ambient were taken from average monthly values for the UK Meteorological Office station at Lyonshall. Feedstock materials were considered to be at ambient temperatures. It was assumed that the buffer tank, digester and pasteuriser were always full and maintained at operating temperatures. 3.8. Energy used in digestate transport and application The energy required for transport and application of the digestate to land was 34,350 kWh and was calculated from actual vehicle trips made, taking into account the vehicle types used and mileages covered. Vehicle fuel efficiencies were based on emissions factors from the EU Environment Agency’s Corinair database (EEA, 2002). Energy used in application of the digestate to land was calculated using tractor power efficiency conversions and estimated hours run and gave a value of 17 MJ tonne1 , which agrees with that quoted in Berglund and Borjesson (2006). More detail on the land application calculations is given in Banks et al. (in review). 3.9. Overall energy balance Table 6 summarises the overall energy balance for the processing of feedstock, including delivery to the recipient sites and application to land during the study period based on the above data. 4. Conclusions The specific methane yield of food waste was 98 m3 tonne1 wet weight or 402 m3 tonne1 VS, and productivity remained high throughout the study period. The nitrogen content led to high ammonia concentrations that buffered VFA accumulation. Net recoverable energy was 405 kWh tonne1 wet weight, including digestate transport and utilisation. The mass balance was 90.4% (wet weight), and 95.7% (VS basis) allowing for loss of volatile components. Since study ended the plant has continued in successful commercial operation and provides a sustainable route for recovery of products from domestic food waste. Acknowledgements Funding for this project was provided by Advantage West Midlands and by Defra from the New Technologies Demonstrator Programme. The authors also gratefully acknowledge the support and assistance of Biocycle South Shropshire Ltd. References APHA, 2005. Standard Methods for the Examination of Water and Wastewater, 21st ed. American Public Health Association, American Water Works Association, Water Environment Federation, Washington, USA. Angelidaki, I., Boe, K., Ellegaard, L., 2005. Effect of operating conditions and reactor configuration on efficiency of full-scale biogas plants. Water Sci. Technol. 52 (1– 2), 189–194. Archer, E., Baddeley, A., Klein, A., Schwager, J., Whiting, K., 2005. MBT: A Guide for Decision Makers–Processes, Policies and Markets. Juniper Consulting Ltd., Uley, Gloucestershire, UK. Table 3 Mass balance for study period (VS). Parameter Unit Value Food waste input kg WW 3936,504 Food waste VS kg VS kg1 WW 0.244 Total input kg VS 959,209 Methane kg VS 275,177 Carbon dioxide kg VS 451,473 Digestate (includes separated and whole digestate) kg WW 3969,080 kg VS kg1 WW 0.029 kg VS 115,521 Fibre kg WW 39,240 kg VS kg1 WW 0.179 kg VS 7040 Reject kg WW 35,820 kg VS kg1 WW 0.244 kg VS 8728 Total output kg VS 857,938 Wet tanks kg WW 92,433 kg VS kg1 WW 0.029 kg VS 2690 Stored material kg WW 30,000 kg VS kg1 WW 0.179 kg VS 5382 Total storage kg VS 8072 Balance kg VS 93,198 % 90.3% Table 4 Electrical and heat outputs from the CHP plant during the study period. Parameter Unit Value % CHP gross energy a kWh 2781,481 100.0 CHP gross electrical output kWh 890,074 32.0 CHP parasitic electrical requirement kWh 37,728 1.4 CHP net electrical output kWh 852,346 30.6 CHP gross heat output kWh 1891,407 68.0 CHP recoverable heat output kWh 1474,185 53.0 CHP waste heat kWh 417,222 15.0 a Calculated as electrical output divided by conversion efficiency taken as 32%. Table 5 Breakdown of process energy requirements. Parameter Unit Value CHP electrical parasitic kWh 37,728 Rest of plant parasitic kWh 232,694 Total electrical parasitic a kWh 270,422 % of gross electrical output 30.4% Heat requirement to raise feedstock temperature kWh 202,674 Heat requirement for pasteurisation kWh 150,709 Heat requirement to maintain tank temperatures kWh 92,951 Total parasitic heat requirement kWh 446,334 % of recoverable heat 30.3% a Total electrical parasitic, divided by CHP gross electrical output from Table 4. Table 6 Overall energy balance. Parameter Unit Value CHP net electrical output kWh 852,346 Parasitic electrical requirement of process plant kWh 232,694 Net energy output as electricity kWh 619,652 Recoverable heat output from CHP kWh 1474,185 Parasitic heat requirement of plant kWh 446,334 Net energy output as heat kWh 1027,851 CHP natural gas used kWh 18,413 Energy required for digestate use kWh 34,350 Total potentially recoverable energy (heat and electricity)a kWh 1594,740 Total potentially recoverable energy per wet tonne of food waste kWh 405 a Includes heat energy generated but not used at the time of the study. C.J. Banks et al. / Bioresource Technology 102 (2011) 612–620 619
620 CJ.Banks et al /Bioresource Technology 102 (2011)612-620 Arnold,R.Banks C.J Chesshire,M.,Foxall,M..Stoker,A.,2010.Defra Demonstration EI Hadi.T.B..Astals,S.Gali,A,Mace,S_Mata-Alvarez,J..2009.Ammonia influence Project:Biocycle South Shropshire Biowaste Digester.Defra New Technologies in anaerobic digestion of OFMSW.Water Sci.Technol.59(6).1153-1158. Programme.Final Report (accessed 01.07.2010) in anaerobic digestion plants resulting from nitrogen in MSW.Waste Manag.27 Banks.CJ.Chesshire,M..Heaven,S.,Arnold,R.Lewis,L.in review.Biocycle (1).30-43. anaerobic digester:performance and environmental benefits.In:Proceedings of Hartmann.H..Ahring.B.K..2005.A novel process configuration for anaerobic the Institution of Civil Engineers-Waste and Resource Management. digestion of source-sorted household waste using hyper-thermophilic post- Banks.C.J..Chesshire,M..Stringfellow.A.2008.A pilot-scale trial comparing treatment.Biotechnol.Bioeng.90(7).830-837. mesophilic and thermophilic digestion for the stabilisation of source segregated Mata-Alvarez.J.(Ed.).2003.Biomethanization of the Organic Fraction of Municipal kitchen waste.Water Sci.Technol.58 (7).1475-1480. Solid Wastes.IWA Publishing.London. Neilsen,H.B..Angelidaki,I.2008.Strategies for optimizing recov ery of the biogas process following ammonia inhibition.Bioresour.Technol.99 (17).7995-8001. 69-76. Neiva Correia,C..Vaz,F,Torres,A..2008.Anaerobic digestion of biodegradable Banks.CJ Zhang.Y.2010.Technical Report:Optimising inputs and outputs from waste-operational and stability parameters for stability control.In:5th IWA anaerobic digestion processes.Defra project Code WR0212 Stretton-Maycock,D..Merrington,G..2009.The use and application to land of MBT (accessed01.07.2010) compost-like output -review of current European practice in relation to Berglund,M.Borjesson.P.,2006.Assessment of energy performance in the life- environmental protection.Science Report SC030144/SR3.Environment cycle of biogas production.Biomass Bioenerg.30(3).254-266. Agency.Bristol.UK. Chesshire.M.,2007.The South Shropshire biowaste digester.UK.Proceedings of Ripley.LE..Boyle,W.C..Converse.J.C..1986.Improved alkalimetric monitoring for the Institution of Civil Engineers -Waste and Resource Management 160 anaerobic digestion of high strength wastes.Journal of the Water Pollution (1).19-26. Control Federation 58 (5).406-411. EEA.2002.European Environment Agency EMEP/CORINAIR Emission Inventory Wang.ZJ.Banks.C.J..2003.Evaluation of a two stage anaerobic digester for the Guidebook,third ed.Technical Report No.30.EEA. treatment of mixed abattoir wastes.Process.Biochem.38(9).1267-1273
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