lable at ScienceDirect

International Biodeterioration & Biodegradation 95 (2014) 176e180

Contents lists avai

International Biodeterioration & Biodegradation

journal homepage: www.elsevier .com/locate/ ibiod

A prototype single-stage anaerobic digester for co-digestion of foodwaste and sewage sludge from high-rise building for on-site biogasproduction

C. Ratanatamskul a, *, G. Onnum a, K. Yamamoto b

a Department of Environmental Engineering, Chulalongkorn University, Bangkok 10330, Thailandb Environmental Science Center, University of Tokyo, Tokyo 113, Japan

a r t i c l e i n f o

Article history:Received 16 January 2014Received in revised form15 June 2014Accepted 16 June 2014Available online 9 July 2014

Keywords:Prototype single-stage anaerobic digesterAnaerobic co-digestionFood wasteSewage sludgeHigh-rise buildingBiogas production

* Corresponding author. Tel.: þ66 816148623.E-mail addresses: dr_chawalit@yahoo.com,

(C. Ratanatamskul).

http://dx.doi.org/10.1016/j.ibiod.2014.06.0100964-8305/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

At present, there has been less results from pilot-scale experiments on anaerobic co-digestion system,especially as an on-site system for high-rise building application. The objective of this study was todevelop a prototype single-stage anaerobic digester for the co-digestion of food waste (FW) and sewagesludge (SWS) from high-rise building for on-site biogas production. Here, the prototype system wasoperated at different hydraulic retention times (HRTs) of 27, 22 and 19 days, corresponding to organicloading rates of 7.9, 10.8 and 14.0 kgCOD/m3 d. The feed mixed waste ratio (FW/SWS) of 10:1 by weightwas selected from the previous laboratory-scale experiment. The results of this study indicate that highergas production rate was obtained at shorter hydraulic retention time (HRT) of 19 days; however, highermethane content of the biogas was obtained at longer HRT of 27 days. Therefore, enhancement ofmethane production from the co-digestion could be achieved with sufficient operating HRT. Moreover,the proposed prototype system could reduce total volatile solid up to 70 percent at HRT of 27 days. Up tonow, the biogas from on-site production has been utilized for cooking at Chulalongkorn University as amodel case study for high-rise building application.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Recently, anaerobic digestion of organic solid wastes has beenpromoted greatly in Thailand to produce alternative renewableenergy source from biogas production. The amount of solid wastegeneration is reported approximately 67,577 tons/day of MSW(municipal solid waste) (Pollution Control Department, 2012). Foodwaste is one of the largest waste stream and accounts for 39.25% oftotal MSW. Due to the characteristics of food waste with 70e90%moisture content and high organic content of volatile solid (VS)(Tchobanoglous et al., 1993; Zhang et al., 2007), improper wastetreatment would cause serious environmental problems, such asodor, leachate production and groundwater contamination (Slacket al., 2005). Anaerobic digestion has been recognized as an envi-ronmentally friendly technology to convert organic solid wastesuch as animal manure, food waste, and organic fraction of MSWinto renewable energy in biogas form (Mata-Alvarez et al., 2000;

Chavalit.R@eng.chula.ac.th

Digman and Kim, 2008). Anyhow, digestion process tends to fail,when one readily degradable organic matter is used as sole sub-strate without external nutrients and buffering agent (Demirel andScherer, 2007).

Chulalongkorn University has started the government projecton “Development of energy recovery system from solid organicwastes for high-rise building application” underMinistry of NaturalResource and Environment (MONRE), Thailand with the financialsupport from JICA-Water Intro Project, Japan. The main objective ofthis research work is to develop a prototype anaerobic co-digestionsystem of food waste and sewage sludge for on-site biogas pro-duction and energy recovery together with organic waste reduc-tion. Here, the single-stage anaerobic digester has been designedand constructed as a compact prototype system with fibreglassstructure, having total volume of 2500 L to receive food waste thatis source separated from a canteen in Julajakrapong Building, whichis located inside Chulalongkorn University and sewage sludge froma conventional wastewater treatment plant, located at the samebuilding. Average food waste amount that generated from thiscanteen was found to be 80 kg per day. Before starting our anaer-obic digestion project, food waste was considered as a waste ma-terial to be disposed of to landfill. Indeed, food waste from this

Table 1Characteristics of food waste (FW) and sewage sludge (SWS) used in this study.

Parameters Food waste (FW) Sewage sludge (SWS) Mixed waste (FW/SWS)

TS, mg/L 80,676 39,100 76,897SS, mg/L 72,410 38,968 69,371TVS, mg/L 78,823 32,400 74,603COD, mg/L 232,795 17,208 213,196VFA, mg/L 2957.8 504.3 2734.8pH 4.7 6.7 4.5TP, mg/L 926 281 867TKN, mg/L 6275 1254 5818.5

C. Ratanatamskul et al. / International Biodeterioration & Biodegradation 95 (2014) 176e180 177

canteen has high moisture content in the range of 80e85%. Thiscould result in significant greenhouse gas emission from wastedegradation in landfill. Thus, food waste is our main concern fororganic solid waste from this high-rise building. Also, sewagesludge from building's wastewater treatment plant can be consid-ered as a potential biowaste for co-digestion with food waste foron-site biogas production. Nowadays, sewage sludge fromhigh-risebuilding in Thailand is usually dewatered before disposal as do-mestic solid waste to landfill. Anaerobic co-digestion can minimizethe problem of bad practice in sewage sludge management.

The benefits of anaerobic co-digestion process can include:dilution of potential toxic compounds, improved balance of nutri-ents, synergistic effects of microorganisms, increased load ofbiodegradable organic matter and better biogas yield due to syn-ergisms (Sosnowski et al., 2003;; Alvarez and Liden, 2008). Someprevious studies (Murto et al., 2004; Serrano et al., 2014; Estevezet al., 2014) have suggested that anaerobic co-digestion process isalso expected to provide more balanced nutrients for efficientdigestion and high biogas production; thus, it could be used to helpachieve higher digestion efficiency.

At present, there has been less results from pilot-scale experi-ments on anaerobic co-digestion system, especially as an on-sitesystem for high-rise building application. From available litera-ture, the previous studies on co-digestion of sewage sludge anddiverse array of biowastes (eg.municipal solid waste, microalgae,food waste, cattle manure, corn straw) were performed mostly atlaboratory scale (Sosnowski et al., 2003; Costa et al., 2012; Garciaand Perez, 2013; Zhou et al., 2013). Thus, the research work ondevelopment of economically self-sustainable pilot-scale systemfor anaerobic co-digestion process is still needed. In this study,reactor design and tank configuration of the pilot-scale system isalso different from a previously published pilot-scale study, con-ducted by Kim et al. (2011) since our reactor design is a horizontal-shaft paddle digester with fiberglass fabricated structure as an on-site prototype system. The advantage of an on-site prototype sys-tem is that organic wastes can be separated at source, and then feddirectly to the anaerobic digester. Furthermore, this proposed tankconfiguration has plug flow character that can reduce short-circuitflow through the system. Here, effects of hydraulic retention time(HRT) andmixedwaste ratio (Foodwaste: sewage sludge) on biogasproduction using the single-stage anaerobic co-digestion of foodwaste and sewage sludge were investigated. Up to now, the pro-totype system has produced biogas on-site that can be utilized forcooking at Chulalongkorn University as a prototypemodel for high-rise building application.

2. Material and methods

2.1. The characteristics of feeding substrates

Here, food waste was collected from a canteen in JulajakrapongBuilding, Chulalongkorn University, which was composed of foodresidues, grain, fruits, vegetables, starch, and grease, etc. The foodwaste was shredded into 5e10 mm size pieces by food grinder.Sewage sludge was collected from an on-site wastewater treatmentplant, located at the same building, which is a conventional acti-vated sludge process, having a maximum treatment capacity of80 m3/day. In the previous laboratory-scale experiment, by varyingthemixing ratios of the feeding substrates (FW/SWS) to 1:1, 6:1 and10:1 by weight, we observed that the highest amount of biogascould be obtained at themixing ratio of 10:1 (data not shown). Also,the enhancement of methane production could be achieved by thissubstrate mixing ratio. Thus, the mixing ratio of 10:1 for feedingsubstrates was selected for further operating the prototype single-stage anaerobic digester. Here, the characteristics of FW, SWS and

the mixed waste (FW/SWS) used in this study, in terms of totalsolids (TS), SS, TVS, chemical oxygen demand (COD), VFA, pH, totalphosphorus (TP), and total Kjehldahl nitrogen (TKN) are presentedin Table 1.

2.2. Development of a prototype single-stage anaerobic digester forco-digestion system

A prototype single-stage anaerobic digester for co-digestion offoodwaste and sewage sludgewas newly designed and constructedfor tropical application at Julajakrapong Building, a high-risebuilding inside Chulalongkorn University campus. The schematicdiagram of a prototype single stage anaerobic digester used in thisstudy is illustrated in Fig. 1. The volume of the reactor was 2500 Lwith a working volume of 1975 L. The digestion system was thusperformed in semi-continuous mode of feeding and operated atmesophilic temperature (35 ± 2 �C). The reactor was operated athydraulic retention time (HRT) of 27, 22 and 19 days, correspondingto organic loading rates of 7.9, 10.8 and 14.0 kgCOD/m3 d. Thefeeding substrate was initially prepared by mixing food waste withsewage sludge at the selectedmixing ratio by themixing apparatus.Then, the screw conveyor was utilized to move the mixed wasteinto the single-stage anaerobic digester. A paddle-type mixer wasprovided for slow mixing at short period after waste feeding to thedigester tank. The biogas generated from the anaerobic activity waskept in the biogas holding tank and sent through the gas pipelinefor further utilized in canteen for cooking purpose. In this study,effect of hydraulic retention time (HRT) at 19,22 and 27 days onsystem performance and biogas production using the prototypesingle-stage anaerobic co-digestion system was investigated forhigh-rise building application.

2.3. Analytical methods

The anaerobic digestion process was evaluated by measuringthe following parameters: total solids, total suspended solids, vol-atile solids, COD, VFA, pH, Alkalinity, total phosphorus (TP), totalKjehldahl nitrogen (TKN), and daily biogas production. TS, SS, TVS,VFA, pH, alkalinity, TP and TKN were determined in accordancewith standard methods (APHA, AWWA, WPCF, 2005). The dichro-mate Chemical Oxygen Demand (COD) was analyzed by CODanalyzer (HACH, DR 2500, USA). The biogas was collected andanalyzed for gas composition. CH4 and CO2 compositions weredetermined through a TRACE GC (Thermo Finnigan) equipped witha thermal conductivity detector.

3. Results

3.1. pH, VFA and alkalinity variation during the anaerobic co-digestion

It has been recognized that anaerobic digestion includes nor-mally three steps; hydrolysis, acidogenesis and methanogenesis.

Fig. 1. Schematic diagram of the prototype single-stage anaerobic digester for co-digestion of food waste and sewage sludge from high-rise building (1. Mixing apparatus, 2. Screwconveyor, 3.Motor, 4. Anaerobic digester, 5. Biogas holding tank, 6. Biogas utilization for canteen 7. Manometer).

C. Ratanatamskul et al. / International Biodeterioration & Biodegradation 95 (2014) 176e180178

The production of a large amount of volatile fatty acid (VFA)through hydrolysis and acidogenesis can lead to a decrease of so-lution pH when alkalinity in the anaerobic digester is insufficient.Non-methanogenic microorganisms responsible for hydrolysis andacidogenesis can be adapted to low pH while methanogens loseactivity at low pH. Thus methanogenesis can be inhibited signifi-cantly at low pH. pH and VFA are important parameters during theco-digestion process to operate the prototype single-stage anaer-obic digester in a stable state with good performance. These pa-rameters were measured in this study to evaluate the stability ofthe single-stage anaerobic digestion system for co-digestion of FWand SWS. The result of pH variation is illustrated in Fig. 2. Also, theVFA and alkalinity of the single-stage digester are shown in Table 2.VFA and alkalinity together are recognized as the good indicatorsfor evaluation of the system stability for operation of anaerobicdigester.

As can be seen from Table 2, the VFA concentrations resultedfrom the co-digestion systemwith HRTs at 27, 22, and 19 days were2734.83, 2971.30 and 3545.25 mg/L,respectively. These values areobviously higher than 1500mg/L, which is commonly considered to

Fig. 2. Effect of HRT on pH change in the co-digestion using the prototype single-stageanaerobic digester.

be the limit for allowing stable operation of one biogas digester(Angelidaki et al., 2005). However, this single-stage anaerobic co-digestion system still could be maintained in normal operation atsuch high VFA concentrations for the operating HRTs of 22 and 27days. This could be attributed to the strong buffering capacity as thehigh alkalinity of above 4900 mg CaCO3/L without any alkalineaddition to the digester tank. It can be seen from Fig. 1 that the pHsin the digester tank with HRTs of 22 and 27 days were still able tomaintain between 6.6 and 7.6, which are in the range for normaloperation of anaerobic digestion process. However, when operatingHRT was decreased to 19 days, the digester pH dropped signifi-cantly to approximately 6.0, indicating an unstable performance ofmethanogenesis inside the digester tank. Moreover, the VFA/Alka-linity ratio shown in Table 2 also implies that the value of nearly 0.8could cause system failure immediately. Therefore, the operatinghydraulic retention time is one of key parameters to be concernedfor operation of the prototype single-stage anaerobic co-digestionsystem.

3.2. COD removal by the anaerobic co-digestion

Effluent COD removal efficiencies by the prototype single-stageanaerobic digestion system for different hydraulic retention time ofthe FW/SWS co-digestion process. The results are illustrated inTable 3 and Fig. 3. The highest COD removal efficiency of 73.71%could be achieved with the operating HRT of 27 days, while rela-tively low COD removal efficiency was found with the operatingHRT of 19 days. The treated COD in the effluent could be reduced to56,068 mg/L with the operating HRT of 27 days. Therefore, the

Table 2VFA and alkalinity of the prototype single-stage anaerobic digester for the co-digestion process.

HRT (days) VFA (mg/L) Alk (mg CaCO3/L) VFA/Alk

27 2734.83 ± 22.91 5146.26 ± 27.17 0.53 ± 0.00522 2971.30 ± 24.86 4906.00 ± 28.82 0.61 ± 0.00619 3545.25 ± 31.48 4443.95 ± 36.22 0.79 ± 0.010

Table 3Summary on COD removal efficiencies with the co-digestion at different HRT.

HRT (days) Influent COD (mg/L) Effluent COD (mg/L) COD removal (%)

27 213,196 ± 4753.49 56,068 ± 1378.60 73.71 ± 0.1522 237,991 ± 5565.12 79,388 ± 2982.26 66.71 ± 0.7919 266,827 ± 3.171.11 129,417 ± 3190.22 51.53 ± 0.83

Fig. 4. TVS reduction through the anaerobic co-digestion operation.

Table 4Summary on the TVS removal efficiencies at different operating HRTs.

HRT (days) Influent TVS (mg/L) Effluent TVS (mg/L) TVS removal (%)

27 74,603 ± 1255.33 23,172 ± 503.25 68.93 ± 0.4322 77,196 ± 1020.85 35,689 ± 862.66 53.81 ± 0.6519 80,254 ± 1217.29 44,715 ± 698.05 44.28 ± 0.19

Table 5TS removal at different operating hydraulic retention time (HRT).

HRT (days) Influent TS (mg/L) Effluent TS (mg/L) TS removal (%)

27 76,897 ± 1175.00 26,625 ± 439.86 65.51 ± 0.1622 80,455 ± 1349.59 36,640 ± 779.31 54.58 ± 0.3319 83,705 ± 1201.73 48,566 ± 1352.61 42.12 ± 1.04

C. Ratanatamskul et al. / International Biodeterioration & Biodegradation 95 (2014) 176e180 179

systemwith longer hydraulic retention time seems to have reliableCOD removal performance in the anaerobic co-digestion process.

3.3. Degradation of organic solid content

Biogas is recognized to be produced through the conversion oforganic matters by anaerobic microorganisms. With the conversionof organic matters into biogas, the amount of organic dry matterswould be reduced accordingly. Based on mass balance, the re-movals of TS and TVS are illustrated in Fig. 4 and Tables 4 and 5. Itwas observed that the removal rates for both TVS and TS wereslightly affected by hydraulic retention time for the selected FW/WS at 10:1. TS removal rates of 65.5%, 54.6% and 42.1% were ach-ieved with the applying hydraulic retention time at 27,22 and 19days, respectively. The TVS content in the digester was found ratherstable for the system with operating HRTs of 27 and 22 days.Therefore, the methanogenesis was still in stable performance,compared to lower efficiency in TVS reduction with operating HRTof 19 days.

3.4. Biogas production from anaerobic co-digestion

The daily biogas production observed at different hydraulicretention time of the co-digestion of FW/WS for the same source ofmixed wastes are illustrated in Fig. 5. It appears that the influenceof the hydraulic retention time on biogas production is significant.The biogas production rate (GPR) of the digester with operatingHRTs of 27, 22 and 19 days were 1045 ± 52.81, 1386.85 ± 25.32, and1662.58± 37.32 L/day, respectively. The comparison of biogas yieldsper gCOD, gTS, gTVS and gSS removed by the co-digestion processat different HRTs are also shown in Table 6. From Fig. 6, themethanecontents of biogas produced by the co-digestion at HRTs of 27, 22and 19 days were 76.8, 62.4, and 44.0 percents, respectively. Thelonger operating HRT, less amount of total biogas production wasobtained; however, methane content was higher in biogascomposition at longer operating HRT. This implies that longeroperating HRT can improve methanogenesis activity until the ul-timate level. Moreover, shorter operating HRT could decreasereactor pH, resulting in system instability for long run operation.

4. Discussion

The co-digestion of the feeding substrates containing foodwaste(FW) and sewage sludge (SWS) at the mixing ratio of 10:1 was

Fig. 3. Effect of HRT on COD removal by the prototype single-stage anaerobic digester.

performed using the developed prototype single-stage anaerobicdigester. The time needed to reach steady state condition lasted 20days to achieve stable performances of COD and TVS reduction andalso stable biogas production as shown in Figs. 3, 4 and 5. In anenvironment where mixed cultures coexist for co-digestion, theoptimal pH range is recommended in the range of 6.8e7.6 as shownin Fig. 2. Although, the feed into the digester was not at neutral pH,it was still possible that the co-digestion could take place withoutthe addition of alkaline to adjust the digester pH since there was anatural buffering action in the anaerobic co-digestion processthrough the dissolution of the carbon dioxide produced during theanaerobic degradation pathway. This could overcome the effect offeeding the digester with a low pH mixed waste (pH ¼ 4.5). Inanaerobic co-digestion process, the drop in pH is often caused by

Fig. 5. Biogas production rate at different operating hydraulic retention time (HRT).

Table 6Biogas yields in the co-digestion of food waste and sewage sludge under differentoperating hydraulic retention times.

Parameter Operating hydraulic retention times (HRTs)

27 days 22 days 19 days

Total biogas production pergCOD removed (L/g COD removed) 0.101 0.158 0.268g TS removed (L/g TS removed) 0.316 0.663 1.283g TVS removed (L/g TVS removed) 0.309 0.607 1.039g TSS removed (L/g TSS removed) 0.359 0.566 0.954Methane yield pergCOD removed (L/g COD removed) 0.077 0.099 0.118g TS removed (L/g TS removed) 0.243 0.413 0.564g TVS removed (L/g TVS removed) 0.237 0.379 0.457g TSS removed (L/g TSS removed) 0.276 0.353 0.420

Fig. 6. Effect of hydraulic retention time (HRT) on methane content of biogas producedby the prototype single-stage anaerobic digester.

C. Ratanatamskul et al. / International Biodeterioration & Biodegradation 95 (2014) 176e180180

the accumulation of VFAs and excessive generation of carbondioxide.

The solid content of the waste is one of main parametersaffecting the system operation. Here, the prototype single stageanaerobic digester treated the mixed waste (FW and SWS) havinghigh-solid content at around 77 g/L as presented in Table 1. Whileallowing a high hydraulic throughput, the biomass inside thereactor might not be well maintained. In a single-stage anaerobicdigester, methanogens are very slow growing microorganisms,then these group could be in danger of being washed out from thedigester, when applying a short hydraulic retention time (HRT lessthan 20 days) for the single-stage digester operation. HRT is one ofimportant design parameters in this anaerobic co-digestion system.As can be seen from the results in Fig. 5, it appears that the increasein the amount of biogas production at decreasing HRT due to moreorganic loadings to the digester. On the other hand, the methanecontent decreased sharply at the HRT below 22 days, that wasresulted from the pH drop inside the digester as shown in Figs. 2and 6, which was accompanied with an increase in the processvolatile fatty acid (VFA) as presented in Table 2. Longer HRTs arerequired to minimize wash out of slow-growing methanogens.Therefore, HRT of at least 20e22 days is recommended, whenoperating the co-digestion of the mixed wastes by the single-stageanaerobic digester. Biogas presents a valuable energy source forcooking and electricity generation. At present, the biogas from on-site production has been utilized for cooking at ChulalongkornUniversity as a model case study for high-rise building application.

5. Conclusions

In this study, the anaerobic co-digestion of food waste andsewage sludge operated in the novel prototype single-stageanaerobic digester at longer hydraulic retention time could givehigher percentage of methane yield in the biogas composition. Theoptimal methane yield was achieved with hydraulic retention timeof 27 days. The methane content was approximately 76.8%. More-over, reduction in total solid and total volatile solid concentrationswere approximately in the range of 65e70%. And COD removal wasin the range of 72e74%. It indicates that anaerobic co-digestion ofsewage sludge and food waste from canteen by the developedprototype single-stage anaerobic digester can be challenging forhigh-rise building application as an alternative renewable energysource from conversion of solid organic wastes into biogas.

Acknowledgment

This research work was financially supported by Japan Inter-national Cooperation Agency (JICA) under Research and Develop-ment of Water Reuse Technology in Tropical Region (Water InTro)Project.

References

Alvarez, R., Liden, G., 2008. Semi-continuous co-digestion of solid slaughterhousewaste, manure, and fruit and vegetable waste. Renew. Energy 33, 726e734.

Angelidaki, I., Boe, K., Ellegaard, L., 2005. Effect of operating conditions and reactorconfiguration on efficiency of full scale biogas plants. Water Sci. Technol. 52,189e194.

APHA, AWWA, WPCF, 2005. Standard Methods for the Examination of Water andWastewater. American Public Health Association, American Water Works As-sociation and Water Pollution Control Federation, New York.

Costa, J.C., Goncalves, P.R., Nobre, A., Alves, M.M., 2012. Biomethanation potential ofmicroalgae Ulva spp. And Gracillaria spp. And in co-digestion with wasteactivated sludge. Bioresour. Technol. 114, 320e326.

Demirel, B., Scherer, P., 2007. Production of methane from sugar beet silage withoutmanure addition by a single-stage anaerobic digestion process. Biomass Bio-energy, 71e77.

Digman, B., Kim, D.S., 2008. Review: alternative energy from food processingwastes. Environ. Prog. 27, 524e534.

Estevez, M.M., Sapci, Z., Linjordet, R., Morken, J., 2014. Incorporation of fish by-product into the semicontinuous anaerobic co-digestion of pre-treated ligno-cellulose and cow manure, with recovery of digestate’s nutrients. Renew. En-ergy 66, 550e558.

Garcia, K., Perez, M., 2013. Anaerobic co-digestion of cattle manure and sewagesludge: influences of composition and temperature. Int. J. Environ. Prot. 3, 8e15.

Kim, H.W., Nam, J.Y., Shin, H.S., 2011. A comparison study on the high-rate co-digestion of sewage sludge and food waste using a temperature-phasedanaerobic sequencing batch reactor system. Bioresource 102, 7272e7279.

Mata-Alvarez, J., Mac�e, S., Llabr�es, P., 2000. Anaerobic digestion of solid wastes. Anoverview of research achievements and perspectives. Bioresour. Technol. 74,3e16.

Murto, M., Bjornsson, L., Mattiasson, B., 2004. Impact of food industrial waste onanaerobic co-digestion of sewage sludge and pig manure. J. Environ. Manag. 70,101e107.

Pollution Control Department, 2012. Thailand State of Pollution Report. Thailand.Serrano, A., Siles, J.A., Chica, A.F., Martin, A., 2014. Improvement of mesophilic

anaerobic co-digestion of agri-food waste by addition of glycerol. J. Environ.Manag. 140, 76e82.

Slack, R.J., Gronow, J.R., Voulvoulis, N., 2005. Sci. Total Environ. 337, 119e137.Sosnowski, P., Wieczorek, A., Ledakowicz, S., 2003. Anaerobic co-digestion of

sewage sludge and organic fraction of municipal solid wastes. Adv. Environ. Res.7, 609e616.

Tchobanoglous, G., Theisen, H., Vigil, S., 1993. Integrated Solid Waste Management.McGraw-Hill, New York, USA.

Zhang, R., El-Mashad, H.M., Hartman, K., Wang, F., Liu, G., Choate, C., 2007. Char-acterization of food waste as feed stock for anaerobic digestion. Bioresour.Technol. 98, 929e935.

Zhou, A., Guo, Z., Yang, C., Kong, F., Liu, W., Wang, A., 2013. Volatile fatty acidsproductivity by anaerobic co-digesting waste activated sludge and corn straw:effect of feed stock composition. J. Biotechnol. 142, 655e662.