Production of biogas from solid organic wastes through anaerobic digestion: a review
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Applied Microbiology andBiotechnology ISSN 0175-7598Volume 95Number 2 Appl Microbiol Biotechnol (2012)95:321-329DOI 10.1007/s00253-012-4152-7
Production of biogas from solid organicwastes through anaerobic digestion: areview
Ismail Muhammad Nasir, Tinia I.MohdGhazi & Rozita Omar
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Production of biogas from solid organic wastesthrough anaerobic digestion: a review
Ismail Muhammad Nasir & Tinia I. Mohd Ghazi &Rozita Omar
Received: 16 December 2011 /Revised: 3 May 2012 /Accepted: 3 May 2012 /Published online: 24 May 2012# Springer-Verlag 2012
Abstract Anaerobic digestion treatments have often beenused for biological stabilization of solid wastes. These treat-ment processes generate biogas which can be used as arenewable energy sources. Recently, anaerobic digestion ofsolid wastes has attracted more interest because of currentenvironmental problems, most especially those concernedwith global warming. Thus, laboratory-scale research on thisarea has increased significantly. In this review paper, thesummary of the most recent research activities coveringproduction of biogas from solid wastes according to itsorigin via various anaerobic technologies was presented.
Keywords Anaerobic digestion . Biogas . Methane .
Million tons of solid waste are produced annually frommunicipal, industrial, and agricultural sources. The indis-criminate decomposition of these organic wastes results inlarge-scale contamination of land, water, and air. Of all theforms of solid organic waste, the most abundant is animaldung primarily from small farms, and it is from these farmsthat the pollution problem originating from waste disposal ismore intense. Research continues to focus on the treatmentof cattle dung for biogas production and possible optimiza-tion methods which could be used to enhance the productionfor practical applicability of the technology. Omar et al.(2008) observed an improvement in biogas yield up to
0.207 m3 kg1 VS added with average methane content of65 % in the anaerobic treatment of cattle manure by additionof palm oil mill effluent as an inoculum in a laboratory scalebioreactor. In another study, Ounaar et al. (2012) obtainedbiogas production of 26.9 m3 with an average methanecontent of 61 % during the anaerobic digestion of 440 kgof cow dung with an energy equivalent of 164.5 kWh. Theseresults are encouraging for the use of animal waste availableto produce renewable energy and clean environment.
According to Yu et al. (2002), decomposition of 1 MT ofgrass waste can possibly release 50110 m3 of carbon diox-ide and 90140 m3 of methane into the atmosphere. Meth-ane is an important greenhouse gas with the ability of globalwarming 25 times greater than that of carbon dioxide, andits atmospheric concentration has been increasing in therange of 1 to 2 % per year (IPCC 2007). Conventionalmunicipal solid waste (MSW) management has been mainlydisposal by land filling (Sosnowski et al. 2003). However,waste from landfills has been identified as the major sourceof anthropogenic methane emission and an essential con-tributor to global warming (IPCC 1996). Therefore, theincreased production of MSW accompanied with environ-mental and economic difficulties facing the conventionalmethods of disposal have resulted in great efforts to findalternative methods of disposal (Zsigraiova et al. 2009). Themost promising alternative to incinerating and compostingthese solid wastes is to digest its organic matter employingthe anaerobic digestion (Bouallagui et al. 2005).
Anaerobic digestion for biogas production has become aworldwide focus of research, because it produces energythat is renewable and environmentally friendly. Special em-phasis was initially focused on anaerobic digestion of MSWfor bioenergy production about a decade ago (Braber 1995;Kiely et al. 1997). Anaerobic biological treatment can be anacceptable solution because it reduces and stabilizes solid
I. Muhammad Nasir : T. I. Mohd Ghazi (*) : R. OmarDepartment of Chemical and Environmental Engineering,Faculty of Engineering, Universiti Putra Malaysia,43400 Serdang, Selangor, Malaysiae-mail: firstname.lastname@example.org
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wastes volume, produces biogas comprising mainly meth-ane and carbon dioxide, and traces amount of other gases(Stroot et al. 2001). In addition to biogas, a nutrient-richdigestate is also produced which provide either fertilizer orsoil conditioner properties. Biological treatment of MSW tobiogas by anaerobic digestion processes including source-sorted and mechanically sorted MSW has been previouslydiscussed (Gunaseelan 1997).
Literature is available about the applications and impor-tance of the anaerobic digestion treatments for solid wastetreatment, especially focusing on promoting process effi-ciency and performance. Therefore, the main objective ofthis review paper is to summarize the research activities onthe effects of both operational and process performanceparameters, covering the anaerobic conversion of varioussolid waste substrates via various anaerobic systems.
Production of biogas by anaerobic digestion processes
Many research papers have been published regarding theperformance of different anaerobic technological systemsdigesting organic solid wastes. Most of them concentrateon the concept of anaerobic digestion of the organic fractionof municipal solid waste (OFMSW). Anaerobic treatment ofOFMSW has been an attractive feedstock for biogas pro-duction. Nevertheless, pretreatment of MSW before thedigestion is the initial stage, and these wastes are character-ized by a high percentage of moisture and VS above 90 %with high biodegradability. Rao et al. (2000) referred tothese wastes as municipal garbage, which is the main con-stituent of MSW (4045 wt.%), emanating from differentsources as food waste (FW) such as households, fruit andvegetable markets, canteens, hotels, etc., and they are rich inorganic matter and can be used for biogas generation byanaerobic digestion. Anaerobic digestion of solid organicwastes has been studied recently, attempting to developtechnology that offers waste stabilization accompanyingresources recovery. About 90 % of the full-scale plant pres-ently in operation in Europe for anaerobic digestion ofOFMSW relies on one-stage system, and this are dividedinto wet and dry digestion (De Baere 2000). A likely reasonfor this is that the industrialists prefer a one-stage systemover the two-stage or multistage systems because a simplerdesign system suffers less frequent technical failures and areeconomical. The dry digestion systems digest waste asreceived, while the wet digestion systems need to slurrythe waste with water to about 12 % TS (Vandevivere et al.2002). However, from a technical point of view, the drydigestion systems appear more robust as regular technicalfailures are reported with wet systems due to sand, plastics,wood, and stones. Many researchers have already reportedvarious studies on laboratory scale, pilot-scale, and full-
scale anaerobic digestion for the treatment of organic solidwaste. From a review of literature on the preliminary designprocedure for anaerobic digesters for the treatment of MSWfor biogas production, Igoni et al. (2008) noted that properreactor size reduction must be considered for the anaerobicdigestion of organic wastes. They further explained that themost important aspect of digestion processes, such as, tem-perature, hydrogen ion concentration, carbon nitrogen (C/N)ratio, organic loading rate (OLR), moisture content, and heatcontent, need to be manipulated so as to achieve optimalperformance for anaerobic digester. They recommend thatthe batch digestion system should be increasingly employedbecause it is cost-effective and economical for treating theever abundant MSW to useful energy. Therefore, a summaryof the anaerobic digestion processes employed for thesewastes will be showed in this section, and the overview ofthe studies are also presented in an orderly manner in Table 1.
A laboratory scale batch anaerobic digestion of municipalgarbage was studied by Rao et al. (2000) at temperatures of25 C and 29 C, with a concentration range between 45 and135 g TS/L. They found out that the methane content fromthe biogas varied between 62 and 72 %, and a conversionefficiency of about 85 % was obtained. In a similar study,Rao and Singh (2004) investigated the batch digestion ofmunicipal garbage under room temperature (264 C) toestimate its bioenergy potential and conversion efficienciesat an HRT of 15 days. They reported a high yield of 0.56 m3
biogas kg1 VS added with 70 % methane content and a VSreduction of 76.3 %. These results demonstrated that mu-nicipal garbage has a high potential to be a bioenergysource. Lpez and Espinosa (2008) evaluated the effect ofpretreating OFMSW with lime in the anaerobic digestionprocess. The laboratory scale experiment was carried out ina completely mixed reactor operated on a batch basis. Themaximum yield of methane obtained under the anaerobicdigestion of the pretreated waste was 0.15 m3 kg1 VSadded. This result is nearly 172 % increase in the methaneyield over the control without pretreatment. In addition,under the same condition, soluble COD and VS removalwere 93 and 94 %, respectively. The outcome implied thatthe chemical pretreatment with lime, followed by anaerobicdigestion, gives the best result for OFMSW stabilization.Elango et al. (2007) reported data on the influence of do-mestic sewage on the biogas production from municipalsolid waste using the anaerobic digestion process. Theyoperated a batch reactor at temperatures from 26 to 36 Cwith a fixed HRT of 25 days and different OLR in the rangeof 0.5 to 4.3 kg VS m3 day1. They obtained a maximumamount of biogas production of 0.36 m3 kg1 VS added atOLR of 2.9 kg VS m3 day1. This OLR was referred to asthe optimum OLR because the maximal removal of TS(87 %), VS (88.15), and COD (89.3 %) occurred at thisstage. They concluded that the disposal problem of MSW
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and domestic sewage can be resolved substantially. Anexperiment was performed by Fernandez et al. (2008) toinvestigate the influence of substrate concentration on drymesophilic anaerobic digestion of the OFMSW. The exper-iment was conducted in a batch reactor at 35 C, during aperiod of 8595 days at solid concentrations of 20 % and30 % TS. Experimental results indicated that the reactorwith 20 % TS achieved a higher yield of 0.11 m3 CH4kg1 VS removed compared to 0.07 m3 CH4 kg
1 VSremoved achieved for 30 % TS reactor. Also, the 20 % TSdigestion attained the highest performance with high dis-solve organic carbon (DOC) removal (80.69 %), comparedto the 30 % TS digestion (69.05 %). Therefore, they con-cluded that the initial substrate concentration during theanaerobic digestion of OFMSW affects the process clearly.In a similar study, Fernandez et al. (2010) investigated themesophilic anaerobic degradation of OFMSW in discontin-uous lab reactors with two different initial concentrations of20 % TS and 30 % TS. The anaerobic treatment was favoredwhen it was conducted with a 20 % TS content in compar-ison to a similar process with 30 % TS. Results showed ahigher level of organic matter removed, in terms of DOCand VFA, 18.18 % and 8.09 %, respectively, in the 20 % TSsystem. Also, the kinetics parameters demonstrated higheractive biomass and a higher coefficient for the production ofmethane at the 20 % TS concentration.
Guendouz et al. (2010) found similar biogas and methaneyields of around 0.211 m3 kg1 VS added with 40 % VSreduction conducted in a laboratory scale high-solid batchdigestion test of MSW under mesophilic conditions for a15-day HRT. The results obtained compared well to a largerpilot-scale reactor operation with a yield of 0.205 m3 meth-ane kg1 VS added. Parawira et al. (2004) examined batchanaerobic codigestion in an experiment with different mix-tures of potato waste and beet leaves. They reported anenhanced yield of 0.68 m3 methane kg1 VS added with amixing ratio of (24:16%TS), and another yield of 0.42 m3
methane kg1 VS added from potato waste alone. Theprocesses were both operated under mesophilic conditionsat an HRT of 14 days. The codigestion result showed anoptimum mixing ratio for successful operation with a higheryield and a shorter HRT. Hence, the batch anaerobic diges-tion can be applied where low cost and low technology areneeded.
Macias-Coral et al. (2008) investigated the applicabilityof a two-phase pilot-scale anaerobic codigestion system forthe treatment of OFMSW, dairy cow manure (CM), andcotton gin waste (CGW). Results showed that the singlewaste digestion of OFMSW and CM produced 0.03 and0.08 m3 CH4 kg
1 VS added. Meanwhile, the codigestionof OFMSW and CM produced a yield of 0.1 m3 kg1 VSadded, in addition to the CGW and CM which obtained thehighest yield of 0.19 m3 kg1 VS added. They concludedT
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that codigestion of the combined wastes resulted in highmethane yield as compared to the single waste digestion.Fernandez et al. (2005) study the potential of anaerobicdigestion for the treatment of fats of different originsthrough codigestion with simulated OFMSW. In the study,a pilot plant operating semicontinuously in the mesophilic(37 C) temperature and HRTof 17 days was employed. Thebiogas and methane yields obtained from the simulatedOFMSWat steady state were 0.8 and 0.5 m3 kg1 VS added,respectively. On the other hand, no significant differencewas observed in the performance of the anaerobic codiges-tion when animal fat was changed with vegetable fat. Also,the yields of biogas and methane were similar to those foundwith OFMSW, methane content being a bit higher in thepresence of fat. They concluded that both fat from animal orvegetable origin are highly degradable (94 % for animal fatand 97 % for vegetable fat) during codigestion withOFMSW. Therefore, this indicates that anaerobic digestionappears to be a suitable technology for the treatment of suchwastes and obtainment of a renewable energy source. Astudy was performed by Nguyen et al. (2007) to evaluatethe effect of prestage flushing and microaeration in additionto the effect of leachate percolation in methanogenesis stageenhancement in order to develop combined batch anaerobicdigestion systems. The study was conducted in a high solidpilot-scale reactor in two runs. In run 1, prestage flushingand microaeration were carried out to find out their effect interms of enhancing hydrolysis and acidification in ambientcondition. Whereas in run 2, following prestage, new con-dition was provided in order to enhance the start-up ofmethanogenesis stage by adjusting the pH to 6.5 and followedby inoculum addition at mesophilic condition (37 C). Theresult showed that the application of microaeration achieved75 % biogas conversion and 61 % VS degradation on thehydrolysis and acidification enhancement. Also, the prestageflushing with lesser volume of water (29.3 l/kg TS) was foundto be significant in removing the organic matter from wastebed in preparation for methanogenesis stage. The leachatepercolation during methanogenesis stage demonstrated anenhanced biomethanization when compared to the reactorswithout leachate percolation. The methane yield observedduring the process was 0.26 m3 kg1 VS added with 75 %process efficiency obtained during the process.
Hartmann and Ahring (2005) investigated the anaerobicdigestion of OFMSW in two thermophilic (55 C) wet labscale reactor systems. In the first reactor, the OFMSW wascodigested with manure with a subsequent higher concen-tration of OFMSW at HRT of 1418 days and OLR of 3.34 kg VS m3 day1. In the second reactor, codigestion ofOFMSW:manure ratio of 50 % (VS/VS) was maintained ascontrol and also the recirculation of 100 % MSW. Resultsshowed that both the codigestion process and the treatmentof 100 % OFMSW with recirculation of process liquid
demonstrated stable operation in spite of fluctuations inthe feed volume. They reported a biogas yield in the rangeof 0.630.71 m3 kg1 VS in both configurations, with VSreduction up to 74 % when treating the 100 % OFMSW.Linke et al. (2006) digested the potato processing wasteanaerobically for biogas production in a CSTR at thermo-phillic condition. They found out that with an increase of theOLR in the range of 0.83.4 kg VS m3 day1, the biogasyield decreases. In addition, biogas yields with its methanecomposition were obtained to be 0.85 to 0.65 m3 kg1 VSand 58 to 50 %, respectively. They concluded that specialimportance should be positioned on the reactor performanceat steady state and at OLR that does not result in reactorfailure. Glass et al. (2005) operated a CSTR and an anaer-obic filter (AF) for biogas production from steam-treatedMSW wastewater. The study reported the CSTR productionto be between 0.02 and 0.29 m3 CH4 kg
1 day1, while theAF production ranged from 0.04 to 0.47 m3 CH4 kg
1 day1.They concluded that the difference in the performance ofboth systems is because the CSTR received wastewatercontaining suspended solids, while the AF received waste-water free from the suspended solids. Sosnowski et al.(2003) studied the anaerobic codigestion of sewage sludgeand OFMSW in a laboratory scale two-phase anaerobicsystem operated in a quasicontinuous mode, with a CSTRas the acidogenic reactor operated at 56 C and a UASBreactor as the methanogenic reactor operated at 36 C. Theyreported a higher specific methane yield as 0.024 m3 kg1
VS added. Also, Fongsatitkul et al. (2010) operated a labo-ratory scale, two-phase, mesophilic anaerobic treatment todetermine the digestibility of OFMSW through codigestionwith varying amounts of return activated sludge (RAS).They found out that increasing the amount of RAS mixedwith OFMSW did not enhance its digestibility, as showedby a decrease in the %VS removal and specific biogasproduction. They concluded that the optimum ratioappeared at 100 % OFMSW (8 % TS) with specific biogasproduction of 0.73 m3 kg1 VS and a VS removal of about87 % at an HRT of 28 days.
According to Bouallagui et al. (2003), the maximumOLR for the mesophilic anaerobic digestion of fruit andvegetable waste (FVW) in a tubular reactor is 6 % of TSwith a high yield of 0.707 m3 biogas kg1 VS added for a20-day HRT. These results compared well with that reportedby Hartmann and Ahring (2005). Thus, it can be deducedthat the tubular reactor behaves in a similar way to a CSTRwith high stability and process economy. Further, in a sim-ilar study, Bouallagui et al. (2004) made a comparisonbetween the performances of the tubular anaerobic digestersoperated under thermophilic conditions and those underpsychrophilic and mesophilic conditions. At an OLR of6 % TS (4, 6, 8, and 10 % on dry weight), the yield in thepsychrophilic and mesophilic digesters was almost the same
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(Table 1), while the performance of the thermophilic digest-er was higher than that of the mesophilic and psychrophilicdigesters. Also, the highest biogas was obtained in thethermophilic digester at a HRT of 10 days. Hence, theseresults established a phenomenon for OLR selection withthe capability of upgrading the existing mesophilic digestersto the thermophilic range, thereby improving the perfor-mance of the digesters.
Zhang et al. (2007) conducted a batch anaerobic diges-tion test to investigate the biodegradability of FWat an HRTof 10 and 28 days. In the study, the highest methane yield of0.435 m3 kg1 VS was obtained at the end of the 28-daysdigestion with VS removal of 81 %, which is followed by0.348 m3 kg1 VS at the end of 10-day digestion. Theseresults indicated that FW was a good alternative substratefor anaerobic digestion because of its high degradability andbiogas yield. In another study, Forster Carneiro et al.(2008b) experimentally study the biomethanization proce-dure of FW in six reactors with three different total solidpercentages (20 %, 25 %, and 30 % TS) and two differentinoculum percentages (2030 % of mesophilic sludge). Thestudy was designed to select the initial performance param-eters (total solid and inoculum contents) in a lab-scalereactor and later, to validate the optimal parameters in alab-scale batch reactor. The best performance for FW treat-ment and the methane generation was the reactor with 20 %TS and 30 % of inoculum. They observed a methane yieldof 0.49 m3 kg1 VS added between 20 and 60 days duringthis operation. In addition, the lab-scale batch reactor showsa classical waste removal with high value of methane yieldof 0.22 m3 kg1 VS added. Finally, they proposed a protocolto improve the start-up phase for dry thermophilic anaerobicdigestion of FW. The study of Bouallagui et al. (2009)observed a maximum OLR of 1.24 kg VS m3 day1 oper-ating a thermophilic anaerobic sequencing batch reactor(ASBR) treating FVW with a 15-day HRT. They achieveda high biogas yield of 0.48 m3 kg1 VS added with 60 %methane content and a 79 % VS reduction. In a similarstudy, Bouallagui et al. (2009) examined the effect of HRTand temperature variations under mesophilic and thermo-philic conditions on the performance of an ASBR treatingabattoir waste water with FVW. They observed a volatilesolid removal between 73 and 86 % and a biogas yield ofabout 0.30.73 m3 kg1 TVS added at OLRs up to 2.56 kgTVS m3 day1 in the ASBR codigestion process. Theresults of the digesters' performances showed that the vari-ation in HRT under mesophilic conditions had no significantinfluence on the organic matter removal. However, thebiogas yield at 20 days of HRT was significantly increasedby increasing the temperature from 35 C to 55 C. At 55 C,the HRT of 10 days resulted in overloading and subsequentfailure of the digestion of the AWand the codigestion process.This could possibly be due to the outlet pH and alkalinity
values observed to increase with the increases of the OLR andtemperature, which fail the biogas production.
Alvarez and Liden (2008) experimentally investigatedthe potential of a semicontinuous mesophilic anaerobictreatment of solid slaughterhouse waste, fruitvegetablewastes, and manure in a codigestion process. Anaerobiccodigestion resulted in methane yields of 0.3 m3 kg1 VSadded, with methane content between 54 and 56 % at OLRsin the range 0.31.3 kg VS m3 day1. However, the biogasproduction was observed to decrease with an increase inorganic loading and subsequent reduction in the methaneyield indicating organic overload or insufficient bufferingcapacity. They concluded that a combined treatment ofvarious waste types like manure (cattle and swine), solidslaughterhouse wastes (rumen, paunch content, and bloodfrom cattle and swine), and FVW in a mesophilic codiges-tion process establishes the possibility of treating the waste,which cannot be successfully treated separately.
Schober et al. (1999) examined the influence of one-stageand a two-stage continuous bioreactor treating kitchen re-fuse under mesophilic and thermophilic conditions. Theresult of the treatment of this kitchen refuse, containing ahigh amount of easily biodegradable organics, especiallylipids, led to a similar yield of biogas of 0.83 m3 kg1 VSadded at thermophilic temperatures and 0.80 m3 kg1 VSadded at mesophilic temperatures. On the other hand, a totalVS reduction of 91 % was achieved with a 7-day HRT in atwo-stage plant using a concentration unit. They concludedfrom main the result that the digestion of MSW should becarried out in a two-stage system with a concentration unitbetween the two stages. By achieving these optimized pro-cess conditions, a turnover of the organic matter of 90 %with low retention time could be accomplished. Parawira etal. (2006) studied the performance of two types of bioreac-tors comparatively treating potato waste leachate. Theyreported that the upflow anaerobic sludge blanket reactor(UASB) demonstrated a stable process with 66 % methanecontent in biogas and a high OLR. This proved superior tothe anaerobic packed-bed reactor (APB) with 59 % methanecontent in the biogas at a lower OLR. The authors found outthat both reactors gave a comparably high performancewhen treating organic matter.
Angelidaki et al. (2006) evaluated how different opera-tional strategies could minimize the process inhibition dueto ammonia accumulation during the anaerobic digestion ofSource-Sorted Organic Fraction of Municipal Solid Waste(SS-OFMSW) in a thermophilic CSTR. At an OLR of11.4 kg VS m3 day and a 15-day HRT, a stable perfor-mance was observed when SS-OFMSW was diluted withfresh water and when process water was recirculated afterammonia stripping. Methane (CH4) yields from both treat-ments were 0.4 and 0.43 m3 CH4 kg
1 VS added, respec-tively. These results were in accord with the yield reported
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by Hartmann and Ahring (2005) during wet digestion of SS-OFMSW at 55 C with effluent recirculation. Davidsson etal. (2007) conducted a study to investigate the methaneyields from the thermophilic pilot-scale digestion of 17types of domestically SS-OFMSW from households. Theyreported a VS reduction of about 80 % and a methane yieldof 0.30.4 Nm3 CH4 kg
1 VS added with a 15-day HRT,corresponding to about 70 % of the potential measuredduring 50-day batch digestion. They concluded that a sort-ing and collection system does not significantly affect theamount of methane produced. Forter-Carneiro et al. (2008a)evaluated the performance of two laboratory-scale reactorstreating OFMSW, SS-OFMSW, and mechanically selectedOFMSW (MS-OFMSW). In the study, discontinuous reac-tors operated at thermophilic temperature and dry digestion(20 % TS) was used. The reactor treating the SS-OFMSWshowed a fast start up beginning within 05 days and 2030 days with a subsequent stabilization phase. AVS reduc-tion of 45 % with a cumulative biogas of 0.120 m3 wasreported in 60 days. Moreover, the MS-OFMSW treatmentindicated a methanogenic phase during the whole period ofexperiment (60 days). They obtained a higher VS removalof 56 % and cumulative biogas of 0.082 m3. These resultsindicated that both digestion treatments were accomplishedand a high level of cumulative methane production wasachieved in less than 60 days.
A study was conducted by Maroun and EL Fadel (2007)in a CSTR to investigate the mesophilic anaerobic digestionof SS-OFMSW. They reported a biogas yield ranging be-tween 0.2 and 0.56 m3 kg1 VS at an OLR of 2.03 kg VSm3 day1. More so, Kim et al. (2006) investigated thedigestibility of FW in a three-stage semicontinuous systemat thermophillic condition. They reported 67 % methanecontent in the biogas at an HRT of 12.4 days. Forster-Carneiro et al. (2008c) conducted a batch experiment todetermine the digestibility of shredded organic fraction ofthe municipals solid waste (SH-OFMSW) and FW separate-ly. Their results indicated that SH-OFMSW is a viablesubstrate for anaerobic digestion with a high VS removalof 74 % and methane yield of 0.05 m3 kg1 VS added, whilethe FW demonstrates the smallest VS removal (32 %) andhigh methane yield of 0.18 m3 kg1 VS added. On the otherhand, the OFMSW indicated the highest VS removal(79.5 %) and achieved a methane yield of 0.08 m3 kg1
VS added. Therefore, they concluded that the nature oforganic substrate has a significant effect on the biodegrada-tion process and methane yield. Also, pretreatment of wasteis not necessary for OFMSW. In another study, Forster-Carneiro et al. (2007) evaluated the effect of different inoc-ulum sources on the performance of laboratory scale reac-tors treating separately collected organic fraction ofmunicipal solid waste (SC-OFMSW). The experimentalconditions selected were: 25 % of inoculums and 30 % of
total solids operated under single phase thermophilic (55 C)temperature. Results showed that the highest methane yieldwas obtained for sludge inoculums reactor, followed byswine/sludge inoculums reactor, and swine inoculums reactor,with values of 0.29, 0.27, and 0.18 m3 kg1 VS added,respectively. However, a lower yield of methane was obtainedfor the reactor treating restaurant waste digested with ricehulls, followed by corn silage, and then cattle waste withvalues of 0.11, 0.22, and 0.03 m3 kg1 VS added, respectively.They concluded that sludge is the best inoculum for thethermophilic anaerobic digestion of SC-OFMSW at dry con-dition (30%TS) achieving 44 % and 43 % COD and VSremoval, respectively.
Sharma et al. (2000) investigated the applicability of aplug flow reactor (PFR) to develop an effective treatment ofsemisolid waste mixed with sewage sludge. Operating underthe mesophilic conditions at 33.7 days HRT, 71 % of VSdestruction was reported with a methane yield of0.7 m3 kg1 VS added which was equivalent to 1.05 m3
biogas kg1 VS added. However, at a short HRT of22.5 days, the methane yield did not show any decrease;thus, it can be operated with a short HRT, thereby reducingreactor volume and cost. Consequently, Lastella et al. (2002)investigated the effect of using different bacteria inoculumson the anaerobic treatment of semisolid organic waste avail-able from the ortho fruit market in a PFR under mesophilicconditions. They obtained 68 % methane content in thebiogas and a VS reduction of 72 %. These results weresimilar to that reported by Sharma et al. (2000) during thetreatment of semisolid waste at 37 C. In another study,Bolzonella et al. (2006) investigated the biogas yield andprocess performance of two full-scale reactors for the treat-ment of differently sorted municipal organic wastes. Theyreported methane yield of 0.4 m3 kg1 VS added for thereactor treating SS-OFMSW operated under a temperatureof 36.7 C at HRT of 4060 days. While the reactor treatingmixed materials consisting of gray wastes, mechanically sortedorganic fraction of municipal solid waste (MS-OFMSW), andsludge showed a methane yield of 0.13 m3 kg1 VS addedmaintained at a temperature of 38.6 C at HRT of 5070 days.They concluded that the strategy of waste collection affects thecharacteristics of the organic waste, hence, affecting the reactoryield even under similar operational conditions. Zupancic et al.(2008) investigated the applicability a full-scale experiment onthe codigestion of organic waste from domestic refuse andmunicipal sludge. In the experiment, the OLR of the digestertreating municipal sludge at HRT of 20 days was raised by25 % to 1 kg VS m3 day1 by the addition of organic waste. Itwas observed from the experiment that the biogas productionincreases by 80 % with an increased yield from 0.39 m3 kg1
VS added before the experiment to 0.6 m3 kg1 VS added,peaking at 0.89 m3 kg1 VS added. They concluded that theexperiment achieved high degradation efficiency with virtually
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all the organic waste degraded. They recommend the applica-tion of such practice as it will assist in handling organic wastein the future.
Anaerobic digestions of organic solid wastes studiedhave shown to be a renewable energy source that can gen-erate biogas with high methane content. Most of the studieson the anaerobic digestion of solid wastes were conductedon different types of anaerobic reactors at various ranges ofoperating parameters such as temperature, OLR, and HRT.The effect of these parameters on process performance isvery important. The information originating from the litera-ture can be used to draw the following conclusions:
Anaerobic digestion of municipal garbage showed ahigh performance in the CSTR and two-stage bioreac-tors than batch and ASBR with a methane yield in therange of 0.10.7 m3 kg1 VS added and a VS destruc-tion >80 % with HRT ranging from 7 to 25 days.
Conventional batch, single-stage, and two-stage anaer-obic digestion processes have been employed to pro-duce biogas from different solid types of substrates suchas municipal solid waste, FVW, FW, etc. The two-phasesystems have shown good stability and optimum biogasproduction. Therefore, more attention should be direct-ed towards the utilization of a two-phase system foroptimum bioenergy recovery. However, the operationof the single-phase in the treatment of solid wastes tobiogas in the rural areas is another alternative for re-newable energy production, especially for developingcountries as well as for the developed countries.
Acknowledgments The authors would like to thank Universiti PutraMalaysia for the financial assistance and facilities, and the NationalFeedlot Corporation for support.
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Production of biogas from solid organic wastes through anaerobic digestion: a reviewAbstractIntroductionProduction of biogas by anaerobic digestion processesReferences