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Page 1: Anaerobic digestion of municipal solid waste as a treatment prior to landfill

Bioresource Technology 98 (2007) 380–387

Anaerobic digestion of municipal solid waste as a treatmentprior to landWll

P.H.L. Nguyen, P. Kuruparan, C. Visvanathan ¤

Environmental Engineering and Management Program, Asian Institute of Technology, P.O. Box 4, Klong Luang, Pathumthani 12120, Thailand

Received 20 August 2005; received in revised form 14 December 2005; accepted 17 December 2005Available online 9 February 2006

Abstract

Anaerobic digestion of organic fraction of municipal solid waste was conducted in pilot-scale reactor based on high-solid combinedanaerobic digestion process. This study was performed in two runs. In Run 1 and Run 2, pre-stage Xushing and micro-aeration were con-ducted to determine their eVect in terms of enhancing hydrolysis and acidiWcation in ambient condition. In Run 2, after pre-stage, themethane phase (methanogenesis) was started-up after pH adjustment and inoculum addition in mesophilic condition. AcidiWed leachateproduced in pre-stage was used for percolation during active methane phase. At the end of methane phase, air Xushing was conductedbefore unloading the digesters. Hydrolysis and acidiWcation yield of 140 g C/kg TS and 180 g VFA/kg TS were achieved, respectively inpre-stage. Micro-aeration exhibited an equivocal result in terms of enhancing hydrolysis/acidiWcation; however it showed a positive eVectin methane phase performance and this needed further investigation. Leachate percolation during methane phase showed an enhancedmethanization when compared to the reactors without leachate percolation. After 60 days, 260 l CH4/kg VS was obtained. Based on thewaste methane potential, 75% biogas conversion and 61% VS degradation were achieved.© 2006 Elsevier Ltd. All rights reserved.

Keywords: Anaerobic digestion; Flushing; Micro-aeration; Hydrolysis; AcidiWcation; Methane phase

1. Introduction

Direct landWlling of municipal solid waste (MSW) wasknown to create lasting detrimental impacts to the environ-ment. Among the major issues associated with landWlls arethe consequential emissions to the atmosphere, hydro-sphere, and pedosphere; risk in landWll stability; and scar-city of land. Since landWll was regarded as an integral partof solid waste management in Asia, it was realized thatwaste treatment prior to landWll is indispensable. In thisregard, biological pre-treatment of waste like anaerobicdigestion is an attractive method especially in Asian coun-tries, because of its suitable waste characteristics. Accord-ing to Visvanathan et al. (2004), municipal solid wastestream in Asian cities is almost similar, composed of highfraction of biodegradable material of more than 50% with

* Corresponding author. Tel.: +66 2 524 5640; fax: +66 2 524 5625.E-mail address: [email protected] (C. Visvanathan).

0960-8524/$ - see front matter © 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.biortech.2005.12.018

high moisture content, and the generation rate is increasingwith time. For example, in Thailand, the organic fraction inMSW consist of food waste (50%), paper (10%), and yardwaste (5%) and the remaining inorganic fraction is com-posed of plastics (14%), glass/stone/can (5%), wood (4%),metals (3%), textile (3%), rubber/leather (2%), and soil/other (4%).

Anaerobic digestion of organic fraction municipalsolid waste (OFMSW) has been studied in recent decades,trying to develop a technology that oVers waste stabili-zation with resources recovery. In the complex process ofanaerobic digestion, hydrolysis/acidiWcation and methano-genesis are considered as rate-limiting steps. However, it ispossible to increase the hydrolysis rate with the applica-tion of micro-aerophilic conditions (Capela et al., 1999;Wellinger et al., 1999). Moreover, Dayanthi et al. (2004)studied the leaching experiment on organic fraction ofMSW showed that Xushing the waste bed could enhancehydrolysis and acidiWcation. SpeciWc features of high-solid

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batch anaerobic digestion process such as simple designand process control, lower investment cost, lesser waterconsumption, etc. make them particularly attractive fordeveloping countries.

Temperature signiWcantly inXuences anaerobic diges-tion process, especially in methanogenesis wherein thedegradation rate is increasing with temperature. Thereaction temperatures with maximum activity are meso-philic and thermophilic (Mata-Alvarez, 2003). Micro-organisms operating in mesophilic range are more robustand can tolerate greater changes in the environmentalparameters than thermophilic condition. The stability ofthe mesophilic process makes it more popular in currentanaerobic digestion facilities due to the fact that thermo-philic bacteria are more sensitive to toxicants and tem-perature Xuctuation outside the optimum range (Bieyet al., 2003). Since this study is in early stage of investiga-tion, ambient condition in pre-stage and mesophilic con-dition in methane phase was selected to investigate theparameters.

The objective of this research was to study the eVect ofXushing and micro-aeration in pre-stage as well as the eVectof leachate percolation in methane phase enhancement inorder to develop a combined anaerobic digestion process inbatch systems.

2. Methods

2.1. System design

This study was performed in pilot scale, double-walledstainless steel anaerobic digesters with a total volume of 375 l.The designated volume for waste bed was 260 l, leaving theavailable headspace and bottom space for biogas generationand gravel support, respectively. The reactors were equippedwith top removable cover for waste loading and unloading ineach batch. Anaerobic condition was ensured by completelyclosing the reactor by placing rubber gasket and silicone seal-ant in between the lids. Temperature inside the digester wascontrolled by a temperature controller wherein hot waterfrom water bath was pumped in the water jacket to maintainthe temperature of 37°C in methane phase. Each reactor wasprovided with 200 l and 60 l leachate tanks for pre-stage andmethane phase leachate storage, respectively. The operationof pumps and air compressor were automatically controlledat certain interval by setting the timers. Fig. 1 represents theschematic diagram of the experimental set-up. Biogas pro-duced in main-stage was directed to a “U” tube for gas sam-pling before reaching wet gas meter for daily gas productionmeasurement. This was to ensure the biogas sample taken atthe “U” tube was not aVected by water in the wet gas meter.

Fig. 1. Schematic diagram of anaerobic digestion experimental set-up.

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2.2. Process features

Fresh market waste were loaded into the reactors andXushed with tap water to produce eZuent in the form ofleachate. The purpose of Xushing was to leach out pollu-tant in order to reduce the organic load from the wastebed and at the same time to reduce the constraint of highorganic loading in high solid batch system. Fig. 2 illus-trates the three stages involved in combined anaerobicdigestion process. (1) Pre-stage: volatile fatty acids(VFA) and other dissolve organic compounds producedby the fresh waste were Xushed into leachate. Flushingand micro-aeration at diVerent strategies were providedto optimize the hydrolysis/acidiWcation process at ambi-ent temperature. (2) Methane phase: biogas productionat mesophilic temperature. Start-up was conducted withpH adjustment and inoculum addition. An active meth-ane phase was indicated by the presence of about 50%methane in biogas and the pH of the system around 7. (3)Final stage: the waste bed was Xushed with fresh air toremove the remaining biogas in digester before unload-ing the waste.

2.3. Feedstock characteristics and preparation

The substrate used was market waste collected fromBangkok, Thailand. The waste was characterized formoisture content (MCD 90%), volatile solids (VSD 79%),and total solids (TSD 10%). Fresh waste was manuallysegregated to remove bulky and inert materials. Thesorted waste was subjected to size reduction to <60 mm byusing mechanical pulverizer. The waste was loaded intothe reactor together with bamboo cutlets (10% volume ofthe loaded waste) as bulking agent. The purpose ofemploying bulking material was to create void space inorder to facilitate the distribution of Xushing water andaeration. Bamboo cutlets were separated manually when

the waste was unloaded from the reactor and the digestedwaste sample was taken for characterization at the end ofthe process.

2.4. Experimental set-up

Three digestion systems ran in parallel to optimize thepre-stage and methane phase. Pre-stage Xushing and micro-aeration conditions were examined in Run 1 and Run 2whereas methane phase was conducted only in Run 2. Thepre-stage operational sequence is schematized in Fig. 3.

In Run 1, the reactors were loaded with fresh wasteto a compaction density of 500 kg/m3 with bulking agent.Pre-stage was conducted for seven days. AcidiWed leachatewas removed daily and was replaced by another 200 l oftap water. Daily water replacement during pre-stage underambient condition was found to enhance leaching (Dayan-thi et al., 2004). Initially Xushing at a rate of 5 l/min for 4 hrun/4 h stop was performed followed by micro-aeration at arate of 1 l/min (0.4 l/kg h) for 2 h run/4 h stop. It should benoted that reactor 1 was non-micro-aerated while reactors2 and 3 was only provided with micro-aeration for the Wrstthree and seven days of operation, respectively. The resultsof pre-stage performance in Run 1 are illustrated in Fig. 4.In general, it could be deduced that after Wve days of Xush-ing in three reactors, the additional removal of DOC loadof only 64% was very low. In this regard, Wve days of Xush-ing could be enough.

In Run 2, pre-stage was conducted for only Wve days.Daily water replacement was applied in reactor 1, whereasin reactors 2 and 3, water was replaced after day 1 and onday 3 of operation. As a result, a total of 1000 l water (48.8 l/kg TS) was used for reactor 1, and only 600 l water (29.3 l/kg TS) was used for reactors 2 and 3. In reactor 3, Wve daysof micro-aeration was provided at a rate of 1 l/min (0.4 l/kg h) for 2 h run/4 h stop; while in reactors 1 and 2, micro-aeration was not applied. The reactors were loaded initially

Fig. 2. Schematic diagram of the three-stage anaerobic digestion system.

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of 150 kg of waste with bulking agent. Around 30 kg offresh waste was added after two and four days of Xushing.This additional amount of waste corresponded to the avail-able reactor’s headspace resulting from waste settlementduring pre-stage, also an optimum density of 500 kg/m3 wasmaintained in the new layer of waste.

Following pre-stage, new condition was provided inorder to enhance the start-up of methane phase. The pH ofthe system was adjusted to 6.5 and was followed by inocu-lum addition. Mixture of cow dung, stabilized/digestedwaste and anaerobic sludge was used as seeding materialtotally accounting for 16% VS of the loaded waste. Percola-tion was performed for two days to distribute inoculumsthroughout the waste bed. Reactors were incubated to atemperature of 37 °C. DiVerent strategies were applied forthree digesters. In reactors 1 and 3, leachate percolationwas only practiced by the time the reactor shifted to activemethane phase (CH4D50%) and that was on day 40 and30, respectively. However, in reactor 2, leachate percolationwas not provided. Pre-stage leachate was percolated at a

Fig. 4. Variation of DOC, pH, and alkalinity in pre-stage leachate of Run 1(broken lines corresponding to primary Y-axis, continuous line correspond-ing to secondary Y-axis); (–�–) reactor 1 (non-micro-aerated); (–�–) reactor2 (three-day micro-aerated); (–�–) reactor 3 (seven-day micro-aerated).

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rate of 0.2 l/min for 4 h run/4 h stop and replaced in batchmode. At the end of methane phase, the waste bed wasXushed with fresh air for one day before unloading thedigester in order to remove the remaining biogas in thereactor for safe unloading.

2.5. Analytical method

Waste characteristic before and after digestion wasexamined in order to determine the extent of waste degra-dation (%VS loss). In addition, waste samples were deter-mined in terms of MC, TS, and VS. The BMP test wasconducted on fresh waste based on the method establishedby Hansen et al. (2004). During pre-stage, the representa-tive leachate sample from three reactors were collecteddaily from the leachate tank and analyzed for dissolvedorganic carbon (DOC); total kjeldahl nitrogen (TKN);VFA including acetic acid (HAc), propionic acid (HPro),butyric acid (HBu), and valeric acid (HVa); pH; andalkalinity. All parameters were determined based on theanalytical procedures in standard methods (APHAet al., 1998). Daily biogas production was determinedfrom wet gas meter and biogas was analyzed in volumet-ric composition (CO2, CH4, O2 and N2) by using Gas chro-matography.

3. Results and discussion

3.1. Pre-stage

Fig. 4 exhibits the variation of DOC, pH, and alkalinityof leachate during pre-stage. In three digesters, highestDOC concentration (3.5 g/l) was noted in the Wrst day ofXushing. The result showed the DOC concentration in dailyleachate reduced sharply with run time to around 0.5 g/l inWve days. Similar trends were observed for TKN and VFA.The VFA concentration during Wrst few days of Xushingwas about 3–4 g/l and decreased to around 0.7 g/l on day 5.Although, Xushing could enhance acid production, itdiluted the produced acids from the digester too early. Thelow pollutant concentration in leachate from day 5 wasdue to the early extraction of hydrolyzed materials and the

Fig. 3. Pre-stage optimization.

Reactor 1 Reactor 2 Reactor 3

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dilution of waste bed by Xushing. There was no furthersigniWcant organic removal was noticed after Wve days. Asmentioned, the purpose of pre-stage operation was topartly remove the organic fraction of the waste bed in theform of leachate in preparation for methane phase; thissuggested that Xushing was insigniWcant after day 5. Theobservation demonstrated that the short duration wasneeded for Xushing.

Moreover, pH value in three reactors was low in therange of 5–6 (Fig. 4). This might be due to high VFA pro-duction and low buVering capacity (alkalinity) of the waste.The pH value in reactor 3 (with seven days micro-aeration)was quite high due to bicarbonate buVering capacity causedby the formation of carbon dioxide in aerobic metabolism.AcidiWcation occurs strongly at the early Xushing periodbecause acidogens are known to be fast-growing bacteriawith a minimum doubling time of around 30 min and arecapable of fermenting part of the soluble fraction oforganic refuse to produce VFA in a short time interval(Mosey, 1983). Since hydrolysis is not rate limiting, acidsare produced quickly. At the end of Xushing, a maximumVFA yield of 180 g VFA/kg TS was achieved in reactor 1.Lower acid yield of 160 g VFA/kg TS was observed in reac-tors 2 and 3.

The cumulative load of single VFA distribution duringpre-stage in three digesters did not vary much. Generally,acetic acid dominates over propionic, butyric, and valericacids. Since acetic acid is the direct substrate for methano-gens, its higher concentration shows that methanogenesisdo not occur during pre-stage. A comparison of DOC andDOC equivalent of TVFA (Table 1) exhibited that overhalf of the soluble organic carbon in leachate was acidiWedinto VFA. This showed that acidiWcation was strong overhydrolysis. The high fraction of VFA in leachate favoredthe proposal of feeding it back into the digester duringmethane phase by leachate percolation. Early extraction ofVFA in pre-stage leachate would prevent imbalancebetween acidogenesis and methanogenesis in methanephase which was normally considered to cause instability inhigh-solid digestion system (Mata-Alvarez, 2003).

Low concentration of VFA at the end of pre-stage sug-gested that strict separation of acidogenesis and methano-genesis could not be maintained at this point and it wouldbe better to shift the reactor to a new stage. In this regard,

in Run 2, pH adjustment and inoculum addition was per-formed after pre-stage. Since it was also observed from Run1 that VFA concentration in leachate was low on day 3 (lessthan 2 g/l), it was possible to reduce the amount of waterused without causing VFA inhibition. Thus, in Run 2, theamount of Xushing water was varied; 1000 l used in reactor1, and 600 l was supplied in reactors 2 and 3. Also, addi-tional 30 kg of waste added into the digesters in Run 2showed an insigniWcant diVerence in terms of pollutantload that could be extracted from the waste bed into leach-ate. Optimum compaction density of 500 kg/m3 in the newwaste layer was hypothesized to be the reason for a com-parable yield.

3.2. Methane phase

Fig. 5a and b show biogas composition and cumulativeproduction in methane stage, respectively. During start-up,biogas production was low and methane content increasedslowly to 50% in reactors. The system was successfullystarted-up after 25 days. Gas production rate in reactor 3was higher than reactors 1 and 2. The possible explanationwas due to early micro-aeration in pre-stage which mighthave resulted in better hydrolysis/acidiWcation during start-up of methanization period providing substrate for metha-nogens.

DiVerent behaviors could be observed after start-up(Fig. 5b). In reactor 3, it was observed that cumulative gasproduction increased immediately after lag phase, whenmethane composition was stable and leachate percolationwas practiced. The curve implied that the waste bed wassuYciently inoculated and buVered so that the methano-gens were activated. During batch 1 of leachate percolationbiogas production kept increasing but there was insigniW-cant change in VFA in reactor 3 (Fig. 6a and b). The reasoncould be due to the continuous production of VFA fromthe waste bed. Thus, based on this observation, the phasechange from acidic to methanogenic stage had not yet com-pleted. The cumulative methane yield rapidly increased andthe high methane content (>60%) indicated a balancedmethane fermentation.

Regarding the cumulative gas production, reactor 3gained the highest biogas production of about 5000 l (256 l/kg TS) after 60 days of operation. This implied that the

Table 1Load of DOC and DOC equivalent of VFA in pre-stage leachate (Run 1 and Run 2)

Note: Unit expressed in g C/kg TS.

R1 (without micro-aeration) R2 (3 days micro-aeration) R3 (7 days micro-aeration)

Run 1 (7 days pre-stage)DOC 140 127 119DOC equivalent of TVFA 86 72 70

R1 (1000 l water) R2 (600 l water) R3 (600 l water)Run 2 (5 days pre-stage)DOC 140 128 129DOC equivalent of TVFA 83 80 88

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reactor reached a mature phase in which the hydrolyzedand acidiWed products were almost consumed so that along term reduction of daily gas production was observed.In reactor 2, only 2700 l of biogas (146 l/kg TS) was pro-duced after 60 days because leachate percolation was notpracticed that caused the gas production rate to increasevery slowly. It reached a highest daily gas production of100 l/day on day 60. This rate was obtained earlier in reac-tors 1 and 3 at day 47 and 30, respectively. This suggestedthat leachate percolation during methane phase could pos-itively enhance biogas production. This was also proven tobe an eVective means of mixing, provided moisture(Ghosh, 1985; Chanakya et al., 1992; O’Keefe et al., 1993)and appropriate pressure to help release biogas (Moheeand Ramjeawon, 2003). On the other hand, Chan et al.(2002) demonstrated that leachate recirculation was notonly eVective in enhancing the degradation rate of wasteand gas production but also in the reduction of the overallleachate loading.

In reactor 3, at the commencement of leachate percola-tion, the daily gas production was increasing (Fig. 6a). Thisshowed that leachate percolation had a beneWcial eVect inenhancing biogas generation. However, after nine days ofleachate percolation (batch 1), the daily gas productionreduced suddenly without reducing VFA concentration. Itwas observed that propionic acid in leachate kept on increas-ing and reached nearly 4 g/l (Fig. 6b) and might positivelycause the sudden decrease of biogas from 160 l to 110 l. Itcould be likely that there was an inhibition caused by highconcentration of this acid, as reported by other authors(Gourdon and Vermande, 1987; Mawson et al., 1991; Inancet al., 1996; Pullammanappallil et al., 2001). New batch ofpre-stage leachate with low propionic acid concentrationwas percolated causing daily gas production to increase backto the level of 150 l/day. The daily gas production was stablefor a week before it started to decrease again. This time, thedrop in biogas production was not accompanied by highconcentration of propionic acid. The reduction of acetic acid,

Fig. 5. (a) Biogas composition and (b) cumulative biogas production (reactors 1 and 3: leachate percolation on day 40, 30; reactor 2: no leachate percola-tion).

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a direct substrate for methanogens, was the reason for bio-gas production to decrease. Third batch of leachate wasreplaced, in order to supply VFA for digester. The systemkept on producing biogas but not as high as observed beforeand the rate was slowly dropping. Long-term decreasingtrend of biogas production was observed showing thathardly biodegradable waste remained. In reactor 1, similarbehavior of biogas production was observed, daily biogasgeneration started to increase at the commencement ofleachate percolation.

In reactor 3, propionic acid was the highest accumu-lated acid in the system during Wrst batch of leachate per-colation. It reached the level of nearly 4 g/l as compared to2 g/l of acetic acid. Butyric and valeric acids concentrationwere low. Due to a favorable environmental condition foracid formation, mainly high pH, other acids started toappear. The stable concentration of acetic acid indicatedthe balance between the rate of acid consumption andproduction. Propionic acid and other acids were producedmore while acetic acid was converted to methane and car-bon dioxide. According to Mrz.-Viturtia et al. (1994), thelow acetic acid concentration compared with propionicacid indicated a stable methane performance. In maturephase, most acids started to decrease revealing that the

system had stabilized and the leachate was mature. ThepH value increased from 5 (pre-stage) to a range of 7.3–7.8(methane phase).

3.3. Air Xushing stage

In Run 2, the reactor 3 reached a mature phase after 60days and air Xushing was practiced to Xush out the remain-ing biogas before unloading the digester. After one day ofaeration, methane and carbon dioxide content droppedto <5%. Thus, one day of aeration was suYcient to removethe remaining biogas and the reactor could be safelyunloaded.

3.4. BMP and actual methane yield

In reactor 3, approximately 5 m3 of biogas with meth-ane content of 55% was obtained, which was equivalentto 230 l CH4/kg VS. The actual methane yield was 230 lCH4/kg VS as compared to 300 l CH4/kg VS in lab-scaleBMP test. This highlighted that almost 75% methaneconversion was obtained in the pilot scale system underthe inXuence of pre-stage (Wve days micro-aeration,Xushing with 600 l of water) in ambient condition and

Fig. 6. (a) Daily gas production and (b) single VFA concentration (reactor 3).

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methane phase (with leachate percolation) in mesophiliccondition.

4. Conclusions

The application of micro-aeration during pre-stageshowed an equivocal result on hydrolysis/acidiWcationenhancement. Flushing for short duration of Wve days withlesser volume of water (29.3 l/kg TS) was found beneWcial topartly remove the organic matter from waste bed in pre-paration for methane phase. Hydrolysis yield of 140 g/kgTS which is equivalent to 30% of C from the waste bedwas removed in leachate form and acidiWcation yield of180 gVFA/kg TS were obtained. The importance of leach-ate percolation during methane phase was recognized toenhance biogas production. Nevertheless, micro-aerationduring pre-stage may have a positive eVect in methanogene-sis since an active methane phase was reached early com-pared to other reactors without micro-aeration. This mighthave resulted in better hydrolysis/acidiWcation during thestart-up of methanization period providing substrate formethanogens. Thus, micro-aeration exhibits an equivocaleVect in terms of enhancing anaerobic digestion processand this needs further investigation. The actual methaneyield of waste (260 l CH4/kg VS) was compared with theresult of BMP test (300 l CH4/kg VS), it revealed that 75%process eYciency can be obtained in this process.

Acknowledgements

The authors are grateful to the Swedish InternationalDevelopment Co-operation Agency (SIDA) for Wnancingthis research, which is a part of the Sustainable Solid WasteLandWll Management in Asia under the Asian RegionalResearch Program on Environmental Technology.

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