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Waste Management 28 (2008) 1827–1834

A pilot plant two-phase anaerobic digestion system forbioenergy recovery from swine wastes and garbage

Chuanping Feng a,*, Sadoru Shimada b, Zhenya Zhang c, Takaaki Maekawa c

a School of Water Resource and Environmental Science, China University of Geosciences, No. 29, Xueyuan Road, Haidian District, Beijing 100083, Chinab Doctoral Program in Graduate School of Life and Environmental Science, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan

c Graduate School of Life and Environmental Science, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan

Accepted 17 August 2007Available online 27 September 2007

Abstract

A pilot plant bioenergy recovery system from swine waste and garbage was constructed. A series of experiments was performed usingswine feces (SF); a mixture of swine feces and urine (MSFU); a mixture of swine feces, urine and garbage (MSFUG); garbage and amixture of urine and garbage (AUG). The system performed well for treating the source materials at a high organic loading rate(OLR) and short hydraulic retention time (HRT). In particular, the biogas production for the MSFUG was the highest, accountingfor approximately 865–930 L kg�1-VS added at the OLR of 5.0–5.3 kg-VS m�3 day�1 and the HRT of 9 days. The removal of VSwas 67–75%, and that of COD was 73–74%. Therefore, co-digestion is a promising method for the recovery of bioenergy from swinewaste and garbage. Furthermore, the results obtained from this study provide fundamental information for scaling up a high-perfor-mance anaerobic system in the future.� 2007 Elsevier Ltd. All rights reserved.

1. Introduction

In Japan, 94 million tonnes of wastewater generatedfrom livestock and 20 million tonnes of domestic andindustrial garbage discharged annually are a major causeof water pollution, due to their high content of ammoniaand other organic substances. The methods for treatingthe wastewater from livestock include field dispersal, com-posting, activated sludge, and methane fermentation; in thetreatment of garbage, composting, methane fermentationand incineration have been used. In an effort to preventenvironmental destruction and resource exhaustion, someregulations for managing waste and reusing resources havebeen stipulated, with increasing focus on the reuse oforganic wastes. The recent oil crisis and the consequentprice rises have spawned considerable interest in the explo-ration of renewable energy sources (Gunaseelan, 1997).

0956-053X/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.wasman.2007.08.009

* Corresponding author. Tel.: +86 10 82322281; fax: +86 10 64882672.E-mail addresses: fengcp@hotmail.com, fengchuanping@gmail.com

(C. Feng).

Bioenergy has been regarded as the most significant renew-able energy from the viewpoint of reusing and recyclingorganic sources. Anaerobic treatment is recognized as auseful, well-established pretreatment method for medium-and high-strength organic wastewater and solid waste dueto its ability to produce methane without much energyexpenditure (Sekiguchi et al., 2001). It is estimated thatup to 20% reduction of global warming may be achievedby utilizing discarded biomass and waste for the produc-tion of biofuels and chemicals (Vieitez and Ghosh, 1999).

Municipal organic waste (Vieitez and Ghosh, 1999; Heldet al., 2002; Rodriguez-Iglesias et al., 1998; Moller et al.,2004), manure from livestock (Moller et al., 2004) and var-ious industrial wastewaters (Fukuzaki and Nishio, 1997;Kida et al., 1999) for methane fermentation have beenextensively studied. The success of anaerobic wastewatertreatment technology can be attributed to the introductionof innovative bioreactors (Uemura and Harada, 2000). Theanaerobic degradation of organic matter is a multi-phaseprocess involving acidogenesis and subsequent methano-genesis. In the first phase, complex organic materials, car-

1828 C. Feng et al. / Waste Management 28 (2008) 1827–1834

bohydrates, amino acids, long-chain fatty acids, and alco-hol are degraded to short-chain fatty acids, which aremetabolized in the subsequent phase (Yu et al., 2002).

The two-phase anaerobic digestion system proposed byPohland and Ghosh (1971) has several advantages overthe traditional single-phase system, e.g., shorter detentiontime, higher gas conversion efficiency, and higher methaneconcentration in the produced gas (Yu et al., 2002). Fur-thermore, it may allow a reduction in total reactor volume(Ince, 1998). This system has been extensively researchedfor treatment of fruit and vegetable waste (FVW) (Boualla-gui et al., 2005), olive mill solid waste (OMSW) (Borjaet al., 2002), spent tea leaves (Goel et al., 2001), wastewaterfrom a fish meal processing factory (Guerrero et al., 1999)and dairy wastewater (Ince, 1998). Moreover, it has beendemonstrated that the anaerobic baffled reactor (ABR)and anaerobic sequencing batch reactor (ASBR) were effec-tive for treatment of high-solids waste (e.g., animal waste)(Boopathy, 1998; Angenent et al., 2002; Masse et al., 2003;Barber and Stuckey, 1999). However, a long retention timeis necessary to reduce solids washout caused by high gasproduction and a larger front compartment (Barber andStuckey, 1999). In this study, we constructed a pilot plantbioenergy recovery system in which a two-phase anaerobicdigestion system was developed. The developed two-phasedigestion system consisted of an acidification reactor and amethane fermentation reactor of an ABR. In order to pro-vide fundamental data for practical application of the sys-tem, the performance of the two-phase digestion systemwas evaluated using swine waste and garbage.

Swine urines

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2. Materials and methods

2.1. Process description

The pilot plant bioenergy recovery system is schemati-cally illustrated in Fig. 1. The bioenergy recovery systemwas comprised of a swine waste separator, a garbage grin-der, a two-phase anaerobic digestion system, and an elec-trochemical treatment system. The source materials usedin the present study were pretreated by the separator orthe grinder, and then anaerobically treated in the two-phase anaerobic digestion system; finally, the digested efflu-ent was treated by the electrochemical treatment system.The two-phase anaerobic digestion system consisted of anacidification reactor (0.4 m3), a methane fermentation reac-tor (2.5 m3), and a digested effluent tank. A biogas holder(1.0 m3) was combined with the methane fermentationreactor, which was divided into five compartments withwalls of rock wool (thickness 30 cm) that could also serveas carriers due to their porosity (Li et al., 1998). Further-more, swine waste contains a large portion of small parti-cles (diameter <0.21 mm), accounting for more than 50%of the potentially available methane (Boopathy, 1998). Itwas considered that the walls could catch these small par-ticles and maintain them in the reactor long enough fordegradation. An alternative hole (diameter 10 cm) wasopened on the upper or under side of the walls, allowingflow to the next compartment and preventing biomass loss.This system was similar to an anaerobic baffled reactor, inwhich bacteria gently rises and settles due to flow charac-

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plant bioenergy recovery system.

C. Feng et al. / Waste Management 28 (2008) 1827–1834 1829

teristics and gas production (Barber and Stuckey, 1999).The methane fermentation reactor could withstand severehydraulic and organic shock loads, intermittent feedings,and temperature changes (Barber and Stuckey, 1999).The acidification reactor also served as a buffering tankbecause the source materials were fed once a day.

The source materials were pumped into the acidificationreactor and forced from the acidification reactor into themethane fermentation reactor by the produced biogas pres-sure, which varied with the biogas consumption of the elec-tric generator. Consequently, the final compartmentreceived the lowest substrate concentration, and then flo-wed into the digested effluent tank. A check valve con-nected the acidification reactor with the methanefermentation reactor, which was also connected with thedigested effluent tank by another check valve. To preventexcessive accumulation of biomass in the digested effluenttank and to float the sludge settled in the fore compart-ments, the flow was reversed periodically by a pump fromthe bottom of the digested effluent tank to the first com-partment of the methane fermentation reactor. Moreover,a part of the biogas was periodically circulated to the bulksolution in the methane fermentation reactor to improvemixing.

Four thermometers were installed, one in the acidificat-ion reactor, two in the sides of the methane fermentationreactor, and one in the digestion effluent tank. A pressuregauge was installed on the biogas line to monitor head-gas pressure in the methane fermentation reactor. Thebiogas flowed through an H2S scrubber, and finally intothe electric generator. The exteriors of the acidificationreactor and the methane fermentation reactor were coatedwith a 5 cm layer of polyurethane foam for heat insula-tion, and the temperature was maintained at 36 ± 1 �Cby circulating warm water from a hot water tank heatedby the generator through the pipe coils installed on thefermentation reactor. Several ports were installed on theside walls of the fermentation system for withdrawingsamples.

2.2. Source materials

In this study, the swine waste was conveyed from aswine-raising farm, and the garbage came from a hospitalor a school meal center. In general, the swine waste wasseparated into feces and urine; the feces were composted,and the urine was treated by the biological treatmentmethod in Japan. However, a large amount of harmfulgases, malodorous compounds, and greenhouse gases wereemitted during composting of the swine feces (Kurodaet al., 1996). Therefore, the methane fermentation for theswine feces was also discussed in this study. Each week apart of the feces was transported by a plastic containerand the urine by a vacuum car, and then transferred to astorage tank at the experiment field. To investigate the per-formance completely in this study, the source materialswere fed into the system in five stages:

Stage 1. Swine feces (SF): The feces was diluted by tapwater at a ratio of 1–1, and mixed well with the use ofa mixer. Afterwards, in order to prevent plugging ofthe influent line, the separator (screen diameter0.8 mm) was used to separate the mixed feces solutionwith a separating ratio of approximately 85%. The sep-arated solids could be composted or carbonized.Stage 2. Mixture of swine feces, and urine (MSFU): Ingeneral, the feces and urine were collected separatelyin a swine-raising farm in Japan. Therefore, the MSFUwas prepared by mixing the feces solution obtained inStage 1 with the urine in a ratio of 4–1.Stage 3. Mixture of swine feces, urine and garbage

(MSFUG): In Japan, rural areas coexist with cities.Therefore, the co-digestion of swine waste and garbagewas suitable to obtain high-performance fermentation.The garbage was brought from a school meal centerdaily. During the summer vacation, it was carried froma hospital. Although the compositions in the garbagefrom the school meal center differed day-by-day due tothe differences in the menu, the garbage was not sortedin this study to simulate practical use in the future.The garbage was ground by the developed grinder(screen diameter 0.5 cm) with a 1:3 of garbage to tapwater to form garbage slurry. The feces solution wasprepared as indicated in Stage 1. Aliquots of the garbageslurry and the feces solution were mixed in a tank toform MSFUG.Stage 4. Garbage: The garbage was prepared asdescribed in Stage 3.Stage 5. Addition of urine to the garbage (AUG): The gar-bage slurry prepared in Stage 3 was mixed with swineurine to eliminate the dramatic pH drop in the methanefermentation reactor because the pH in the urineexceeded 8.5.

Table 1 gives the wastewater characteristics. It can beseen that the pH was 7.2–7.3 in the swine wastewater,and that it was decreased by adding garbage because theground garbage was souring before feeding during sum-mer. Furthermore, supposing that 58% of the total nitro-gen in the source materials would be converted toammonia (Held et al., 2002), the ammonia concentrationsin the source materials were adjusted to 1300–1631 mg L�1

by adding tap water to prevent the ammonia from inhibit-ing methane fermentation. The C/Ns in various sourcematerials were 10.2–11.1, which were suitable for methanefermentation.

2.3. Experiment design

In the case of SF (Stage 1), the OLR was increased from1 to 4.9 kg-VS m�3 day�1; the HRT at 3.6 kg-VS m�3

day�1 was 16 days, and that at 4.9 kg-VS m�3 day�1 was12 days. The OLR and HRT in MSFU (Stage 2) were3.1 kg-VS m�3 day�1 and 11 days, respectively. ForMSFUG (Stage 3), the OLR was raised from 2.9 to

Table 1Characteristics of the source materials fed

Source materials pH TS (%) VS (%) NHþ4 -N (mg L�1) T-N (mg L�1) C/N

Swine feces (SF) 7.3 ± 0.11 7.2 ± 0.21 5.0 ± 0.11 1116 ± 12 3600 ± 8 10.2 ± 0.10Mixture of swine feces and urine (MSFU) 7.2 ± 0.10 5.0 ± 0.15 3.6 ± 0.10 1631 ± 15 3600 ± 11 11.0 ± 0.12Mixture of swine feces urine and garbage (MSFUG) 6.4 ± 0.10 5.7 ± 0.18 4.5 ± 0.12 1300 ± 5 3500 ± 6 10.8 ± 0.15Garbage from school meal center 4.9 ± 0.12 6.6 ± 0.13 5.6 ± 0.14 75 ± 1 2100 ± 9 11.1 ± 0.17

1830 C. Feng et al. / Waste Management 28 (2008) 1827–1834

9.3 kg-VS m�3 day�1 by changing the proportions of theswine feces, urine, and garbage. Four kinds of MSFUGwere prepared with ratios of 4:3:4 (OLR 2.9 kg-VS m�3

day�1), 6:4.5:4 (OLR 5.0 kg-VS m�3 day�1), 4:3:2 (OLR9.3 kg-VS m�3 day�1), and 2:1:1 (OLR 5.3 kg-VS m�3

day�1). The OLR was increased from 3.1 to 5.4 kg-VS m�3 day�1 for garbage (Stage 4), and from 2.2 to2.3 kg-VS m�3 day�1 for AUG (Stage 5). The ratios ofthe garbage and urine were 2.5:2 (OLR 2.2 kg-VS m�3

day�1) and 5:2 (OLR 2.3 kg-VS m�3 day�1) in Stage 5.During the experiments, the source material was fed into

the methane fermentation system once a day. Subse-quently, the biogas line was shut to raise the pressure inthe methane fermentation reactor, and the digested effluentequal with the source materials fed into it was discharged atonce.

2.4. Analytical methods

Biogas volume was measured automatically by a gasflow meter every day. Biogas analysis was performed bya gas chromatograph (TCD detector, Shimalite Q andPorapak Q column, argon carrier gas), and H2S contentin the biogas was analyzed with a detector using a standardglass tube with a range of 0–10,000 ppm. Samples weretaken once a week from the acidification reactor, the meth-ane fermentation reactor, and the digested effluent tank.The concentrations of TS, VS, COD, and NHþ4 -N weremeasured according to the Japanese Standard Methodsfor Sewage and Wastewater (Japan Sewage Works Associ-ation, 1997). The N and C contained in the source materi-als and the sludge were measured by a CHNS/O Analyzer(Series II, Perkin–Elmer).

3. Results and discussion

3.1. Start-up of the two-phase methane fermentation system

The two-phase methane fermentation system was inocu-lated with anaerobic digester sludge (1300 L) obtained froma municipal wastewater treatment plant in Tsuchiura City,Japan, and initiated by adding the swine wastewater in astepwise manner after acclimatizing for 1 month. Unfortu-nately, the startup failed after 2 months because the ammo-nia concentrations in the acidification reactor and themethane fermentation reactor exceeded 3000 mg L�1, indi-cating inhibition of methane fermentation. Therefore, cattlefeces was diluted by tap water, mixed to form slurry, sepa-

rated by the separator, and subsequently fed the liquid partat a rate of 100 kg day�1 for 2 months to restore the methanefermentation. The source materials were then fed accordingto the experiment design.

After the start-up period, the source materials were fedinto the system. The corresponding OLR, HRT, pH,COD, biogas production, and gas composition are depictedin Fig. 2.

3.2. Stage 1: SF (days 1–116)

In this stage, the SF was fed into the system. The initialOLR was 1 kg-VS m�3 day�1 over a period of 10 days andgradually increased to 1.8 kg-VS m�3 day�1, at which itwas maintained for 17 days. The objective of this lowOLR was to develop a suitable biomass of flocculentorganisms in the methane fermentation reactor before thehigher OLR was performed (Fischer et al., 1981). Duringthis start-up phase of operation, the biogas productionincreased gradually from 600 to 2500 L day�1, and meth-ane content increased from 62% to 76%. Ammonia concen-tration in the methane fermentation reactor increasedgradually from 500 to 1500 mg L�1 with an increase ofthe ammonia concentration in the fed source materials,higher than that in the source material from day 20. TheTS, VS, and COD in the digested effluent varied with thosein the source materials, but remained less than those in thesource materials.

Considering that the fed cattle manure in the methanefermentation reactor was replaced completely by the SFin a period of 27 days, the OLR was increased to 3.6 kg-VS m�3 day�1, maintaining for 34 days an equivalent oftwo times HRT (16 days) from day 28. This period wasconsidered as the transitional phase, representing the tran-sition from the establishment of a flocculent biomass to acommercially acceptable loading rate (Boopathy, 1998).Biogas production increased steadily to 468 L kg�1-VSadded from day 42. Ammonia concentration in the meth-ane fermentation reactor increased to above 2000 mg L�1

from day 60, and methane content in the biogas was 76%and remained constant in this period. The TS and VS inthe digested effluent changed remarkably with those inthe source materials, and approximately 60% of TS andVS were removed. However, COD in the digested effluentwas slightly changed with the source material; the removalof COD was 65% (Table 2).

From day 62, the OLR was raised to 4.9 kg-VS m�3 day�1, and the HRT was shortened to 12 days,

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Fig. 2. Operational performance of the two-phase digestion system for treating swine wastes and garbage.

C. Feng et al. / Waste Management 28 (2008) 1827–1834 1831

maintaining the OLR for 55 days. However, biogas pro-duction was not observed to increase until day 100; it actu-ally decreased from 3700 to 2800 L day�1 from days 80 to100. A large amount of foam was observed in the biogasline, and consequently the way of the biogas was blocked,resulting in the pressure rise in the methane fermentation

reactor, and as a result, the solution was washout. It wasconsidered that the shorter HRT and higher organic matterin the source material could cause foaming, as insufficienttreatment time and bacterial washout would cause incom-plete biological conversion, which would produce manyintermediate surface active organic compounds. Hill and

Table 2Performance of the two-phase digestion system for treating the swine wastes and garbage

Source materials HRT (day) OLR (kg-VS m�3 day�1) Biogas production(L kg�1-VS added)

CH4 (%) VS removal (%) COD removal (%)

SF 16 3.6 468 76 60 6512 4.9 423 76 50 55

MSFU 11 3.1 579 76 58 59

MSFUG 13 2.9 924 73 57 619 5.0 865 62 75 746 9.3 481 67 ND ND9 5.3 930 73 67 73

Garbage 13 3.1 845 64 81 8410 5.4 511 58 91 89

MGU 20 2.3 676 62 90 8811 2.2 551 62 93 88

1832 C. Feng et al. / Waste Management 28 (2008) 1827–1834

Bolte (2000) proposed that foaming occurred at a loadingrate of 7.5 kg-VS m�3 day�1 in conventional anaerobic fer-mentation for low-solid-concentration liquid swine waste.However, this foaming problem was observed at a loadingrate of 3.6 kg-VS m�3 day�1, and it became serious at4.9 kg-VS m�3 day�1 in the present study. This might havebeen due to the fact that no mechanical mixing was pro-vided in the fermentation reactor in this study; thereforethe foam produced could not be immediately destroyed.To solve this problem, we installed a foam-destroyingapparatus between the methane fermentation reactor andthe biogas line. The foam-destroying apparatus was con-structed by PVC pipe (diameter 18 cm), in which threepieces of barrier plate were settled for destroying the foam.Afterwards, biogas production increased to 423 L kg�1-VSadded from day 100; 50% of VS and 55% of COD wereremoved. The methane content in the biogas remained con-sistent with the OLR of 3.6 kg-VS m�3 day�1 (Table 2).

The theoretical methane production was 516 L kg�1-VSadded for swine wastes (Moller et al., 2004). However, inthis study the methane production was 356 L kg�1-VSadded at an OLR of 3.6 kg-VS m�3 day�1 and 321 L kg�1

-VS added at an OLR of 4.9 kg-VS m�3 day�1. Theseresults were in accordance with those reported in the previ-ous work (Moller et al., 2004). The methane content of thebiogas was relatively high because of high carbon dioxidedissolution into bicarbonate form, which enhanced the buf-fering capacity of the methane fermentation reactor. Inaddition, the methane content in the biogas was higher,accounting for 50–80% in a two-phase system comparedto 40–60% in a single-phase system (Yu et al., 2002). Inthe whole period of Stage 1, the pH in the methane fermen-tation reactor increased significantly from 7.2 to 8.2,regardless of the pH in the source material.

3.3. Stage 2: MSFU (days 117–153)

To investigate the performance for MSFU treatment, theexperiment was performed at an OLR of 3.1 kg-VS m�3 day�1 and HRT of 11 days in this stage. The biogasproduction decreased gradually during the initial period,

and then increased to 579 L kg�1-VS added. The methanecontent of 76% was the same as in Stage 1. The removal ofVS was 58% and that of COD was 59% (Table 2). Theremoval of COD in Stages 1 and 2 was relatively lower thanthat reported in previous work (Boopathy, 1998). This isprobably because the digested effluent was dischargedimmediately after the source material was fed in, and thisresulted in that a part of the digested sludge floated intothe digested effluent. The pH in the fermentation reactordecreased slightly, but was kept at 8.0 in this stage.

3.4. Stage 3: MSFUG (days 154–229)

In this stage, the MSFUG was fed at an OLR of 2.9 kg-VS m�3 day�1 in the first 14 days, and later the OLR wasincreased to 5.0 kg-VS m�3 day�1 at the HRT of 9 daysfor 39 days. To investigate the maximum capacity of themethane fermentation system, the OLR was increased to9.3 kg-VS m�3 day�1 at an HRT of 6 days. The biogas pro-duction increased stably to 865 L kg�1-VS at 5.0 kg-VS m�3 day�1. However, the biogas production decreaseddramatically to 481 L kg�1-VS after a 6-day feeding periodat 9.3 kg-VS m�3 day�1. The experiment was then stoppedsince it was considered that the fermentation would fail ifthe experiment continued. Subsequently, the OLR wasdecreased to 5.3 kg-VS m�3 day�1 to restore the perfor-mance of the fermentation system, with 67% of VS removaland 73% of COD removal (Table 2). The methane contentin the biogas was 62%, lower than in Stages 1 and 2, whilethe biogas production in this stage was larger than that inStages 1 and 2. At 5.0 kg-VS m�3 day�1 the removal of VSwas 75% and that of COD was 74% (Table 2). The pH inthe methane fermentation reactor increased consistentlyto approximately 9; then it decreased until day 200 andincreased again in this stage, as a result of the increase inthe ammonia concentration.

3.5. Stage 4: Garbage (days 230–279)

In this stage, garbage was fed at an OLR of 3.1 and5.4 kg-VS m�3 day�1. For an OLR of 3.1 kg-VS m�3

C. Feng et al. / Waste Management 28 (2008) 1827–1834 1833

day�1, the biogas production decreased and then becamestable from day 245. The specific biogas production valuewas 845 L kg�1-VS added, while the methane content was64%. The removal of VS was 81%, and that of COD was84% (Table 2). The OLR was increased to 5.4 kg-VS m�3 day�1 from day 272, and the biogas productiondecreased to 511 L kg�1-VS added. The pH decreased shar-ply, especially in the acidification reactor. On the otherhand, the pH in the methane fermentation reactordecreased gradually from day 240 and then decreased rap-idly from day 280, indicating inhibition of organic acid dueto the high OLR, but remarkable decrease in pH was notobserved from days 230 to 272. These results were proba-bly due to the fact that: (1) the garbage was from the hos-pital from days 230 to 272, and contained larger quantitiesof pork than that from the school meal center; (2) thesludge (biomass) saved in the fore stages remained stillfrom days 230 to 272, but washed out little-by-little fromday 272; and (3) the OLR was relatively high from day 272.

3.6. Stage 5: AUG (days 280–340)

To prevent organic acid inhibition in Stage 4, swineurine was added to the garbage slurry at an OLR of2.3 kg-VS m�3 day�1 (HRT 20 days) and 2.2 kg-VS m�3 day�1 (HRT 11 days) in this stage. The pHs inthe acidification reactor and the methane fermentationreactor increased with the same tendency. The pH in themethane fermentation reactor exceeded 7.5 from day 310,indicating that the fermentation was stable. The specificbiogas production values were 676 L kg�1-VS added at2.3 kg-VS m�3 day�1 (HRT 20 days) and 551 L kg�1-VSadded at 2.2 kg-VS m�3 day�1 (HRT 11 days), while themethane content was 62%. Similarly, Sterling et al. (2001)reported that the addition of urea led to nearly immediateincreases in pH and alkalinity during anaerobic digestionof dairy cattle manure. Excess ammonia probably contrib-uted to the increased alkalinity in two ways. First, ammo-nia increased the bicarbonate concentration in the digestersby forming an ammonium salt with bicarbonate takenfrom dissolved CO2 (Georgacakis et al., 1982):

NHþ4 þOH��NH3 þH2O ð1ÞCO2 þH2O�Hþ þHCO�3 ð2ÞNHþ4 þOH� þHþ þHCO�3 � NHþ4 þHCO�3

� �saltþH2O

ð3Þ

Second, a high ammonia concentration inhibited boththe hydrolytic and acetogenic groups of bacteria, therebyreducing VFA concentrations in the methane fermentationreactor. In general, the organic acid inhibition in the meth-ane fermentation for garbage was regarded as a seriousproblem; so it was necessary to add an alkali such asNaOH (Rao et al., 2000) or NaHCO3 (Rao and Singh,2004) to make the pH suitable for fermentation, resultingin a high operating cost. Therefore, in this study a cost-sav-ing method of adding of swine urine was proposed.

Comparing the performances in various stages, the spe-cific biogas production was the largest in Stage 2 and leastin Stage 1, while the methane content was the highest inStage 1. These results indicate that the addition of garbagecould raise the biogas production and lower the methanecontent; the performance of the methane fermentationcould be improved by the co-digestion of the swine manureand the garbage. The methane content in the swine wastewas higher than that in the presence of garbage probablydue to the higher concentration of ammonia in the swinewaste, as shown in Eqs. (1)–(3). As Fig. 2 depicts, the vary-ing tendency of ammonia concentration was in agreementwith that of the methane content. In contrast, the removalof VS and COD was higher with the addition of garbagethan without, because the garbage contained more degrad-able organic substances than the swine waste did. The H2Sin the biogas was 2000–3000 ppm throughout all the stages,similar to the general methane fermentation of the swinewaste (Livestock Industry’s Environmental ImprovementOrganization, 2001). Therefore, 99% of the H2S wasremoved before it was supplied to the electric generatorin this study.

Matsumoto et al. (2003) investigated the co-digestion ofswine manure and kitchen garbage, using a continuouslystirred tank reactor (CSTR) at an OLR of 2.47 kg-VS m�3 day�1 and HRT of 30 days (far longer than in thisstudy); specific biogas of 900 L kg�1-VS was obtained.Throughout the experiments, increasing the OLR andshortening the HRT led to decreased biogas production,while the developed two-phase anaerobic digestion systemindicated a high-performance at a high OLR and a shortHRT, compared with the conventional ones. The acidifi-cation reactor also served as a buffer tank because thesource materials were fed once a day. Therefore, the two-phase anaerobic digestion system is suitable for practicaluse, considering that the discharge and compositions ofthe swine manure and garbage fluctuate daily.

4. Conclusions

A pilot plant scale system for recovering bioenergy fromswine waste and garbage was constructed. A series ofexperiments were performed to evaluate the performanceof the two-phase methane fermentation system using differ-ent kinds of source materials. The results showed that themethane fermentation system had high-performance fortreating swine waste and garbage at a high OLR andHRT. In particular, the specific biogas production washigh for the MSFUG, 865–930 L kg�1-VS added at anOLR of 5.0–5.3 kg-VS m�3 day�1 and HRT of 9 days inour study. In addition, the removal of VS for the MSFUGtreatment was from 67% to 75%, and that of COD wasfrom 73% to 74%. These findings suggested that the co-digestion of swine manure and garbage was a promisingmethod for recovering bioenergy from industrial wastes.Moreover, adding swine urine effectively improved theanaerobic treatment performance of garbage. This study

1834 C. Feng et al. / Waste Management 28 (2008) 1827–1834

indicated that the two-phase methane fermentation systemwas suitable for treating swine manure and garbage withconstantly varying quantity and quality. The resultsobtained from this study provide useful fundamental infor-mation for scaling up high-performance anaerobic systemsin the future.

Acknowledgement

This study was funded by the Ministry of Education,Culture, Sports, Science and Technology of Japan.

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