Bioresource Technology 102 (2011) 5498–5503

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Bioresource Technology

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Treatment of fresh leachate with high-strength organics and calciumfrom municipal solid waste incineration plant using UASB reactor

Jiexu Ye a,⇑, Yongjie Mu b, Xiang Cheng b, Dezhi Sun b,⇑a State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, 202 Haihe Road, Harbin 150090, Chinab College of Environmental Science and Engineering, Beijing Forestry University, 35 Tsinghua East Road, Beijing 100083, China

a r t i c l e i n f o

Article history:Received 31 July 2010Received in revised form 31 December 2010Accepted 3 January 2011Available online 8 January 2011

Keywords:Fresh leachateMunicipal solid wasteIncineration plantUASB reactorCalcium carbonate

0960-8524/$ - see front matter � 2011 Published bydoi:10.1016/j.biortech.2011.01.001

⇑ Corresponding authors. Tel./fax: +86 10 6233 659E-mail addresses: yejiexu8292@163.com (J. Ye), sd

a b s t r a c t

Treatment of a fresh leachate with high-strength organics and calcium from municipal solid waste(MSW) incineration plant by an up-flow anaerobic sludge blanket (UASB) reactor was investigated undermesophilic conditions, emphasizing the influence of organic loading rate (OLR). When the reactor was fedwith the raw leachate (COD as high as 70,390–75,480 mg/L) at an OLR of 12.5 kg COD/(m3 d), up to�82.4% of COD was removed suggesting the feasibility of UASB process for treating fresh leachates fromincineration plants. The ratio of volatile solids/total solids (VS/TS) of the anaerobic sludge in the UASBdecreased significantly after a long-term operation due to the precipitation of calcium carbonate in thegranules. Scanning electron microscopy (SEM) observation shows that Methanosaeta-like species werein abundance, accompanied by a variety of other species. The result was further confirmed by polymerasechain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) and sequencing.

� 2011 Published by Elsevier Ltd.

1. Introduction

During the past decades, the amount of municipal solid waste(MSW) has been drastically increasing around the world as a resultof the growing urbanization. Incineration is an effective technologyof MSW disposal for its advantages in reducing the volume andmass of MSW (by �90% and �75%, respectively), detoxification,sterilization and energy production (Chimenos et al., 1999; Chouet al., 2009), especially in the cities with developed economies,high population and limited space for landfill.

In China, the number of commercial-scale MSW incineratorsconstructed plus under construction will possibly reach 100 by2010 (Ni et al., 2009). As the calorific value of China’s MSW(4000–7000 kJ/kg) is in general lower than a half of those in devel-oped countries (8400–17,000 kJ/kg), resulted from the high mois-ture and high proportion of kitchen waste in the MSW (Nie,2008), the fresh MSW in general needs to be stored in storage bun-kers for 3–7 days before incineration. A considerable amount offresh leachate can be generated during this period. The leachatewarrants special attention as it contains various contaminantssuch as organics, refractory compounds, ammonium, heavy metals,etc. Fresh leachate is usually sprayed into the incinerator furnacesin European where MSW is of high Lower Heating Value (LHV) andof low moisture (Chen and Christensen, 2010). However, it wouldnot be applicable in most incineration plants in China because

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6.zlab@126.com (D. Sun).

the low-LHV MSW produces large amounts of fresh leachate,which could severely affect the incineration reaction when beingall introduced to the furnaces. Exploring an effective treatmentmethod for the MSW fresh leachate is therefore necessary to meetthe discharge standard.

Anaerobic treatment has been demonstrated to be the mostuseful technology for the treatment of high-strength organicwastewaters. The processes perform well at high organic loadingrates (OLR) with low operating costs and also produce usable bio-gas (Zhou et al., 2007). Various anaerobic processes were studied inrecent years for the treatment of MSW landfill leachate, such asanaerobic digester (Kheradmand et al., 2010), fluidized bed reactor(Gulsen and Turan, 2004), sequencing batch reactor (Timur andÖzturk, 1999), up-flow anaerobic sludge blanket (UASB) reactor(Calli et al., 2006; Parawira et al., 2006; Agdag and Sponza, 2005;Kennedy and Lentz, 2000), anaerobic membrane bioreactor (Zayenet al., 2010; Trzcinski and Stuckey, 2009), etc. Among these pro-cesses, UASB was the most widely investigated in different scalesand applied in many real projects. The reports showed that COD re-moval efficiencies of >75% was achieved at OLRs ranging from 6 to23.5 kg/(m3 d) for MSW leachate with COD concentrations of3210–47,800 mg/L. In fresh leachate from MSW incinerationplants, the concentrations of COD, biological oxygen demand(BOD5), metals (e.g., calcium) can be much higher than those inlandfill leachate (Chen and Christensen, 2010). By now, little workwas done on the treatment of the fresh leachate with extremelyhigh concentrations COD of 50,000–80,000 mg/L. Although UASBis widely accepted as an efficient process for landfill leachate

J. Ye et al. / Bioresource Technology 102 (2011) 5498–5503 5499

treatment, its effectiveness for MSW fresh leachate need to bereevaluated.

This paper studied the feasibility of treating fresh leachate fromMSW incineration plants by using an UASB reactor under meso-philic conditions. Effects of operational parameters on the COD re-moval, methane production and VFAs accumulation wereinvestigated and discussed. Polymerase chain reaction-denaturinggradient gel electrophoresis (PCR-DGGE) fingerprinting andsequencing were conducted to identify the archaeal structureand composition of the granular sludge.

2. Methods

2.1. Experimental set-up

Continuous treatment of the fresh leachate was carried out in aplexiglass UASB reactor. The reactor had a working volume of 3 Lwith inside diameter of 55 mm in the reaction zone, 105 mm inthe settling zone and a total height of 900 mm. A gas–liquid–solidsseparator was installed at the top of the reactor. Six sampling portswere installed along the height of the reactor. A peristaltic pumpwas used to feed the reactor from the bottom. The temperatureof the UASB was kept at 35 ± 1 �C using a thermostatic heater.

The operation time of the reactor was divided into two periods.OLR was gradually increased by reducing HRT or increasing theinfluent concentration of COD. For period I (days 0–124), theCOD concentration of the feed was increased stepwise throughdilution of the raw leachate to desired COD concentrations withtap water. Period II lasted for 46 days, when the reactor was fedwith the raw leachate and the HRT varied in the range of 4.5–5.8 d.

2.2. Fresh leachate

The fresh leachate used as the feed in this experiment was ob-tained from a MSW-to-energy incineration plant in Beijing, China.The water composition of the leachate is shown in Table 1. Thefeed solution was adjusted to pH 7.1 ± 0.2 by adding 1 mol/L NaOHor 1 mol/L HCl solution and was stored at 4 �C in a refrigerator.

2.3. Seed sludge

The UASB reactor was inoculated with 0.75 L of anaerobic gran-ular sludge taken from the wastewater treatment system of QingheFood Company, Beijing, China. The specific methanogenic activity(SMA), total solids (TS) and volatile solids (VS) concentrations ofthe seed sludge were 0.55 gCH4-COD/gVSS d, 59.9 g/L and 43.1g/L, respectively. The diameter of the seeded sludge was in therange of 0.5–2.0 mm.

Table 1Characteristics of leachate from the MSW incineration plant (unit: mg/L, except forpH).

Item Value Item Value

CODcr 70,390–75,480 Zn 13.9–36.5BOD5 39,250–46,458 Cu 0.05–0.22NHþ4 -N 642–895 Mn 4.56–43.50TN 1330–2179 Ca 3275–5827TP 104.6–165.7 Mg 463–1598pH 3.9–6.4 Fe 59.1–679.9SS 7760–27,215 Pb 1.11–7.61Cl� 3978–7153 Co 0.24–0.3

SO2�4

833–1662 Ni 0.91–2.3

VFA 4806–15,900 Cd 0.01–0.02

2.4. Analytical methods

COD, alkalinity, BOD5, TS, VS, TN, TP and NHþ4 -N were deter-mined by standard methods (APHA et al., 1998). VFAs was mea-sured by titration (Anderson and Yang, 1992). pH was measuredwith a Thermo Orion 3-Star pH meter. Cl� and SO2�

4 were analyzedusing Dionex-4500i Ion Chromatogram, and the metal ions wereanalyzed using an inductively coupled plasma optical emissionspectrometry (ICP-OES) system (Optima 5300DV, PerkinElmer).

Biogas and methane production were measured by two wet gasmeters and were normalized to 0 �C and 1 atm (STP). The volumeof total biogas was measured by the first gas meter. After passingthrough a 5 mol/L of NaOH solution, the remained gas was mea-sured by the second wet gas meter and was counted as methane.

2.5. Specific methanogenic activity

SMA of the sludge was measured in triplicate in 125 mL serumbottles with a total liquid volume of 100 mL at 35 �C under strictanaerobic conditions, following the procedure reported by Kettun-en et al. (1996). Acetic acid solution (4.5 gCOD/L) was used as themedium after being adjusted to pH 7.0–7.2 by 1 mol/L NaOH.Methane production was measured every 2 h using a glass gas dis-placement device filled with a 3 mol/L of NaOH solution. SMA wascalculated from the linear range of the specific methane productionrate curve using linear regression.

2.6. Scanning electron microscopy

The sludge samples were fixed for 1.5 h at 4 �C by immersing in2.5% glutaraldehyde with 0.1 mol/L phosphate buffer (pH 6.8). Thesamples were then washed three times by 0.1 mol/L phosphatebuffer solution (pH 6.8). Afterwards, dehydration was carried outstepwise with a graded series of ethanol solutions (v/v, 50%, 70%80%, 90% and 100%). Then the samples were sequentially exposedto an ethanol/isoamyl acetate mixture (v/v, 1:1) and 100% isoamylacetate. The samples were dried at 35 �C for 12 h and were sputter-coated with a thin layer of gold before being observed by a scan-ning electron microscope (JSM-7401F, JEOL, Japan).

2.7. X-ray diffraction

X-ray diffraction (XRD) patterns were recorded over a 2h from10� to 90� in a scan rate of 3�/min using nickel-filter Cu Ka radia-tion (k = 0.15418 nm) with a graphite monchrometer at 40 kVand 120 mA (D/MAX-RB, Rigaku, Japan). The sludge samples werewashed thoroughly by de-ionized water, dried at 105 �C andground before analysis.

2.8. DNA extraction, PCR-DGGE and sequencing

DNA was extracted from 0.25 g (wet weight) of samples with aPowerSoil DNA kit (MoBio Laboratories, CA, USA) following themanufacturer’s instructions. The granular sludge was sampled atday 0 (seed sludge) and day 148 of the UASB operational period.

The 16S rRNA genes of the archaeal community were amplifiedwith the primers ARC344F-GC (Raskin et al., 1994) and ARC915R(Stahl and Amann, 1991). PCR reaction systems were performedin 25 lL reaction volumes as described by Zhao et al. (2010). ThePCR conditions were as follows: initial denaturation at 94 �C for5 min, followed by 30 cycles of denaturing at 94 �C for 1 min,annealing at 55 �C for 1 min and extension at 72 �C for 1 min, witha final elongation step at 72 �C for 10 min. The purity and quantityof the PCR products were examined on a 1% (w/v) agarose gel, com-paring their brightness to the quantitative marker DL2000 (Takara,Dalian China).

5500 J. Ye et al. / Bioresource Technology 102 (2011) 5498–5503

DGGE was performed in 8% (w/v) polyacrylamide gels contain-ing a 30–60% denaturing gradient (100% denaturant contains7 mol/L of urea and 40% (v/v) de-ionized formamide) using a Bio-Rad Dcode system (Bio-Rad, Hercules, CA, USA). Electrophoresiswas performed for 8 h at 140 V at a constant 60 �C. The gel wasthen stained with silver (Caetano-Anollés and Gresshoff, 1994).

Dominant bands were separately excised from the DGGE geland were eluted with TE buffer. The eluted DNA was re-amplifiedusing the primers of ARC344F and ARC915R. The re-amplifiedproducts were purified using the gel recovery kit (Watson, Shang-hai, China), and were then cloned into the pMD19-T vector (Takara,Dalian, China) using Escherichia coli DH5a (Takara, Dalian, China)as a host. Positive clones were selected by blue/white screeningand PCR methods and were subsequently transferred to 800 lL ofamplicilin containing LB medium and grown overnight at 37 �C. Fi-nally, the samples were submitted to company (Qinke, Beijing, Chi-na) for sequencing. The obtained sequences were compared withother sequences in the National Center for Biotechnology Informa-tion (NCBI) database using the Basic Local Alignment Search Tool(BLAST).

3. Results and discussion

3.1. Performance of the UASB reactor

Fig 1 shows the operational conditions and performance of theUASB reactor for 170 days, including HRT, OLR, concentrations ofCOD, NHþ4 -N and VFA in the influent and effluent and the CODremoval efficiencies. The pH in the reactor was in the range of

Fig. 1. Performance of the UASB reactor: (a) COD concentration and COD removalefficiency; (b) OLR and HRT; (c) VFA and (d) ammonium concentration.

7.2–7.8 throughout the runs except for the period from day 149to day 170, when the pH was 6.4–6.7.

The UASB reactor was started up with a COD concentration of4000 mg/L and a HRT of 3 d (Fig. 1a and b). The COD removal effi-ciency increased to 92.6% after 12 days of operation. Afterwards,the OLR was increased stepwise from 2 to 16.5 kg COD/(m3 d)through increases in COD concentration of the feed and decreasesin HRT (days 13–82). The results indicate that 92.9% of COD and>95% of BOD5 could be efficiently removed at an OLR of 12.5 kgCOD/(m3 d) and a COD concentration of the influent of�25,000 mg/L. When a higher OLR of 16.5 kg COD/(m3 d) wasintroduced by lowering the HRT from 2 to 1.5 d (days 75–82),the average COD removal efficiency declined to �85% and the VFAsin the effluent rose to >800 mg/L (Fig. 1c). The subsequent decreasein the OLR to 8 kg COD/(m3 d) resulted in a rapid recovery of thesystem revealed by the COD removal efficiency of 93.7% and signif-icant reductions in the effluent VFA (<300 mg/L). The OLR was thenkept at 12.5 kg COD/(m3 d) to study the UASB performance at high-er concentrations of influent COD (days 83–148). It was found thatthe COD removal efficiency gradually decreased from 91.5% to82.4% as the concentration of influent COD stepwise increasedfrom 35,000 to 73,000 mg/L. Higher concentrations of the VFAs inthe effluents were observed when the reactor was fed with undi-luted leachate at 12.5 kg COD/(m3 d). The result was probablyattributed to the methanogenesis inhibition from high concentra-tions of ammonium in the reactor and large amounts of toxins inthe raw fresh leachate, as well as a limited conversion of VFAs tomethane. In addition, it was reported that the SMA of the sludgedecreased and the mass transfer became low as the severe calciumprecipitation in the granules (Yu et al., 2001; Van Langerak et al.,2000). Thus, calcium precipitation in the sludge could be anotherreason and would be discussed latter.

When the OLR was further increased to 16 kg COD/(m3 d) dur-ing the period from day 149 to day 170, the performance of thereactor considerably deteriorated with the efficiency of COD re-moval sharply dropped to �66.9% and the methane content fell.At the same time, a serious accumulation of VFAs was observedwhen the VFAs in the effluent suddenly increased from 3000 mg/L to �14,000 mg/L, leading to a high ratio of VFAs/alkalinity (upto �1.8). Anaerobic processes can be considered to be operatingfavorably without acidification risk when the VFAs/alkalinity ratiois less than 0.3–0.4 (Leitão et al., 2006). The high VFAs/alkalinityratio suggests that the buffering capacity in the reactor was poorand did not suffice to maintain a proper pH level. pH in the reactordeclined to 6.4–6.7, which could have further inhibited the meth-anogenesis. The severe accumulation of VFAs also indicated thatthe reactor was overloaded. Therefore, an OLR less than 12.5 kgCOD/(m3 d) is suggested to achieve an efficiency of COD removalof >80% and a stable UASB system.

Fig 1d shows that the NHþ4 -N concentration was always higherin the effluent than that in the influent. It is similar to the resultreported by Liu et al. (2010). The concentration of NHþ4 -N in theeffluent reached >1000 mg/L after the reactor was fed with theraw leachate. The increase in the ammonium concentration couldbe attributed to the decomposition of the nitrogenous organiccompounds in the leachate. This led to high concentrations offree ammonia (FA) in the reactor as ammonium concentration isthe only influencing factor on FA level at a fixed pH and tempera-ture. In order to investigate the inhibitory effect of FA on methano-genesis, the sludge withdrawn on day 145 was evaluated in termsof SMA under different concentrations of ammonium at pH 7.5. Asshown in Fig. 2, SMA decreased from 0.4 to 0.16 gCH4-COD/(gVSS d) as the ammonium concentration increasing to 2000 mg/L and the concentration of the corresponding FA being 86.8 mg/L.Thus, the high concentration of ammonium in the reactor wasone of the reasons for the decrease in the COD removal efficiency.

Fig. 2. Effects of free ammonia on SMA.

Fig. 3. Relationship of volumetric methane production rate with (a) OLR and (b)

J. Ye et al. / Bioresource Technology 102 (2011) 5498–5503 5501

To avoid ammonia inhibition, leachate dilution can be the simplestmethod when practical. Except dilution, pretreatment methods,such as chemical precipitation or ammonia stripping, could alsobe possible options. However, ammonia stripping requires a highpH (>10.5) which is scaling problem. Struvite precipitation hasbeen demonstrated to be an effective way for the removal ofammonia and part of COD from landfill leachate (Kabdasli et al.,2008). It would be a working method to eliminate the FA inhibitionon the followed anaerobic units during the treatment of freshleachate. In addition, the ammonia inhibition could be diminishedby lowering the influent pH (Calli et al., 2006). More research iswarranted in real cases to determine an appropriate inlet pH forpreventing FA inhibition because the effects of low pH of the influ-ents on the process were related to reactor configuration, OLR andwastewater characteristics.

COD removal rate.

Table 2Metals contents (mg/g TS) and VS/TS ratio of the granular sludge in the UASB reactorat selected days.

Day Ca Mg Fe VS/TS

0 0.259 0.525 0.749 0.7250 – – – 0.5575 121.890 0.023 0.085 0.32100 – – – 0.26135 213.267 0.005 0.018 0.23

All samples were taken from the sampling port near the bottom.

3.2. Methane production rate

The methane content in the biogas in volume was measured tobe 65–80%, which was comparable to the values generally ob-served in leachate treatment (70–90%) (Kettunen and Rintala,1998). However, the methane content fell to 56% during the periodfrom day 149 to day 170. This could be a result of the reduction inSMA of the sludge at a high OLR of 16 kgCOD/(m3 d).

Gas production in anaerobic treatment has been found to be re-lated to the OLR. As shown in Fig. 3, when the reactor was operatedwith an OLR of 12.5 kg COD/(m3 d) or lower, the volumetric meth-ane production rate (VMPR) at STP increased linearly versus theOLR and COD removal rate with slopes of 0.27 and 0.31, respec-tively. The average methane yield was 0.31 L CH4/gCODremoved(STP).88.6% of the removed COD was converted to methane as the theo-retical methane production rate was 0.35 L CH4/gCODremoved (Timurand Özturk, 1999) and the rest (11.4%) was presumably trans-formed to biomass, which was close to the results reported by Chenet al. (2008). The slope of VMPR versus OLR of 0.27 indicates that77.1% (0.27/0.35 � 100%) of the COD loaded was converted tomethane, with the rest COD (22.9%) in the form of synthesized bio-mass and COD residue.

3.3. Characteristics of anaerobic granular sludge

The VS/TS ratio of the sludge in the near-bottom zone of theUASB reactor rapidly decreased during the whole period, whilethat in the top zone of the sludge bed decreased relatively slower(Table 2). On day 135, the VS/TS ratio of the sludge at the top

was 0.35, higher than that at the near-bottom (0.23). Similarresults were reported by Kettunen and Rintala (1998). The granulesfrom the bottom of the reactor (day 135) consisted of mostly inor-ganic precipitates as revealed by SEM observations (pictures notshown). For a long-term operation, the sludge at the bottom shouldbe removed/replaced to maintain the working volume and biolog-ical activity of the UASB system. Leachate pretreatment, such aschemical precipitation, is also a viable alternative for metals re-moval prior to the anaerobic processes.

The decrease in VS/TS was mainly attributed to the formation ofcalcium precipitates in the granular sludge. As shown in Table 2,the calcium content reached to 213.267 mg/g TS on day 135, whichwas all from the feed: the average concentration of calcium in theinfluent was >550 mg/L from day 23, and kept >3000 mg/L whenthe reactor was fed with the raw leachate. No accumulation of ironor magnesium was observed. This is consistent with the result of

Table 316S rRNA gene sequence-based phylogenetic affiliations of DGGE bands using a BLASTsearch of the NCBI database.

DGGEband

Phylogenetic affiliation (NCBI accession number) Similarity(%)

A1 Uncultured Methanoculleus sp. clone Ar3113(HQ141849)

95

A2 Uncultured Methanobacterium sp. clone WA1(EU888017)

99

A3 Methanosaeta concilii (NR028242) 99A4 Methanosaeta concilii (NR028242) 99A5 Methanosaeta concilii (NR028242) 99A6 Uncultured Methanosarcinales archaeon clone

QEEI1BA091 (CU916172)96

A7 Methanosaeta concilii (NR028242) 99

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XRD analysis (figure not shown). Only characteristic peaks for cal-cium carbonate (calcite, JCPDS CARD NO.: 5-586) were seen in theXRD pattern, suggesting that calcium carbonate was the major pre-cipitate. The precipitation of calcium carbonate in anaerobic sludgewas reported previously (Yu et al., 2001; Lozecznik et al., 2010).CO2 produced from the anaerobic degradation of organics couldbe used for the calcite precipitation. Yu et al. (2001) reported thathigher input of calcium and COD and a higher efficiency of COD re-moval resulted in more deposition calcium carbonate.

Van Langerak et al. (2000) found that treatment of calcium-richwastewater (780–1560 mg/L) led to the development of anaerobicsludge with a high ash content, and the presence of non-acidifiedsubstrate retarded cementation of the sludge bed compared withthe treatment of completely acidified wastewater. In this study,the partially acidified fresh leachate might account for the fact thatonly slight cementation occurred at the bottom of the sludge bed atthe end of the experiment (day 170).

3.4. Microbial community

SEM images of the granules show that Methanosaeta-like(bamboo-shaped) microorganisms were predominant in the coresection of the granule when the reactor was stable (pictures notshown). Methanosaeta concilii (formerly Methanothrix soehngenii(Rincón et al., 2006)) plays a crucial role in the initial granuledevelopment (Hulshoff Pol et al., 2004) and is able to outcompeteother methanogens when acetate concentration is low as is in theinterior of granule (Gujer and Zehnder, 1983). In the exterior layerof the granule, the Methanosaeta-like bacteria were also in abun-dance, but accompanied by a diverse population of cocci, thin fila-ments and rods.

The archaeal community of the granules was analyzed by PCR-DGGE and gene sequencing when the reactor was close to a steadystate (after 148 days of operation) and was compared with that of

Fig. 4. DGGE pattern for Archaea of the seed sludge and granules sampled on day148.

the seed sludge. As illustrated in Fig. 4, the strength of two DGGEbands (A1 and A3) for the granules on day 148 was enhanced com-pared with those for the seed sludge. Four bands from A4 to A7 didnot show obvious difference. Band A2 was not observed in the seedsludge. In order to further identify the composition of the archaealcommunity, the major bands (A1–A7) were recovered and se-quenced. The obtained sequences were compared with those inthe NCBI database and the similarities are presented in Table 3.Band A1 showed a 95% sequence similarity to hydrogenotrophicuncultured Methanoculleus sp. The sequence of band A2 was closelyrelated to uncultured Methanobacterium sp. (99% similarity), whichis able to produce methane from H2/CO2 and formate. Sequences ofbands A3–A5 and A7 have a 99% similarity to that of M. concilii. Thisagrees with the result of SEM observation. The prevalence ofacetoclastic M. concilii, known to play an important role inthe granules formation, could have contributed to the stableperformance of the USAB.

4. Conclusions

The UASB process was feasible to treat the fresh leachate fromincineration plant. An average COD removal efficiency of 82.4%was achieved when the reactor was fed with raw leachate (CODas high as 70,390–75,480 mg/L) at OLR of 12.5 kg COD/(m3 d) un-der mesophilic conditions. Due to the high calcium concentrationin the raw leachate, VS/TS ratio of the sludge decreased signifi-cantly after a long-term operation with the precipitation of calciumcarbonate. The average methane yield was 0.31 L CH4/gCODremoved

(STP) and 88.6% of the removed COD was converted to methanewith an OLR of 12.5 kgCOD/m3 d or lower.

Acknowledgements

This work was supported by Co-construction Program ofBeijing Education Committee (2008) and Open Project of StateKey Laboratory of Urban Water Resource Environment, HarbinInstitute of Technology (No. QAK201007).

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