Waste Management 34 (2014) 2278–2284

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Waste Management

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Anaerobic co-digestion of food waste and landfill leachatein single-phase batch reactors

http://dx.doi.org/10.1016/j.wasman.2014.06.0140956-053X/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding authors. Address: School of Environmental Science and Engi-neering, Huazhong University of Science & Technology, 1037 Luoyu Road, Wuhan430074, China. Tel./fax: +86 27 87792401.

E-mail addresses: jpzhuhust@163.com (J. Zhu), liaoli2003@126.com (L. Liao).

Liao Xiaofeng, Zhu Shuangyan, Zhong Delai, Zhu Jingping ⇑, Liao Li ⇑School of Environmental Science and Engineering, Huazhong University of Science & Technology, Wuhan 430074, China

a r t i c l e i n f o

Article history:Received 28 February 2014Accepted 16 June 2014Available online 22 July 2014

Keywords:Anaerobic digestionFood wasteLandfill leachateAmmonia nitrogenFluorescence excitation–emission matrixspectroscopyUV–vis spectroscopy

a b s t r a c t

In order to investigate the effect of raw leachate on anaerobic digestion of food waste, co-digestions offood waste with raw leachate were carried out. A series of single-phase batch mesophilic (35 ± 1 �C)anaerobic digestions were performed at a food waste concentration of 41.8 g VS/L. The results showedthat inhibition of biogas production by volatile fatty acids (VFA) occurred without raw leachate addition.A certain amount of raw leachate in the reactors effectively relieved acidic inhibition caused by VFA accu-mulation, and the system maintained stable with methane yield of 369–466 mL/g VS. Total ammonianitrogen introduced into the digestion systems with initial 2000–3000 mgNH4–N/L not only replenishednitrogen for bacterial growth, but also formed a buffer system with VFA to maintain a delicate biochem-ical balance between the acidogenic and methanogenic microorganisms. UV spectroscopy and fluores-cence excitation–emission matrix spectroscopy data showed that food waste was completely degraded.

We concluded that using raw leachate for supplement water addition and pH modifier on anaerobicdigestion of food waste was effective. An appropriate fraction of leachate could stimulate methanogenicactivity and enhance biogas production.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

As one of the most effective systems for biological treatment oforganic waste, anaerobic digestion technology has a number ofbenefits, such as solid reduction and biogas production, whichmakes it an attractive technology (Klavon et al., 2013). However,anaerobic digestion sometimes tends to be inefficient when someorganic waste such as manure or food waste is used as sole sub-strate (Yamashiro et al., 2013). Relatively low biodegradabilityand biogas yield of dairy manure make it usually unfavorable forenergy production. Compared to dairy manure, food waste hashigher biogas potentials. Due to its extremely high biodegradabil-ity, accumulated volatile fatty acids (VFA) often makes methaneproduction suppressed.

As an alternative, co-digestion which simultaneously uses morethan one organic waste stream as substrates, seems promising forimproving digestion efficiencies. With an improved balance ofnutrients, and the synergy effect between organic substrates, co-digestion can improve process performance (Viotti et al., 2004).

According to the study of Bouallagui et al. (2009), during the diges-tion of fruit and vegetable waste, additions of abattoir wastewateror activated sludge could enhance biogas yields by 51.5% and43.8%, respectively. The added wastewater or sludge loweredcarbon to nitrogen ratio and enhanced biogas yields. Co-digestionof dairy manure and more degradable wastes is effective forimproving the economics of dairy digesters by increasing thebiogas production rate (El-Mashad and Zhang, 2010).

Anaerobic co-digestion processes require the proper conditionswith regard to substrates. In the batch experiments of Li et al.(2009), a mixing ratio of 3:1 was optimal for co-digestion of cattlemanure and kitchen waste, with methane yield of 233 ml/g VS. Forfood waste digestion, it is important to maintain suitable pHduring the anaerobic process and maintain the balance betweenVFA and methane production. Co-digestion of food waste withother organic waste would provide an improved balance of nutri-ents and the development of synergistic microbial consortia(Sosnowski and Ledakowicz, 2003; Hartmann and Ahring, 2005).Manure, sludge are often used as the co-digestion substrate of foodwaste (Ye et al., 2013; Kim et al., 2013; Yamashiro et al., 2013).Studies showed that food waste co-digested with mixture ofmanure resulted in increased biogas production rate and biogasyield compared to digesting manure or food waste alone (Liet al., 2010). Wan reported that using Chinese silver grass (CSG)as a co-substrate in food waste anaerobic digestion system is a

Table 1Characteristics of raw leachate and food waste used in the co-digestion experiments.

Parameters Raw leachate Parameters Food waste

pH 8.54 Protein (%) 3.37NH4–N (mg/L) 3625 Fat (%) 4.43Salinity (g/L) 8.18 Carbohydrate (%) 21.37DOC (mg/L) 1298 Salinity (%) 0.84COD (mg/L) 2500 Moisture (%) 69.52BOD5 (mg/L) 345 C/N 25.39TOC VS/TS 95.7

TOC (g/kg TS) 450

X. Liao et al. / Waste Management 34 (2014) 2278–2284 2279

potential simple method to convert CSG into renewable energy andto simultaneously improve food waste treatment (Wan et al.,2013).

Shahriari et al. (2012) studied the effect of using untreatedleachate for supplement water addition and liquid recirculationon anaerobic digestion of food waste. An appropriate fraction ofrecycled leachate and fresh water could stimulate methanogenicactivity and enhance biogas production. Landfill leachate has oftenbeen used as the co-digestion substrate for sewage sludge, septage,or domestic wastewater treatment (Kawai et al., 2012;Montusiewicz and Lebiocka, 2011). However, when amount ofleachate exceeds some extent, high-strength ammonia nitrogenmight inhibit methanogenic activities.

In this paper, anaerobic digestion of food waste was studied anduntreated raw leachate was used as co-digestion substrate. Theobjective of this work was to study the impact of addition of differ-ent amounts of raw leachate on anaerobic digestion of food wasteand provide insight into the impact of co-digestion on biogas pro-duction and stabilization of food waste treatment process. Becauseit was difficult to distinguish the decomposition rate or methaneproductivity of each substrate separately (Mata-Alvarez et al.,2000; Hartmann and Ahring, 2005), most studies about co-diges-tion of landfill leachate were seldom concerned about the changeof substrates before and after co-digestion.

Dissolved organic matter (DOM) is a heterogeneous mixture ofhumic substances, hydrophilic acids, proteins, lipids, carbohydrates,carboxylic acids, amino acids, and hydrocarbons (Leenheer andCroue, 2003). Multiplicate analytical methods are used to distinguishthe characteristics of DOM in wastewater (Huo et al., 2008; Seo et al.,2007). For example, ultraviolet–visible (UV–vis) spectrometry, fou-rier transform infrared (FT-IR) spectroscopy, and fluorescence excita-tion–emission matrix (EEM) spectroscopy have been employed todetermine the molecular size magnitude, structures, and specificpollutants quantificationally and qualitatively (Kim et al., 2006;Hudson et al., 2007). However, few works have been reportedabout the characteristics of DOM in slurry samples after anaerobicdigestion using spectral spectroscopy.

Fluorescence EEM spectroscopy can provide considerabledetailed information about fluorescence properties of DOM thatmay reveal important information about its composition (Burdigeet al., 2004; Baker, 2005; Marhuenda-Egea et al., 2007). In thisstudy, UV–Vis spectroscopy analysis and fluorescence excitation–emission matrix (EEM) analysis were used to analyze characteris-tics of slurry samples after the reaction in order to obtain moreinformation about the decomposition of leachate and co-digestedsubstrate.

2. Materials and method

2.1. Substrates

The raw leachate was sampled from the Chenjiachong Munici-pal Solid Waste Landfill, Wuhan, China, which has been in opera-tion since 2007. Food waste was collected from canteens ofHuazhong University of Science & Technology, Wuhan, China.The chief constituents of the food waste were classified as cookedrice, vegetables and meat and was homogenated using a waste dis-posal unit (France) before use. Characteristics of the middle-agedraw leachate and food waste were summarized in Table 1.

2.2. Anaerobic digestion

Co-digestions of food waste and raw leachate experiments wereconducted with HRT (35 days) in single-stage batch reactors with aworking volume of 1500 mL. The reactors were in a water bath formaintaining a mean mesophilic temperature of 35 ± 1 �C. A 6 L gas

container was attached to each reactor for biogas collection. Thegas volume was calculated daily based on the downward displace-ment of water.

The initial OLR for the reactors 1–7 was 41.8 g VS/L and differ-ent amounts of raw leachate were added into these reactors(shown in Table 2). A blank digesters (the reactor 8) that containedinoculum and raw leachate only were also incubated at the sametemperature to test for the biogas produced from the inoculumand raw leachate. The inoculum was anaerobic granular sludgetaken from an anaerobic fermentation bioreactor in a food plantin Wuhan, China. The seed sludge consisted of well-settled blackgranules, with about 90% showing the size >1.5 mm in diameter.The volatile suspended solid (VSS) content was 79,680 mg/L, corre-sponding to about 71% of the total suspended solid (SS).

2.3. Analytical methods

CH4 and CO2 concentrations were measured by a Biogas 5000analyzer (Geotech, England). Samples were collected every twodays and were analyzed. pH value was measured with a PB-10pH-meter (Sartorius, Germany). NH4–N concentration was mea-sured with a HT 93733 Ammonia detector (Hanna, Italy). TotalVFA concentration was determined colorimetrically (ThermoUV–Vis spectrophotometer).

For determination of total solids, the samples were dried at105 �C for 24 h, and total solid contents were calculated from thedifferences between weights before and after drying. The driedmatters were heated at 550 �C for 4 h, and organic matter contentswere calculated from the losses on ignition.

For determination of total nitrogen, a 5 g sample was weighedinto a Kjeldahl digestion apparatus; 20 mL of concentrated H2SO4

and 0.2 g CuSO4 (Kjeldahl catalyst) were added. The Kjeldahl diges-tion apparatus was heated for 2 h until all nitrogen was conversedto ammonium sulphate. After cooling, the digested sample wastransferred into 100 mL volumetric flasks and diluted with distilledwater. After distillation of the digested solution, the distillate wastitrated against 0.1 mol/L HCl.

For determination of dissolved organic carbon (DOC), after fil-tration through 0.45 lm membrane to remove suspended materi-als, samples was determined by a multi N/C 2100 TOC analyzer(Jena, Germany).

2.4. UV–Vis spectral analysis

Specific ultraviolet light absorbance (SUVA254) calculated asA254/DOC is a measure of the contribution of aromatic structuresto DOC and is usually used to define the aromaticity of dissolvedorganic matter (DOM) (Weishaar et al., 2003).

The ratio of UV absorbance at 253 nm to that at 203 nm (A253/A203) reflects the degree of substitution of the aromatic ring andthe kind of the substitution (Korshin et al., 1997). A253/A203 islow for unsubstituted aromatic ring structures. When the aromaticrings are highly substituted, the value is high. The changes in

Table 2The amount of raw leachate and fresh water added in the reactors.

Reactors 1 2 3 4 5 6 7 8

Organic load of food waste (g TS/L) 40 40 40 40 40 40 40 0Leachate (ml) 0 142 284 426 568 710 852 1200Fresh water (ml) 1031 889 747 605 463 321 179 0NH4–N (mg/L) 0 500 1000 1500 2000 2500 3000 0

2280 X. Liao et al. / Waste Management 34 (2014) 2278–2284

A253/A203 suggest that aromatic rings substituted with various fac-tional groups are structurally altered by treatment.

After filtration through 0.45 lm membrane to remove sus-pended materials, slurry samples were diluted and UV–Vis spectrawere measured using a Thermo UV–Vis spectrophotometer. Super-Q water was used as a blank. SUVA254 and A253/A203 values forslurry samples were calculated.

2.5. Fluorescence EEM spectral analysis

Fluorescence EEM spectroscopy can provide considerabledetailed information about fluorescence properties of DOM thatmay reveal important information about its composition. With thistechnique, a three-dimensional picture is generated of fluores-cence intensity as a function of excitation and emissionwavelength.

Fluorescence regional integration (FRI), a quantitative tech-nique that integrates volumes beneath different excitation–emission regions in EEM spectra, is used to quantitatively analyzeEEM spectra (He et al., 2011). According to the reports (Chen et al.,2003), consistent excitation and emission wavelength boundariesfor each EEM horizontal and vertical lines were drawn to dividethe EEM into five regions. Peaks at shorter excitation wavelengths(200–250 nm) and shorter emission wavelengths (280–380 nm)were related to simple aromatic proteins such as tyrosine (RegionI) and tryptophan (Region II) (Ahmad and Reynolds, 1999). Peaks atexcitation wavelengths (250–300 nm) and shorter emission wave-length (280–380 nm) were related to soluble microbial byproduct-like material (Region IV) (Ismaili et al., 1998; Reynolds and Ahmad,1997). Peaks at excitation wavelengths (200–250 nm) and emis-sion wavelengths (380–550 nm) were related to fulvic acid-likeorganics (Region III) (Mounier et al., 1999). Peaks at excitationwavelengths (250–450 nm) and emission wavelengths (380–550 nm) were related to humic acid-like organics (Region V). FRItechnique was used to integrate the volume beneath EEM withineach region (Ui). The cumulative volume beneath the EEM (UT) iscalculated as UT =

PUi. All UT and Ui values were normalized to

the DOC concentration and the projected excitation–emission areawithin that region to get Ui,n and UT,n.

The EEMs of slurry samples were measured by a F-4600 fluores-cence spectrometer (Hitachii, Japan). A xenon excitation sourcewas used in the spectrometer, and the excitation and emission slitswere set to a 5 nm band-pass. Each EEM spectrum was generatedby scanning excitation wavelengths from 200 to 450 nm at 5 nmsteps, and detecting the emission fluorescence between 250 and550 nm at 2 nm steps. The scan speed was set at 1200 nm/min.The spectrum of Super-Q water was recorded as blank. All sampleswere adjusted to pH 7 with HCl prior to measurement. UT,n valuesof the samples were obtained. The percent fluorescence responsein a specific region (Pi,n) was calculated as Pi,n = Ui,n/UT,n � 100%.

3. Results and discussion

Table 3 showed characteristics of slurry samples after digestion.The samples from the reactors 1–4 had high BOD5/COD values,while those from the reactors 5–7 had low biodegradability.

3.1. pH and VFA concentration change during co-digestion

As shown in Fig. 1, initial pH values in all the reactors were >7.With the hydrolysis of food waste, total VFA concentrationsincreased greatly, which induced pH decline rapidly. In the reactor1, the accumulation of VFAs led to irreversible acidification whichinhibited methanogenic microorganisms’ activities.

With different amount of raw leachate added in the reactors2–7, changes of pH and VFA concentrations were different duringthe whole process. Acid accumulation with varying degrees wasobserved in the reactors 2–4. On the other hand, the systems inthe reactors 5–7 showed stronger regulation abilities for VFA. After5 days, pH values in the reactors 5–7 increased gradually to>7.5 and VFA concentrations were maintained at a relatively lowlevel, which showed that the buffer effect of ammonia nitrogenintroduced by raw leachate mitigated acid accumulation effec-tively and pH values remained in a range which was suitable formethanogenic microorganisms’ activities.

3.2. Gas production

In Fig. 2, little gas (1.03 L) was produced in the reactor 8, show-ing refractory of raw leachate to biodegradation. The gas yield ofthe reactors 1–4 were only 14.95%, 9.56%, 14.85% and 33.33% ofthat of the reactor 5, and gas production stopped at day 23.

In conventional wet anaerobic digestion of food waste withoutbuffers, OLR usually remained insufficient (<20 g VS/L) to maintaina stable operation with high methane yield 364–489 mL/g VS(Zhang et al., 2009). In this study, raw leachate added in the reac-tors 5–7 effectively relieved acidic inhibition caused by VFA accu-mulation, and the system of OLR 41.8 g VS/L maintained stablewith methane yield of 369–466 mL/g VS (Table 4).

In this study, OLR of the system was much higher than thatreported in the literatures (Zhang et al., 2009). Rapid hydrolysisand acidification of food waste caused acid accumulation in thereactors 2–4, where the buffer effect of raw leachate for pH regu-lation was relatively poor. On the other hand, TAN introduced intothe digestion systems with initial 2000–3000 mgNH4–N/L in thereactors 5–7 was important for the maintenance of a delicate bio-chemical balance between the acidogenic and methanogenicmicroorganisms. Because the growth of bacteria consumed a cer-tain amount of nitrogen, no suppression of high-strength ammonianitrogen on anaerobic digestion was observed.

3.3. Fluorescence EEMs

In Fig. 31–4, fluorescence in the region of emission wavelength<300 nm, showed the existence of protein-like substances, andreflected an incomplete degradation of food waste in the reactors1–4. In addition, fluorescence peaks in the region of excitationwavelength 250–300 nm and emission wavelength 280–320 nmwas related to soluble microbial byproduct-like materials. Com-bined with Fig. 1, we deferred that VFA was decomposed furtherand used as nutrients for bacterial growth. In Fig. 35–8, fluores-cence was mainly in the region of longer emission wavelength>300 nm, showing the existence of humic-like and fulvic-like sub-

Table 3Characteristics of slurry samples after anaerobic digestion.

Parameters Reactors

1 2 3 4 5 6 7 8

pH 5.02 4.98 5.1 5.42 8.24 8.32 8.25 8.06NH4–N (mg/L) 2813 125 600 625 1200 1375 1100 1600COD (mg/L) 12,250 10,000 11,500 7750 5500 7250 7000 1200BOD (mg/L) 13,000 12,500 16,600 11,500 37 65 85 76DOC (mg/L) 8630 8332 8225 7187 1113 1563 1604 935BOD5/COD 1.061 1.25 1.443 1.483 0.007 0.009 0.012 0.063

0 5 10 15 20 25 30 35

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

9.0

reactor 1 2 3 4 5 6 7 8

pH

Time (Days)

0 5 10 15 20 25 30 350

5000

10000

15000

20000

25000

30000

35000

40000

45000

reactor 1 2 3 4 5 6 7 8

VFA

conc

entra

tion

(mg/

L)

Time (Days)

Fig. 1. Change of pH and VFA in slurry samples from the reactor 1–8 duringanaerobic digestion.

0 5 10 15 20 25 30 350

10000

20000

30000

40000

50000

Gas

yie

ld (m

l)

Time (Days)

reactor 1 2 3 4 5 6 7 8

Fig. 2. Gas production during anaerobic digestion in the reactor 1–8.

Table 4Biogas and methane production of anaerobic digestion.

Parameters Reactors

1 2 3 4 5 6 7

Biogas production (mL/g VS) 131 84 130 293 878 817 740Methane production (mL/g VS) 1.1 5.6 22 91 466 423 369

X. Liao et al. / Waste Management 34 (2014) 2278–2284 2281

stances. To some extent, it reflected an complete degradation offood waste.

UT,n of the samples from the reactors 5–7 were larger than thosefrom the reactors 1–4 (Table 5), showing the existence of fluores-cent compounds, such as humid acid and fulvic acid in a largerproportion in the former.

The percent fluorescence response in a specific region (Pi,n) wascalculated as Pi,n = Ui,n/UT,n � 100%. Variability in Pi,n values may beused as surrogates for dominant structures in the samples. Pi,n ofsamples from the reactors 5–7 resembled to that of the reactor 8,and their combined PIII,n (fulvic-like) and PV,n (humic-like) valueswere larger than 0.6, which indicated that fluorescent propertiesof these samples were similar to raw leachate. Their small UT,n

could be ascribed to the contribution of non-fluorescent sub-

stances to DOC. The results were similar to that He et al. reported(He et al., 2011).

In the reactor 1–4, the combined percentage (Pi,n) of Region Iand Region II was higher than PIII,n, showing protein-like domi-nance. This quantitative result was consistent with visual analysisof the location of EEM fluorescence.

3.4. UV spectral characteristics

With incomplete decomposition of food waste in the reactors1–4, large quantities of organic compounds are dissolved in thewater and decomposed to smaller molecules. Food waste in the reac-tors 5–7 has been decomposed more completely and more organiccompounds have been transformed into biogas. A254 values of thesamples from the reactors 5–7 were larger than those from the reac-tors 1–4, showing that there were more unsaturated groups in theformer samples. Because of the difference in DOC values, the differ-ences in SUVA254 values calculated are greater (Fig. 4).

The larger SUVA254 values, which meant the existence of a largerportion of aromatic compounds, may be attributed to the formationof complex molecules during food waste biodegradation, followedby the formation of more polycondensed humic structures(Jouraiphy et al., 2005). In addition, saturated organic compoundsin added raw leachate may be decomposed and formed unsaturatedcompounds during co-digestion in the reactors 5–7, which contrib-uted to SUVA254 values. In reactor 8 where only raw leachate wasadded, the change of UV spectra characteristic (SUVA254 and A254

values) showed the formation of more aromatic substances.

250 300 350 400 450 500 550200

250

300

350

400

450

Em (nm)

E x (n

m)

10.0020.8043.2589.96187.1389.1809.216833500

1

250 300 350 400 450 500 550200

250

300

350

400

450

Em (nm)

E x (n

m)

10.0021.1544.7294.57200.0422.9894.418914000

2

200

250

300

350

400

450 10.0019.9439.7679.29158.1315.3628.712542500

3

250 300 350 400 450 500 550200

250

300

350

400

450

E x (n

m)

10.0022.0148.43106.6234.5516.1113624995500

4

200

250

300

350

400

450 10.0019.7338.9476.85151.7299.3590.611652300

5

250 300 350 400 450 500 550200

250

300

350

400

450 10.0017.7831.6256.23100.0177.8316.2562.31000

6

200

250

300

350

400

450 10.0018.1933.1060.21109.5199.3362.6659.61200

7

200

250

300

350

400

450 10.0019.9439.7679.29158.1315.3628.712542500

8

Em (nm)

Em (nm)

E x (n

m)

250 300 350 400 450 500 550

Em (nm)

E x (n

m)

E x (n

m)

250 300 350 400 450 500 550

Em (nm)

E x (n

m)

250 300 350 400 450 500 550

Em (nm)

E x (n

m)

250 300 350 400 450 500 550

Em (nm)

Fig. 3. Fluorescence EEM spectra for slurry samples from the reactor 1–8 after the reaction.

2282 X. Liao et al. / Waste Management 34 (2014) 2278–2284

A253/A203 of slurry in the reactor 5–7 was higher than those inthe reactor 1–4 (Fig. 4), showing an increase in the ratio of unsat-urated groups such as carbonyl, carboxyl or others on the aromatic

rings. The compounds in the slurry from the reactors 1–4 mainlyconsisted of aliphatic chains, and the content of aromatic com-pounds was lower. Altogether, UV spectra analysis indicated after

Table 5The percentage distribution of fluorescence response (Pi,n) and UT,n for slurry samples.

Reactors Pi,n (%)P

UT,n (�10�6) (AU-nm2-(mg/L C)�1)

Region I Region II Region III Region IV Region V

Raw leachate 8 14 47 17 14 242.31 17 23 27 21 12 29.52 18 20 29 21 12 19.33 14 23 32 19 12 27.34 23 22 24 23 8 45.75 2 17 48 17 16 159.16 3 18 50 14 15 132.87 4 16 53 12 15 146.68 4 16 54 11 15 277.3

0 1 2 3 4 5 6 7 80

5

10

15

20

25

The reactor number

SUVA

254

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

SUVA 254 A 254 A 253 /A 203

A25

4 and

A25

3/A

203

Fig. 4. SUVA245, A254 and A253/A203 values of slurry samples after the reaction. ‘‘0’’was raw leachate.

X. Liao et al. / Waste Management 34 (2014) 2278–2284 2283

digestion, slurry samples in the reactors 5–7 was more refractorythan raw leachate.

4. Conclusions

The raw leachate added in the reactors 5 effectively relievedacidic inhibition caused by VFA accumulation, and the system ofOLR 41.8 g VS/L maintained stable with methane yield of 466mL/g VS. UV–Vis spectral analysis showed characteristics of slurrysamples after the reaction. Both A254 and A253/A203 values showedan increase in the ratio of unsaturated groups in the samples fromthe reactors 5 after AD.

Co-digestion of more than one organics stream can improvebalance of nutrients, and the synergy effect between organic sub-strates. In this study, addition of an appropriate amount of rawleachate is beneficial to food waste digestion by effective regula-tion of pH at high OLR. The formation of buffer system with ammo-nia nitrogen introduced by the addition of raw leachate and VFA iseffective for maintaining a biochemical balance between the acido-genic and methanogenic microorganisms.

Acknowledgement

This work was supported by the National Sci-Tech Support Planof China (No. 2012BAC18B02).

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