Effects of exogenous aerobic bacteria on methane production and biodegradation of municipal solid waste in bioreactors

Effects of exogenous aerobic bacteria on methane production and biodegradation of municipal solid waste in bioreactors

Waste Management xxx (2015) xxx–xxx Contents lists available at ScienceDirect Waste Management journal homepage: www.elsevier.com/locate/wasman Eff...

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Waste Management xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

Waste Management journal homepage: www.elsevier.com/locate/wasman

Effects of exogenous aerobic bacteria on methane production and biodegradation of municipal solid waste in bioreactors Sai Ge a,b, Lei Liu c, Qiang Xue c,⇑, Zhiming Yuan a,⇑ a

Key Laboratory of Agricultural and Environmental Microbiology, Wuhan Institute of Virology, Chinese Academy of Sciences, Wuhan 430071, China University of the Chinese Academy of Sciences, Beijing 100039, China c State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan 430071, China b

a r t i c l e

i n f o

Article history: Received 13 May 2015 Revised 11 November 2015 Accepted 11 November 2015 Available online xxxx Keywords: Exogenous aerobic bacteria Municipal solid waste Methane production Biodegradation Bioreactors

a b s t r a c t Landfill is the most common and efficient ways of municipal solid waste (MSW) disposal and the landfill biogas, mostly methane, is currently utilized to generate electricity and heat. The aim of this work is to study the effects and the role of exogenous aerobic bacteria mixture (EABM) on methane production and biodegradation of MSW in bioreactors. The results showed that the addition of EABM could effectively enhance hydrolysis and acidogenesis processes of MSW degradation, resulting in 63.95% reduction of volatile solid (VS), the highest methane production rate (89.83 L kg 1 organic matter) ever recorded and a threefold increase in accumulative methane production (362.9 L) than the control (127.1 L). In addition, it is demonstrated that white-rot fungi (WRF) might further promote the methane production through highly decomposing lignin, but the lower pH value in leachate and longer acidogenesis duration may cause methane production reduced. The data demonstrated that methane production and biodegradation of MSW in bioreactors could be significantly enhanced by EABM via enhanced hydrolysis and acidogenesis processes, and the results are of great economic importance for the future design and management of landfill. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction In the past century, as the world’s population grew to be more concentrated in urban areas and affluent, waste production has risen tenfold (Hoornweg et al., 2013). Global municipal solid waste (MSW) production reached 1.3 billion tons per year in 2010 and it is expected to increase to 2.2 billion tons per year by 2025 (Hoornweg and Bhada-Tata, 2012). Disposal of MSW is of growing importance and the landfill technology is the most common and efficient way to achieve this objective. 54% of all solid waste produced in the United States was transported to landfill in 2010 (US EPA, 2012). While in 2011, 77% of all solid waste produced in China went to landfills (National Bureau of Statistics, 2012). However, the landfill gas (LFG) produced by microorganisms within a landfill, mostly methane and carbon dioxide, could contribute to the greenhouse effect if it released into the air directly. Alternatively, methane can be collected and used as an excellent fuel to

Abbreviations: EABM, exogenous aerobic bacteria mixture; WRF, white-rot fungi; LFG, landfill gas. ⇑ Corresponding authors. E-mail addresses: [email protected] (Q. Xue), [email protected] (Z. Yuan).

generate electricity and heat. By the year of 2012, there are approximately 50 MSW to energy facilities in operation in China, with an installed capacity of 100 MW, but maintenance is costly (Zheng et al., 2014). Therefore, it is of great economic and social importance to effectively manage landfill to support maximum methane production within limited time. The landfill gas is produced via chemical and biological reactions of microorganisms with the waste, and the production rate is depended on the waste composition, landfill geometry, and microbial populations. Currently, numerous studies have focused on the effects of various physical and chemical operational parameters to enhancing methane production and biodegradation of MSW. It has been reported that leachate recirculation, recirculation volume, waste shredding, waste compaction, control of moisture content and temperature, pH adjustment, aeration and addition of sludge, nutrient and gravel can be used to enhance biological degradation of the waste in bioreactors (Reinhart et al., 2002; Valencia et al., 2009; Warith, 2002). However, there are limitations to existing technology in field application and the conditions of landfills are difficult to modify after construction. In recent years, there has been increasing interest in studying the effect of microorganisms on substrate degradation and

http://dx.doi.org/10.1016/j.wasman.2015.11.024 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Ge, S., et al. Effects of exogenous aerobic bacteria on methane production and biodegradation of municipal solid waste in bioreactors. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.11.024

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S. Ge et al. / Waste Management xxx (2015) xxx–xxx

methane production in landfill (Divya et al., 2015). It has been reported that in the landfill, the degradation of solid waste results from various physical, chemical and biological reactions occurring simultaneously with interactive relations (Barlaz, 1997), and bacteria play a crucial role among those complicated reactions (Sawamura et al., 2010). The process for organic waste decomposition generally takes place in four stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis (Rich et al., 2008). The organic wastes such as protein and lipids are firstly hydrolyzed by aerobic microorganisms to monosaccharides, amino acids and fatty acids and then these hydrolysis products were broken down into H2, CO2 and organic acids (Sang et al., 2012). Finally, methanogenic microorganisms convert these products into methane in anaerobic condition. Therefore, the optimization of microbial population in landfill is regarded as rational method to enhance the solid waste degradation. For instance, researchers have observed increased methane production when dungs containing highly active microbial communities are introduced in food waste as an additive (Dhamodharan et al., 2015; Eze and Agbo, 2010). Some enzymes like lignin peroxidases, polyphenol oxidases, manganese dependent peroxidases, and laccases were used as additives to ligninrich MSW to enhance methane production (Jayasinghe et al., 2012; Schroyen et al., 2014; Shah et al., 2015). Methane generation of MSW begins with the degradation of organic matter, thus we investigated whether methane production can be enhanced by promoting the degradation of the organic matter. Therefore, some aerobic bacterial strains with high protein and lipid enzyme activity were isolated and added in MSW to promote methane production. Lignin can be only partly degraded to monomeric compounds by hydrolysis and is mostly degraded by attack on the C–C bonds. It was shown that the side chain and aromatic rings of lignin model compounds were oxidatively cleaved via aryl cation radical and phenoxy radical intermediates in reactions mediated by lignin peroxydase (Lip)/H2O2, and laccase (LA)/O2 mediator (Higuchi, 2006). And therefore increase the methane production (Liu et al., 2014; Shah et al., 2015; Taherzadeh and Karimi, 2008). We expected the addition of WRF would help to enhance the methane production of MSW in bioreactors. The aim of this work is to study the effects and the role of EABM and WRF on the biodegradation and methane production of MSW in laboratory-scale bioreactors, and then provide a practical method for the design and management of landfills. The results showed that the EABM can promote the degradation of organic matter, enhance the hydrolysis and acidogenesis process, accelerate the acetogenesis and methanogenesis, and increase the cumulative methane production almost three times more than the control. The results obtained are of great importance for the future design and maintenance of landfill.

WRF mycelia pellets were harvested directly from culture by centrifugation (3000 rpm, 15 min). The collected MSW from a landfill site in Huangjinkou, Wuhan were sorted as food residual, paper and leaves, plastic, cloths, metal, glass and stone by weight. The artificial MSW used in this study was prepared according to the method described in previous studies (Ag˘dag˘ and Sponza, 2005; Chiemchaisri et al., 2002; Šan and Onay, 2001), with 69% food residual, 14% paper and leaves, 2% plastic, 8% cloth, 2% metal, 3% glass and 2% stone. In order to accelerate waste degradation and to facilitate compaction, solid waste was shredded to diameter of 5 cm and well mixed (Jayasinghe et al., 2011). 2.2. Experiment setup The bioreactors were made up of acrylic material with 8 mm thickness, internal diameter of 0.2 m and height of 0.66 m (Fig. 1). Each bioreactor consisted of leachate collection, sampling, recirculation, and gas collection and MSW sampling pot. Every bioreactor was filled with 10 kg gravel stones at the bottom (about 10 cm height), above that was 10 kg MSW (about 45 cm) with 2 kg soil layer (about 5 cm) covered on the top (Table 1). An opening was created to each bioreactor on one side for sampling (Fig. 1). All bioreactors were kept and operated in a laboratory at room temperature. Leachate was collected at the bottom of the bioreactor for sampling and was recirculated back to the bioreactor through the pipe by pumping. 2.3. Experiment procedure The control bioreactors R1 and R2 were sprayed and well mixed with 1 kg water and 1 kg culture medium, respectively. The EABM was added at the ratio of 1.0, 1.0, 0.5 and 0.25 kg per 10 kg of MSW in R3, R4, R5 and R6, respectively (Table 1), and additional WRF was supplemented in R4 for evaluating its effect on lignin decomposition. The water content of each sample was adjusted by spraying water accordingly to maintain the same moisture content during the experiment in all bioreactors. The bioreactor experiments lasted 55 days until no methane production could be recorded. 2.4. Sampling and analytical methods Biogas was collected every day, leachate and MSW samples were collected and measured every two days for pH, COD, BOD5,

2. Materials and methods 2.1. Materials Preparation of aerobic bacterial mixture: Five aerobic bacteria strains with high protein and lipid enzyme activity were screened and isolated from MSW in previous work and identified as Bacillus cereus, B. subtilis, Staphylococcus saprophyticus, Staphylococcus xylosus and Pantoea agglomerans (data unpublished). A white-rot fungal (WRF) strain Phanerochaete chrysosporium was stored at 4 °C in our lab. All bacteria strains were grown in medium composed of agricultural byproduct (soybean meal 2.4%, starch 1.6% and fish meal 1%, pH 7.0) at 30 °C, 200 rpm overnight, and WRF was cultured in the potato medium at 30 °C, 200 rpm for 72 h. The aerobic bacterial mixture composed of five strains was obtained by mixing the five individual bacterial liquid cultures at the ratio of 1:1:1:1:1.

Fig. 1. Scheme of bioreactors.

Please cite this article in press as: Ge, S., et al. Effects of exogenous aerobic bacteria on methane production and biodegradation of municipal solid waste in bioreactors. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.11.024

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S. Ge et al. / Waste Management xxx (2015) xxx–xxx Table 1 Configuration and features of bioreactors.

a b c

Experiments

Quantity of wet waste (kg)

Moisture contenta (mean ± SD) (%)

VSa (mean ± SD) (%)

Leachate recirculation

Supplement

R1 R2 R3 R4 R5 R6

10 10 10 10 10 10

39.82 ± 1.14 40.46 ± 1.13 41.37 ± 1.01 39.86 ± 1.72 40.31 ± 1.34 40.19 ± 1.69

68.4 ± 1.45 67.8 ± 0.93 68.9 ± 0. 89 68.8 ± 1.30 68.9 ± 1.37 68.6 ± 1.25

Yes Yes Yes Yes Yes Yes

1 kg water 1 kg medium 1 kg EABMb 1 kg EABM and 200 g WRFc 0.5 kg EABM 0.25 kg EABM

The values are the averages with standard deviation of triplicate measurement. EABM: Exogenous aerobic bacteria mixture. WRF: White-rot fungi mycelia pellets.

VS measurements in all bioreactors during 55 days. Biogas was collected by connecting a Tedlar bag to the gas port of the bioreactor and measured by liquid displacement, containing 2% (v/v) H2SO4 and 10% (w/v) NaCl (Sponza and Ag˘dag˘, 2004). The methane production in biogas was measured using infrared methane gas analyzer Gasboard-3200L (Cubic Optoelectronics China Ltd, China). The pH in leachate samples was measured immediately after sampling using a pH meter (Toledo Instruments Ltd, United Kingdom). The chemical oxygen demand (COD) was measured Digestion System DRB200 and DR890 (HACH, USA). The biochemical oxygen demand (BOD5) was measured by Quick-Determinator LY-06 (LvYu Environment Protection China Ltd, China) (Liu et al., 2015). All the measurements were performed according to the procedure provided by producers. Moisture content was determined by heating the ground sample at 105 °C for 24 h to a constant weight. VS was determined by ashing the dry sample at 550 °C to a constant weight in a muffle furnace. Before the organic components were analyzed, the solid waste sample was first dried at 105 °C, and then shredded. Protein was determined as Kjeldahl N times a factor of 6.25 (Rice et al., 2012). Lipid was determined through extraction using AOAC, 1990, method no. 920.39 (Chemists, 1990; Zhang et al., 2012). Lignin was determined according to TAPPI, T 222 om-11standard procedure (Tang et al., 2013). All the measurements were triplicated and the average values with stand deviation were presented in tables and used to draw curves by GraphPad Prism 5.

3. Results and discussion 3.1. Methane production In all bioreactors, the daily methane production increased progressively. The peak methane production in R1, R2, R3, R4, R5 and R6 were 7.3 L on 35th, 8.4 L on 35th, 20.8 L on 25th, 23.8 L on 25th, 11.3 L on 28th, 10.3 L on 33th day, respectively (Fig. 2). As expected, the daily methane production decreased sharply after the peak production. The addition of EABM in R3 increased the peak methane production almost a three-fold compared to R1. While the highest peak methane production was observed in R4 supplemented with both WRF and EABM, with 13.42% higher production than R3, which only supplemented EABM alone. However, the quantity of EABM affected the methane production; less production were recorded in R5 and R6 than in R3 and R4, but surely higher than the two control bioreactors R1 and R2. The trends of cumulative methane production in all bioreactors were similar. The production increased rapidly after a initial lag phase, and reaching stabilized until the last day (55 days) of the study, with the cumulative methane production in R1, R2, R3, R4, R5, and R6 was 127.1 L, 150.3 L, 362.9 L, 409.5 L, 192.4 L and 157.1 L, respectively (Fig. 2). Medium used in R2 helped to generate 18.25% more methane compared to R1. The production in R3 showed about almost a three-fold increase over that in R1. Cumu-

Fig. 2. Methane production: (A) Daily methane production; (B) Cumulative methane production; (C) Concentration of methane production.

lative methane production in R4, with the addition of WRF, was greater than R3 by 12.84%. The production in R5 and R6 were less than R3 but greater than R1 and R2. The amount of methane production per kilogram of organic matter stabilized is taken to be an indicator of the degree of waster stabilization (Sponza and Ag˘dag˘, 2004). Table 2 shows the time (days) to accomplish methane production rate from 10 to 100 L kg 1 organic matter. The methane production rate at the end of study in R1, R2, R3, R4, R5 and R6 was 30.87 L kg 1 organic matter, 37.23 L kg 1 organic matter, 89.83 L kg 1 organic matter,

Please cite this article in press as: Ge, S., et al. Effects of exogenous aerobic bacteria on methane production and biodegradation of municipal solid waste in bioreactors. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.11.024

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S. Ge et al. / Waste Management xxx (2015) xxx–xxx

Table 2 Methane production rate. Methane production rate a

10 20a 30a 35a 40a 45a 50a 80a 90a 100a Total methane production (L) L kg 1 dry waste L kg 1 VS a

R1

R2

R3

R4

R5

R6

25 32 39 – – – – – – – 127.1

23 30 36 39 – – – – – – 150.3

13 17 20 21 22 23 24 31 – – 362.9

13 17 19 20 21 22 23 29 32 – 409.5

19 24 28 30 32 34 – – – – 192.4

22 28 32 35 – – – – – – 157.1

21.12 30.87

25.24 37.23

61.90 89.83

68.09 98.96

32.23 46.78

26.27 38.29

Time (days) to accomplish methane production rate from 10 to 100 L kg

1

VS.

have a positive effect on biodegradation of municipal solid waste. The addition of WRF leads 12.84% cumulative methane production increased in R4 than in R3. It proves the point that the WRF could enhance methane production of municipal solid waste (Liu et al., 2014; Shah et al., 2015; Taherzadeh and Karimi, 2008). Although, the methane concentrations in all the bioreactors share almost the same trend, significant differences were observed among each one of them (Fig. 2). In this study, the average methane concentration in R1, R2, R3, R4, R5, R6 was 19.21%, 22.33%, 42.73%, 44.03%, 31.76%, 28.30%, respectively. The highest methane concentration observed was 69.2% in R3 and R4 on 25th day. Lower methane concentration in R1 indicated the inability of the system to develop an active methanogenic population and enhance waste stabilization (Šan and Onay, 2001). The results show that the exogenous aerobic bacteria playd a positive role in maintaining higher methane production.

3.2. Leachate quality

Fig. 3. pH variation in all bioreactors with time.

98.96 L kg 1 organic matter, 46.78 L kg 1 organic matter and 38.29 L kg 1 organic matter, respectively. The lowest and highest methane production rate was observed in R1 on 39th day and R4 on 32th day. The methane production in R3 and R4 share almost the same trend in the first 30 days, which was clearly higher and quicker than any other bioreactors. At the end of study, the methane production rate in R4 was higher by 10.17% compared to which in R3. Furthermore, the methane production rate has been reported for all kinds of combinations of bioreactor operational parameters between 57.27 L kg 1 organic matter and 79.28 L kg 1 organic matter (Ag˘dag˘ and Sponza, 2005; Chiemchaisri et al., 2002; Šan and Onay, 2001), which is lower than what was achieved in R3 and R4. Cumulative methane production and daily methane production in experimental bioreactors show that exogenous aerobic bacteria

3.2.1. pH The initial pH of those six bioreactors was 5.88, 6.13, 6.54, 6.61, 6.25, and 6.17 (Fig. 3). The pH variation appears to be similar that it dropped to 4.85–5.24, which indicated the accumulation of organic acids occurred. Leachate pH in R1 and R2 slowly increased to 7.13 and 7.24, respectively. On the other hand, pH in R3, R4, R5 and R6 increased quickly over 7, accompanied by an increase in methane production and stayed to 7.31–7.69 at the end of study. The process of MSW degradation is very complex and the optimum pH range for the anaerobic digestion changes in the process. Previous studies showed that the range between 6.8 and 7.2 is the optimum pH for methanogenesis (Chugh et al., 1998; Warith, 2002), while pH 5–6.5 favors the hydrolysis and acidolysis processes, pH 6.4– 7.2 favors the methanogenesis in the anaerobic condition (Kruempelbeck and Ehrig, 1999). Moreover, in R1 and R2 we discovered the lowest pH value 4.85 and 5.01 on 18th day. Although the variations of leachate pH were similar, significant differences were observed between each one of them. The pH value during degradation was one of the parameters to differentiate the various phases of degradation (Christensen and Kjeldsen, 1989). The acidogenesis phase was marked by a decrease of pH in all bioreactors. A neutral pH, a characteristic of acetogenesis and methanogenesis (Francois et al., 2007), was firstly recorded on 22th day in R3 and R4, while on 24th, 34th, 36th and 40th day in R5, R6, R2 and R1, respectively. It is clear that EABM enhanced the hydrolysis and acidogenesis processed in R3, therefore acetogenesis and methanogenesis took place 18 days earlier compared to R1. Recently, Valencia (Valencia et al., 2009) declared that pH was the possible ‘driving force’ to trigger all processes and the degradation rate of solid waste was decreased due to lower pH in anaerobic bioreactor, which increased the time period of waste

Fig. 4. COD (A) and BOD5 (B) variation in all bioreactors with time.

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S. Ge et al. / Waste Management xxx (2015) xxx–xxx Table 3 Organic fractions degradation in all bioreactors. VS (%) a

R1 R2 R3 R4 R5 R6 a

Protein (%) a

a

Lipid (%) a

a

Lignin (%) a

Initial (mean ± SD)

Final (mean ± SD)

Initial (mean ± SD)

Final (mean ± SD)

Initial (mean ± SD)

Final (mean ± SD)

Initiala (mean ± SD)

Finala (mean ± SD)

68.4 ± 1.45 67.8 ± 0.91 68.9 ± 0.89 68.8 ± 1.30 68.9 ± 1.37 68.6 ± 1.25

48.3 ± 0.78 46.5 ± 0.95 24.8 ± 0.98 21.9 ± 1.11 32.6 ± 0.69 39.8 ± 1.01

18.1 ± 1.58 17.5 ± 1.55 18.6 ± 2.19 18.4 ± 1.98 18.8 ± 1.01 18.2 ± 1.59

7.5 ± 0.97 6.5 ± 1.06 2.8 ± 1.46 3.3 ± 1.03 4.3 ± 1.76 4.7 ± 0.76

16.8 ± 1.36 16.3 ± 1.60 17.4 ± 1.81 17.6 ± 1.67 17.5 ± 1.69 17.3 ± 1.55

9.2 ± 1.09 8.6 ± 1.51 4.5 ± 1.37 3.8 ± 1.75 6.5 ± 1.77 7.4 ± 1.09

10.5 ± 1.68 10.3 ± 2.45 10.6 ± 1.23 9.8 ± 1.73 11.3 ± 1.39 10.8 ± 1.46

9.4 ± 1.41 9.2 ± 1.35 9.2 ± 1.07 6.4 ± 1.41 10.0 ± 1.04 9.6 ± 0.98

The values are the averages with standard deviation of triplicate measurement.

Fig. 5. VS (A), lipid (B), protein (C), and lignin (D) degradation in all bioreactors with time.

treatment without biogas formation. Therefore, the lower pH value observed in R1 may cause methane production reduced compared to that in other bioreactors. 3.2.2. Chemical oxygen demand (COD) and biochemical oxygen demand (BOD5) The COD concentrations of the leachate we collected at the bottom of bioreactors are given in Fig. 4. The COD concentration in R3 and R4 increased to 12,000 mg/L from initial 5000 mg/L by the first 10–15 days, and the maximum values of 8000 mg/L and 10,000 mg/L were recorded on 20th day in R5 and R6 respectively. By contrast, the COD concentration in R1 and R2 increased to the maximum values at about 6000 mg/L on 30th day. After reaching the maximum values, the COD concentration in all bioreactors began to decrease sharply until the end of study. The COD leachates made it possible to distinguish the phases of degradation. During acidogenesis, the degradation involved a strong release of organic compounds with weak chains (acetic acid and propionic acid), which resulted in a high COD (Francois et al., 2007). As indicated above, the highest COD value found in each bioreactor was on 30th day, 28th day, 12th day, 10th day, 20th

day and 20th day in all bioreactors, respectively. This suggests that the macromolecular organic compounds in R3 and R4 are more easily and quickly converted to soluble molecules during hydrolysis and acidogenesis compared to others, indicating the addition of exogenous aerobic bacteria could shorten the time required for stabilization of COD. The variation of BOD5 concentration showed similar trends as COD concentration in all bioreactors (Fig. 4). The BOD5/COD ratio is an important parameter to reflect the biodegradability of leachates and an indirect indicator of the stability of landfill bioreactors (Alvarez-Vazquez et al., 2004; Kjeldsen et al., 2002). Although the changes of BOD5/COD ratio with time in all bioreactors were similar but at the end of study, the BOD5/ COD ratio in different bioreactor was 0.41, 0.38, 0.19, 0.16, 0.29 and 0.34, respectively. This suggested that R3 and R4 (BOD5/COD ratio less than 0.2) had reached a more stable state, and less nondegraded organic materials existed in R3 and R4 compared to R1. 3.3. Stabilization of MSW The degradation of the organic mainly occurs in the stage of hydrolysis (Bareither et al., 2013). The initial and final VS and three

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organic fractions in all bioreactors were determined (Table 3) and their changes during the stage of hydrolysis (until 18th day) were monitored (Fig. 5). The results showed that VS dry weight basis were reduced by 20.1%, 21.3%, 44.1%, 46.9%, 36.3% and 29.1% in bioreactors R1, R2, R3, R4, R5 and R6, respectively at the end of study. Higher protein and lipid degradation was exhibited in R3 and R4. 74.1% lipid and 84.9% protein degraded in R3 and 78.4% lipid and 82.0% protein degraded in R4. Besides, the highest lignin degradation was found in R4, with a degradation rate of 34.5%, while others were between 10.4% and 12.7%. Cumulative methane production in R4 was greater than R3 by 12.84%, which could support the point that WRF may enhance methane production through higher lignin degradation rate (Amirta et al., 2006). 4. Conclusions The results obtained in this study suggest that the methane production of MSW in bioreactors could be significantly increased by exogenous aerobic bacteria. The cumulative methane production in R3 (362.9 L) showed almost a three-fold increase over that in R1 (127.1 L). The methane production rate in R3 (89.83 L kg 1 organic matter) and R4 (98.96 L kg 1 organic matter) were higher than literature reported. Moreover, WRF helped to generate 13.42% more methane production by a three-fold increase in lignin degradation. Hence, the EABM would be a way for laboratory and field trials even landfill management practices with great advantages including environment and economic benefits. Acknowledgement This study was supported by the National Basic Research Program of China (973 Program) (2012CB719802). References Ag˘dag˘, O.N., Sponza, D.T., 2005. Effect of alkalinity on the performance of a simulated landfill bioreactor digesting organic solid wastes. Chemosphere 59, 871–879. Alvarez-Vazquez, H., Jefferson, B., Judd, S.J., 2004. Membrane bioreactors vs conventional biological treatment of landfill leachate: a brief review. J. Chem. Technol. Biotechnol. 79, 1043–1049. American Association of Chemists, 1990. Official Methods of Analysis, vol. I, 15th ed.. AOAC, Arlington, VA. Amirta, R., Tanabe, T., Watanabe, T., Honda, Y., Kuwahara, M., Watanabe, T., 2006. Methane fermentation of Japanese cedar wood pretreated with a white rot fungus, Ceriporiopsis subvermispora. J. Biotechnol. 123, 71–77. Bareither, C.A., Wolfe, G.L., McMahon, K.D., Benson, C.H., 2013. Microbial diversity and dynamics during methane production from municipal solid waste. Waste Manage. 33, 1982–1992. Barlaz, M.A., 1997. Microbial studies of landfills and anaerobic refuse decomposition. Manual Environ. Microbiol., 541–557 Chiemchaisri, C., Chiemchaisri, W., Nonthapund, U., Sittichoktam, S., 2002. Acceleration of solid waste biodegradation in tropical landfill using bioreactor landfill concept. 5th Asian Symposium on Academic Activities for Waste Management, pp. 9–12. Christensen, T.H., Kjeldsen, P., 1989. Basic biochemical processes in landfills. In: Sanitary Landfilling: Process, Technology, and Environmental Impact. Academic Press, New York, pp. 29–49 (9 fig, 3 tab, 34 ref). Chugh, S., Clarke, W., Pullammanappallil, P., Rudolph, V., 1998. Effect of recirculated leachate volume on MSW degradation. Waste Manage Res. 16, 564–573. Dhamodharan, K., Kumar, V., Kalamdhad, A.S., 2015. Effect of different livestock dungs as inoculum on food waste anaerobic digestion and its kinetics. Bioresour. Technol. Divya, D., Gopinath, L., Christy, P.M., 2015. A review on current aspects and diverse prospects for enhancing biogas production in sustainable means. Renew. Sustain. Energy Rev. 42, 690–699.

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Please cite this article in press as: Ge, S., et al. Effects of exogenous aerobic bacteria on methane production and biodegradation of municipal solid waste in bioreactors. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.11.024