The purposes of this study were to investigate the effect of waste leachate (WL) addition on batch anaerobic digestion of food waste (FW), and to examine the influence of mixture ratio on the co-digestion process. The results showed that anaerobic digestion of FW was greatly enhanced by WL addition, as indicated by the higher methane yield, higher methane content, more volatile solids (VS) destruction, and higher stability. Although WL was rich in volatile fatty acids (VFA), its addition did not cause VFA inhibition. It was found that WL addition was beneficial to accelerate the start-up and shorten the long reaction time of the batch anaerobic process. The time to reach the peak methane yield was reduced by 1, 2, and 4 days with WL addition. The optimum FW to WL ratio was 77.9:22.1 with the highest methane yield (416 mL/g VS), the highest methane content (64.3%), the greatest VS removal (77.6%), and stable performance. These results confirmed the positive effects of WL addition on methane fermentation from FW.

INTRODUCTION

With the explosive growth of food waste (FW) production and the aggravation of environmental pollution, FW treatment has raised a lot of social and environmental concerns. At present the annual yield of FW of China has reached 82.8 million tons (Uçkun Kiran et al. 2014). In China, due to the mixed collection of municipal solid waste (MSW), plenty of FW was mixed into unsorted waste, which greatly increased the difficulty of MSW treatment due to the features of FW (high moisture and organic matter content and easy decay). In addition, because of the profit that could be gained, a good deal of FW was used for producing illegal cooking oil and feeding pigs, which could threaten human health or induce animal epidemics. It is still a challenge to develop an environmentally friendly and economical method for FW treatment.

Recently, anaerobic digestion of FW has attracted much attention due to great environmental, economic, and societal benefits in terms of high removal efficiency of organics, valuable products, low pollutant emission, and low consumption of energy and capital (Zhang et al. 2014). Biogas is a clean and versatile fuel, which could be burned for heat and electricity or upgraded to bio-methane for various alternative uses. Moreover, the digestate rich in nutrient substance could be used as fertilizer. In addition, anaerobic digestion is feasible in large-scale or small applications, which makes its application possible for developing countries and rural areas, where energy supply might be insufficient. Thus, FW anaerobic digestion is significant to ensure energy supply and environmental safety.

Although anaerobic digestion of FW is attractive, some critical problems greatly limited its application including volatile fatty acids (VFA) inhibition (Dearman & Bentham 2007), ammonia inhibition (Banks et al. 2012), long-chain fatty acids inhibition (Cirne et al. 2007), and trace elements deficiency (Zhang & Jahng 2012). It is still an urgent task to develop a high-efficiency anaerobic process for FW.

Co-digestion was considered as an important strategy for anaerobic digestion owing to some attractive advantages including improved buffering capacity, cost reduction by the sharing of equipment, dilution of toxic compounds, balanced nutrient composition, synergistic effect of micro-organisms, detoxification based on co-metabolism process, and increased organic loading (Mata-Alvarez et al. 2014). Waste leachate (WL) from waste storage bunkers of waste incineration plant is another organic waste stream. Recently MSW incineration has developed rapidly in China due to its excellent performance in waste reduction, and the percent of incinerated MSW had reached 24.7% in 2012 (China National Bureau of Statistics 2013). However, the high moisture content and low calorific value of MSW both showed that it could not be incinerated effectively without fuel addition or meticulous separation. Thus, in order to improve the calorific value and reduce the moisture content of MSW, it should be stored in storage bunkers for 3–7 days before incineration. During this period, plenty of WL is produced. The WL has abundant organics, VFA, ammonia nitrogen, and metal elements. With regard to the WL, its high alkalinity and abundant trace elements are favorable to anaerobic digestion. Thus, co-digesting with WL may be a feasible method to improve FW anaerobic digestion. Compared to the high investment and maintenance cost and the strict requirements for complex operation and process control of continuous digesters, it is easier for batch digestion to obtain satisfactory performance, especially in developing countries and rural areas, where capital and technology might be not sufficient. So far, batch co-digestion of FW and WL from waste incineration plant has not been reported, which should therefore be investigated. This was the reason for initiating this research. In the present study, the effect of WL addition on batch methane fermentation from FW was investigated under different mixture ratios.

MATERIALS AND METHODS

Material preparation and analytical methods

The FW was obtained from a restaurant of Dalian University of Technology, Dalian, China. The WL was obtained from a waste storage bunker of an MSW incineration plant, Dalian, China. An electrical blender was employed to homogenize the FW, then the FW was screened using a 14-mesh screen. The FW and WL samples were stored at 5 °C before use. The seed sludge was obtained from a digester of a sewage sludge treatment plant, Dalian, China. The characteristics of FW, WL and seed sludge are shown in Table 1.

Table 1

Characteristics of FW, WL and seed sludge

Parameter Unit FW WL Seed sludge Parameter Unit FW WL 
pH – 4.4 ± 0.1 6.6 ± 0.1 7.5 ± 0.1 Ca mg/L a 3,801 
TS (total solids) g/L 232.0 ± 1.8 57.7 ± 1.4 35.1 ± 0.4 Na mg/L 1,875 1,079 
VS (volatile solids) g/L 217.0 ± 1.5 31.7 ± 1.2 29.3 ± 0.6 mg/L 680.538 648.2 
Lipid g/L 64.9 ± 1.3 – – Mg mg/L 190.506 1,192 
Ammonia-N mg/L 230 ± 10 2,290 ± 130 400 ± 30 Fe mg/L 38.908 206 
Ash %TS 6.5 ± 0.1 – – Zn mg/L 13.033 17.93 
%TS 49.5 ± 0.2 – – Ni mg/L 3.551 2.327 
%TS 7.0 ± 0.1 – – Mn mg/L 3.171 20.24 
%TS 2.4 ± 0.1 – – Cu mg/L 1.858 1.289 
Ob %TS 34.6 ± 0.2 – – Mo mg/L 0.360 2.259 
Acetate mg/L – 7,118 ± 332 n.d. Co mg/L 0.041 0.419 
Propionate mg/L – 3,197 ± 19 n.d. Al mg/L – 35.7 
iso-Butyrate mg/L – 333 ± 15 n.d. Cr mg/L – 1.342 
Butyrate mg/L – 9,085 ± 178 n.d. Pb mg/L – 0.785 
iso-Valerate mg/L – 398 ± 18 n.d. Cd mg/L – 0.052 
Valerate mg/L – 5,346 ± 149 n.d.     
Total VFA mg/L – 25,476 ± 239 –     
Parameter Unit FW WL Seed sludge Parameter Unit FW WL 
pH – 4.4 ± 0.1 6.6 ± 0.1 7.5 ± 0.1 Ca mg/L a 3,801 
TS (total solids) g/L 232.0 ± 1.8 57.7 ± 1.4 35.1 ± 0.4 Na mg/L 1,875 1,079 
VS (volatile solids) g/L 217.0 ± 1.5 31.7 ± 1.2 29.3 ± 0.6 mg/L 680.538 648.2 
Lipid g/L 64.9 ± 1.3 – – Mg mg/L 190.506 1,192 
Ammonia-N mg/L 230 ± 10 2,290 ± 130 400 ± 30 Fe mg/L 38.908 206 
Ash %TS 6.5 ± 0.1 – – Zn mg/L 13.033 17.93 
%TS 49.5 ± 0.2 – – Ni mg/L 3.551 2.327 
%TS 7.0 ± 0.1 – – Mn mg/L 3.171 20.24 
%TS 2.4 ± 0.1 – – Cu mg/L 1.858 1.289 
Ob %TS 34.6 ± 0.2 – – Mo mg/L 0.360 2.259 
Acetate mg/L – 7,118 ± 332 n.d. Co mg/L 0.041 0.419 
Propionate mg/L – 3,197 ± 19 n.d. Al mg/L – 35.7 
iso-Butyrate mg/L – 333 ± 15 n.d. Cr mg/L – 1.342 
Butyrate mg/L – 9,085 ± 178 n.d. Pb mg/L – 0.785 
iso-Valerate mg/L – 398 ± 18 n.d. Cd mg/L – 0.052 
Valerate mg/L – 5,346 ± 149 n.d.     
Total VFA mg/L – 25,476 ± 239 –     

aNot available.

b%O = 100%-(%C + H + N + ash).

n.d., not detectable.

A pH meter (PB-10, Sartorius, Germany) was employed to measure the pH value. The total solids (TS) and volatile solids (VS) were determined using the standard method (APHA 2005). Ammonia-N was analyzed using a spectrophotometer (DR-5000, Hach, USA). A Soxhlet extractor (SXT-06, Shanghai, China) was employed to determine lipid content. The elemental composition (C, H, O, N) was analyzed using an elemental analyzer (Vario EL III, Elementar, Germany). The metal elements were determined with an ion coupled plasma–atomic emission spectrometer (Optima 2000DV, PerkinElmer, USA). The VFA concentration was determined by a gas chromatograph (GC-7900, Techcomp, Shanghai) (Zhang et al. 2015). The composition of biogas was analyzed with a gas chromatograph (GC-7800, Beijing) and the methane volume was calculated according to the change of molar ratio of CH4 or CO2 to N2 (Zhang et al. 2015).

Experimental procedures

The batch experiments were operated in 500-mL Schott Duran bottles with silica gel stoppers at mesophilic temperature (37 °C). Every reactor had a 300-mL effective volume with 225 mL of seed sludge and 75 mL of substrate. The proportions of WL (on VS basis) in feedstock of R1, R2, R3, and R4 were 0%, 5.6%, 11.2%, and 22.1%, respectively. The organic loading of R1–R4 were 20.0 g VS/L, 20.1 g VS/L, 20.3 g VS/L, and 20.5 g VS/L, respectively. The ratio of inoculum to substrate (on VS basis) was 1.1. The control was filled with 225 mL of seed sludge and 75 mL of distilled water for background CH4 production of seed sludge. Nitrogen gas was injected into the bottles for 5 minutes to eject air. The reactors were incubated in a shaking incubator at 150 rpm. The batch experiments lasted for 20 days. Each experiment was carried out in triplicate.

RESULTS AND DISCUSSION

Effect of WL addition on biogas production

Biogas profiles of batch anaerobic co-digestion of FW and WL under different mixture ratios are shown in Figure 1(a)1(c). As shown in Figure 1(a), after a short lag period of 2–3 days, the CH4 yields of R1–R4 gradually increased to the peak on Day 13, 11, 10, and 9, respectively. The long reaction time of batch anaerobic digestion might be an important limiting factor for process efficiency. It should be noted that with the increase of WL addition in R1–R4, it took less time to reach the peak of CH4 yield. Compared to R1, the time to reach the peak of methane yield in R2–R4 was reduced by 1 day, 2 days, and 4 days, respectively. It might be attributed to the characteristics of WL. As shown in Table 1, the pH value (6.6 ± 0.1) and the high VFA concentration (25,476 ± 239 mg/L) of WL both suggested that it was in the acid-fermented stage. Plenty of VFA of WL could be utilized by acetogens and methanogens without hydrolysis and acidogenesis, which might accelerate the anaerobic reaction. The result indicated that WL addition was beneficial to accelerate the start-up and shorten the long reaction time of the batch process.

Figure 1

The process performance of R1–R4: CH4 production rate (a); cumulative CH4 production (b); cumulative biogas production and the average CH4 content (c); VS removal (d); pH (e); VFA in R1 (f); VFA in R2 (g); VFA in R3 (h); VFA in R4 (i).

Figure 1

The process performance of R1–R4: CH4 production rate (a); cumulative CH4 production (b); cumulative biogas production and the average CH4 content (c); VS removal (d); pH (e); VFA in R1 (f); VFA in R2 (g); VFA in R3 (h); VFA in R4 (i).

All anaerobic reactions in R1–R4 showed good performances under the present organic loading condition (20–20.5 g VS/L). As shown in Figure 1(b), after 20 days reaction, the cumulative CH4 yield of mono-digestion of FW in R1 reached a high level of 358 mL/g VSadded. It was attributed to the excellent biodegradability and high methane potential of FW, which was rich in easily biodegradable organic matters including rice, vegetables, meat, and oil. Especially, lipids mainly from meat and cooking oil were reported as attractive substrates for anaerobic digestion owing to its much higher methane potential (1,014 L/kg) than proteins (496 L/kg) and carbohydrates (415 L/kg) (Browne & Murphy 2014). The FW used in this research was rich in lipids, which reached 29.9% of the VS.

Figure 1(b) shows that about 85% of the final methane yield was produced in the initial 15 days and total methane yield increased gradually with the increased proportion of WL from R1 to R4. After 20 days of reaction, the cumulative CH4 yields of R2, R3, and R4 reached 361 mL/g VSadded, 394 mL/g VSadded, and 416 mL/g VSadded, respectively. Compared to R1, the CH4 yield was increased by 1, 10, and 16% by WL addition in R2–R4. The highest methane yield was obtained in R4 where the FW/WL ratio is 77.9:22.1, accepted as the optimum FW/WL ratio. Figure 1(c) clearly shows that WL addition increased the biogas yield and the methane content of mono-digestion of FW. Compared to anaerobic mono-digestion of FW, the cumulative biogas yield was increased from 589 to 646 mL/g VSadded with the addition of WL (5.6%, 11.2%, and 22.1% on VS basis, respectively), and the corresponding CH4 content was enhanced from 60.8% to 64.3%. The results of batch anaerobic co-digestion clearly showed the positive effect of WL addition on biogas production of anaerobic mono-digestion of FW.

Effect of WL addition on organic matter degradation

The VS removals, which indicate the organic matter degradation in R1–R4, are presented in Figure 1(d). It could be observed that the VS removals of R1–R4 increased slowly at the beginning (Day 1–4) and at the end of the experiments (Day 16–20). The greatest enhancement of VS removal was obtained in the middle stage from Day 7 to Day 13. These results were in line with our analytical results that the fastest growth of methane yields in R1–R4 occurred in Day 7–13 since plenty of organics were decomposed and converted to biogas via hydrolysis, acidogenesis, acetogenesis, and methanogenesis in this stage. The rapid degradation of organics led to a great enhancement of methane yield. After 20 days of reaction, the VS removal of R1 reached 72.8%, which indicated that most organic matters were degraded and converted to biogas. The VS removal of FW obtained in this study was lower than the value reported by El-Mashad & Zhang (2010), who obtained the VS removal of 82%. It should be mentioned that the anaerobic experiment of El-Mashad & Zhang (2010) was operated under the organic loading of 2 g VS/L, which was much lower than this study (20 g VS/L). In addition, their experimental time (30 days) was much longer than this study (20 days). These might be the reasons for the different results. Compared to R1, co-digestion of FW and WL in R2, R3, and R4 showed better performances of organic degradation, and the VS removals reached 74.1%, 75.9%, and 77.6%, respectively. These results were in line with our analytical results that the greater organics removals resulted in the higher cumulative methane and biogas yields of R2–R4 compared to R1. The results also confirmed the good anaerobic biodegradability of FW and WL and clearly indicated that WL addition increased the organic degradation of FW. It was also found that the higher VS removals coincided with the greater proportion of WL addition.

Changes of pH and VFA during batch experiments

It is known that organic acids are the main intermediary products in the metabolic pathway of methane fermentation. The concentration of VFA is an essential consideration for good performance of a digester. Moreover, pH was considered as one of the most important parameters, which greatly affects the activity of anaerobic bacteria. Suitable pH value was reported as an indispensable condition for maintaining good performance. Figure 1 shows the pH and VFA during anaerobic digestion.

As is shown in Figure 1(e), the initial pH of R1–R4 was 7.2, 7.3, 7.3, and 7.4, respectively. The more WL addition, the higher the initial pH in digesters. It was attributed to the higher pH of WL (6.6) than that of FW (4.4). The WL maintained relatively high pH in the presence of plenty of VFA. The result showed the high buffering capacity of WL. The extremely low pH of FW might have a negative effect on the anaerobic process. The addition of WL could effectively increase the pH and enhance the buffering capacity of the anaerobic digestion system, thus maintaining process stability. During the initial 2–3 days, the total VFA (TVFA) concentrations of R1–R4 increased rapidly and reached a peak. Meanwhile, the pH of every reactor declined rapidly. It was attributed to the rapid hydrolysis of the complex organic matters to soluble compounds and the acidogenesis of soluble compounds to VFA. However, because the generation time of methanogens was much longer than that of acidogens and the VFA had to be oxidized to carbon dioxide, acetate and hydrogen to become available substrates for the methanogens, the degradation rate of VFA was much lower than its generation rate. Thus, abundant VFA could not be decomposed immediately, and accumulated to a high concentration. Then, the VFA accumulation consumed the alkalinity and reduced the buffering capacity of the anaerobic system, which led to the rapid decline of pH in digesters. During the same period, as mentioned above, every reactor had a short lag phase of CH4 production. Nevertheless, no strong VFA inhibition in R1–R4 was observed, as indicated by the following rapid increase of CH4 production rates from Day 3. As time passed, the VFA concentrations of the four reactors experienced a course of rapid decline (Days 3–10) and slowing down (Days 11–20). Meanwhile, after the initial drop, the pH of R1–R4 gradually increased to above 7.0 due to the consumption of VFA by acetogenesis and methanogenesis. The corresponding CH4 production rates of R1–R4 increased to the peak on Days 9–13 and then gradually declined (Figure 1(a)). The CH4 production rate was associated with the VFA conversion rate. The great CH4 production rate resulted from the rapid degradation of VFA. It should be noted that the pH of R2–R4 was always higher than R1 during the experimental period. After the experiment, the final pH of R1–R4 reached 7.3, 7.4, 7.5, and 7.5, respectively. The high ammonia-N concentration of WL (2,290 ± 130 mg/L) might play an important role in the stabilization of the pH of the anaerobic system, especially for FW, which had a high C/N ratio. The results indicated that WL addition contributed to the high buffering capacity and the suitable pH of the anaerobic system was against the inhibition of VFA accumulation.

Also, it was found that acetate, propionate, and butyrate appeared as the main components of VFA in R1–R4 at the beginning. However, as the reaction continued, the concentrations of propionate became much higher than other acids during Days 10–15. It might be attributed to the much lower degradation rate of propionate than those of acetate and butyrate. Wang et al. (1999) suggested that the VFA decomposition rates were sorted in the following order: butyrate >(acetate, valerate, iso-butyrate) > (propionate, iso-valerate). The decreased concentrations of acetate and butyrate were accompanied by their rapid degradation for CH4 production, but the concentration of propionate remained high. Interestingly, from Day 16, the concentration of propionate gradually declined, but that of acetate increased step by step. The result could be because propionate was oxidized by acetogens into acetate, H2, and CO2, accumulating acetate to a high concentration, which was the desirable substrate for methanogenesis. The WL was rich in VFA, but its addition did not cause VFA inhibition and it also improved the performance of the batch anaerobic digestion of FW.

Table 2 reviews the batch anaerobic co-digestion of FW with other biomass reported in literature as compared to this study, including experimental conditions, results, improvement of co-digestion, and influencing factors (Heo et al. 2003; Kim et al. 2003; Heo et al. 2004; Li et al. 2009; El-Mashad & Zhang 2010; Lin et al. 2012; Li et al. 2013; Ye et al. 2013; Zhang et al. 2013; Zhou et al. 2014). In this study, the methane yield, methane content, and VS removal of R4 were 416 mL/g VSadded, 64.3%, and 77.6%, respectively, relatively high levels. Co-digestion with WL was proved to be a potential method for enhancement of FW anaerobic digestion. As is shown in Table 2, the complementary advantages of anaerobic co-digestion of FW with other biomass reported in literature mainly related to three aspects: improvement of buffering capacity, adjustment of C/N ratio, and micronutrients supplementation, which should therefore be the factors for inducing improvement of co-digestion. In this study, except for high alkalinity and buffering capacity, trace elements from WL might be another important factor. From Table 1, compared to FW, WL contained greater amounts of metal elements, especially trace metal elements (Fe, Co, Mo), which were verified to be indispensable to synthesis of enzyme cofactors during methane production. This feature of WL was connected with the various sources and complex compositions of MSW. Previous researches suggested that FW was deficient in trace elements, and trace metal elements as micronutrients played a significant role in the performance and stability of anaerobic digestion of FW (Banks et al. 2012; Zhang & Jahng 2012). In this study, trace metal element supplementation from WL might also contribute to the improvement of co-digestion.

Table 2

Review of batch anaerobic co-digestion of FW with other biomass as compared to this study

  Experimental condition
 
Result
 
      
Co-substrates Mixture ratio (on VS basis) Ratio of inoculum to substrate (on VS basis) Organic loading (g VS/L) Reaction time (day) T (°C) Methane yield (mL/g VS) Methane content (%) VS destruction (%) Improvement of co-digestion Influencing factor Reference 
FW + WL 77.9:22.1 1.1:1 20.5 20 37 416 64.3 77.6 Increase methane yield and content; accelerate the start-up of process; improve VS removal – This study 
FW + waste activated sludge 90:10 5:1 – 30 37 423 71 – Increase methane yield Increase nutrient concentration Heo et al. (2003)  
FW + waste activated sludge 90:10 5:1 – 40 35 407 – – Increase methane yield C/N ratio Heo et al. (2004)  
FW + pulp and paper sludge 1:1 – – 50–60 37 256 – – Increase methane yield; improve organics removal, a more stable process High buffering capacity; C/N ratio Lin et al. (2012)  
FW + sewage sludge 1:1 – 16 55 280 – – Increase methane yield C/N ratio Kim et al. (2003)  
FW + dairy manure 48:52 – 30 35 311 59 68 Increase methane yield and content; improve VS removal – El-Mashad & Zhang (2010)  
FW + pig manure + rice straw 0.4:1.6:1 – 54 45 37 383.9 56.92 55.76 Increase methane yield and content; improve VS removal C/N ratio; sufficient micronutrients; avoid VFA inhibition Ye et al. (2013)  
FW + cattle manure 2:1 – 12 27 35 388 – – Increase methane yield Enhance buffering capacity; C/N ratio; high biodegradation of lipids; trace element supplementation Zhang et al. (2013)  
FW + cattle manure 1:1 – 20 45 35 310.8  65.8 Increase methane yield; stable performance Synergetic effect Li et al. (2009)  
FW + corn stover – – 45 50 35 311.8 – – Increase methane yield C/N ratio Zhou et al. (2014)  
FW + chicken manure 1:1 1:1.5 28 37 491 – – Increase methane yield Better buffering capacity; C/N ratio; synergistic effect Li et al. (2013)  
  Experimental condition
 
Result
 
      
Co-substrates Mixture ratio (on VS basis) Ratio of inoculum to substrate (on VS basis) Organic loading (g VS/L) Reaction time (day) T (°C) Methane yield (mL/g VS) Methane content (%) VS destruction (%) Improvement of co-digestion Influencing factor Reference 
FW + WL 77.9:22.1 1.1:1 20.5 20 37 416 64.3 77.6 Increase methane yield and content; accelerate the start-up of process; improve VS removal – This study 
FW + waste activated sludge 90:10 5:1 – 30 37 423 71 – Increase methane yield Increase nutrient concentration Heo et al. (2003)  
FW + waste activated sludge 90:10 5:1 – 40 35 407 – – Increase methane yield C/N ratio Heo et al. (2004)  
FW + pulp and paper sludge 1:1 – – 50–60 37 256 – – Increase methane yield; improve organics removal, a more stable process High buffering capacity; C/N ratio Lin et al. (2012)  
FW + sewage sludge 1:1 – 16 55 280 – – Increase methane yield C/N ratio Kim et al. (2003)  
FW + dairy manure 48:52 – 30 35 311 59 68 Increase methane yield and content; improve VS removal – El-Mashad & Zhang (2010)  
FW + pig manure + rice straw 0.4:1.6:1 – 54 45 37 383.9 56.92 55.76 Increase methane yield and content; improve VS removal C/N ratio; sufficient micronutrients; avoid VFA inhibition Ye et al. (2013)  
FW + cattle manure 2:1 – 12 27 35 388 – – Increase methane yield Enhance buffering capacity; C/N ratio; high biodegradation of lipids; trace element supplementation Zhang et al. (2013)  
FW + cattle manure 1:1 – 20 45 35 310.8  65.8 Increase methane yield; stable performance Synergetic effect Li et al. (2009)  
FW + corn stover – – 45 50 35 311.8 – – Increase methane yield C/N ratio Zhou et al. (2014)  
FW + chicken manure 1:1 1:1.5 28 37 491 – – Increase methane yield Better buffering capacity; C/N ratio; synergistic effect Li et al. (2013)  

CONCLUSIONS

The batch anaerobic mono-digestion of FW was enhanced by co-digestion with WL, as indicated by the higher methane yield, higher methane content, more VS destruction, and higher stability. Compared to anaerobic mono-digestion of FW, the cumulative CH4 yield was increased by 1, 10, and 16% with the addition of WL (5.6%, 11.2%, and 22.1% on VS basis, respectively). The corresponding CH4 content was enhanced from 60.8 to 64.3% and the VS removal was improved from 72.8 to 77.6%. Although WL was rich in VFA, its addition did not cause VFA inhibition. It was found that WL addition was beneficial to accelerate the start-up and shorten the long reaction time of the batch anaerobic process. The better performances of batch anaerobic co-digestion coincided with the greater proportion of WL addition. The FW/WL ratio of 77.9:22.1 was the optimum in terms of the highest methane yield (416 mL/g VSadded), the highest methane content (64.3%), the greatest VS removal (77.6%), and stable performance. These results very clearly exhibited the positive effects of WL addition on batch anaerobic digestion of FW.

ACKNOWLEDGEMENTS

This research was supported by the National Natural Science Foundation of China (NSFC) (No. 51208075), National Key Technology R&D Program (No. 2012BAC05B04), and Liaoning Province Education Administration (L2012023).

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