Co-digestion Of Food Waste Research Articles

Dosing effect of nano zero valent iron (NZVI) on the dark hydrogen fermentation performance via lake algae and food waste co-digestion

The effect of nano zero valent iron (NZVI) on hydrogen production by the co-digestion of dry lake algae and food waste was investigated. The results showed that 10 mg.gTS−1 addition of NZVI improved the performance of the co-digestion system, with the cumulative hydrogen increased by 29.20% to 40.04 mL.gVS−1, compared to that of the control. However, the cumulative hydrogen yield decreased to 14.77 and 5.37 mL.gVS−1 when the NZVI dosage was increased to 20 and 40 mg.gTS−1, respectively. The oxidation reduction potential (ORP) of the reaction system was decreased dramatically to -331∼-490 mV at the initial reaction stage. The degradation rates of carbohydrate and protein also reached their peaks at the NZVI dosage of 10 mg.gTS−1. The concentration of end product in the liquid phase and the dehydrogenase activity was first increased then decreased when enhancing the NZVI dosage. Analysis of the transmission electron microscopy images revealed that excessive NZVI dosage could destroy the cells of microorganisms.

Thermophilic co-digestion of food waste and sewage sludge

Due to the rising concerns regarding food waste disposal as well as the increasing demand for renewable energy nowadays, a number of European countries have adopted anaerobic co-digestion, a technology that deals with food waste and sewage sludge. As stated in the HKSAR Government’s Policy Address 2016, the feasibility of using existing sewage treatment facilities for co-digestion of food waste and sewage sludge has been under exploration. A trial scheme has been commenced in 2019. This paper aims to compare the efficiency of biogas production and volatile solid reduction in co-digestion of food waste and sewage with mono-digestion in laboratory scale.

Effects of Initial pH on Two-Stage Biogas Production and Hydrolysis in a Co-Digestion Process of Food Waste and Waste Activated Sludge

Batch tests were performed to investigate the effects of initial pH on the two-stage hydrogen and methane production process by co-digestion of food waste and waste activated sludge. Results show that the two-stage process had good pH adaptive capacity. Hydrogen production was similar at initial pH of 5.5–11.0 with hydrogen yields of 37.3–42.0 mL-H2/g-VSadded, but was inhibited at initial pH of 4.0. The highest hydrogen production rate was obtained at a stable pH of 4.5–5.7. The dominant acidification products at the end of hydrogen production was acetate at initial pH of 4.0 and butyrate at initial pH of 11.0, and the subsequent methane production was likely to be affected by the component of volatile fatty acids (VFA) at the end of hydrogen fermentation. The residuals produced from hydrogen fermentation process at initial pH of 5.5–10.0 had similar methane yields (MY) of 313.6–318.7 mL-CH4/g-VSadded at a fermentation time of 24.8 days. By contrast, methane production in the second stage was enhanced at extreme alkaline initial pH of 11.0 in the first stage, but partly inhibited at extreme acidic initial pH of 4.0. In the test of initial pH of 11.0, the lowest content of VFA and propionate throughout methane production stage were beneficial for MY and methane production rate. The hydrolysis efficiencies were also influenced by the initial pH in hydrogen production stage, which followed the descending order of alkaline, neutral, and acidic condition for hydrogen fermentation and the whole two-stage process, but followed the opposite orders for methane fermentation.

Understanding the fate and impact of capsaicin in anaerobic co-digestion of food waste and waste activated sludge

Anaerobic co-digestion is an attractive option to treat food waste and waste activated sludge, which is increasingly applied in real-world situations. As an active component in Capsicum species being substantially present in food waste in many areas, capsaicin has been recently demonstrated to inhibit the anaerobic co-digestion. However, the interaction between capsaicin and anaerobic co-digestion are still poorly understood. This work therefore aims to deeply understand the fate and impact of capsaicin in the anaerobic co-digestion. Experiment results showed that capsaicin was completely degraded in anaerobic co-digestion by hydroxylation, O-demethylation, dehydrogenation and doubly oxidization, respectively. Although methane was proven to be produced from capsaicin degradation, the increase in capsaicin concentration resulted in decrease in methane yield from the anaerobic co-digestion. With an increase of capsaicin from 2 ± 0.7 to 68 ± 4 mg/g volatile solids (VS), the maximal methane yield decreased from 274.6 ± 9.7 to 188.9 ± 8.4 mL/g VS. The mechanic investigations demonstrated that the presence of capsaicin induced apoptosis, probably by either altering key kinases or decreasing the intracellular NAD+/NADH ratio, which led to significant inhibitions to hydrolysis, acidogenesis, and methanogenesis, especially acetotrophic methanogenesis. Illumina Miseq sequencing analysis exhibited that capsaicin promoted the populations of complex organic degradation microbes such as Escherichia-Shigella and Fonticella but decreased the numbers of anaerobes relevant to hydrolysis, acidogenesis, and methanogenesis such as Bacteroide and Methanobacterium.

Comparison of bio-hydrogen and bio-methane production performance in continuous two-phase anaerobic fermentation system between co-digestion and digestate recirculation

The effect of co-digestion of food waste (FW) and cow dung (CD) with different ratios, and digestate recirculation with different recirculation ratios (RR) on the substrate degradation and energy production in continuous two-stage anaerobic fermentation system was investigated. Results from experiments indicated that co-digestion and digestate recirculation could promote the hydrogen production rate (HPR) and the methane production rate (MPR). Maximum HPR and MPR of 3.3 and 3.1 L/L/d were achieved for two-stage fermentation with recirculation system at RR of 0.4. Meanwhile, both co-digestion and digestate recirculation technology could reduce the amount of alkali addition to maintain pH in the hydrogen-reactor. Compared to digestate recirculation, co-digestion of FW and CD promote much more energy production, 654.9 and 4854.8 kJ/kgVSr were obtained from the co-digestion of FW and CD with the ratio of 2:1 in the hydrogen reactor and the methane reactor.

Anaerobic digestion of food waste at varying operating conditions

Food waste is one of the major problems contributing to the degradation of the environment, and thus needs serious attention. Among different options, anaerobic digestion is possibly the most effective technique for managing degradable waste, and produce renewable energy and fertilizer. Despite multiple-use and benefits of the technology, its application is limited due to a few technical challenges. This study focuses on the anaerobic digestion of food waste with the addition of different percentages of digested cow manure as inoculum to it, at different total solid content in ambient temperature. Anaerobic digestion of food waste in batch and semi-continuous processes were carried out in three different trials at an average temperature range of 20-26℃: Food waste with 20% inoculum, food waste with 50%, 100%, and 200% inoculum and 10% total solid content in batch process and food waste with 20% inoculum with 6% and 10% total solids content in the semi-continuous process. During each trial, some amount of gas production was observed, however, the gas composition showed a negligible amount of methane production (maximum 13% of CH4). There were two common problems detected in each trial: the inability to complete the methanogenesis process, and instability of the overall process due to the high degradability and acidic nature of food waste. Therefore, this study suggests that the mono digestion of food waste is not a suitable option. However, anaerobic co-digestion of food waste with different organic substrate might provide a favorable condition for stable anaerobic digestion as seen from preliminary results.

Effect of Mixture Ratio of Food Waste and Vetiver Grass on Biogas Production

The objectives of this research were to study the optimum percentage ratios of food waste and vetiver grass (Vetiveria zizaniodes (L.) Nash) for biogas production and process stability in anaerobic digestion system. The nine mixture ratios of food waste (FM) and vetiver grass (VG) were 100:0, 80:20, 75:25, 66.67:33.33, 50:50, 33.33:66.67, 25:75, 20:80 and 0:100. The biogas production was subjected to anaerobic batch with working volume of 1.8 L and had digestion time 60 d at 35 ± 2 °C. High specific methane yield of 0.30 L g-1 VS removed was obtained from the fermentation at ratio of FW:VG at 80:20 with C/N ratio of 28.20. The specific methane yield of the single digestion of food waste (100:0) and single digestion of vetiver grass (0:100) were only 0.18 and 0.11 L g-1 VS removed, respectively. It was found that the cumulative methane production of FW: VG ratio of 80:20 was increased 34.89 % and 96.93 % compared to single substrate of food waste and vetiver grass, respectively. The results also showed the highest COD, VS and TS removal with a percentage ratio of FW: VG at 80:20, while the single vetiver grass digestion was the lowest COD, VS and TS removal. VFAs/Total alkalinity ratio of all ratio of food waste to vetiver at digestion time 40 d remained in 0.0895±0.0007 to 0.1944±0.0027 were steadied for this digester. It can be concluded that co-digestion of food waste and vetiver improve the biogas yield and degradation efficiency.

Advanced methanogenic performance and fouling mechanism investigation of a high-solid anaerobic membrane bioreactor (AnMBR) for the co-digestion of food waste and sewage sludge

Disposal of the increasingly huge amounts of sewage sludge (SeS) has become an impending problem worldwide. To solve this problem, a high-solid anaerobic membrane bioreactor (AnMBR) was used for the anaerobic co-digestion (AcoD) of SeS and food waste (FW). This study investigated the effects of SeS ratio on the methanogenic performance of the AcoD with a gradual increase value from 0 to 25%, 50%, 75% and 100% (total solids based). The results showed that the highest methanogenic performance was achieved at mono FW digestion. As for the co-digestion, the optimal FW/SeS ratio for methanogenic performance was 75%:25% among all the mixing ratios. The COD based biogas yield and methane yield were 0.498 L-biogas/g-CODfed and 0.295 L-CH4/g-CODfed at this optimal mixing ratio, which were 67.7% and 67.6% higher than those of the mono SeS digestion, respectively. The upgraded values were attributed to the improved hydrolysis ratio (by 8.14%) and the balanced carbon-to-nitrogen (C/N) ratio by co-digestion with FW, which synergistically stimulated methanogenesis ratio by 81.0%. The continuous membrane filtration property was investigated and four typical trans-membrane pressure (TMP) variation curves at different fouling degrees were determined. The membrane could sustainably operate at a flux of 6 L/m2/h (LMH) at the mixed liquor total solids (MLTS) concentration of 25 - 30 g/L. The combination of continuous membrane filtration property, particle size distribution of the mixed liquor and the external forces analysis was firstly applied to unravel the membrane fouling mechanism of a high-solid AnMBR. The result of this study will contribute to the establishment of an efficient FW and SeS treatment strategy.

Methane Gas Production from different composition of Food Waste and Chicken Manure

High carbohydrate content in food waste is one of the characteristic suitable acted as a co-substrate for fermentation to produce methane gas. In this study, co-digestion of chicken manure (CM) and food waste (FW) was used for fermentation process in methane gas production. The different effects of the independent variables (ratio, pH and temperature) is the most significant parameters of methane gas fermentation of CM and FW were investigated. Based on the analytical study showed a good fit between the experimental and the predicted data as the R2 values of 0.991 with methane yield to be 537 mL CH4/g VS at ratio 80:20 (CM:FW); temperature 35 °C; and initial pH 7.11 in 20 days of fermentation. The experiment was then performed based on optimized parameters of pH, temperature and ratio at the highest yield of methane obtained from optimum parameters. After optimization, the result showed that the pH 7.11 at 35 °C temperature and the ratio of 80:20 with methane production yield of 1560.5 mL CH4/g VS for the 21 days of digestion process.

Effect of calcium peroxide and sodium hydroxide on hydrogen and methane generation during the co-digestion of food waste and cow dung

Anaerobic co-digestion of food waste, cow dung, and sludge solution was experimentally evaluated to produce hydrogen and methane considering the effect of calcium peroxide (CaO2) and sodium hydroxide (NaOH) addition with the feedstock. The various substrate to inoculum ratios (v/v) of 1:1(S1), 1:2(S2), 1:3(S3), 1:4(S4), and 1:5(S5) were evaluated in separate fermentative and methanogenic reactors. The optimum hydrogen and methane production was observed in S3 compared to the other samples. The biogas generation was increased up to 17% with an enhancement of methane concentration up to 10% by using NaOH compared to CaO2. The cumulative methane yield was increased by 12% due to the addition of NaOH as compared to CaO2. The maximum hydrogen concentration was improved by 7% with a substantial enhancement of cumulative hydrogen yield up to 37% by adding CaO2 with the feedstock as compared to NaOH. It was evident from the experiment that the production of hydrogen was significantly enhanced by adding CaO2 while the methane generation was influenced by the addition of NaOH during the co-digestion.

Biogas production by co-digestion of municipal wastewater and food waste: Performance in semi-continuous and continuous operation.

Municipal wastewater has recently attracted interest in relation to anaerobic wastewater treatments. Biogas production with co-digestion of mixed substrates has been proposed and has many potential benefits for municipal wastewater. The aim of this research was to assess the performance of biogas production during co-digestion of municipal wastewater and food waste under semi-continuous and continuous operation with various hydraulic retention times (HRTs). A laboratory-scale continuous stirred tank reactor (CSTR) at room temperature operation (27-32°C) was employed in this research. The highest methane yields of 167.41±66.52, 194.35±85.44, and 214.81±85.44ml/g-Vs were found in semi-continuous mode, respectively, at 30, 10, and 10days of HRTs with a 10:90 ratio of municipal wastewater to food waste (based on TS). Result finding of optimum condition (10days of HRTs) presented a methane yield of 485.58±82.35ml/g with continuous operation. Hence, food waste has practical implications for use as a co-substrate with the optimization condition of HRT and operation mode for biogas production from municipal wastewater. PRACTITIONER POINTS: Food waste has good potential for use as a co-substrate for biogas production from municipal wastewater. HRT reduction from 30 to 10days in semi-continuous, biogas production from municipal wastewater and food waste increased by 59%. Co-digestion of municipal wastewater and food waste with continuous mode and 10days of HRT was the effective biogas production.

Effects of long-term acclimatization on the optimum substrate mixture ratio and substrate to inoculum ratio in anaerobic codigestion of food waste and cow manure

The effects of long-term acclimatization on the optimum food waste to cow manure ratio (FW/CM) and substrate to inoculum ratio (S/I) in anaerobic codigestion with FW and CM were investigated by batch trials. For the unacclimated sludge, the highest CH4 yields of 646.6 and 653.4 mL/g VS were achieved under the optimum FW/CM (2.5 VS/VS) and S/I (0.07 VS/VS) ratios, respectively. After more than 550 days of acclimatization, the optimum FW/CM and S/I of the acclimated sludge were improved to 3.4 and 0.68 VS/VS with more anaerobic digestion enzymes and lignocellulose, respectively. Based on high-throughput sequencing analysis, the microbial community structures of bacteria, fungi, and archaea were changed, which was the main reason for the change in the optimum FW/CM and S/I. Therefore, the FW/CM and S/I should be periodically optimized during the long-term operation of codigestion to improve the codigestion efficiency for biogas production.

Highly efficient anaerobic co-digestion of food waste and horticultural waste using a three-stage thermophilic bioreactor: Performance evaluation, microbial community analysis, and energy balance assessment

The thermophilic anaerobic co-digestion of food waste and horticultural waste with a three-stage bioreactor was investigated to produce renewable energy, focusing on performance evaluation, microbial community analysis, and energy balance assessment. The results of this study indicated that the average methane yield (0.42 ± 0.06 L/gVS) in the bench-scale three-stage thermophilic bioreactor was approximately 45% and 31% higher than that of the one and two-stage bioreactors, respectively. The average volatile solid (VS) reduction achieved in the bench-scale three-stage thermophilic bioreactor was 63.0 ± 8.2%, which was 61% and 32% higher than that of the one and two-stage bioreactors, respectively. Pyrosequencing analysis of microbial communities in simulated thermophilic three-stage co-digestion experiments showed that bacterial phyla Firmicutes, Thermotogae and Bacteroidetes dominated in all three types of bioreactors. However, significant differences were observed at the genus level of bacterial communities between the one, two and three-stage bioreactors. Indeed, due to selective enrichment, the dominant methanogenic pathways in the three-stage bioreactor shifted from hydrogenotrophic methanogenesis at a low organic loading rate (OLR) to a balance of hydrogenotrophic and acetoclastic pathways at high OLR. Furthermore, assessments of the energy balance and the economic viability of the bench-scale system demonstrated that the three-stage thermophilic co-digestion could be a promising process for the treatment of food waste and horticultural waste as well as for bioenergy recovery at an industrial scale.

Performance of Anaerobic Membrane Bioreactors for the Co-digestion of Sewage Sludge and Food Waste

A laboratory-scale anaerobic membrane bioreactor (AnMBR) was used for the co-digestion of sewage sludge and food waste to investigate its organic matter removal characteristics, biogas production performance, and microbial community composition. The results showed that the degradation rate of volatile solids (VS) increased from 17.5% for a single digestion to 40% for the total digestion, and that the COD removal was 95.3% when the organic loading rate (OLR) was stabilized at 0.59-0.64 kg·(m3·d)-1. The solids content of the digested sludge increased by a factor of 3.9. The final CH4 content was 60% and the CH4 yield was 78.7 mL·g-1 of CODadded. The transmembrane pressure (TMP) and average flux were maintained at between -3.1 and -2.7 kPa and 0.106 L·(m2·h)-1, respectively, and membrane fouling was not serious. According to an analysis of the microbial diversity using 16S rRNA, the anaerobic bacterium in the AnMBR were mainly phylum Proteobacteria, Bacteroidetes, and Cloacimonetes, and the dominant methanogens included the Methanobacterium family, Methanosaeta genus, and Methanolinea genus. This study provides a strong theoretical basis for research into the stability and performance of AnMBRs for the co-treatment of sludge and other high-solid waste streams, and provided an effective solution for biomass resource utilization and the energy crisis.

Batch anaerobic co-digestion of food waste and sludge: a multi criteria decision modelling (MCDM) approach

The present study investigates the application of Multi Criteria Decision Modelling (MCDM) on batch anaerobic co-digestion of kitchen food waste (FW) and the fresh septic tank sludge (STS) for robust output. The batch experiment was carried out at a temperature of 37 ± 2 °C. Different ratio of FW and STS (FW: STS:100:0; 75:25; 60:40; 50:50; 25:75; 0:100) were adopted based on volatile solid (VS) basis as co digestion strategy for 90 days digestion period. Results revealed that experimentally, mono-digestion of FW showed maximum biogas yield (544 ± 65 mL/g VS) followed by blending ratio of 75:25 (FW: STS). However, mono-digestion of STS showed only 150 ± 15 mL/g VS of biogas yield and also showed negative effect on co digestion with FW for every blending on VS basis as STS was high in minerals. MCDM revealed that the FW is the best possible alternative amongst the other blends with STS considering pH, chemical oxygen demand, VS reduction, alkalinity, C/N and biogas yield as output.

Optimization of Cattle Manure and Food Waste Co-Digestion for Biohydrogen Production in a Mesophilic Semi-Continuous Process

Biohydrogen production from organic solid waste has shown particular advantages over other methods owing to the combination of waste reduction and bioenergy production. In this study, biohydrogen production from the co-digestion of cattle manure and food waste was optimized in a mesophilic semi-continuous process. To maximize hydrogen production, the effects of the mixing ratio (the proportion of food waste in the substrate), substrate concentration, and hydraulic retention time (HRT) on the co-digestion were systematically analyzed using a Box–Behnken design. The results showed that strong interactive effects existed between the three factors, and they had a direct effect on the responses. Hydrogen was primarily produced via the butyrate pathway, which was accompanied by the competing heterolactic fermentation pathway. Propionate and valerate produced from lipids and proteins, respectively, were obtained along with butyrate. The optimal process parameters included a mixing ratio of 47% to 51%, a substrate concentration of 76 to 86 g L−1, and an HRT of 2 d. Under these optimal conditions, the hydrogen production rate and hydrogen yield were higher than 1.00 L L−1 d−1 and 30.00 mL g−1 VS, respectively, and the predicted results were consistent with the experimental data. The results indicate that the co-digestion of cattle manure and food waste is a practical and economically promising approach for biohydrogen production.

Biogas yield assessment from the anaerobic co-digestion of food waste and cymbopogon citratus

This study, aimed at maximizing the biogas yield from the co-digestion of food waste (FW) and cymbopogon citratus (CC) was done in reactors (Rs) I–V at temperature of 36 ± 2 °C. Rs I, II, III, IV and V with respective FW: CC of 100:0; 80:20; 60:40; 40:60; 0:100 had mean biogas yield of 0.41 ± 0.02, 0.68 ± 0.05, 0.49 ± 0.02, 0.29 ± 0.01, and 0.22 ± 0.03 L/gVS, respectively. From the modeling result, while R I had the least latency (λ) of 3.9, RV had the highest value of 11.2 days. RII also had a reasonably low latency (λ) of 5.2 days. Top maximum specific biogas yield (Rm) of 0.48 L/gVS/day and maximum biogas production potential (A) of 19.9 L/gVS were obtained from RII. The high R2 values of 0.99–0.89 obtained from the simulation analysis showed a good fit with the modified Gompertz model. The study has revealed that FW: CC of 80: 20% could be most appropriate for optimal biogas production.

Optimization of methane production parameters during anaerobic co-digestion of food waste and garden waste

Anaerobic digestion is an alternative process to organic waste treatment, which enables energy production from biogas. Additionally, the volume of total waste that goes to landfills decreases, resulting in an increase in landfills lifespan. When considering different organic waste sources, food waste is highly important as it is generated and usually disposed of in large amounts. In this context, the goal of this research was to evaluate to which extent the addition of lignocellulosic waste (also known as garden waste – GW) influences the performance of food waste (FW) anaerobic digestion, focusing on process optimization as a function of the quantity and quality of the produced biogas. The experiment was conducted in two pilot-scale reactors. They were operated at the same time and contained a volume of 500 L each. Both reactors were adapted with agitators (30 rpm) and temperature controllers (36 °C). The reactor’s feeding was performed in a semi-continuous process with a gradual increase in the Organic Loading Rate (OLR), from 0.24 to 0.54 kg VS m−3d−1. Operational control parameters, such as pH, alkalinity, VFA, and process performance, characterized by volatile solids, biogas production, and methane content, were carried out to monitor the anaerobic co-digestion. The results showed that a substitution of 20% of the food waste OLR to the lignocellulosic substrate improved both the biogas production and specific methane yield. Also, the organic material conversion efficiency, related to volatile solids (VS), reached 83%. These results indicate that the FW and GW co-digestion can be used as an alternative to organic waste treatment. The process also assists in the control of the monitoring parameters, such as pH, alkalinity and volatile fatty acids, during anaerobic digestion.

Impact of total solids content on anaerobic co-digestion of pig manure and food waste: Insights into shifting of the methanogenic pathway

Dry anaerobic digestion (AD) has advantages over wet AD in treating high-solid organic wastes like livestock and food wastes, but an elevated total solids (TS) content would affect the AD performances. In this study, methane production of digesters co-digesting pig manure (PM) and food waste (FW) at different TS contents (R1, TS 5%; R2, TS 10%; R3, TS 15%; and R4, TS 20%) was assessed. The results showed the specific methane yield had no significant difference with the increase of TS contents from 5% to 15% (278.8–291.7 NmL/g VSadded), while it was reduced at a 20% TS content (259.8 NmL/g VSadded). Two peaks of total volatile fatty acids and daily methane production were observed in the high-solid digesters (R2-R4), while only one peak occurred in wet AD (R1). A new kinetics model was developed to describe the two-peak methane production behavior at high TS contents. The analysis on the microbial community structure clearly showed the different evolutions of methanogenic pathways in low and high solids content systems. In dry AD (R4), there was a general shifting from the acetoclastic pathway, to mixotrophic pathway and hydrogenotrophic pathway, with the dominance of mixotrophic and hydrogenotrophic methanogens.

The effect of microwave pretreatment on anaerobic co-digestion of sludge and food waste: Performance, kinetics and energy recovery

This paper studied the effect of microwave (MW) pretreatment on anaerobic co-digestion of sludge (SS) and food waste (FW). Using SS and FW as digestive substrates, the MW pretreatment method was used to determine the changes in the substrate matrix by means of batch anaerobic digestion at 37 °C. The kinetics of methane production were calculated, and the changes in organic matter during anaerobic co-digestion, the properties of the anaerobic-digested effluent, and the net energy output of the co-digestion system were determinated. The results showed that MW pretreatment was beneficial to the dissolution of organic matter, conversion of protein to NH4+-N, cumulative methane production, unit biomethane yield, and reaction rate of methane production in the SS and FW anaerobic co-digestion system. The highest cumulative methane production in the co-digestion system reached 3446.3 ± 172.3 mL (35 days), which was 19.93% higher than that of the control. Furthermore, MW pretreatment significantly increased the accumulation of VFAs and the content of butyric acid in the anaerobic-digested effluent, which was beneficial to the methanogenesis process. The MW pretreatment of all co-digested substrates produced a greater net energy output than the control, and the MW-SS + MW-FW group yielded the highest net energy output, which was 76.25 kJ/g Fed VS. The results indicated that MW pretreatment prior to SS and FW anaerobic co-digestion is an effective way to improve the anaerobic digestion efficiency and energy recovery rate.