Thermophilic Methane Research Articles

Enhancing thermophilic methane production from oil palm empty fruit bunches through various pretreatment methods: A comparative study

This study investigated the effects of various pretreatment methods on the anaerobic digestibility of oil palm empty fruit bunches (EFB) for methane production. Pretreatment methods included weak alkaline (2 % Ca(OH)2), weak acid (2 % acetic acid), acidified palm oil mill effluent (POME), biogas effluent, hydrothermal (180 °C, 190 °C, and 200 °C), and microwave pretreatments. All pretreatment methods enhanced methane yield compared to untreated EFB (189.45 mL-CH4/g-VS), with weak alkaline pretreatment being the most effective (277.11 mL-CH4/g-VS), followed by hydrothermal pretreatment at 180 °C (244.33 mL-CH4/g-VS) and biogas effluent pretreatment (238.32 mL-CH4/g-VS). The enhanced methane yield was attributed to increased cellulose content (45.5 % for weak alkaline pretreatment), reduced hemicellulose (18.0 % for hydrothermal pretreatment at 200 °C), and lignin contents (19.0 % for hydrothermal pretreatment at 200 °C), decreased crystallinity index (40.0 % for hydrothermal pretreatment at 200 °C), and increased surface area. Weak alkaline pretreatment also showed the highest net energy balance (8.73 kJ/g-VS) and a short break-even point (2 years). Microbial community analysis revealed that weak alkaline pretreatment favored the growth of syntrophic acetate-oxidizing bacteria and hydrogenotrophic methanogens, contributing to improved methane yield. This study demonstrates the potential of EFB pretreatment, particularly weak alkaline and biogas effluent pretreatment, for enhancing methane production and sustainable management of palm oil mill waste.

Thermophilic methane oxidation is widespread in Aotearoa-New Zealand geothermal fields.

Geothermal areas represent substantial point sources for greenhouse gas emissions such as methane. While it is known that methanotrophic microorganisms act as a biofilter, decreasing the efflux of methane in most soils to the atmosphere, the diversity and the extent to which methane is consumed by thermophilic microorganisms in geothermal ecosystems has not been widely explored. To determine the extent of biologically mediated methane oxidation at elevated temperatures, we set up 57 microcosms using soils from 14 Aotearoa-New Zealand geothermal fields and show that moderately thermophilic (>40°C) and thermophilic (>60°C) methane oxidation is common across the region. Methane oxidation was detected in 54% (n = 31) of the geothermal soil microcosms tested at temperatures up to 75°C (pH 1.5-8.1), with oxidation rates ranging from 0.5 to 17.4 μmol g-1 d-1 wet weight. The abundance of known aerobic methanotrophs (up to 60.7% Methylacidiphilum and 11.2% Methylothermus) and putative anaerobic methanotrophs (up to 76.7% Bathyarchaeota) provides some explanation for the rapid rates of methane oxidation observed in microcosms. However, not all methane oxidation was attributable to known taxa; in some methane-consuming microcosms we detected methanotroph taxa in conditions outside of their known temperature range for growth, and in other examples, we observed methane oxidation in the absence of known methanotrophs through 16S rRNA gene sequencing. Both of these observations suggest unidentified methane oxidizing microorganisms or undescribed methanotrophic syntrophic associations may also be present. Subsequent enrichment cultures from microcosms yielded communities not predicted by the original diversity studies and showed rates inconsistent with microcosms (≤24.5 μmol d-1), highlighting difficulties in culturing representative thermophilic methanotrophs. Finally, to determine the active methane oxidation processes, we attempted to elucidate metabolic pathways from two enrichment cultures actively oxidizing methane using metatranscriptomics. The most highly expressed genes in both enrichments (methane monooxygenases, methanol dehydrogenases and PqqA precursor peptides) were related to methanotrophs from Methylococcaceae, Methylocystaceae and Methylothermaceae. This is the first example of using metatranscriptomics to investigate methanotrophs from geothermal environments and gives insight into the metabolic pathways involved in thermophilic methanotrophy.

Biogas Production and Energy Balance in a Two-Stage Anaerobic Digestion of Fruit and Vegetable Waste: Thermophilic versus Mesophilic

This study aimed to investigate biogas production and energy balance in a two-stage anaerobic digestion system of fruit/vegetable waste under thermophilic and mesophilic conditions. Firstly, the feedstock was hydrolyzed and acidified in an acidic reactor at 37 °C with a retention time of 5 d. Then, the liquid hydrolysate was collected and pumped into an up-flow methane reactor under a mesophilic temperature with a retention time of 5 d and a thermophilic condition with a retention time of 3 d. The experimental results showed that in the thermophilic methane reactor, the COD removal, biogas yield, and methane concentration were 96.3%, 492 mL/g-VS, and 70.4%, respectively. These values were 3%, 10%, and 3% higher, respectively, than those obtained in the mesophilic methane reactor. In terms of energy, the mesophilic and thermophilic methane reactors consumed the same thermal energy demand for temperature control. They were much lower than the heat values produced by the power engine. The two-stage anaerobic digestion system using a thermophilic methane reactor obtained a gross energy of 11.20 kJ/g-VS and a net energy of 9.83 kJ/g-VS. These values were 13.2% and 14.8% higher, respectively, than those obtained by the system with a mesophilic condition. Moreover, the use of a thermophilic reactor helped reduce the reactor volume by 40%, leading to significant investment cost savings.

PH buffering additives mitigate the inhibition of thermophilic dry methane fermentation

pH buffering additives mitigate the inhibition of thermophilic dry methane fermentation

Thermophilic biodigestion of fermented sugarcane molasses in high-rate structured-bed reactors: Alkalinization strategies define the operating limits

Energetic exploitation of sugarcane-derived byproducts via anaerobic digestion (AD) has recently been highlighted as an alternative to the conventional ethanol-producing approach. In this context, the thermophilic (55 °C) methane production from fermented molasses was assessed in the long-term (>200 d) continuous operation of two anaerobic structured-bed reactors. Two types of alkalinization strategies, namely, NaHCO3 dosing (RM1) and NaOH dosing coupled to effluent recirculation (RM2) were compared, as well as different levels of organic loading rate (OLR) and hydraulic retention time (HRT) were applied to reach limiting operating conditions. Both alkalinization strategies provided an equivalent buffer control up to an OLR of 7.5 kg-CODt m−3 d−1, from which unwanted accumulation of organic acids was observed in RM2. NaHCO3 dosing enabled doubling the OLR (15.0 kg-CODt m−3 d−1) without performance deterioration, i.e., organic matter removal > 80.0% and maximum methane yield values. A detailed assessment of the food-to-microorganism ratio demonstrated the maintenance of stable operating conditions at excess substrate availability (>3.0 g-CODt g−1VSS d−1) in the feeding chamber (FDC) of the reactors, in which substrate conversion exceeded 70.0%. Organic overloads occurred only when biomass retention reached a saturation level in the FDC, hampering the uptake of volatile organic acids, regardless of the operating condition. In any case, the wide range of operating conditions (OLR = 2.5–15.0 kg-CODt m−3 d−1 and HRT = 24.0–36.0 h) associated with maximized methane production characterized a highly flexible process, which enables varied design arrangements in full-scale AD plants.

Improvement of coffee grounds high solid thermophilic methane fermentation by co-digestion with in-situ produced waste activated sludge: Performance and stability

The feasibility of in-situ stabilization in the co-digestion of coffee grounds (CG) and waste activated sludge (WAS) was investigated. Two lab-scale thermophilic continuous stirred tank reactors (CSTR), R1 and R2 were operated with substrates that contained different WAS ratios, S1 (WAS% = 20%) and S2 (WAS% = 30%). During the whole process, there was no external supply of ammonia and trace elements. The volatile solid (VS) removal efficiency of R1 and R2 was comparable, and the biogas yield of R1 (0.467 ± 0.100 L/g-VSin) was slightly higher than R2 (0.408 ± 0.020 L/g-VSin). The total ammonia nitrogen (TAN) of R1 and R2 was 482 ± 32 and 884 ± 24 mg/L, respectively. The stoichiometry formulas of co-digestion were established to calculate the theoretical microbial yield coefficients and the requirements of microorganism reproduction. A comparison between the theoretical requirements and experimental values showed that co-digestion with WAS could avoid supply for an external supply of minerals. For the net energy production, R1 and R2 could generate 6342 and 5069 kWh of electricity daily, respectively.

Batch-Mode Analysis of Thermophilic Methanogenic Microbial Community Changes in the Overacidification Stage in Beverage Waste Treatment.

Biogasification by methane fermentation is an important and effective way to utilize beverage wastes. Beverage wastes are good feedstocks for methane fermentation because of their richness in sugars and proteins, although overacidification and inhibition of methane production caused by high substrate loading often become problematic. This study investigated changes in microbial communities in the overacidification state of the thermophilic methane fermentation process with beverage waste by establishing a simulated batch culture. We assessed 20 mL-scale batch cultures using a simulant beverage waste mixture (SBWM) with different amounts of addition; high cumulative methane production was achieved by adding 5 mL of SBWM (11358 mg—chemical oxygen demand—COD/L of organic loading), and overacidification was observed by adding 10 mL of SBWM (22715 mg—COD/L of organic loading). The results of 16S rRNA amplicon sequence analysis using nanopore sequencer suggested that Coprothermobacter proteolyticus, Defluviitoga tunisiensis, Acetomicrobium mobile, and Thermosediminibacter oceani were predominantly involved in hydrolysis/acidogenesis/acetogenesis processes, whereas Methanothrix soehngenii was the major acetotrophic methane producer. A comparison of microbial population between the methane-producing cultures and overacidification cultures revealed characteristic population changes especially in some minor species under 0.2% of population. We concluded that careful monitoring of population changes of the minor species is a potential indicator for prediction of overacidification.

Thermophilic Methane Production from Hydrothermally Pretreated Norway Spruce (Picea abies)

Norway spruce (Picea abies) is an industrially important softwood species available in northern Europe and can be used to produce bio-methane after proper pretreatment to overcome its recalcitrant complex structure. Hot water extraction (HWE) pretreatment at two different conditions (170 °C for 90 min (severity 4.02) and 140 °C for 300 min (severity 3.65)) was applied to extract hemicellulosic sugars from Norway spruce for thermophilic anaerobic digestion (AD) of the hydrolysate. The methane yield of hydrolysate prepared at the lower pretreatment severity was found to be 189 NmL/gCOD compared to 162 NmL/gCOD after the higher pretreatment severity suggesting higher pretreatment severity hampers the methane yield due to the presence of inhibitors formed due to sugars and lignin degradation and soluble lignin, extracted partially along with hemicellulosic sugars. Synthetic hydrolysates simulating real hydrolysates (H170syn and H140syn) had improved methane yield of 285 NmL/gCOD and 295 NmL/gCOD, respectively in the absence of both the inhibitors and soluble lignin. An effect of organic loadings (OLs) on the methane yield was observed with a negative correlation between OL and methane yield. The maximum methane yield was 290 NmL/gCOD for hydrolysate pretreated at 140 °C compared to 195 NmL/gCOD for hydrolyate pretreated at 170 °C, both at the lowest OL of 6 gCOD/L. Therefore, both pretreatment conditions and OL need to be considered for efficient methane production from extracted hydrolysate. Such substrates can be utilized in continuous flow industrial AD with well-adapted cultures with stable organic loading rates.

Acclimation Improves Methane Production from Molasses Wastewater with High Salinity in an Upflow Anaerobic Filter Reactor: Performance and Microbial Community Dynamics.

This study evaluated the performance of an upflow anaerobic filter (UAF) reactor in the thermophilic methane fermentation of hypersaline molasses wastewater. The high salinity (~ 45mS/cm) of the undiluted wastewater completely inhibited the biogas production. An acclimation strategy involving gradient dilution of the molasses wastewater was implemented to gradually increase the salt stress. Consequently, the biogas production was recovered, inhibited only slightly by the high salinity of the undiluted wastewater. The reactor steadily achieved a high total organic carbon (TOC) loading rate of 5g/L/day, with approximately 60% TOC removal efficiency. Acclimation to the gradually increased salt stress leads to a relative abundance of some halotolerant microbes, such as bacteria from Arcobacter, Tissierella, and Ruminococcaceae, which increased as their hydrolytic and acidogenic abilities adjusted to the incremental increase in salinity. Additionally, hydrogenotrophic methanogens, especially Methanoculleus, showed greater resistance to hypersalinity than aceticlastic methanogens. These results suggest that acclimation of the fermentation microbial community to hypersalinity is an effective strategy to improve methane production from hypersaline molasses wastewater in thermophilic UAF reactors.

Comparison of mesophilic and thermophilic methane production potential of acids rich and high-strength landfill leachate at different initial organic loadings and food to inoculum ratios

Landfill leachate (LL), which can contaminate both ground and surface water is a major global environmental issue. The aim of the present study was to investigate the biomethane potential (BMP) of a high-strength LL with low pH (5.0), high solids concentration (16%), and high organic matter (170 g/L of chemical oxygen demand (COD); 55 g/L of volatile fatty acids (VFA)) with ammonia nitrogen (NH3-N) (17 g/L). We investigated the BMP of LL at four different initial organic loadings (IOL) of 170 g/L, 85 g/L, 42.5 g/L and 21 g/L of COD and Food to inoculum (F/I) ratios of 0.5; 1; 2 and 3 at mesophilic (35 ± 2 °C) and thermophilic temperatures (55 ± 2 °C).We found that the highest cumulative CH4 could be obtained at an IOL of 42.5 g/L of COD regardless of the F/I ratio and temperature. The highest methane content results in biogas at an IOL of 42.5 g/L were 72% and 74% at mesophilic and thermophilic temperatures respectively. About 80–100% of cumulative methane was produced within 15 days in thermophilic reactors, and 40–72% in mesophilic reactors. The kinetic study revealed a fourfold reduction of lag phase in thermophilic compared to mesophilic reactors. The methane yield and organic matter removal rate increased as the concentration of IOL in LL decreased from 170 g/L to 21 g/L regardless of temperature. There exists an inverse correlation between IOL and organic matter removal efficiency. About 80% COD reduction was obtained at mesophilic temperature, and 90% at thermophilic temperature, at an IOL of 42.5 g/L and 21 g/L of COD. The modified Gompertz model showed a good fit to the experimental data, with R2 > 0.98 in all cases. Overall, the findings of this study conclude that treatment of acids rich and high-strength LL both at mesophilic and thermophilic temperature is feasible at an optimum IOL of 42.5 g/L of COD. However, treatment of LL at thermophilic temperature outperformed compared to mesophilic over the digestion time.

Biodegradability and methane fermentability of polylactic acid by thermophilic methane fermentation

Polylactic acid (PLA) is considered to be a promising polymer to replace petroleum-based plastics. The aim of this study was to evaluate the biological decomposability of PLA by thermophilic methane fermentation (TMF) and to analyse the microorganisms involved in PLA decomposition. Small- and large-scale batch tests were carried out using disposable cutlery as PLA products and pure poly L-lactic acid (PLLA) pellets. It was found that 70–77% of PLA was decomposed by TMF, based on weight changes. The yield of CH4 was 321–343 mL CH4/g PLA consumed. The addition of bacteria doubled the physicochemical degradation rate of PLA. It was determined that PLA decomposition by TMF was efficient, and that it was enhanced by microbial activity. In particular, lactic acid-consuming bacteria were found to play an important role in PLA decomposition by TMF. Thus, we can conclude that TMF is potentially suitable for PLA treatment and biofuel generation.

Effect of organic loading rate on thermophilic methane fermentation of stillage eluted from ethanol fermentation of waste paper and kitchen waste

Thermophilic methane fermentation was a valid approach for handling the stillage eluted from ethanol fermentation of waste paper and kitchen waste. The wide organic loading rate (OLR) range (2-14g VTS/(L d)) for stable performance and relatively high energy recovery efficiency (79.0%) were achieved, and OLR of 8g VTS/(L d) was optimum for achievement of highest biogas evolution and VTS removal efficiency. Microbial community analysis revealed that hydrolysis of cellulose was the critical step for methane production from the stillage.

Biogas Recovery from Hyper-Thermophilic Anaerobic Co-Digestion of Thickened Waste Activated Sludge, Organic Fraction of Municipal Solid Waste and Fat, Oil and Grease

The use of organic fraction of municipal solid waste and Fat Oil and Grease (FOG) as co-substrates for thickened waste activated sludge anaerobic digestion has the potential to improve the biodegradation process and significantly enhance biogas production and methane yields. This will not only help convert these potential waste streams from landfills increasing the longevity of existing landfills, but also provide a sustainable waste to energy waste management method. In this study the anaerobic co-digestion of organic fraction of municipal solid waste, with thickened waste activated sludge (50:50, w/w based on total volatile solids) was investigated using anaerobic digestion thermophilic and hyper-thermophilic biochemical methane potential (BMP) assays. The hyper-thermophilic BMP assays outperformed the thermophilic BMP assays by providing faster biogas production rates, higher cumulative biogas productions and methane yields. Additionally, 10, 20 and 30% FOG (based on total volatile solids) were added to the co-digestion mixtures in order to boost the biogas production and methane yield in three hyperthermophilic assays. 30% FOG in the co-digestion mixture enhanced the biogas methane content for sample TWAS:OFMSW:30%FOG(H) to 66.4% compared to 60.1% for the control sample TWAS(T), and accordingly improved the methane yield to be 84.4% higher than the methane yield of the control.

Fermentation inhibition of thermophilic methane fermentation with coffee grounds and its mitigation methods

Fermentation inhibition of thermophilic methane fermentation with coffee grounds and its mitigation methods

Thermophilic Dry Methane Fermentation of Distillation Residue Eluted from Ethanol Fermentation of Kitchen Waste and Dynamics of Microbial Communities.

Thermophilic dry methane fermentation is advantageous for feedstock with high solid content. Distillation residue with 65.1% moisture content was eluted from ethanol fermentation of kitchen waste and subjected to thermophilic dry methane fermentation, after adjusting the moisture content to 75%. The effect of carbon to nitrogen (C/N) ratio on thermophilic dry methane fermentation was investigated. Results showed that thermophilic dry methane fermentation could not be stably performed for >10weeks at a C/N ratio of 12.6 and a volatile total solid (VTS) loading rate of 1g/kg sludge/d; however, it was stably performed at a C/N ratio of 19.8 and a VTS loading rate of 3g/kg sludge/d with 83.4% energy recovery efficiency. Quantitative PCR analysis revealed that the number of bacteria and archaea decreased by two orders of magnitude at a C/N ratio of 12.6, whereas they were not influenced at a C/N ratio of 19.8. Microbial community analysis revealed that the relative abundance of protein-degrading bacteria increased and that of organic acid-oxidizing bacteria and acetic acid-oxidizing bacteria decreased at a C/N ratio of 12.6. Therefore, there was accumulation of NH4+ and acetic acid, which inhibited thermophilic dry methane fermentation.

Acid-Tolerant Moderately Thermophilic Methanotrophs of the Class Gammaproteobacteria Isolated From Tropical Topsoil with Methane Seeps

Terrestrial tropical methane seep habitats are important ecosystems in the methane cycle. Methane oxidizing bacteria play a key role in these ecosystems as they reduce methane emissions to the atmosphere. Here, we describe the isolation and initial characterization of two novel moderately thermophilic and acid-tolerant obligate methanotrophs, assigned BFH1 and BFH2 recovered from a tropical methane seep topsoil habitat. The new isolates were strictly aerobic, non-motile, coccus-shaped and utilized methane and methanol as sole carbon and energy source. Isolates grew at pH range 4.2–7.5 (optimal 5.5–6.0) and at a temperature range of 30–60°C (optimal 51–55°C). 16S rRNA gene phylogeny placed them in a well-separated branch forming a cluster together with the genus Methylocaldum as the closest relatives (93.1–94.1% sequence similarity). The genes pmoA, mxaF, and cbbL were detected, but mmoX was absent. Strains BFH1 and BFH2 are, to our knowledge, the first isolated acid-tolerant moderately thermophilic methane oxidizers of the class Gammaproteobacteria. Each strain probably denotes a novel species and they most likely represent a novel genus within the family Methylococcaceae of type I methanotrophs. Furthermore, the isolates increase our knowledge of acid-tolerant aerobic methanotrophs and signify a previously unrecognized biological methane sink in tropical ecosystems.

Production of biofuels from sweet sorghum juice via ethanol–methane two-stage fermentation

Sweet sorghum juice is rich in fermentable sugar. Combining ethanol fermentation with methane fermentation to convert sweet sorghum juice to biofuels not only maximizes the energy recovery, but also reduces the environmental load. A two-stage fermentation, consisting of continuous ethanol fermentation and thermophilic methane fermentation, was developed to convert sweet sorghum juice to productions of ethanol and methane. The results of batch ethanol fermentation indicated that it was essential to supplement the feedstock with nutrients in order to improve the ethanol yield. Continuous ethanol fermentation could be performed at 35°C without decreasing the ethanol yield at a dilution rate of 0.3h−1, and ethanol yield and productivity of 88.5% and 20.3g/L/h were obtained, respectively. In contrast, the productivity was improved to 27.4g/L/h by increasing the dilution rate to 0.4h−1 at a fermentation temperature of 33°C. The stillage eluted from the ethanol production process was subjected to thermophilic methane fermentation. After adjusting the C/N ratio of the stillage to 40, a total organic carbon (TOC) removal efficiency of 87.0% and gas evolution rate of 1200mL/g-TOC were achieved, even at a high TOC loading rate of 10g/L/d by adding (NH4)2SO4 of 0.1g/L-stillage.

Characterization of methanogenesis, acidogenesis and hydrolysis in thermophilic methane fermentation of chicken manure

The thermophilic methane fermentation of chicken manure (CM) with a total solids (TS) of approximately 10% was investigated with regard to ammonia inhibition. A gradual increase in total ammonia nitrogen to 6000mg/L was observed in the case of raw CM fermentation. A distinct rise in VFA accumulation combined with a low methane production of 0.29L/gVS occurred at a TAN of 4000∼5000mg/L. Biogas production completely ceased when TAN reached 8000mg/L after the addition of NH4HCO3. The high sensitivity of methanogenesis to TAN and free ammonia (FA) was quantitatively confirmed. The IC50 of TAN for methanogenesis, acidogenesis and hydrolysis was 5058, 5305 and 5707mg/L at a pH value of 8.1±0.2. Similar results but with a lower IC50 were obtained for FA inhibition during fermentation. The microbial community analysis revealed significant differences in hydrogenotrophic methanogens and aceticlastic methanogens before and after inhibition.

Microbial community shifts and biogas conversion computation during steady, inhibited and recovered stages of thermophilic methane fermentation on chicken manure with a wide variation of ammonia

The thermophilic methane fermentation of chicken manure (10% TS) was investigated within a wide range of ammonia. Microbiological analysis showed significant shifts in Archaeal and Bacterial proportions with VFA accmulation and CH4 formation before and after inhibition. VFA accumulated sharply with lower methane production, 0.29 L/g VS, than during the steady stage, 0.32 L/g VS. Biogas production almost ceased with the synergy inhibition of TAN (8000 mg/L) and VFA (25,000 mg/L). Hydrogenotrophic Methanothermobacter thermautotrophicus str. was the dominate archaea with 95% in the inhibition stage and 100% after 40 days recovery compared to 9.3% in the steady stage. Aceticlastic Methanosarcina was not encountered with coincided phenomenal of high VFA in the inhibition stage as well as recovery stage. Evaluation of the microbial diversity and functional bacteria indicated the dominate phylum of Firmicutes were 94.74% and 84.4% with and without inhibition. The microbial community shifted significantly with elevated ammonia concentration affecting the performance.

Trace metals requirements for continuous thermophilic methane fermentation of high-solid food waste

The amount of iron, cobalt and nickel required to perform thermophilic methane fermentation using high-solid food waste was examined in this study. To determine this, we devised and applied a theoretical consideration method. Three runs were performed using a completely stirred tank reactor. During the first run, start-up was carried out by decreasing the HRT from 100 to 50 and finally to 30days, which corresponds to a COD loading rate of 1.9, 3.8 and 6.3kgm−3day−1, respectively. Four typical operational parameters—the gas production rate, methane content, pH and total volatile fatty acid (VFA) concentration—were stable except a temporary rise in VFA was observed when the HRT was decreased from 50 to 30days, indicating that the system performed well with COD loading rate of 6.3kgm−3day−1. After day 57 (HRT: 30days, COD loading rate: 6.3kgm−3day−1), the reactor efficiency deteriorated due to the trace metals limitation. A combination of iron, cobalt and nickel (at 10, 1 and 1mgL−1, respectively) was added to the reactor approximately every 45days during the second run. The reduced gas production, methane content and pH were all restored to stable levels after regularly adding the trace metals. During the third run, trace metals were added to the substrate, and no limitation was noted at all. Based on the theoretical consideration and the experimental results, the most suitable values for Fe/COD, Co/COD, Ni/COD are 276, 4.96 and 4.43mgkg−1COD-removed, respectively, for high-solid thermophilic methane fermentation of food waste.