Average Specific Methane Yield Research Articles

Startup performance and microbial communities of a decentralized anaerobic digestion of food waste

The aim of this work was to evaluate the feasibility of treating food waste generated from a hawker centre in a pilot-scale anaerobic digester operating on site in an urban area of Singapore. For this purpose, a 10.4 m3 digester was housed within two 20 feet containerized systems and sited adjacent to the hawker centre. The system reported in this work was during the startup phase, for over 71 days of real and varying food wastes loading rate. The results demonstrated that the decentralized system had an average specific methane yield of 0.55 ± 0.04 m3CH4/kgVS, with methane concentrations of 56.6 ± 2.3%. For the power generation output, the energy assessment revealed an average of 2.05 ± 0.57 m3/kW h consumption rate. Accordingly, on the last day of startup phase, the inoculum of the digester was richer in organisms from the phylum Thermoplasmatota, i.e., genera Candidatus_Methanogranum and Candidatus_Methanoplasma, alongside with other dominant abundance from phyla Bacteroidota, Firmicutes, Synergistota, and Verrucomicrobiota. This study provides new insights into pilot scale decentralized anaerobic digestion with varying food waste relate to the characterizations of digester microbial communities, as well as turning in a typical integrated anaerobic digestion of food waste-to-energy system a reality.

Pilot-scale anaerobic co-digestion of food and garden waste: Methane potential, performance and microbial analysis

Among the organic waste sources used for energy recovery through anaerobic digestion, food waste can be emphasized, as they are the result of the growing demand for food in the world. Additionally, lignocellulosic waste is one of the major energy sources available in urban and rural areas. In this context, the objective of this research was to evaluate the effects of food and garden waste co-digestion in the anaerobic process performance and also in the microbial community involved, by comparing two pilot scale reactors (500 L). To enable the comparison between mono-digestion and co-digestion, one reactor was operated only with food waste as a substrate while in the other, 20% of the ORL from the food waste was substituted by garden waste. The results showed a better performance in co-digestion, with a removal of 83% in VS, an average specific methane yield of 0.47 LCH4 gVS−1, a biogas production rate of 86 L d−1, with an average methane content of 67% and a delay in the system acidification when an OLR of 0.24 kgVS m−3d−1 was applied. The SEM images showed that the microorganisms were attached to garden waste particles as support medium for biofilm formation, favoring the development of certain species of microorganisms more resistant to external variations and, consequently, delaying the acidification of the reactor. The analysis of the microbial community in the reactors indicated a higher presence of hydrogenotrophic methanogenic archaea (Methanospirillum, Methanobacterium, Methanobrevibacter and Methanoculleus).

Accelerated Biomethane Potential assay for straw with artificially flocculated sludge and defined ‘synthetic manure’

A Biomethane Potential (BMP) assay for lignocellulosic material was elaborated with milled wheat straw (WS) as reference substrate. The generally used cow manure as co-substrate was replaced with a buffering salt solution having trace elements called ‘synthetic manure’. The inoculum sludge was artificially flocculated and thickened to accelerate the BMP assay and to favour microbial syntrophy. The chemical stoichiometric formula for WS was estimated to be C3.71H6.04O2.79N0.044S0.005 and this was used to calculate the theoretically possible 100% methane yield being 293.4 mLSTP CH4 gVS−1, after the non-degradable lignin portion (23.05%) was subtracted. Experimentally, an average specific methane yield of 287.1 mLSTP CH4 gVS−1 was obtained at about 98%. This result indicated a 100% degradation, if the microbially generated biomass during the BMP assay was considered. Methanization was completed after 15 days only, instead of mostly 60 days as shown by novel in-situ methane sensors.

Biomass and methane yield of giant reed (Arundo donaxL.) as affected by single and double annual harvest

AbstractThe replacement of silage maize with giant reed as energy crop has been proposed as a mean for reducing the need of irrigation water as well as monetary and environmental costs of cultivation. Little is known about giant reed response to within‐season harvesting, and its effect on the methane production in anaerobic digestion. The effect of three harvest schedules on yield, biomass composition and methane production of giant reed was evaluated at one site of the Po Valley, northern Italy, for three consecutive years. In a completely randomized block design with four replicates the treatments applied annually were: (i) double harvest 1: first cut at the end of June + second cut at the beginning of October (DH1); (ii) double harvest 2: first cut at the end of July + second cut at the beginning of October (DH2); (iii) single harvest at the beginning of October (SH). The crop stand was established in the year 2015 and treatments were repeatedly applied in the years 2016, 2017 and 2018. The SH treatment determined the highest average annual dry matter (DM) yield (59.0 Mg DM ha−1). The DM yield for treatments with double harvest was significantly lower, that is, −30% for DH1 and −15% for DH2. In terms of specific methane yield there was little advantage in harvesting biomass twice during the growing season. The average specific methane yield varied substantially between years (i.e. from 144 to 233 ml CH4 g−1 VS), and in every year the values tended to decrease with the ageing of harvested biomass. Methane yield per hectare, however, was driven by DM yield, thus SH also determined the highest average value (9110 m3 CH4‐STP ha−1). In conclusion, the single annual harvest at the end of the growing season is an ideal strategy for maximizing methane production from giant reed.

A novel integrated single-stage anaerobic co-digestion and oxidation ditch-membrane bioreactor system for food waste management and building wastewater recycling

This study is to develop a novel integrated single-stage anaerobic co-digestion and oxidation ditch membrane bioreactor (SAC/OD-MBR) for food waste and building wastewater recycling. The co-digestion of food waste (FW) from a canteen with waste sludge (WS) from OD-MBR was performed with the proportion of FW:WS at 10:1 by weight. The liquid digestate from the co-digestion process was further co-treated with building wastewater in the OD-MBR system for water reuse purpose. Maximum methane content of 65.2% in biogas as well as average specific methane yield of 0.24 gCH4/gVS could be obtained with anaerobic co-digestion of food waste and waste sludge from OD-MBR with HRT of 24 h and horizontal flow velocity of 0.3 m/s. The observed main methanogen species in this co-digestion process were Methanoculleus bourgensis and Methanoculleus palmolei. For co-treatment of liquid digestate and building wastewater with the OD-MBR, it was found that HRT of 24 h and horizontal flow velocity of 0.3 m/s could achieve highest COD and nitrogen removal efficiencies. HRT can be considered as a main key parameter to promote nitrification activity inside the OD-MBR system. Moreover, treated effluent from the SAC/OD-MBR could comply with the water reuse standard for garden and landscape application in the university campus. Furthermore, the techno-economic analysis indicates that this proposed system has a high potential of total cost savings and other indirect benefits. Therefore, the prototype SAC/OD-MBR can be an alternative system for food waste management and wastewater recycling for building application.

The Performance of Mesotrophic Arrhenatheretum Grassland under Different Cutting Frequency Regimes for Biomass Production in Southwest Germany

Biogas production is a key renewable energy pathway for a more sustainable future bioeconomy. However, there is a crucial trade-off between biomass productivity and social-ecological sustainability of available biogas cropping systems. Permanent grassland has been frequently promoted as a promising perennial cropping system for biomass production. Three- and four-cut regimes are usually the highest-yielding and thus preferable for biogas production. A three-year field trial in southwest Germany investigated biomass yield and biochemical composition of mesotrophic Arrhenatheretum grassland under three cutting regimes (two-, three- and four-cut). For the three-cut regime, a preliminary biogas batch test was conducted. The three-cut regime had the highest annual accumulated dry matter yield (11.8–14.8 Mg ha−1), an average specific methane yield of 0.289 m3N kg−1 volatile solids−1 and an accumulated annual methane yield of 3167–3893 m³N ha−1. The four-cut regime performed least favorably due to a lower dry matter yield than the three-cut regime, the highest ash content and the highest nitrogen content. Thus, the three-cut regime promises the best yield performance, whereas the two-cut regime can potentially provide more ecosystem services such as biodiversity conservation and wild-game protection. Consequently, the two-cut regime could help improve the social-ecological sustainability of biogas crop cultivation.

Combined cultivar and harvest time to enhance biomass and methane yield in sorghum under warm dry conditions in Pakistan

Methane from agricultural biomass contributes to achieving self-reliance and sustainability in alternative energy sources. In this framework, a two-year experiment was set up in Punjab, Pakistan, combining four cultivars (CV) of biomass sorghum with five harvest times (HT) (from 60 to 120 days after sowing) (DAS). HT and CV, both alone and together strongly influenced morphological, compositional and yield traits. Dual purpose (grain and biomass) CV (Jawar-2011 and JS-263) attained higher dry biomass yield (DBY) than fibre CV (Hagari and JS-2002) (+22%), possibly due to a stronger sink for assimilates at late growth stages. Delayed harvest (120 vs. 60 DAS) determined the largest differences in all traits: increase in DBY (+48% DBY), hemi-cellulose (+80%), cellulose (+85%) and lignin (+41%), and concurrent decrease in protein (−22%) and ash (−28%). These compositional changes indicate a strengthening in plant structure with advancing maturity that negatively reflected on specific methane yield (SMY) (−17%). However, average SMY obtained with sorghum in the warm dry conditions of Pakistan (290 lN kg−1 VS) is consistent with data observed at higher latitudes under temperate climate. The significant HT × CV interactions for DBY, SMY and their product, methane yield per hectare, were fitted by means of functions describing CV trends in time. Based on this, suitable combinations of HT and CV can be envisaged to balance methane yield and bio-degradability over a reasonable harvest window. On concluding, data obtained in this experiment disclose favourable prospects for sorghum as bio-methane feedstock in warm dry areas of the world.

Radiation use efficiency, chemical composition, and methane yield of biogas crops under rainfed and irrigated conditions

For biomethane production, the cup plant (Silphium perfoliatum L.) is considered a promising alternative substrate to silage maize (Zea mays L.) due to its high biomass potential and associated ecological and environmental benefits. It has also been suggested to grow cup plant on less productive soils because of its presumed drought tolerance, but robust information on the impact of water shortage on biomass growth and substrate quality of cup plant is rare. Therefore, this study assesses the effects of soil water availability on the chemical composition and specific methane yield (SMY) of cup plant. Furthermore above-ground dry matter yield (DMY) was analysed as a function of intercepted photosynthetic active radiation (PAR) and radiation use efficiency (RUE). Data were collected in a two-year field experiment under rainfed and irrigated conditions with cup plant, maize, and lucerne-grass (Medicago sativa L., Festuca pratensis Huds., Phleum pratense L.). The cup plant revealed a slight decrease of −6% in the SMY in response to water shortage (less than 50% of plant available water capacity). The average SMY of cup plant [306 l (kg volatile solids (VS))−1] was lower than that of maize [362 l (kg VS)−1] and lucerne-grass [334 l (kg VS)−1]. The mean drought-related reduction of the methane hectare yield (MHY) was significantly greater for cup plant (−40%) than for maize (−17%) and lucerne-grass (−13%). The DMY reduction in rainfed cup plant was mainly attributed to a more severe decrease in RUE (−29%) than for maize (−16%) and lucerne-grass (−12%). Under water stress, the mean cup plant RUE (1.3 g MJ−1) was significantly lower than that of maize (2.9 g MJ−1) and lucerne-grass (1.4 g MJ−1). Compared to RUE, the reduced PAR interception was less meaningful for DMY in rainfed crops. Hence, the cup plant is not suitable for growing on drought prone lands due to its high water demand required to produce reasonably high MHYs.

Methane yield performance of amaranth (Amaranthus hypochondriacus L.) and its suitability for legume intercropping in comparison to maize (Zea mays L.)

The use of amaranth as a new crop in biogas crop rotations has raised scientific interest Europe-wide over the past decade. However, the results of studies on its overall performance are somewhat contradictory. This study aims to investigate both the performance of amaranth (Amaranthus hypochondriacus L.) as a biogas crop in comparison to maize (Zea mays L.) and to test its suitability for intercropping with two different legume species. For this purpose, field trials were conducted in southwest Germany in 2014 and 2015. The two legumes common bean (Phaseolus vulgaris L.) and white clover (Trifolium repens L.) were chosen for intercropping with the main crops amaranth and maize. Each year, aboveground fresh matter yield, dry matter content (DMC), ash content and specific methane yield (SMY) were determined. The average methane yield per hectare (MYH) of all amaranth treatments was 3030.6±187.3m3N ha−1 in 2014 and 2265.6±243.4m3N ha−1 in 2015, approximately half that of maize each year. In 2015, the low MYHs stemmed from low dry matter yields (DMYs) caused by drought conditions. This effect was much stronger for maize than for amaranth. The average SMY over the two years was much lower for amaranth (266.0±1.7lNkg−1 of volatile solids (VS)) than for maize (330.0±1.5lNkg−1 of VS) due to higher contents of both ash (>13% of VS) and lignin (>6% of VS). Intercropping with common bean did not significantly affect the DMY and SMY of either main crop in either year. By contrast, the establishment of white clover was successful in both main crops and years. However, white clover also significantly decreased the DMY of both main crops in 2015. Overall, it was shown that amaranth and maize appear equally suitable for legume intercropping, but new amaranth genotypes and optimized cultivation techniques are required to render it a more productive biogas crop.

Anaerobic Co-digestion of Pig Manure with Dried Maize Straw

The anaerobic co-digestion of pig manure (PM) with dried maize straw (DMS) was studied at 35 °C with a volatile solid (VS) ratio (VSPM/VSDMS) of 1:2 in a continuously stirred tank reactor, and the digestions of mono-PM and mono-DMS were evaluated under the same conditions. The organic loading rates (OLRs) of 2, 3, and 4 g VS/L/d were studied and found to correspond to hydraulic retention times (HRTs) of 60, 40, and 30 d, respectively. Under the condition of long HRT and low OLR, PM could be degraded completely. The co-digestion of PM with DMS showed the most stable performance in TAN, whereas TAN in mono-PM increased with the increase of OLR. The specific methane yield (SMY) did not have a linear correlation with OLR, since HRT changed with different OLR. The maximum average SMY in the co-digestion reactor was 272 mL/g VS-fed at an OLR of 3 g VS/L/d and an HRT of 40 d. The SMY in mono-DMS digestion was the lowest and it decreased with the increase of OLR.

Starting-up low temperature dry anaerobic digestion of cow feces and wheat straw

Psychrophilic dry anaerobic digestion (PDAD) of animal manures and agriculture residues is of high interest in cold-climate regions. This paper reports the results of a start-up experiment (113 days) of PDAD of cow feces and wheat straw mixture (at two total solids (TS) of 18 and 21%) in laboratory scale sequence batch reactor (SBR) at 20 °C. An average specific methane yield (SMY) of 96.1 ± 5 L of CH4 per kg of volatile solid (VS) corrected to standard pressure and temperature (101.3 kPa and 273 K) (NL CH4 kg−1 VS) has been achieved for a feed with TS of 18% along with an organic loading rate (OLR) 4.0 g total chemical oxygen demand (TCOD) kg−1 inoculum day−1 and a treatment cycle length (TCL) of 21 days. An average SMY of 149.9 ± 14 NL CH4 kg−1 VS with a maximum daily CH4 production rate of 7.2 ± 0.7 NL CH4 kg−1 VS day−1 have been obtained for a feed with total solid of 21% along with an average daily inoculum OLR of 4.2 g TCOD kg−1 inoculum day−1 and TCL of 21 days. The rapid decrease in volatile fatty acids concentration after 7 days of treatment and their low concentration thereafter indicated that hydrolysis was the reaction limiting step. The results indicate that PDAD of cow feces and wheat straw is feasible at feed TS of 21%.

High rate psychrophilic anaerobic digestion of undiluted dairy cow feces

Novel high rate psychrophilic (20°C) anaerobic digestion (PAD) of undiluted cow feces (11.5–13.5% total solids) was demonstrated using sequence batch reactor in long-term operation with successive cycles of 21days treatment cycle length (TCL). At organic loading rates (OLR) 9.0, 10.0, 11.0 and 12.0g TCOD kg−1 inoculum d−1 average specific methane yield (SMY) was 154.0±11.7, 152.1±12.2, 126.0±2.8 and 116.0±6.1NL CH4 per kg of VS fed, respectively. Volatile solids removal averaged around 31.7±3.3%, 32.2±1.0%, 27.9±2.2% and 23.4±0.5%, respectively. Substrate-to-inoculum ratio (SIR; wet-mass basis) ranged between 1.17±0.06 and 1.43±0.05. Concentration of volatile fatty acids in the bioreactors during the TCL indicated that hydrolysis was the rate limiting reaction. High rate PAD of undiluted cow feces is possible at OLR (g TCOD kg−1 inoculum d−1) 9.0 and 10.0 with a TCL of 21days; however, OLR of 11.0 and 12.0 are also possible but require longer TCL to maintain the SMY.

Biogas and Methane Yield from Rye Grass

Biogas production in the Czech Republic has expanded substantially, including marginal regions for maize cultivation. Therefore, there are increasingly sought materials that could partially replace maize silage, as a basic feedstock, while secure both biogas production and its quality.Two samples of rye grass (Lolium multiflorum var. westerwoldicum) silage with different solids content 21% and 15% were measured for biogas and methane yield. Rye grass silage with solid content of 15% reached an average specific biogas yield 0.431 m3.kg-1 of organic dry matter and an average specific methane yield 0.249 m3.kg-1 of organic dry matter. Rye grass silage with solid content 21% reached an average specific biogas yield 0.654 m3.kg-1 of organic dry matter and an average specific methane yield 0.399 m3.kg-1 of organic dry matter.

High rate psychrophilic anaerobic digestion of high solids (35%) dairy manure in sequence batch reactor

Zero liquid discharge is increasingly adopted as an objective for waste treatment process. The objective of this study was to increase the feed total solids (TS) and the organic loading rate (OLR) fed to a novel psychrophilic (20°C) dry anaerobic digestion (PDAD). Duplicate laboratory-scale bioreactors were fed cow feces and wheat straw (35% TS in feed) at OLR of 6.0g TCOD kg−1 inoculum d−1 during long-term operation (147days consisting of 7 successive cycles).An overall average specific methane yield (SMY) of 151.8±7.9NL CH4 kg−1 VS fed with an averaged volatile solids removal of 42.4±4.3% were obtained at a volatile solids-based inoculum-to-substrate ratio (ISR) of 2.13±0.2. The operation was stable as indicated by biogas and VFAs profiles and the results were reproducible in successive cycles; a maximum SMY of 163.3±5.7NL CH4 kg−1 VS fed was obtained. Hydrolysis was the reaction limiting step. High rate PDAD of 35% TS dairy manure is possible in sequential batch reactor within 21days treatment cycle length.

Psychrophilic Dry Anaerobic Digestion of High Solids Content Dairy Manure: Long-Term Operation

<abstract> <bold><sc>Abstract.</sc></bold> This is the first study on successful long-term psychrophilic (20°C) dry anaerobic digestion (PDAD) of cow feces with wheat straw at 27% total solids (TS) in feed. Experimental results demonstrated the feasibility of cow feces and wheat straw PDAD in long-term operation of a laboratory-scale sequencing batch reactor. An average specific methane yield (SMY) of 182.9 ±16.9 _NL CH₄ kg^-1 VS fed during 12 successive cycles (273 days) was obtained for feed TS of 27% at an organic loading rate 3.0 g TCOD kg^-1 inoculum d^-1 and treatment cycle length (TCL) of 21 days. A maximum SMY of 219.2 ±18 _NL CH₄ kg^-1 VS fed with a maximum CH₄ production rate of 10.2 ±0.8 _NL CH₄ kg^-1 VS d^-1 was achieved. The low levels of volatile fatty acid concentrations in the bioreactor indicated that hydrolysis was the reaction-limiting step. The SMY, methane production rate, and their reproducibility during 12 successive cycles indicate that PDAD of cow feces and straw (27% TS in feed) is as efficient as mesophilic DAD.

Psychrophilic dry anaerobic digestion of dairy cow feces: Long-term operation

This paper reports experimental results which demonstrate psychrophilic dry anaerobic digestion of cow feces during long-term operation in sequence batch reactor. Cow feces (13–16% total solids) has been anaerobically digested in 12 successive cycles (252days) at 21days treatment cycle length (TCL) and temperature of 20°C using psychrotrophic anaerobic mixed culture.An average specific methane yield (SMY) of 184.9±24.0, 189.9±27.3, and 222±27.7 NLCH4kg−1 of VSfed has been achieved at an organic loading rate of 3.0, 4.0, and 5.0gTCODkg−1inoculumd−1 and TCL of 21days, respectively. The corresponding substrate to inoculum ratio (SIR) was 0.39±0.06, 0.48±.02, 0.53±0.05, respectively. Average methane production rate of 10±1.4 NLCH4kg−1VSfedd−1 has been obtained. The low concentration of volatile fatty acids indicated that hydrolysis was the reaction limiting step.

Methane yield through anaerobic digestion for various maize varieties in China

The methane potential of nine varieties of fresh maize harvested at three different times and of maize silage was experimentally determined in batch assays. The ultimate methane productivity in terms of volatile solids (VS) was determined as 213.94–313.63, 195.88–334.81mL/g VS from several fresh and silage maize in three stages, respectively. The average specific methane yield of wax ripeness stage for fresh maize and full ripeness stage for silage maize were higher than that of other stages, respectively. The high-oil varieties of fresh maize and silage varieties of ensiling maize could produce more methane than general varieties in the same ripeness stage. Methane yield of ensiled materials was higher than fresh material. The methane yields of fresh and silage maize in full ripeness stage were ranged 5656–7956 and 4633–8915m3/ha, respectively. The corresponding maximum of methane yield came from fresh HO5580 and silage CAU No. 4.

Methane yield from switchgrass and reed canarygrass grown in Eastern Canada

Methane yields from silage made from switchgrass- and reed canarygrass-seeded plots with two N application rates and three harvest dates were assessed in Eastern Canada. The average specific methane yield from reed canarygrass-seeded plots (0.187NLCH4gVS−1) was less than from switchgrass-seeded plots (0.212NLCH4gVS−1). Switchgrass did not establish well and made up only a small proportion of the DM yield. As a consequence, the average methane yield per hectare from reed canarygrass-seeded plots (1.37GLCH4ha−1) was significantly greater than switchgrass-seeded plots (0.91GLCH4ha−1). Increased N fertilization reduced specific methane yields but increased methane yield per hectare, primarily because of increased DM yield. Delaying harvest resulted in decreased methane yields per hectare and specific methane yields, particularly for reed canarygrass. Further long-term research could help identify important factors influencing methane yields from crops during a complete stand life cycle.

Biogas production from boreal herbaceous grasses – Specific methane yield and methane yield per hectare

The objective of this study was to determine the specific methane yields of four grass species (cocksfoot, tall fescue, reed canary grass and timothy) cultivated under boreal conditions as well as how harvesting time and year of cultivation affects the specific methane yields per ha. The specific methane yields of all grasses and all harvests varied from 253 to 394 Nl CH 4/kg volatile solids (VS) added. The average specific methane yield of the 1st harvest of all grasses was higher than the 2nd harvests. In this study the methane and energy yields from different harvest years were ranged from 1200 to 3600 Nm 3 CH 4/ha/a, corresponding from 12 to 36 MWh CH 4 /ha/a. The methane yield per hectare of the 1st harvest was always higher than that of the 2nd harvest per hectare because of the higher dry matter yield and specific methane yield. High biomass yield per hectare, good digestibility and regrowth ability after harvesting are important factors when choosing grass species for biogas production. If 30% of fallow and the second harvest of grassland were cultivated grasses and harvested for biogas production in Finland, the energy produced could be 4.9 TWh CH 4 .

CO‐DIGESTION OF GRASS SILAGE AND COW MANURE IN A CSTR BY RE‐CIRCULATION OF ALKALI TREATED SOLIDS OF THE DIGESTATE

Three laboratory, continuously stirred tank reactors (CSTRs) co‐digesting grass silage and cow manure (forming 30% and 70% of substrate volatile solids (VS), respectively) were operated to evaluate the effects of re‐circulating an alkali‐treated and untreated solid fraction of the digestate back to the reactors. The CSTRs were operated at an organic loading rate (OLR) of 2 kg VS m−3 day−1 and hydraulic retention time (HRT) of 20 days with a semi‐continuous mode of feeding. The feasibility of co‐digestion with substrate VS containing 30% VS of crop was reinforced, resulting in average specific methane yield of about 180–185 l CH4 kg−1 VS. Re‐circulation of the solid fraction of digestate back to the reactors in both alkali‐treated and untreated forms decreased the methane yield by 11% and 21%, respectively, and resulted in operational problems such as scum formation and accumulation of the reactor materials. Batch studies were conducted to evaluate (i) the methane potentials of the solid fraction of digestate, and whole digestate with alkali treatments ranging from 20–60 g NaOH kg−1 VS of substrate, and (ii) methane potentials of the accumulated reactor materials as top, middle and bottom layers. The solid fraction of digestate treated with 20 g NaOH kg−1 VS showed higher specific methane yield (340 l CH4 kg−1 VS) than the higher range of alkali treatments. The bottom layers of the control reactor and the reactor fed with alkali‐treated solids gave a higher specific methane yield (93 and 85 l CH4 kg−1 VS, respectively), and all three layers of untreated solids gave similar methane potentials.