Volumetric Methane Production Rate Research Articles

Comparison of three biomass-retaining reactors of the ASBR, the UBF and the USR treating swine wastewater for biogas production

The comparative process performance and efficiency of reactors treating swine wastewater for biogas production at 35 °C were evaluated for three biomass-retaining reactors with different configurations and operational conditions: an anaerobic sequencing batch reactor (ASBR), an upflow anaerobic sludge bed-filter (UBF) and an upflow solid reactor (USR). The maximum volumetric methane production rates of the ASBR, UBF and USR were 1.679, 1.669 and 1.234 L L−1 d−1, respectively, at the OLR of 8 g TS L−1 d−1. Compared with the USR, the ASBR and UBF achieved better performance, which was attributed to more efficient biomass retention indicated by a higher solids residence time-to-hydraulic residence time ratio. The worst performance of the USR was caused by serious washout of sludge. The microorganism distribution profile along with depth showed that methanogenic genera of Methanosaeta and Methanospirillum accounted for 81.37–90.83% and 77.67–88.50% of all archaeal clones in the ASBR and UBF, respectively, and presented non-obvious spatial heterogeneity, while the abundances of methanogenic genera decreased from 93.61 to 4.64% with depth in the USR, instead of an increase in the populations of acid resistant bacteria.

Coproduction of hydrogen and methane in a CSTR-IC two-stage anaerobic digestion system from molasses wastewater.

A continuous hydrogen and methane production system in a two-stage process has been investigated to increase energy recovery rate from molasses wastewater in this study. This system consisted of a continuous stirred-tank reactor for hydrogen production and an internal circulation (IC) reactor for methane production, and was studied under the influent organic loading rate (OLR) of 18, 24, 30 and 36kg COD/(m3·d) (COD: chemical oxygen demand). The maximum volumetric hydrogen production rate of 2.41 L/(L·d) was obtained at the OLR of 30kg COD/(m3·d) with a hydrogen content of 42%, and the maximum volumetric methane production rate of 2.4 L/(L·d) with a methane content of 74.45% was obtained at the OLR of 36kg COD/(m3·d) using the effluents of hydrogen fermentation as substrate. The maximum of 71.06% of the molasses wastewater energy was converted to biogas (hydrogen and methane) at the OLR of 30kg COD/(m3·d).

Increasing capacity of an anaerobic sludge digester through FNA pre-treatment of thickened waste activated sludge

Free nitrous acid (FNA) pre-treatment has been previously demonstrated to be effective in enhancing methane production and volatile solids (VS) destruction in the anaerobic digestion of waste activated sludge for an equivalent hydraulic retention time (HRT). We hypothesise that, due to enhancement of hydrolysis kinetics, FNA pre-treatment will also allow reduction in the HRT while retaining performance. This would allow for improvement of capacity constrained digesters. Two anaerobic sludge digesters (control-experiment) were fed with the same thickened waste activated sludge (TWAS) from a full-scale plant for 6 months. With 24 h pre-treatment of TWAS at an FNA concentration of 6.1 mgN/L (NO2-N = 250 mg/L, pH = 5.0, T = 25 °C), the HRT for the experimental anaerobic digester was progressively reduced from 15 days to 12 days and then to 7.5 days. In comparison, the control reactor was operated at a constant HRT of 15 days, representing typical loading conditions. With the shortened HRTs, the experimental AD reactor achieved VS destruction at 36.9 ± 0.8% (12 days) and 36.8 ± 1.0% (7.5 days), representing 30–40% relative increase in comparison to the control reactor (at 26.5 ± 0.8% and 28.3 ± 0.7%, respectively, in the same two periods). This was supported by a similar (31–35%) increase in the methane production per unit of VS fed. The volumetric methane production rate of the experimental digester was increased by 165% at HRT of 7.5 days compared with the control digester at HRT of 15 days. The results demonstrated that FNA pre-treatment of TWAS can substantially increase the capacity of an anaerobic sludge digester, with a highly favourable economic outcome.

Methane production from thermally pretreated Scenedesmus obtusiusculus biomass in semi-batch reactors at low reaction times

In the pursuit of renewable sources for biogas generation, microalgae biomass constitutes a promising alternative. In this work, thermally pretreated Scenedesmus obtusiusculus was evaluated as source of biomass for methane production, in systems operated under batch and semi-batch regime. In batch experiments, methane production yields ranging from 109.3 to 192.7 mL CH4/g VSadded were observed with thermally-pretreated (98 °C, 6 h) microalgae biomass at concentrations between 1 and 10 g tCOD/L. On the other hand, the system operated under semi-batch mode for 50 days attained a volumetric methane production rate of 1.15 L CH4/L-d with a methane yield of 177.9 mL CH4/g VSadded at an organic loading rate of 10 g tCOD/L-d and a reaction time of 12 h. These results demonstrate that methane production from microalgae biomass in a semi-batch reactor is possible at low reaction time (12 h) and high organic loading rate using a moderate-temperature pretreatment for the microalgae biomass.

Instability diagnosis and syntrophic acetate oxidation during thermophilic digestion of vegetable waste

Effective process monitoring and instability diagnosis are important for stable anaerobic digestion (AD) of vegetable waste (VW). In order to evaluate the performance of thermophilic digestion of VW, to make early diagnosis for instability after organic overload, and to reveal the dynamics of microbial community under different running states, thermophilic AD of VW was carried out under improved organic loading rates (OLR) of 0.5–2.5 g volatile solid (VS)/(L ∙ d) in this study. Gaseous parameters including volumetric methane production rate (VMPR), CH4, CO2, and H2 concentrations, and liquid parameters including pH, oxidation–reduction potential, volatile fatty acid (VFA), and total alkalinity (TA), bicarbonate alkalinity (BA), intermediate alkalinity (IA), and ammonia, were monitored. The coupling parameters, such as the CH4/CO2, VFA/BA, and BA/TA ratios were also used to evaluate stability. The dynamics of syntrophic acetate-oxidizing bacteria (SAOB), acetoclastic methanogens (AM), and hydrogenotrophic methanogens (HM) were analyzed by high-throughput sequencing. The main methanogenic bacteria were HM (Methanothermobacter) during the start-up period of OLR 0.5 gVS/(L ∙ d), while they were AM (Methanosarcina) during the stable period of OLR of 1.0 gVS/(L ∙ d). The VMPR of stable period was about 0.29 L/(L · d) with total VFA concentration below 100 mg/L, CH4/CO2 > 1.3, and BA/TA>0.9. The first instability due to the accumulation of VFA and self-recovery due to syntrophic acetate oxidation occurred at an OLR of 1.5 gVS/(L ∙ d). The syntrophic acetate-oxidizing bacteria probably belong to genus S1 (family Thermotogaceae). The digestion failed at an OLR of 2.0 g VS/(L · d). H2 was only detected during collapsed period instead of instable period. The total ammonia nitrogen loss and bicarbonate alkalinity (BA) reduction were the primary causes for the instability of AD of VW without effluent recirculation. Compared with single parameters, the CH4/CO2 and BA/total alkalinity (TA) ratios are recommended as early warning indicators for engineering applications of thermophilic AD of VW.

Effect of organic loading rate on anaerobic co-digestion of rice straw and pig manure with or without biological pretreatment

In this study, rice straw (RS) and pig manure (PM) mixtures with or without bio-pretreatment were used as the substrates and digested in a 9 L of anaerobic reactor at Organic loading rates (OLRs) of 0.4–3.1 kg COD/(m3 d). The volumetric methane production rate (VMPR), methane yield and anaerobic stability were comparatively investigated. The results showed the co-anaerobic digestion processes of RS and PM mixture after biological pretreatment were very stable at OLRs of 0.4–2.5 kg COD/(m3 d), and its optimal VMPR and methane yield could reach 0.64 L CH4/(L d) and 0.4557 L CH4/g CODremoved at OLR of 2.5 kg COD/(m3 d), which were 62.4% and 37.8% higher than those of the control under the same OLR condition. This study indicated the biological pretreatment with a cellulolytic microbial consortium own great potential in improving the methane yield and productivity of RS and PM wastes.

Continuous hydrogen and methane production from Agave tequilana bagasse hydrolysate by sequential process to maximize energy recovery efficiency

Continuous H2 and CH4 production in a two-stage process to increase energy recovery from agave bagasse enzymatic-hydrolysate was studied. In the first stage, the effect of organic loading rate (OLR) and stirring speed on volumetric hydrogen production rate (VHPR) was evaluated in a continuous stirred tank reactor (CSTR); by controlling the homoacetogenesis with the agitation speed and maintaining an OLR of 44 g COD/L-d, it was possible to reach a VHPR of 6 L H2/L-d, equivalent to 1.34 kJ/g bagasse. In the second stage, the effluent from CSTR was used as substrate to feed a UASB reactor for CH4 production. Volumetric methane production rate (VMPR) of 6.4 L CH4/L-d was achieved with a high OLR (20 g COD/L-d) and short hydraulic retention time (HRT, 14 h), producing 225 mL CH4/g-bagasse equivalent to 7.88 kJ/g bagasse. The two-stage continuous process significantly increased energy conversion efficiency (56%) compared to one-stage hydrogen production (8.2%).

Effect of the organic loading rate on the performance and microbial populations during the anaerobic treatment of tequila vinasses in a pilot‐scale packed bed reactor

AbstractBACKGROUNDPilot‐scale studies focused on evaluating the robustness of biofilm‐based anaerobic digestion processes for further application at full‐scale are scarce. Therefore, the aim of this work was to evaluate the performance of a 445 L packed bed reactor (PBR) operated at different organic loading rates (OLRs between 4 and 12.5 g COD L‐1 d‐1) for the treatment of tequila vinasses. The reactor performance was correlated with the microbial dynamics to elucidate the specific role of the microbial communities in the degradation pathways that govern the process.RESULTSThe PBR was operated for 231 days under different OLRs showing a stable performance. The COD removal and methane yield were maintained throughout the reactor operation at 86–89% and 0.24–0.28 L CH4 g‐1 CODadded, respectively. Meanwhile, the highest volumetric methane production rate of 3.03 L CH4 d‐1 L‐1 was reached at the highest OLR, 12.5 g COD L‐1 d‐1. Regarding microbial dynamics, the Bacteria and Archaea populations were able to adapt to the OLR disturbances, favoring the interactions between syntrophic Bacteria and Methanosaeta at high OLRs.CONCLUSIONThis work contributes to the scarce information regarding anaerobic treatment of tequila vinasses at pilot‐scale and demonstrates that the PBR is a promising and robust configuration that allows treating higher OLRs than currently reported technologies. © 2017 Society of Chemical Industry

Evaluation of different types of anaerobic seed sludge for the high rate anaerobic digestion of pig slurry in UASB reactors

Three different types of anaerobic sludge (granular, thickened digestate and anaerobic sewage) were evaluated as seed inoculum sources for the high rate anaerobic digestion of pig slurry in UASB reactors. Granular sludge performance was optimal, allowing a high efficiency process yielding a volumetric methane production rate of 4.1LCH4L−1d−1 at 1.5days HRT (0.248LCH4g−1COD) at an organic loading rate of 16.4gCODL−1d−1. The thickened digestate sludge experimented flotation problems, thus resulting inappropriate for the UASB process. The anaerobic sewage sludge reactor experimented biomass wash-out, but allowed high process efficiency operation at 3days HRT, yielding a volumetric methane production rate of 1.7LCH4L−1d−1 (0.236LCH4g−1COD) at an organic loading rate of 7.2gCODL−1d−1. To guarantee the success of the UASB process, the settleable solids of the slurry must be previously removed.

Performance Evaluation of a Large-Scale Swine Manure Mesophilic Biogas Plant in China

Abstract. With the rapid growth of large-scale and intensive swine farms have come many ecological and environmental problems associated with the substantially increased and concentrated animal waste production. In this article, a swine manure and flushed slurry to renewable energy management system is present and discussed. This system was installed in a commercial feeder-to-finish swine farm with 18,000 head of swine in Beijing, China, and included two mesophilic upflow solids reactors (USRI and USRII, 500 m3 and 700 m3) and one psychrophilic plug-flow reactor (PFR, 1000 m3). In this study, USRII was monitored throughout a whole year to evaluate the performance of this swine waste to energy system. The biogas plant used mixed solid swine manure and flushed slurry as substrate with a relatively low organic loading rate (OLR) of 0.7 to 1.8 kg volatile solids (VS) m-3 d-1. The hydraulic retention time (HRT) varied from 15 to 22 days depending on the season. Less added water contributed to the longer HRT and more concentrated influent in winter. In winter, the specific methane production (SMP) of the digester was 0.43 m3 CH4 kg-1 VSadded, which was slightly lower than the value reported in Europe (0.45 m3 CH4 kg-1 VSadded) but about 48.3% higher than that in Asia (0.29 m3 CH4 kg-1 VSadded). This indicated that the performance of this USR in winter was stable, with a higher biogas production, and up to 90% of the VS was removed as well. However, the low OLR limited the volumetric methane production rate to only 0.21 to 0.57 m3 m-3 d-1. Keywords: Flushed slurry, Large-scale biogas plant, Monitoring, Performance, Swine manure.

Intermediate ozonation to enhance biogas production in batch and continuous systems using animal dung and agricultural waste

Agricultural waste and animal manure (dung) pose an environmental threat in developing countries. This investigation focused on the possible use of such waste as an energy source in the form of biogas produced via anaerobic digestion (AD). The impact of single and mixed substrates on methane production under controlled batch and continuous experimental setups was considered. The study was extended to investigate the effect of substrate size and the impact of an intermediate ozonation process on enhancing the production of biogas from single and mixed substrates. Cumulative methane production (CMP), ultimate methane yield (UMY), methane production potential (MPP), methane production rate (MPR), and maximum methane production rate (MPRmax) were used as performance indicators of the effectiveness of the anaerobic digestion process. CMP and MPP from mixed substrates were found to be higher than values obtained from a single substrate feeds, which may be attributed to a more balanced nutrient and organic matter found in mixed substrates. The large surface area of fine substrate influenced MPR and MPRmax values in the first 30 days of digestion. In later AD stages, the effect of substrate size was negligible. The MPRmax for fine substrates was 12.3 ± 0.3 LkgVS−1 compared to 8.8 ± 0.2 LkgVS−1 obtained for coarse substrate. Continuous AD with organic loading rate (OLR) of 4 kgVSm−1d−1 showed a %ADefficiency of 62%, an average specific methane production in the range of 98–230 LkgVS−1 and a volumetric methane production rate in the range of 1.94–2.35 m3 m−3d−1. Increasing the OLR increased the accumulation of volatile fatty acids in the system and resulted in decreased methane production. Two-stage AD with an intermediate ozonation process showed a significant increase in CMP and %ADefficiency compared with single-stage AD. The %ADefficiency for two-stage AD ranged from 63% to 83%, and for single-stage AD, it was in the range of 42.2%–64.3%. Anaerobic digestion of mixed agricultural waste improved the filtration, dewaterability, and settling ability of the final substrate, making it suitable for use as a soil fertilizer.

Role of organic loading rate in bioenergy generation from palm oil mill effluent in a two-stage up-flow anaerobic sludge blanket continuous-stirred tank reactor

This contribution presents the technical possibilities for continuous hydrogen and methane production using an optimum organic loading rate of palm oil mill effluent in a two-stage reactor at a thermophilic temperature of 55 °C. The influence of four organic loading rates, namely, 30, 40, 50, and 60 kg COD/(m3 d) for hydrogen production and 8.3, 10.2, 13.1, 15.8 kg COD/(m3 d) for methane production, were investigated. Hydrogen production was controlled in an up-flow anaerobic sludge blanket reactor at a constant hydraulic retention time of 12 h. The maximum hydrogen content, volumetric hydrogen production rate and hydrogen yield were found to be 45%, 2.5 L H2/d and 33.48 mL H2/g COD, respectively, at the organic loading rate of 50 kg COD/(m3 d). The effluent from the hydrogenogenic reactor was further digested into methane in the continuous stirred tank reactor at a hydraulic retention time of 5 d. The maximum volumetric methane production rate and methane yield were 10.58 L CH4/d. and 0.11 m3 CH4/kg COD, respectively, at an organic loading rate of 13.1 kg COD/(m3 d). A total chemical oxygen demand removal of 91% was achieved in this two-stage process. The scientific contribution of this two-stage technology with an optimized organic loading rate may play a significant role in degrading palm oil mill effluent and developing an energy-efficient strategy for waste management.

A model for methane production in anaerobic digestion of swine wastewater

A study was conducted using a laboratory-scale anaerobic sequencing batch digester to investigate the quantitative influence of organic loading rates (OLRs) on the methane production rate during digestion of swine wastewater at temperatures between 15 °C and 35 °C. The volumetric production rate of methane (Rp) at different OLRs and temperatures was obtained. The maximum volumetric methane production rates (Rpmax) were 0.136, 0.796, 1.294, 1.527 and 1.952 LCH4 L−1 d−1 at corresponding organic loading rates of 1.2, 3.6, 5.6, 5.6 and 7.2 g volatile solids L−1 d−1, respectively, which occurred at 15, 20, 25, 30 and 35 °C, respectively. A new model was developed to describe the quantitative relationship between Rp and OLR. In addition to the maximum volumetric methane production rate (Rpmax) and the half-saturation constant (KLR) commonly used in previous models such as the modified Stover–Kincannon model and Deng model, the new model introduced a new index (KD) that denoted the speed of volumetric methane production rate approaching the maximum as a function of temperature. The new model more satisfactorily described the influence of OLR on the rate of methane production than other models as confirmed by higher determination coefficients (R2) (0.9717–0.9900) and lower bias between the experimental and predicted data in terms of the root mean square error and the Akaike Information Criterion. Data from other published research also validated the applicability and generality of the new kinetic model to different types of wastewater.

Treatment of corn ethanol distillery wastewater using two-stage anaerobic digestion

In this study the mesophilic two-stage anaerobic digestion (AD) of corn bioethanol distillery wastewater is investigated in laboratory-scale reactors. Two-stage AD technology separates the different sub-processes of the AD in two distinct reactors, enabling the use of optimal conditions for the different microbial consortia involved in the different process phases, and thus allowing for higher applicable organic loading rates (OLRs), shorter hydraulic retention times (HRTs) and better conversion rates of the organic matter, as well as higher methane content of the produced biogas. In our experiments the reactors have been operated in semi-continuous phase-separated mode. A specific methane production of 1,092 mL/(L·d) has been reached at an OLR of 6.5 g TCOD/(L·d) (TCOD: total chemical oxygen demand) and a total HRT of 21 days (5.7 days in the first-stage, and 15.3 days in the second-stage reactor). Nonetheless the methane concentration in the second-stage reactor was very high (78.9%); the two-stage AD outperformed the reference single-stage AD (conducted at the same reactor loading rate and retention time) by only a small margin in terms of volumetric methane production rate. This makes questionable whether the higher methane content of the biogas counterbalances the added complexity of the two-stage digestion.

Effect of temperature on continuous dry fermentation of swine manure

Laboratory-scale experiments were performed on the dry digestion of solid swine manure in a semi-continuous mode using 4.5 L down plug-flow anaerobic reactors with an organic loading rate of 3.46 kg volatile solids (VS) m−3 d−1 to evaluate the effects of temperature (15, 25 and 35 °C). At 15 °C, biogas production was the poorest due to organic overload and acidification, with a methane yield of 0.036 L CH4 g−1 VS added and a volumetric methane production rate of 0.125 L CH4 L−1 d−1. The methane yield and volumetric methane production rate at 25 °C (0.226 L CH4 g−1 VS added and 0.783 L CH4 L−1 d−1, respectively) were 6.24 times higher than those at 15 °C. However, the methane yield (0.237 L CH4 g−1 VS added) and the volumetric methane production rate (0.821 L CH4 L−1 d−1) at 35 °C were only 4.86% higher than those at 25 °C, which indicated similar results were obtained at 25 °C and 35 °C. The lower biogas production at 35 °C in dry digestion compared with that in wet digestion could be attributed to ammonia inhibition. For a single pig farm, digestion of solid manure is accomplished in small-scale domestic or small-farm bioreactors, for which operating temperatures of 35 °C are sometimes difficult to achieve. Considering biogas production, ammonia inhibition and net energy recovery, an optimum temperature for dry digestion of solid swine manure is 25 °C.

Functional and taxonomic dynamics of an electricity-consuming methane-producing microbial community

The functional and taxonomic microbial dynamics of duplicate electricity-consuming methanogenic communities were observed over a 6months period to characterize the reproducibility, stability and recovery of electromethanogenic consortia. The highest rate of methanogenesis was 0.72mg-CH4/L/day, which occurred during the third month of enrichment when multiple methanogenic phylotypes and associated Desulfovibrionaceae phylotypes were present in the electrode-associated microbial community. Results also suggest that electromethanogenic microbial communities are very sensitive to electron donor-limiting open-circuit conditions. A 45min exposure to open-circuit conditions induced an 87% drop in volumetric methane production rates. Methanogenic performance recovered after 4months to a maximum value of 0.30mg-CH4/L/day under set potential operation (−700mV vs Ag/AgCl); however, current consumption and biomass production was variable over time. Long-term functional and taxonomic analyses from experimental replicates provide new knowledge toward understanding how to enrich electromethanogenic communities and operate bioelectrochemical systems for stable and reproducible performance.

Investigation of anaerobic biodegradability of real compost leachate emphasis on biogas harvesting

Anaerobic treatability of real compost leachate was assessed using laboratory-scale anaerobic sequencing batch reactor at mesophilic conditions. Interventional study was conducted at wide range of organic loading rate 0.93–25 g l−1 day−1 by varying hydraulic retention times 23 and 12 h. Initial chemical oxygen demand (COD) was 1.85–25 g l−1. pH variations; total, soluble (SCOD), readily biodegradable (rbCOD) chemical oxygen demand; volatile fatty acids degradation; biogas production; and methane fraction were considered in this study. The organic matter removal efficiencies were in the range of 76–81 % depending on loading rates applied. The maximum volumetric methane production rate of 5.7 l CH4 l−1 day−1 was achieved at the loading rate of 19.65 g l−1 day−1. About 85 % of removed organic matters during the biodegradation were converted to the methane. The results have shown that the anaerobic batch reactor could be an appealing option for changing compost leachate into the useable products such as biogas and other energy-rich compounds, which may play a serious role in meeting the world’s ever-increasing energy requirements in the future.

Anaerobic co-digestion of cheese whey and the screened liquid fraction of dairy manure in a single continuously stirred tank reactor process: Limits in co-substrate ratios and organic loading rate

Mesophilic anaerobic co-digestion of cheese whey and the screened liquid fraction of dairy manure was investigated with the aim of determining the treatment limits in terms of the cheese whey fraction in feed and the organic loading rate. The results of a continuous stirred tank reactor that was operated with a hydraulic retention time of 15.6days showed that the co-digestion process was possible with a cheese whey fraction as high as 85% in the feed. The efficiency of the process was similar within the range of the 15–85% cheese whey fraction. To study the effect of the increasing loading rate, the HRT was progressively shortened with the 65% cheese whey fraction in the feed. The reactor efficiency dropped as the HRT decreased but enabled a stable operation over 8.7days of HRT. At these operating conditions, a volumetric methane production rate of 1.37m3CH4m−3d−1 was achieved.

Fuzzy optimisation approach on the treatment of palm oil mill effluent (POME) via up-flow anaerobic sludge blanket–hollow centered packed bed (UASB–HCPB) reactor

An optimization study on the mesophilic anaerobic treatment of palm oil mill effluent (POME) in an up-flow anaerobic sludge blanket–hollow centered packed bed (UASB–HCPB) reactor was conducted for the first time by using kinetics modeling. Monod model was opted for the optimization due to its capability of describing the anaerobic digestion and the involvement of adequate amount of parameters in the kinetics model. The best-fit kinetic constants obtained from previous work were incorporated in the optimization model. Fuzzy optimization was adopted and it yielded the optimized chemical oxygen demand (COD) removal of 86.7% and volumetric methane production rate of 0.448 LCH4/L.day at hydraulic retention time (HRT) of 4.33 days, organic loading rate (OLR) of 14.75gCOD/L.day and mixed liquor volatile suspended solid (MLVSS) concentration of 19,130mg/L. A confirmative experiment was conducted and the results validate the legitimacy of the fuzzy optimization model. Further studies are required to improve the reliability of this newly introduced optimization technique in the wastewater treatment industry.

Co-digestion of molasses or kitchen waste with high-rate activated sludge results in a diverse microbial community with stable methane production

Kitchen waste and molasses are organic waste streams with high organic content, and therefore are interesting substrates for renewable energy production by means of anaerobic digestion. Both substrates, however, often cause inhibition of the anaerobic digestion process, when treated separately, hence, co-digestion with other substrates is required to ensure stable methane production. In this research, A-sludge (sludge harvested from a high rate activated sludge system) was used to stabilize co-digestion with kitchen waste or molasses. Lab-scale digesters were fed with A-sludge and kitchen waste or molasses for a total period of 105 days. Increased methane production values revealed a stabilizing effect of concentrated A-sludge on kitchen waste digestion. Co-digestion of molasses with A-sludge also resulted in a higher methane production. Volumetric methane production rates up to 1.53 L L−1 d−1 for kitchen waste and 1.01 L L−1 d−1 for molasses were obtained by co-digestion with A-sludge. The stabilizing effect of A-sludge was attributed to its capacity to supplement various nutrients. Microbial community results demonstrated that both reactor conditions and substrate composition determined the nature of the bacterial community, although there was no direct influence of micro-organisms in the substrate itself, while the methanogenic community profile remained constant as long as optimal conditions were maintained.