Inhibition of anaerobic digestion through accumulation of volatile fatty acids occasionally occurs as the result of unbalanced growth between acidogenic bacteria and methanogens. A fast recovery is a prerequisite for establishing an economical production of biogas. However, very little is known about the microorganisms facilitating this recovery. In this study, we investigated the organisms involved by a novel approach of mapping protein-stable isotope probing (protein-SIP) onto a binned metagenome. Under simulation of acetate accumulation conditions, formations of (13)C-labeled CO2 and CH4 were detected immediately following incubation with [U-(13)C]acetate, indicating high turnover rate of acetate. The identified (13)C-labeled peptides were mapped onto a binned metagenome for improved identification of the organisms involved. The results revealed that Methanosarcina and Methanoculleus were actively involved in acetate turnover, as were five subspecies of Clostridia. The acetate-consuming organisms affiliating with Clostridia all contained the FTFHS gene for formyltetrahydrofolate synthetase, a key enzyme for reductive acetogenesis, indicating that these organisms are possible syntrophic acetate-oxidizing (SAO) bacteria that can facilitate acetate consumption via SAO, coupled with hydrogenotrophic methanogenesis (SAO-HM). This study represents the first study applying protein-SIP for analysis of complex biogas samples, a promising method for identifying key microorganisms utilizing specific pathways.

Flexible biogas production that adapts biogas output to energy demand can be regulated by changing feeding regimes. In this study, the effect of changes in feeding intervals on process performance, microbial community structure, and the methanogenesis pathway was investigated. Three different feeding regimes (once daily, every second day, and every 2 h) at the same organic loading rate were studied in continuously stirred tank reactors treating distiller's dried grains with solubles. A larger amount of biogas was produced after feeding in the reactors fed less frequently (once per day and every second day), whereas the amount remained constant in the reactor fed more frequently (every 2 h), indicating the suitability of the former for the flexible production of biogas. Compared to the conventional more frequent feeding regimes, a methane yield that was up to 14% higher and an improved stability of the process against organic overloading were achieved by employing less frequent feeding regimes. The community structures of bacteria and methanogenic archaea were monitored by terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S rRNA and mcrA genes, respectively. The results showed that the composition of the bacterial community varied under the different feeding regimes, and the observed T-RFLP patterns were best explained by the differences in the total ammonia nitrogen concentrations, H 2 levels, and pH values. However, the methanogenic community remained stable under all feeding regimes, with the dominance of the Methanosarcina genus followed by that of the Methanobacterium genus. Stable isotope analysis showed that the average amount of methane produced during each feeding event by acetoclastic and hydrogenotrophic methanogenesis was not influenced by the three different feeding regimes.I nterest in a demand-driven biogas supply for flexible electricity production with the aim of balancing the supply of electricity generated from sources producing fluctuating amounts of electricity, such as solar and wind sources, has increased recently. Different strategies can be employed to obtain a demand-driven biogas supply, including a strategy involving a conventional biogas plant with biogas storage or a strategy involving a conventional biogas plant with a biogas upgrade to biomethane for subsequent storage in a natural gas grid (1-3). Conventional biogas production with integrated heat and power (CHP) plants are normally run on a semicontinuous substrate feeding regime in order to provide a constant biogas output and electricity generation (4).Alternatively, flexible biogas production that adapts biogas output to energy demand can be implemented by feeding management, including varying the feeding regimes and substrate composition. The production of larger amounts of biogas can be achieved immediately after feeding, and smaller amounts of biogas production are achieved during the nonfeeding period. Compared to the conventional operation of biogas plants with biogas storage, flexible biogas produc...

Background Commercial biogas upgrading facilities are expensive and consume energy. Biological biogas upgrading may serve as a low-cost approach because it can be easily integrated with existing facilities at biogas plants. The microbial communities found in anaerobic digesters typically contain hydrogenotrophic methanogens, which can use hydrogen (H 2 ) as a reducing agent for conversion of carbon dioxide (CO 2 ) into methane (CH 4 ). Thus, biological biogas upgrading through the exogenous addition of H 2 into biogas digesters for the conversion of CO 2 into CH 4 can increase CH 4 yield and lower CO 2 emission. Results The addition of 4 mol of H 2 per mol of CO 2 was optimal for batch biogas reactors and increased the CH 4 content of the biogas from 67 to 94%. The CO 2 content of the biogas was reduced from 33 to 3% and the average residual H 2 content was 3%. At molar H 2 :CO 2 ratios > 4:1, all CO 2 was converted into CH 4 , but the pH increased above 8 due to depletion of CO 2 , which negatively influenced the process stability. Additionally, high residual H 2 content in these reactors was unfavourable, causing volatile fatty acid accumulation and reduced CH 4 yields. The reactor microbial communities shifted in composition over time, which corresponded to changes in the reactor variables. Numerous taxa responded to the H 2 inputs, and in particular the hydrogenotrophic methanogen Methanobacterium increased in abundance with addition of H 2 . In addition, the apparent rapid response of hydrogenotrophic methanogens to intermittent H 2 feeding indicates the suitability of biological methanation for variable H 2 inputs, aligning well with fluctuations in renewable electricity production that may be used to produce H 2 . Conclusions Our research demonstrates that the H 2 :CO 2 ratio has a significant effect on reactor performance during in situ biological methanation. Consequently, the H 2 :CO 2 molar ratio should be kept at 4:1 to avoid process instability. A shift toward hydrogenotrophic methanogenesis was indicated by an increase in the abundance of the obligate hydrogenotrophic methanogen Methanobacterium . ...

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