Influences of the substrate feeding regime on methanogenic activity in biogas reactors approached by molecular and stable isotope methods.

In order to better understand the effects of the substrate feeding regime on methanogenesis during anaerobic digestion in biogas reactors, four continuous stirred tank reactors operated under mesophilic conditions were investigated. In addition to standard physicochemical parameters, the stable isotopic signatures of CH4 and CO2 before and after daily feeding were analyzed. The activity of the methanogens was assessed by methyl coenzyme M reductase alpha-subunit (mcrA/mrtA) gene transcript analysis. Two different feeding regimes i.e. single vs. double consecutive feeding of the otherwise same daily maize silage load were investigated. During the first phase, a single feeding of the whole daily dose increased the biogas production within 70-80 min from around 0.5 to 2.0 L/h. This increase was associated with a transient increase of the acetic acid concentration and a corresponding decrease of the pH. Only moderate increase in biogas yield and VFA concentration (mainly acetate) was observed when the daily substrate was apportioned into two feedings. However, the overall daily gas production was similar in both cases. Regardless of the feeding regime, significantly depleted δ(13)CH4 and minor changes in the CO2 content of biogas were observed after feeding, which were followed by enrichment of δ(13)CH4. This period was associated with detectable changes in activity of methanogenic communities monitored by terminal restriction fragment length polymorphism analysis based on the transcripts of mcrA/mrtA genes. Methanoculleus and Methanobacterium spp. were the predominant methanogens in all reactors, while Methanosarcina spp. activity was only significant in two reactors. The activity of Methanoculleus and Methanosarcina spp. increased after the feeding in these reactors, which was followed by a depletion of δ(13)C in the produced gas. In both reactors, the less depleted isotopic values were detected before the second feeding, when Methanobacterium was the most active genus. Variations in reactor performance and methanogenic community characteristics were attributed to inoculum heterogeneity and stochastic factors during the reactor set up.

Analysis of greenhouse gas emissions from 10 biogas plants within the agricultural sector.

With the increasing number of biogas plants in Germany the necessity for an exact determination of the actual effect on the greenhouse gas emissions related to the energy production gains importance. Hitherto the life cycle assessments have been based on estimations of emissions of biogas plants. The lack of actual emission evaluations has been addressed within a project from which the selected results are presented here. The data presented here have been obtained during a survey in which 10 biogas plants were analysed within two measurement periods each. As the major methane emission sources the open storage of digestates ranging from 0.22 to 11.2% of the methane utilized and the exhaust of the co-generation units ranging from 0.40 to 3.28% have been identified. Relevant ammonia emissions have been detected from the open digestate storage. The main source of nitrous oxide emissions was the co-generation unit. Regarding the potential of measures to reduce emissions it is highly recommended to focus on the digestate storage and the exhaust of the co-generation.

Evaluation of stable isotope fingerprinting techniques for the assessment of the predominant methanogenic pathways in anaerobic digesters.

Laboratory biogas reactors were operated under various conditions using maize silage, chicken manure, or distillers grains as substrate. In addition to the standard process parameters, the hydrogen and carbon stable isotopic composition of biogas was analyzed to estimate the predominant methanogenic pathways as a potential process control tool. The isotopic fingerprinting technique was evaluated by parallel analysis of mcrA genes and their transcripts to study the diversity and activity of methanogens. The dominant hydrogenotrophs were Methanomicrobiales, while aceticlastic methanogens were represented by Methanosaeta and Methanosarcina at low and high organic loading rates, respectively. Major changes in the relative abundance of mcrA transcripts were observed compared to the results obtained from DNA level. In agreement with the molecular results, the isotope data suggested the predominance of the hydrogenotrophic pathway in one reactor fed with chicken manure, while both pathways were important in the other reactors. Short-term changes in the isotopic composition were followed, and a significant change in isotope values was observed after feeding a reactor digesting maize silage. This ability of stable isotope fingerprinting to follow short-term activity changes shows potential for indicating process failures and makes it a promising technology for process control.