Originally published as:Kleyböcker, A., Liebrich, M., Verstraete, W., Kraume, M., Würdemann, H. (2012) ABSTRACTEarly warning indicators for process failures were investigated to develop a reliable method to increase the production efficiency of biogas plants. Organic overloads by the excessive addition of rapeseed oil were used to provoke the decrease in the gas production rate. Besides typical monitoring parameters, as pH, methane and hydrogen contents, biogas production rate and concentrations of fatty acids; carbon dioxide content, concentrations of calcium and phosphate were monitored. The concentration ratio of volatile fatty acids to calcium acted as an early warning indicator (EWI-VFA/Ca). The EWI-VFA/Ca always clearly and reliably indicated a process imbalance by exhibiting a two-to threefold increase 3 to 7 days before the process failure occurred. At this time, it was still possible to take countermeasures successfully. Furthermore, increases in phosphate concentration and in the concentration ratio of phosphate to calcium also indicated a process failure, in some cases, even earlier than the EWI-VFA/Ca.
Microbial community diversity in two thermophilic laboratory-scale and three full-scale anaerobic co-digesters was analysed by genetic profiling based on PCR-amplified partial 16S rRNA genes. In parallel operated laboratory reactors a stepwise increase of the organic loading rate (OLR) resulted in a decrease of methane production and an accumulation of volatile fatty acids (VFA). However, almost three-fold different OLRs were necessary to inhibit the gas production in the reactors. During stable reactor performance, no significant differences in the bacterial community structures were detected, except for in the archaeal communities. Sequencing of archaeal PCR products revealed a dominance of the acetoclastic methanogen Methanosarcina thermophila, while hydrogenotrophic methanogens were of minor importance and differed additionally in their abundance between reactors. As a consequence of the perturbation, changes in bacterial and archaeal populations were observed. After organic overload, hydrogenotrophic methanogens (Methanospirillum hungatei and Methanoculleus receptaculi) became more dominant, especially in the reactor attributed by a higher OLR capacity. In addition, aggregates composed of mineral and organic layers formed during organic overload and indicated tight spatial relationships between minerals and microbial processes that may support de-acidification processes in over-acidified sludge. Comparative analyses of mesophilic stationary phase full-scale reactors additionally indicated a correlation between the diversity of methanogens and the VFA concentration combined with the methane yield. This study demonstrates that the coexistence of two types of methanogens, i.e. hydrogenotrophic and acetoclastic methanogens is necessary to respond successfully to perturbation and leads to stable process performance.
As for other renewable energies in Germany, biogas production has rapidly expanded in recent years, such that the current installed capacity in this country accounts for around half of the European total. Against this background, and recognising the actual research need in the profitability analysis of along the whole supply chains for biogas, this paper carries out an economic analysis of three operational German co-digestion biogas plants, which employ biowaste, sewage sludge and energy crops for electricity production as well as for injection of biomethane into the natural gas grid. The profitability of the considered plants is assessed using the static Profit Comparison and the dynamic Net Present Value methods. From the analysis of each of the plants based on several technical and economic assumptions, the production costs for electricity and biomethane have been derived. Investment-related costs and substrate prices represent the most important financial variables of the considered plants. The electricity production from energy crops appears to be the most lucrative option, with a dynamic pay-back period of 6.7 years. Hence, subsidies and incentive schemes for biogas play a key role for the plants using energy crops as well as for plants using biowaste and sewage sludge. Furthermore, process failures under mesophilic conditions are here briefly described and could affect the biogas yield and thus have an influence on the profitability of the plants. The obtained results represent the foundation for a forthcoming comprehensive energy system analysis of the integration of biogas into the German energy system.
Originally published as:Lienen, T., Kleyböcker, A., Verstraete, W., Würdemann, H. (2014) AbstractThe microbial community composition in a full-scale biogas plant fed with sewage sludge and fat, oil and grease (
Originally published as:Kleyböcker, A., Liebrich, M., Kasina, M., Kraume, M., Wittmaier, M., Würdemann, H. (2012) ABSTRACTFollowing a process failure in a full-scale biogas reactor, different counter measures were undertaken to stabilize the process of biogas formation, including the reduction of the organic loading rate, the addition of sodium hydroxide (NaOH), and the introduction of calcium oxide (CaO). Corresponding to the results of the process recovery in the full-scale digester, laboratory experiments showed that CaO was more capable of stabilizing the process than NaOH. While both additives were able to raise the pH to a neutral milieu (pH > 7.0), the formation of aggregates was observed particularly when CaO was used as the additive. Scanning electron microscopy investigations revealed calcium phosphate compounds in the core of the aggregates. Phosphate seemed to be released by phosphorus-accumulating organisms, when volatile fatty acids accumulated. The calcium, which was charged by the CaO addition, formed insoluble salts with long chain fatty acids, and caused the precipitation of calcium phosphate compounds. These aggregates were surrounded by a white layer of carbon rich organic matter, probably consisting of volatile fatty acids. Thus, during the process recovery with CaO, the decrease in the amount of accumulated acids in the liquid phase was likely enabled by (1) the formation of insoluble calcium salts with long chain fatty acids, (2) the adsorption of volatile fatty acids by the precipitates, (3) the acid uptake by phosphorus-accumulating organisms and (4) the degradation of volatile fatty acids in the aggregates. Furthermore, this mechanism enabled a stable process performance after re-activation of biogas production. In contrast, during the counter measure with NaOH aggregate formation was only minor resulting in a rapid process failure subsequent the increase of the organic loading rate.2
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