Norbert Ács

Improvement of Biogas Production by Bioaugmentation

Biogas production technologies commonly involve the use of natural anaerobic consortia of microbes. The objective of this study was to elucidate the importance of hydrogen in this complex microbial food chain. Novel laboratory biogas reactor prototypes were designed and constructed. The fates of pure hydrogen-producing cultures of Caldicellulosiruptor saccharolyticus and Enterobacter cloacae were followed in time in thermophilic and mesophilic natural biogas-producing communities, respectively. Molecular biological techniques were applied to study the altered ecosystems. A systematic study in 5-litre CSTR digesters revealed that a key fermentation parameter in the maintenance of an altered population balance is the loading rate of total organic solids. Intensification of the biogas production was observed and the results corroborate that the enhanced biogas productivity is associated with the increased abundance of the hydrogen producers. Fermentation parameters did not indicate signs of failure in the biogas production process. Rational construction of more efficient and sustainable biogas-producing microbial consortia is proposed.

Exploitation of the extremely thermophilic Caldicellulosiruptor saccharolyticus in hydrogen and biogas production from biomasses

Caldicellulosiruptor saccharolyticus has attracted considerable attention by virtue of its ability to degrade various polysaccharide, oligosaccharide and monosaccharide substrates at temperatures above 70 °C, while its ability to convert various sugars to hydrogen has led to C. saccharolyticus being selected for the fermentative production of hydrogen. In this study, the utilization of a pure cellulosic substrate and mixed biomasses of plant origin was investigated. Cellulase biosynthesis can be triggered by growing cells on various monomeric carbohydrates, e.g. glucose or fructose. Pretreatment with cellulase‐producing Bacilli improves the hydrogen yield, indicating that C. saccharolyticus alone can only partially decompose cellulosic substrates. The hydrogen‐producing activity of C. saccharolyticus can be exploited in biogas technologies. With appropriate induction of the polymer‐degrading enzymes, C. saccharolyticus may become a prime candidate with which to improve the yield and efficacy of practical hydrogen‐ and biogas‐producing processes.

Metagenome changes in the mesophilic biogas-producing community during fermentation of the green alga Scenedesmus obliquus

A microalgal biomass offers a potential alternative to the maize silage commonly used in biogas technology. In this study, photoautotrophically grown Scenedesmus obliquus was used as biogas substrate. This microalga has a low C/N ratio of 8.5 relative to the optimum 20-30. A significant increase in the ammonium ion content was not observed. The methane content of the biogas generated from Sc. obliquus proved to be higher than that from maize silage, but the specific biogas yield was lower. Semi-continuous steady biogas production lasted for 2 months. Because of the thick cell wall of Sc. obliquus, the biomass-degrading microorganisms require additional time to digest its biomass. The methane concentration in the biogas was also high, in co-digestion (i.e., 52-56%) as in alga-fed anaerobic digestion (i.e., 55-62%). These results may be related to the relative predominance of the order Clostridiales in co-digestion and to the more balanced C/N ratio of the mixed algal-maize biomass. Predominance of the order Methanosarcinales was observed in the domain Archaea, which supported the diversity of metabolic pathways in the process.

Changes in the Archaea microbial community when the biogas fermenters are fed with protein-rich substrates

Terminal restriction fragment length polymorphism (T-RFLP) was applied to study the changes in the composition of the methanogens of biogas-producing microbial communities on adaptation to protein-rich monosubstrates such as casein and blood. Specially developed laboratory scale (5-L) continuously stirred tank reactors have been developed and used in these experiments. Sequencing of the appropriate T-RF fragments selected from a methanogen-specific (mcrA gene-based) library revealed that the methanogens responded to the unconventional substrates by changing the community structure. T-RFLP of the 16S rDNA gene confirmed the findings.

Bioaugmentation of biogas production by a hydrogen-producing bacterium

The rate-limiting nature of the hydrogen concentration prevailing in the anaerobic digester has been recognized, but the associated alterations in the microbial community are unknown. In response to the addition of Enterobacter cloacae cells in laboratory anaerobic digesters, the level of biogas production was augmented. Terminal restriction fragment length polymorphism (T-RFLP) and real-time polymerase chain reaction (Real-Time PCR) were used to study the survival of mesophilic hydrogen-producing bacteria and the effects of their presence on the composition of the other members of the bacterial community. E. cloacae proved to maintain a stable cell number and to influence the microbial composition of the system. Bioaugmentation by a single strain added to the natural biogas-producing microbial community was demonstrated. The community underwent pronounced changes as a result of the relatively slight initial shift in the microbiological system, responding sensitively to the alterations in local hydrogen concentration.

Conversion of H2 and CO2 to CH4 and acetate in fed-batch biogas reactors by mixed biogas community: A novel route for the power-to-gas concept

BackgroundApplications of the power-to-gas principle for the handling of surplus renewable electricity have been proposed. The feasibility of using hydrogenotrophic methanogens as CH4 generating catalysts has been demonstrated. Laboratory and scale-up experiments have corroborated the benefits of the CO2 mitigation via biotechnological conversion of H2 and CO2 to CH4. A major bottleneck in the process is the gas–liquid mass transfer of H2.ResultsFed-batch reactor configuration was tested at mesophilic temperature in laboratory experiments in order to improve the contact time and H2 mass transfer between the gas and liquid phases. Effluent from an industrial biogas facility served as biocatalyst. The bicarbonate content of the effluent was depleted after some time, but the addition of stoichiometric CO2 sustained H2 conversion for an extended period of time and prevented a pH shift. The microbial community generated biogas from the added α-cellulose substrate with concomitant H2 conversion, but the organic substrate did not facilitate H2 consumption. Fed-batch operational mode allowed a fourfold increase in volumetric H2 load and a 6.5-fold augmentation of the CH4 formation rate relative to the CSTR reactor configuration. Acetate was the major by-product of the reaction.ConclusionsFed-batch reactors significantly improve the efficiency of the biological power-to-gas process. Besides their storage function, biogas fermentation effluent reservoirs can serve as large-scale bio CH4 reactors. On the basis of this recognition, a novel concept is proposed, which merges biogas technology with other means of renewable electricity production for improved efficiency and sustainability.

Bioaugmentation of the thermophilic anaerobic biodegradation of cellulose and corn stover

Two stable, thermophilic mixed cellulolytic consortia were enriched from an industrial scale biogas fermenter. The two consortia, marked as AD1 and AD2, were used for bioaugmentation in laboratory scale batch reactors. They enhanced the methane yield by 22-24%. Next generation sequencing method revealed the main orders being Thermoanaerobacterales and Clostridiales and the predominant strains were Thermoanaerobacterium thermosaccharolyticum, Caldanaerobacter subterraneus, Thermoanaerobacter pseudethanolicus and Clostridium cellulolyticum. The effect of these strains, cultivated in pure cultures, was investigated with the aim of reconstructing the defined cellulolytic consortium. The addition of the four bacterial strains and their mixture to the biogas fermenters enhanced the methane yield by 10-11% but it was not as efficient as the original communities indicating the significant contribution by members of the enriched communities present in low abundance.

Biomethane: The energy storage, platform chemical and greenhouse gas mitigation target

Results in three areas of anaerobic microbiology in which methane formation and utilization plays central part are reviewed. a.) Bio-methane formation by reduction of carbon dioxide in the power-to-gas process and the various possibilities of improvement of the process is a very intensively studied topic recently. From the numerous potential methods of exploiting methane of biological origin two aspects are discussed in detail. b.) Methane can serve as a platform chemical in various chemical and biochemical synthetic processes. Particular emphasis is put on the biochemical conversion pathways involving methanotrophs and their methane monooxygenase-catalyzed reactions leading to various small molecules and polymeric materials such as extracellular polysaccharides, polyhydroxyalkanoates and proteins. c.) The third area covered concerns methane-consuming reactions and methane emission mitigation. These investigations comprise the anaerobic microbiology of ruminants and approaches to diminishing methane emissions from ruminant animals.

Improvement of biogas production by biotechnological manipulation of the microbial population

Biogas is a renewable energy carrier and the production of biogas is associated with double benefits: elimination of environmental pollution problems is coupled with the generation of useful energy. Biogas technologies commonly apply natural anaerobic consortia of microbes. This is partly due to the fact that, from a microbiological aspect, this is a very complicated and complex system. Moreover, the population dynamics of the natural ecosystems could not be properly studied before the introduction of molecular biological techniques. Research on the diversity of these microbial communities is needed for the optimization of biogas production technologies as their economic viability is closely related to the efficacy of the concerted microbiological actions. A systematic study, using 5-litre CSTR digesters, disclosed that a key fermentation parameter is the loading rate of organic total solids (OTS) in maintaining an altered population balance.