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Methane fermentation, consisting of anaerobic degradation of organic matters and subsequent methanogenesis, is one of potentially attractive technologies for treatment of wastewater and biological wastes. Because methane fermentation is a cost-effective energy-yielding process, produces far less excess sludge than aerobic wastewater treatment systems, and a large part of the energy stored within organic matters can be recovered as biogas. However, it also has disadvantages, such as poor treating rate, low digestion efficiency and instability in reactor operation. These disadvantages are caused by the difficulty of monitoring in situ microbial community structure and metabolic functions in bioreactors. Methane fermentation is based on a complex community of microorganisms of wide phylogenetic diversities with different metabolic functions. Moreover, only poor proportion of microorganisms have been isolated and cultivated and analyzed. One possible approach to solve these disadvantages and achieve high rate and high efficient methane fermentation processes with stable reactor operation is, thus, to accumulate knowledge of the microbial communities and to use them as landmarks responsible for specific degradation pathways in methane fermentation. Therefore, we constructed continuous anaerobic methane fermentation processes using completely stirred tank reactors (CSTR) fed by specific substrates, such as acetate, propionate, butyrate, long-chain fatty acids, glycerol, protein (bovine serum albumin) and starch, to achieve the chemostat cultivations of microbial communities related to degradation of these substrates. For each microbial community under steady state conditions, we analyzed the structures and metabolic functions by mainly using molecular biological techniques, such as fluorescent in situ hybridization, 16S rRNA gene clone library analyses, quantitative real-time PCR techniques, quantitative RT-PCR and denaturing gradient gel electrophoresis. Even feeding the same substrates, the microbial communities were remarkably different by different dilution rates. For acetate-degrading communities, dilution rate effected the change in community structure, as well as shift of pathway between aceticlastic and non-aceticlastic methanogenesis. Additional Ni^<2+> and Co^<2+> in the wastewater fed into the reactors and operation temperature also affected on the community structure, as well as the reactor performance. Based on the fundamental knowledge in the landmarks of microbial communities for specific degradation pathways of organic matters, we subsequently evaluated the relationship between reactor performance and microbial community structures in bioreactors treating actual wastewaters and biological wastes, such as municipal solid wastes, awamori distillery wastewater, livestock manure and surplus sludge of a wastewater treating plant. Our research results would provide a significant milestone for achievement of stable operational methane fermentation process with high rate and efficiency.
2009-12-25

メタン発酵プロセスに関与する微生物群集

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