Abstract
Bacterial composition of the flocs and the aqueous phase in an anaerobic digester was examined by analysis of fermentation products, enumeration of trophic groups and isolation of predominant bacteria. Digester contents were separated into the floc fraction and the bulk aqueous phase fraction by standing. Starch was converted completely to methane and carbon dioxide by the bacterial floc fraction, but in the free-living bacterial fraction a much smaller amount of methane was produced and propionic, acetic and butyric acids were accumulated. In the floc fraction, three bacterial trophic group populations (cells/g dry weight) were present; starch-degrading bacteria 4.5×107, propionate-producing bacteria 4.5×1010, sulfate-reducing bacteria 1.8×106, propionate-degrading bacteria 4.0×104, butyrate-degrading bacteria 1.0×108, and methanogenic bacteria 1.9×1012. In the free-living bacterial fraction, the bacterial numbers of starch-degrading bacteria, propionate-producing bacteria, sulfate-reducing bacteria and methanogenic bacteria were 5.2×108, 6.0×1011, 6.0×105 and 7.6×1011, respectively. Floc flora comprised a series of bacteria from hydrolytic bacteria to methanogenic bacteria involved in the methane fermentation from starch. However, there were few syntrophic acetogenic bacteria and filamentous acetate-utilizing methanogens in the bulk aqueous phase of the anaerobic digesters, although fermentative bacteria and hydrogen-utilizing methanogens were predominant.
