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In this study, continuous operation of a pilot-scale upflow anaerobic sludge blanket (UASB) reactor for sewage treatment was conducted for 630 days to investigate the physical and microbial characteristics of the retained sludge. The UASB reactor with a working volume of 20.2 m³ was operated at ambient temperature (16-29 °C) and seeded with digested sludge. After 180 days of operation, when the sewage temperature had dropped to 20 °C or lower, the removal efficiency of both total suspended solids (TSS) and total biochemical oxygen demand (BOD) deteriorated due to washout of retained sludge. At low temperature, the cellulose concentration of the UASB sludge increased owing to the rate limitation of the hydrolytic reaction of suspended solids in the sewage. However, after an improvement in sludge retention (settleability and concentration) in the UASB reactor, the process performance stabilized and gave sufficient results (68% of TSS removal, 75% of total BOD removal) at an hydraulic retention time (HRT) of 9.7 h. The methanogenic activity of the retained sludge significantly increased after day 246 due to the accumulation of Methanosaeta and Methanobacterium following the improvement in sludge retention in the UASB reactor. Acid-forming bacteria from phylum Bacteroidetes were detected at high frequency; thus, these bacteria may have an important role in suspended solids degradation. [ABSTRACT FROM AUTHOR]

A thermophilic UASB reactor was operated at 55 deg. C for greater than 470 days in order to investigate the effects of feed composition on the changes in microbial community structure where thermophilic granular sludge was used as the inoculum source. The feed compositions were changed with cultivation days; phase 1 (1-70 days), alcohol distillery wastewater; phase 2 (71-281 days), artificial acetate wastewater; phase 3 (282-474 days), artificial sucrose wastewater. During the first one month of each phase, the methanogenic activity and cell density of methanogens quantified by fluorescence in situ hybridization (FISH) drastically changed as a result of shift in feed composition. When artificial acetate wastewater was used as feed, retained granular sludge was partially disintegrated due to a decrease in the number of symbiotic bacterial community members: acetogens (acidogens) and hydrogenotrophic methanogens. In contrast, when the feed was shifted to sucrose (phase 3), granulation of biomass was promoted by a remarkable proliferation of the symbiotic community. The presence of hydrogen-utilizing methanogens and acetogens (acidogens) are shown to be effective for the enhancement of thermophilic granulation. The cell density of methanogens determined by FISH was strongly correlated with the methane-producing potential of the retained thermophilic granular sludge. [ABSTRACT FROM AUTHOR]