To investigate the mechanism of excess sludge reduction in an activated sludge process (ASP) that incorporates an anaerobic side-stream bioreactor (ASSR), four systems were operated in the laboratory: (a) ASP+ASSR (R1); (b) ASP+aerobic digester (R2); (c) ASP+anaerobic
digester (R3); and (d) ASP without solids wastage (R4). The overall sludge yield of ASP+ASSR (R1) was the lowest among four systems without showing any negative impacts on effluent quality, The net sludge yield of R1 was 24-57% and 4-49% less than the three other systems in phase I and
phase II, respectively. Accounting of biomass in R1 reveals that ASSR works like a conventional anaerobic digester but leads to additional sludge reduction in the main aeration basin. The results from batch digestion tests indicated that both anaerobically and aerobically digestible sludge
degrade in R1. The extracellular polymeric substances (EPS) extraction results showed that release and degradation of base-extractable EPS, thought to be iron and/or aluminum bound floc materials, accounts for lower sludge yield in ASP+ASSR. The bacterial fingerprinting data showed that
there was high similarity of microbial composition between the anaerobic side-stream reactor and the anaerobic digester; however, there were unique microbial cells in the side-stream tank reactor as well, indicating that some microbial cells were enriched in anaerobic side-stream bioreactor
due to the continuous sludge recirculation via minimal sludge wasting.
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