As one of the largest means of achieving pollution control of our waterways, activated sludge is of considerable practical importance. Satisfactory operation of this process, however, has often been hampered by various problems associated with poor settling capability of activated sludge. Despite extensive research, no definitive solutions to these problems exist due to a lack of knowledge of the mechanisms involved in bioaggregation that leads to flocculation. Previous studies have focused on establishing a direct link between operational conditions and physical characteristics of activated sludge and largely ignored the complex microbial community involved. To provide this missing link, four laboratory-scale sequencing batch reactors were operated at SRTs of 4, 8, 16, and 32 days, respectively, for over eight months, during which reactor performance, sludge settling characteristics, and microbial community structure were monitored. All reactors reached steady state after five times the SRTs. In agreement with previous research, a positive correlation was demonstrated between SRT and sludge settling capability at steady state, indicating an impact of SRT on bioaggregation/flocculation. While this is generally thought due to the involvement of protozoa and extracellular polymeric substances in flocculation, this study provides an alternative explanation, i.e. selection of a microbial community that has better aggregation capability. This is supported by two lines of evidence: 1) significantly different microbial community structures between the reactors at steady state; and 2) more diverse bacterial community with an increase in SRT, which might be resulted from less severe “competitive exclusion” due to the improved bioaggregation process. These results provide important insights into the complex relationship between SRT, microbial diversity, and settling characteristics of activated sludge.
Activated sludge, solids retention time, microbial diversity, sludge volume index, flocculation.