A Redefinition of the Role of Microorganisms in Bioaggregation in Activated Sludge

Aggregation of microorganisms is considered an important trait crucial to their ecological success and has significant implications for biological wastewater treatment. It has been thought that floc formation in such systems is primarily mediated by floc-forming bacteria, whereas other organisms do not really contribute to flocculation. This hypothesis was tested here by a combination of culture-dependent and molecular biology techniques to characterize and compare the aggregation capability of bacteria isolated from activated sludge flocs and the coexisting planktonic community, and to assess the microbial diversity and community structure of the two communities. Both results indicate significant similarity and difference between the two communities with respect to species richness and abundance, suggesting that microbial aggregation that leads to flocculation in activated sludge may be contributed by most, if not all, bacteria present in the system. It is also shown that growth conditions of many bacteria play an important role in the development of aggregates. These observations challenge the previous hypothesis and argue for a redefinition of the role of various bacteria in bioaggregation in activated sludge.