Microbial community structure of wastewater treatment subjected to high mortality rate due to ozonation of return activated sludge.

AIMS: This study investigated the effects of return activated sludge (RAS) ozonation, on the bacterial community structure of pilot-scale wastewater treatment systems. METHODS AND
RESULTS: Two parallel activated sludge reactors were operated to treat real municipal wastewater for 98 days. The RAS of one of the reactors was subjected to increasing doses of ozone during the experimental period, which resulted in higher reduction in biosolids waste production and higher bacterial growth rate. The bacterial community structures were investigated by 16S rRNA gene amplicon high-throughput pyrosequencing and fluorescence in situ hybridization (FISH). The structures remained highly similar throughout the experiment despite the ozone treatment. Comparative analyses between pyrosequencing and FISH revealed clear discrepancies in the proportion of some bacterial populations.
CONCLUSIONS: The results suggest that RAS ozonation is not a main environmental factor structuring the community composition. Instead, the parallel drifts and slight convergence of the two community structures indicate that other environmental factors such as influent wastewater composition and temperature may be more important. Care should be exercised in interpreting the proportion of sequence reads as pyrosequencing may be biased as compared to FISH. SIGNIFICANCE AND IMPACT OF THE STUDY: This study provides new insights on the importance of indiscriminate high mortality rates brought by external factors (here ozonation) on microbial community structures of activated sludge system.