The ozonation of activated sludge has been used as a technical measure for bulking control in a high number of full-scale wastewater treatment plants (WWTP), despite a lack of precise
predictions on the level of reduction in filament growth or the lack of knowledge of impact on microbial community from this technique. Ozone is a strong oxidant reacting rapidly with
suspended solids. Various studies have suggested that ozone attacks the bacterial cell surface, alters the permeability of the cell membrane and ultimately results in the leakage of cell
contents. However, the microbes in the sludge form a complex matrix, and ozone may affect bacterial populations at different rates different depending on their locations in the floc or their
capacity for adaptation. Nitrification, a key step of the nitrogen cycle, is the sequential oxidation of ammonia via nitrite to nitrate. This process is catalysed by ammonia-oxidizing bacteria
(AOB) and nitrite-oxidizing bacteria (NOB), whose cooperation is needed to achieve complete nitrification. Although the nitrification process in WWTPs has been investigated in depth, the response of microbial communities are still a focus of considerable interest due to their high sensitivity to inhibitory compounds and environmental factors that results in repeated
breakdowns of nitrification performance. In this study, we focus on two aspects that have not been thoroughly considered in previous studies; the use of ozone for Gordonia foaming
elimination on dynamic population of a nitrifying bacterial community, and the nitrification performance of activated sludge system.