BACKGROUND: The occurrence of emerging contaminants in treated municipal wastewaters is of increasing concern from health and safety points of view. Existing conventional wastewater treatment plants were not designed to remove these types of contaminants. In this work, the efficacy of an ozonation stage integrated with a biological oxidation process treating municipal urban wastewater spiked with MCPA has been assessed. RESULTS: Previous aerobic biodegradation experiments showed that MCPA was barely removed from urban wastewater; just 7.2% MCPA conversion was attained in 7 h. Total elimination of herbicide from municipal wastewater was achieved for an ozone dose of 176.5 mg O-3 g(-1) MLVSS. After ozonation in the activated sludge process, the sludge volumetric index significantly decreased in all cases regardless of the ozonation time applied. An ozone quantity of 85.1 mg O-3 g(-1) MLVSS dramatically reduced the amount of filamentous bacteria (20 min ozone exposure). 40 min of ozone exposure also eliminated filamentous bacteria, although this high applied dose started to affect protozoa viability. CONCLUSIONS: Ozone applied directly in the biodegradation process favours MCPA elimination also improving activated settling properties. Under the conditions used, ozone is a selective disinfectant not affecting the key groups of microfauna responsible for the activated sludge process. (C) 2014 Society of Chemical Industry