Wastewaters generated by many economically relevant industrial activities contain recalcitrant organic compounds which pass unaltered through biological stage of the treatment plant making it difficult to meet the discharge limits currently in force. Therefore, an additional treatment is usually required to remove these compounds. In this study, the application of ozonation together with biological treatment was investigated. In particular, the effectiveness of biological degradation followed by or integrated with ozonation for treating the effluents produced by three environmentally relevant activities (i.e., leather and textile processing and municipal waste landfilling) are compared in the present paper. The results show that biological treatment followed by ozonation does not guarantee depurative levels sufficient for discharge for landfill leachates and tannery wastewater. On the contrary, thanks to the synergy between biological degradation and zonation, integrated treatment significantly improves the process performance for all the investigated wastewaters, thus allowing the discharge limits to be met. (C) 2011 Elsevier Ltd. All rights reserved.