The aim of the project MicroPoll (www.umwelt-schweiz.ch/micropoll) is to compile decision-support information and to develop a strategy for reducing the release of micropollutants from wastewater treatment plants (WWTPs) in Switzerland. Within the project the WWTP of Regensdorf near Zurich (current load 15,000 inhabitants plus 10,000 population equivalents) was upgraded with a full-scale ozonation step after secondary treatment and prior to sand filtration. The method developed for the screening of micropollutants is comprehensive and can also be applied for other enhanced or advanced water treatment processes. The full-scale reactor proves ozonation to be an efficient technique for the elimination of micropollutants from secondary effluent as well as for disinfection at reasonable additional energy consumption. The specific and non-specific toxicity is significantly reduced by ozonation indicating that no further toxic compounds are produced. Additional sand filtration is useful for elimination of N-nitrosodimethylamine and biodegradable compounds formed during ozonation.