Ozone treatment efficiently removes many micropollutants contained in wastewater, but in some cases, it can lead to the formation of toxic byproducts. “Whether wastewater treatment plants should implement ozone treatment depends on the water they treat,” says Urs von Gunten, joint professor at EPFL and Eawag. “When, for example, ozone treatment is used to treat wastewater containing bromide, bromate can be formed, a potentially carcinogenic compound.” Von Gunten and his team developed a five-step test procedure to assess whether the wastewater from a specific plant can be safely treated with ozone. “The idea is that the test, which will assist cantonal agencies and federal authorities in deciding how best to upgrade wastewater treatment plants,” he says. In the coming years about 100 wastewater treatment plants in Switzerland will be upgraded, adding treatment steps specifically designed to eliminate micropollutants.