The present invention relates to a method for treating liquids, comprising the steps of irradiating a flow of air and a flow of the liquid to be treated at the same time in order to create ozone in both the air and the liquid, mixing the ozone-containing air with the liquid to be treated up-streams the liquid irradiating point, irradiating the flow of liquid containing the in-mixed ozone in order to break down the ozone in the liquid for producing free radicals.