The process according to the invention for treating water in a treatment plant comprising a reaction chamber in which a biodegradable material present in the water is biodegraded at least partly by microorganisms, in the course of which, at least temporarily, water is transferred out of the reaction chamber into a separating chamber and the biodegradable material and the microorganisms are separated at least partly from the water therein, a return stream comprising biodegradable material and at least some of the microorganisms flowing out of the separating chamber into the reaction chamber via a recycle line, is characterized in that at least some of the following substances: a) the biodegradable material and b) the microorganisms are treated with ozone (O3), an amount of 15 to 150 milligrams of ozone per kilogram of dry substance of the biodegradable material and of the microorganisms being added per hour in the reaction chamber.