Methods for treating aqueous effluent, which include reducing sludge formation during the biological treatment of an aqueous effluent. The sludge formation is reduced by injecting an ozone containing gas into an aeration tank which contains the effluent. The affluent is in contact with microorganisms in the tank at least once during the method. The ozone containing gas contains at least 2.5 mg of ozone per liter of the gas.