Abstract: Aerobic purification of waste water with intermittent batchwise supply of waste water to the activated sludge tank from which the treated waste water drains continuously is disclosed, in which the nitrate content in the waste water is simultaneously reduced by throttling the aeration of the activated sludge tank, starting with the admission of the waste water to the tank, to values equivalent to less than or equal to 0.5 mg/l dissolved oxygen. In particular, aeration may be off until such time as the nitrate content in the tank has dropped below 5 mg/l, in particular below 1 mg/l. In the case of urban waste waters a cycle time of about 1 hour is expediently employed under these circumstances and the aeration is shut off for about 5 minutes, starting from the rapid admission of the waste water charge. Preferably, inlet charges are less than or equal to 30%, in particular between about 5% and 20%, of the tank capacity.