A modified activated sludge process called contact-stabilization-denitrification makes it possible to eliminate 80% to 90% of the incoming total nitrogen in addition to the usual BOD removal. A correlation between the organics removal and denitrification was made with laboratory data obtained and the data of other investigators. The principle of the process is as follows: Following the contact period, the activated sludge is separated from the clarified liquor by sedimentation. The clarified liquor, low in carbon, is pumped to the trickling filter for nitrification. The nitrified effluent and the sludge loaded with soluble and colloidal organic materials are contacted with each other in the denitrification tank for the stabilization of the organic matter and denitrification. The oxygen requirement for the stabilization of organic matter removed in the contact tank is supplied by the nitrate as the stabilization tank is kept anaerobic. The final effluent obtained after clarification is low, both in carbon and nitrogen.