The Metropolitan Area of Istanbul is the major polluter in the Marmara Sea which shows ail the indications of a sensitive zone. The adopted wastewater management policy involves biological nitrogen and phosphorus removal at all the discharge points in the area. The Riva plant at the Black Sea coast is designed as a two-stage step feeding system. A model simulation shows that the proposed design cannot meet the effluent standard of 10 mgl(-1) total N for the sewage character assessed on the basis of an experimental study, totally different from the one adopted for design. Copyright (C) 1996 IAWQ.