A membrane separation process was coupled to a sequencing batch reactor (SBR) for biological nutrient removal (BNR) processes and a combined system was named a membrane sequencing batch reactor (MSBR). MSBR was used for the treatment of dairy industry wastewater and optimized to increase the treatment efficiency. Since a diffuser-attached module design, subcritical flux operation, and intermittent suction method were adapted to the system, long-term operation was possible, i.e. the system could be operated for more than 110 days with only one membrane washing. BOD removal was high (97-98%) and stable. SS (suspended solid)-free effluent was obtained by membrane separation. Since nitrogen was mainly consumed as nutrient for synthesis of new cells due to the low influent concentration, the removal rate reached 96%. Phosphorus removal was relatively low because of the limit of the biological process, i.e. removal efficiency ultimately depends on the amount of excess sludge wasting. A removal rate of 80% was reached after system optimization. (C) 2003 Elsevier Science Ltd. All rights reserved.