For the prevention of excess sludge production from a membrane bioreactor (MBR), an ultrasonic cell disintegration process was incorporated. The results of this study showed that excess sludge production could be prevented using an ultrasound hybrid (MBR-US) system at an organic loading of around 0.91 kg BOD5/m(3) per day. Under the same organic loading rate, the mixed liquor suspended solid (MLSS) of MBR-US system was maintained at 7000-8000 mg/l while the MLSS of a conventional MBR increased from 7000 to 13,700 mg/l during the experimental period. While sludge production was completely prevented, the effluent quality of the MBR-US system slightly deteriorated. The additional organic loading caused by disintegrated sludge return was considered to be a reason. With sonication the volume of the average particle size of the sludge in the aeration tank decreased from 132 to 95 mum. In the MBR-US system, around 25-30% of total phosphorus removal was achieved without sludge removal from the aeration tank. (C) 2003 Elsevier Ltd. All rights reserved.