The effect of sludge lysis using ultrasonic waves on the sludge yield coefficient was investigated using pilot-scale SBR reactors. Our results revealed that reduction of excess sludge is affected by multiple factors including the energy supplied to sludge mass, the power used for wave generation, duration of wave usage, and percentage of sludge exposed to the waves. Increasing the energy supplied to the sludge mass resulted in more reduction of sludge yield but energies greater than 35,000 kJ/kg VSS showed no significant effect on sludge reduction. Moreover, less power and more retention time showed better performance in sludge reduction. Using ultrasonic waves for reducing sludge yield deteriorated the quality of the reactor effluent and also increasing volume of the sludge lysed by waves at every run brought about decreasing COD removal efficiency. In order to meet standards for effluent discharge to surface and groundwater bodies, a maximum 30% of the system sludge could be lysed resulting in approximately 78% reduction of sludge yield. In this case, the sludge SOUR decreased down to 9 mg DO/g SS.h, but the sludge sedimentation property was improved and SVI was reduced. (C) 2011 Elsevier B.V. All rights reserved.