Previous study showed that sonication was effective to reduce waste activated sludge (WAS) using artificial wastewater. This paper confirms the viability and evaluates the performance of this method in practical wastewater treatment using urban sewage without temperature control. The results showed that sonication significantly lowered the WAS and biomass synthesis, and greatly enhanced the mineralization of sewage organics. The optimal specific energy for sludge lysis was 20.0 kWh/kgDS. Further energy-increase had little benefit on WAS reduction. When the specific energy was 20.0 kWh/kgDS and the sludge recycle ratio was 0.007, the WAS decreased by 54%, the biomass synthesis abated by 59%, and the sewage mineralization ratio increased from 31% to 58%. The effluent COD and nitrogen were stable but phosphorus was higher than that of the control bioreactor. The COD removal was lower but the WAS reduction was higher for urban sewage than for artificial wastewater. The accumulation pattern of heavy metals in sludge was greatly alternated by the sonication-cryptic growth: and different metals behaved differently. The sludge Ni concentration increased by 141% while As decreased by 53%. (C) 2008 Elsevier B.V. All rights reserved.