This paper studied a new method to improve the microbial activity of the activated sludge for wastewater treatment. Concentrated sludge was sonicated in an extra chamber for short period and then returned to the activated sludge system. The results showed that the bio-activity of the activated sludge, expressed as oxygen utilization rate (OUR), could be enhanced by ultrasonic irradiation. Powerful ultrasound (in the magnitude of W/ml) was much more effective than weak ultrasound (in the magnitude of W/L) in stimulating the activated sludge, but too strong sonication (power density higher than 0.5 W/ml) disintegrated the sludge and thus decreased the sludge activity. Low frequency (25 kHz) was more effective than higher ones (80 kHz and 150 kHz), indicating that mechanical effects, instead of free radicals, were responsible for the bio-activity enhancement. The optimal sonication conditions were sound frequency of 25 kHz, power density of 0.2 W/ml and duration of 30 s; under which the sludge OUR increased by 28%, the bio-mass growth rate increased by 12.5%, and the wastewater chemical oxygen demand (COD) and total nitrogen removal efficiency increased by 5-6%. (C) 2007 Elsevier Ltd. All rights reserved.