The effect of ultrasonics in a biological activated carbon membrane reactor on the removal of organic pollutants in micro-polluted water was investigated through parallel experiments. Results showed that ultrasonics at a power of 10 W could enhance biological activity significantly. The effect of ultrasonic treatment could be maintained for 24 h, so the time interval for ultrasonic treatment should be set at 24 h. The ultrasonic effect on biodegradation was studied through the organic load of the bioreactor, 2,3,5-triphenyl tetrazoliumchloride-dehydrogenase activity (TTC-DHA) of biological activated carbon and the apparent molecular size distribution (AMSD) of the organic compounds in influent and effluent water. The ultrasonic field in the reactor was analyzed and the mechanism of biological activity enhancement by ultrasonics was investigated. (c) 2005 Elsevier Ltd. All rights reserved.