The aim of this work was to study the effect of ultra sound treatment on the solid content of sludge and biological activity, and the increase in the soluble chemical oxygen demand (SCOD), proteins and nucleic acids concentrations during sonication. The results showed that sonication effectively degraded and inactivated the sludge. The sludge disintegration and cell lysis occurred continuously while sludge inactivation mainly occurred in the second stage (10-30 min) during sonication. The SCOD, supernatant proteins and nucleic acids concentrations, and sludge mass reduction and inactivation degrees increased with the sonication time and power density increases. Higher energy ultrasound was more efficient than lower energy ultrasound for the sludge treatment. (c) 2005 Elsevier Ltd. All rights reserved.