This study investigates the physical-chemical characteristics Of sludge treated with controlled levels of ultrasound. The results indicate that the energy used for sonication strongly influences the physical-chemical characteristics of sludge. Based on turbidity and settling velocity measurements, 1000 kJ/kg total solids (TSs) is recommended as an optimal specific energy input for improving sludge settling. For purposes of sludge disintegration, applying higher specific energies to the sludge is more efficient than applying lower levels. Nevertheless, a single, optimal energy level for thoroughly disintegrating sludge could not be determined. Possible mechanisms of ultrasonic treatment are also discussed. (C) 2008 Elsevier B.V. All rights reserved.