Waste activated sludge (WAS) is more difficult to digest than primary sludge due to rate limiting cell hydrolysis. High-power ultrasound can effectively disintegrate the bacterial cells and thus enhance the subsequent digestion. This research examines the effectiveness of ultrasound pretreatment on WAS disintegration at different specific energy inputs, ultrasonic densities and total solids (TS) contents. The results show that the cut diameter (d(50)) for WAS with 2% TS content declined nearly 6.5-fold at an ultrasonic density of 0.67 W/ml. For higher TS contents of 4 and 6%, higher densities of 1.03 and 0.86 W/ml, respectively, were needed to achieve the same degree of particle size reduction. The efficacy of ultrasonic disintegration measured as soluble chemical oxygen demand (SCOD) release was primarily governed by ultrasonic density (W/ml); whereas ultrasonic density did not show a significant effect on protein release at all TS levels. SCOD release of about 320 mg SCOD/g TS was obtained at a TS content of 2% and specific energy input of 5 kWs/g TS. The SCOD release, however, decreased to 160 and 90 mgSCOD/g TS at 4 and 6% TS contents, respectively. The highest protein release of 73 mg/g TS was obtained at a TS content of 2% and specific energy input of 10 kWs/g TS. The sludge disintegration efficiency declined significantly at higher TS content. Thus, there is a limiting TS concentration that could be effectively - disintegrated by ultrasound, and this is governed by the capability of an ultrasonic unit in producing cavitation. The degree of disintegration also depends on types of ultrasonic unit used.