In this study, the characteristics of sonicated waste activated sludge (WAS) originating from a nutrient-removal wastewater treatment plant were investigated and evaluated. Different combinations of power inputs, sonication durations and volumes were used for optimization of the sonication conditions. Ultrasound density levels ranged between 0.32 and 3.2 W/mL. Optimal conditions based on soluble COD concentrations (SCOD) after sonication at different ultrasound density levels and sonication durations (1, 5, 10 and 30 min) were determined. An ultrasonic density of 1.6 W/mL and a sonication time of 30 min were identified as optimal sonication conditions in terms of SCOD concentration. The sludge sonicated under these optimal conditions was further investigated to determine the effect of ultrasound treatment on solubilization of organic matter and volatile solids, particle size and biodegradability. Disruption of the sludge structure was confirmed by the increase in the SCOD by 35.5%, reduction in volatile suspended solids by 26% and reduction in particle size by 85%. The availability of COD fractions of sonicated sludge was also tested with oxygen uptake rate and phosphate release data indicating that using sonicated sludge for both subsequent biological treatment and sludge treatment is promising.