In this study, the effects of combination of ultrasonic process with UV light and both classical Fenton (CFP) and modified Fenton (MFP) processes on disintegration of waste sludge were investigated. Fe2+ and Fe-0 dosage, hydrogen peroxide dosage, reaction time, pH, and different UV lamps were optimized to achieve a high degree of disintegration (DD). In addition, kinetic study and toxicity analysis were performed under optimum conditions for all processes. For CFP, the best DD 22.85% was found with optimum pH 3, reaction time 60 min, 7 g/kg TS Fe2+, 35 g/kg TS H2O2 doses, and UV-C light, while this value increased to 37.83% with ultrasound (US) application. For MFP; the best DD was achieved as 25.84% with optimum pH 3, reaction time 60 min, 5 g/kg TS Fe-0, 25 g/kg TS H2O2 doses, and UV-C light; however, it rose to 42.32% in the presence of US. The use of US in all processes increased the germination percentage that expresses the sludge toxicity, up to 100%. In the kinetic study, it was found that all processes are in compliance with zeroth-order kinetics. It was concluded with this study that US has an important synergistic effect on Fenton applications and contributes to sludge disintegration.