Increase in sludge dewatering plays an important role in reducing its treatment, handling and disposal costs. In this work, efficacy of various sludge dewatering techniques such as Fenton (F), Free Nitrous Acid (FNA) and F/FNA was performed under ultrasonication. Experiments were carried out to determine the amount of sludge filtrate and Capillary Suction Time (CST) under various conditions. The optimum pH was found to be 3, 3 and 5 respectively for Fenton, FNA and F/FNA. More amount of sludge filtrate was detected for treated sludge in the order F/FNA (57 mL) > FNA (55 mL) > Fenton (48 mL) and minimum CST was observed in F/FNA (28 s) < FNA (31 s) < Fenton (49 s). SEM and DSC analysis were performed to observe the variation in physical characteristics before and after treatment. The change in functional groups in sludge cake before and after treatment was investigated using FT-IR. Excitation-Emission Matrix (EEM) was used to measure the fluorescence peak and intensities of the released biopolymers into sludge filtrate caused by the disintegration of EPS. In addition, the radical scavenging experiments were also performed to identify the significant roles of hydroxyl (OH·), nitrogen dioxide (NO2 ·), nitric oxide (NO·) and superoxide (O2 ·-) radicals. The experimental results indicated that the increased mass transfer due to the application of ultrasonication resulted in higher dewaterability. The leaching of metals from the sludge cake into the treated sludge filtrate portrays that sludge could be utilized as manure and solid biomass fuel. © 2019 Elsevier B.V.