During the past decades, ultrasonic technology has gained wide interest as an effective mechanical pretreatment method for sludge, due to its excellent performance, good technical and operational stability, compactness, and environmentally friendly processing. This review is intended to provide an overview on the applications of ultrasonics in sludge solubilization and subsequent digestion, sludge sanitization, enhancement of enzymatic activity, resource recovery, extraction of chemicals, ultrasound assisted lysis–cryptic growth, and degradation of hazardous pollutants. Moreover, the working mechanisms behind the treatment and the influence of various process conditions, including power density/intensity, specific energy input, ultrasonic frequency, duration of ultrasonication (i.e., partial and complete ultrasonication, solids concentration, and type of sludge on the ultrasonic processing of sludge is specifically emphasized. Also, a discussion is included on the future research directions, an economical assessment of ultrasonic technique, and feasibility of full-scale implementation.

A rapid and efficient treatment method, using periodate (PI) for sonochemical oxidation of persistent and bioaccumulative perfluorooctanoic acid (PFOA) was developed. With an addition of 45 mM PI, 96.5% of PFOA was decomposed with a defluorination efficiency of 95.7% after 120 min of ultrasound (US). The removals of PFOA were augmented with an increase in PI doses. In all the PI+US experimental runs, decomposition efficiencies were essentially similar to those of defluorination, indicating that PFOA was decomposed and mineralized into fluoride ions. Lower solution pHs resulted in an increase in decomposition and defluorination efficiencies of PFOA due to acid-catalyzation. Dissolved oxygen increased the amount of IO4 radicals produced, which consumed the more effective IO3 radicals. Consequently, presence of oxygen inhibited the destruction of PFOA. The PFOA degradation rates with different gases sparging are in the following order: nitrogen>air>oxygen. Effects of anions follow the Hofmeister effects on PFOA degradation (i.e., Br(-)>none ⩾ Cl(-)>SO4(2)(-)). Br(-) could react with OH to yield radical anion Br2(-) that enhances the PFOA degradation. A reaction pathway was also proposed to describe the PI oxidation of PFOA under US irradiation.