The combination of ultrasonication and chemicals aims at enhancing degradation by the action of chemicals and by the generation of hydroxyl radicals from the chemicals by ultrasonication. It was mentioned that ultrasonication can also cause water sonolysis, which produces radicals. A number of scientists have proposed the Fenton process as a promising option; however, only a small number of studies have reported the improved Fenton process through a combination with other methods, i.e., ULS. The Fenton process combined with the ULS pre-treatment of sludge is also advantageous, due its high destruction of microbial flocs and removal of extracellular polymeric substances. Sludge disintegration under the Fenton-ULS pre-treatment can also increase the release of nutrients, e.g., carbon, nitrogen and phosphorous, which can play a synergistic effect in the biological treatment of sludge. ULS pre-treatment, a mechanical process, has been reported to be an effective sludge pre-treatment method.