Biodegradation of excess sludge is a commonly used method in biological treatment by anaerobic digestion. Ultrasonic pretreatment enhances the subsequent anaerobic digestion resulting in a better degradation of volatile solids and increased production of biogas. Anaerobic digester is an air-tight, oxygen-free container, where anaerobic bacteria digest sewage producing methane and CO₂. It may be of the following types : standard-rate digester, high-rate digester and two-stage Digester. Sonochemical reactions resulting in the formation of highly reactive radicals and hydrogen peroxide have also been reported to contribute to the ultrasonic disintegration of sludge. Another mechanism operative during sonication of sludge is acoustic streaming. Formation of methane in anaerobic digestion proceeds in four different steps and these are hydrolysis, acidogenesis, acetogenesis and methanogenesis. There are several factors that influence the effect of United States on anaerobic digestion, including organic loading rate, sludge characteristics, particle size, sonication time, temperature, pH, specific energy input, operating frequency and sludge retention time.