The activated sludge method is under continuous development and improvements. The slow
degradation rate of sludge in anaerobic digester is due to the rate limiting step of sludge hydrolysis. This is
caused by a low biodegradability of the cell walls and extra cellular biopolymers in sludge. It is important to
reduce the amount of sludge produced and to reduce its residual organic content. There is a urgent need for
the modification in sludge in anaerobic digester for compact, cost effective and efficient process for water
purification system. The review showed that ultrasonic irradiation has improved the process of anaerobic
digestion, solubilization, cell disruption of microorganisms, disintegration of bio-solids, the reduction of flock
size and reduction in weight and volume of sludge. Thus ultrasound treatment has the potential to improve the
anaerobic digestion process and enhance the recovery of valuables. This review paper will discuss the role of
ultrasound in all these processes.