The application of ultrasound to sludge is an efficient method for sludge disintegration in order to accelerate the anaerobic digestion process, but energy balance on the basis of lab tests is disadvantaged due to the inefficiency at this scale. In this paper the energy balance is discussed for five full scale WWTPs equipped with different devices with the aim to investigate the cost-effectiveness of the ultrasonic pre-treatment before digestion. These five experiences show positive energy gains operating at 20 kHz: best performances are attained at short sonication time and high sonication power. Moreover, sludge sonication at 200 kHz is presented as an alternative pretreatment to improve the anaerobic digestibility and to remove potentially organic micropollutants. Lab experiment results of this treatment indicated quite good disintegration effect, in terms of substrate solubilization, with consequent acceleration of the hydrolysis phase and increase of biogas production.