Activated sludge was treated with ultrasonic waves (22 kHz, 24-64 kW/m(2), 12 GJ/kg dry mass, 75-201 kW/m(3)) for 300 s under lab. conditions. A substantial increase in chem. O-2 demand in the supernatant and decrease in content of total solids were found. A considerably increase in the sludge temp. was also obsd. The results agreed with the data given by V. Naddeo et al. (2009).