Abstract: To ascertain the thermal effect of ultrasonic on disintegrating excess sludge， ultrasonic and water bath heating were used to treat organic macromolecules， bacteria and excess sludge. Results show that the TC and TOC content of acid orange are decreased after ultrasonic treatment， which proves the hyperthermia pyrolysis effect. The ultrasonic disintegrating effects on bacteria and excess sludge in three cases can be compared as: ultrasonic under no recirculating cooling water > ultrasonic under recirculation cooling water > water bath heating. The results show that ultrasound thermal effects can promote the disintegration of bacteria and the excess sludge. When the excess sludge is disintegrated， the temperature rise caused by energy from ultrasonic cavitation effect accounts for more than 10% of the total ultrasonic energy. When the temperature is up to 70℃， the contribution of ultrasonic heat to the excess sludge is about 20%.

Key words: ultrasonic, excess sludge, acid orange, bacteria, sludge disintegration, thermal effect