Oily sludge with an initial oil content of 0.130 g g(-1) (dry basis) was mixed with water and treated in an ultrasound cleaning tank. The oil was then separated from the oily sludge by air floatation. Experiments were carried out with and without 28 kHz ultrasonic irradiation at different temperatures. The results show that the minimum oil content, 0.055 g g(-1) (dry basis), was obtained at 40 degrees C after ultrasound irradiation, which was 55.6% less than without ultrasonic irradiation. in addition, this work clearly establishes that 28 kHz ultrasound is superior to 40 kHz ultrasound. The ultrasonic acoustic pressure amplitude with the 28 kHz ultrasound was 0.085 MPa: the 28 kHz ultrasound also exhibited lower oil content than the 40 kHz ultrasound, which yielded 0.120 MPa acoustic pressure amplitude. It can also be concluded that sodium silicate obstructs ultrasound oily sludge deoiling. (c) 2009 Published by Elsevier B.V.