BACKGROUND: Disintegration was developed as a pretreatment process for sludge to accelerate the digestion processes. Ultrasonic treatment may be a good alternative for sludge disintegration. In this study, different specific energy inputs ranged between 0 and 15 880 kJ kg(-1) and very low ultrasonic densities ranged between 0.04 and 0.1 W mL(-1) were applied to biological sludge for disintegration purposes. The potential for improving anaerobic digestion through ultrasonic pre-treatment and the effect of ultrasonic pre-treatment on the filterability characteristics of sludge were also investigated. RESULTS: 9690 W kg(-1) TS of supplied energy and very low power density of 0.09 Wm L-1 are efficient for floc disintegration. For 9690 kJ kg(-1) TS, 44% higher methane production was achieved than with raw sludge as a result of biochemical methane potential assay. The supernatant characteristics of the sludge were also affected by the ultrasonic pre-treatment. For 9690 kJ kg-1 TS, the soluble chemical oxygen demand (SCOD), dissolved organic carbon (DOC), total nitrogen (TN), and total phosphorus (TP) in the sludge supernatant increased by 340%, 860%, 716%, and 207.5%, respectively. CONCLUSION: Ultrasonic pre-treatment is an effective method for biological sludge disintegration even at very low ultrasonic density levels. It leads to increased anaerobic biodegradability but deteriorates the filterability characteristics of biological sludge. (C) 2009 Society of Chemical Industry