This study investigated the potential benefits of ultrasound-conditioned sludge dewatering treatments with specific energy dosages from 0 to 35,000 kJ/kg total solids (TS). Capillary suction time (CST) and specific resistance of filtration (SRF) were used to evaluate sludge dewaterability. Sludge water distribution was measured by the drying test and mechanical separation methods. Both extracellular polymeric substance (EPS) content and sludge particle size were determined in an attempt to explain the observed changes in sludge dewaterability. The results indicated that application of low specific energy dosages (<4400 kJ/kg TS) slightly enhanced sludge dewaterability, but larger specific energy dosages (>4400 kJ/kg TS) significantly deteriorated sludge dewaterability. The optimal specific energy to give maximal dewaterability characteristics was found to be 800 kJ/kg TS, which generated sludge with optimal EPS concentration (400-500 mg/l) and particle size distribution (80-90 mu m diameter). Subjecting sludge to a combination of cationic polymer and ultrasound pretreatments did not present any clear advantages over polymeric conditioning alone for improving sludge dewaterability. The effects of cationic polymer treatment predominated over those of ultrasound pretreatment when both were used together. (c) 2008 Elsevier Ltd. All rights reserved.