Aerobic and anaerobic digestions were compared in reactors fed with sonicated activated sludge. Sonication treatment of activated sludge led to solubilisation of matter and especially of organic compounds. An important improvement of aerobic and anaerobic biodegradability was observed for a sonication treatment of 108,000 kJ kg TS-1 due to the increase of the instantaneous specific soluble COD uptake rate. Sonication led to an increase of biogas production due to the increase of available soluble COD. In this study, sludge sonication prior to aerobic digestion in the aim of enhancing sludge reduction was inconclusive. Under anaerobic conditions, the enhancement of sludge reduction due to sonication depended on the disintegration degree of the sludge. The combination of high disintegration degree of sonicated sludge prior to an anaerobic digestion led to very good results in term of sludge reduction (80%). Energy balance was also studied. (C) 2008 Elsevier B.V. All rights reserved