In this study ultrasonic pretreatment was investigated in order to improve anaerobic digestion. First, the most effective sonication time was selected during the preliminary studies conducted on waste activated sludge samples. Then the optimal time selected was confirmed running batch anaerobic reactors. In the last part of the experiments, the effect of sonication was investigated for different operational conditions of semi-continuous digesters. Preliminary studies showed 15 min of sonication increased 50 mg/L initial soluble COD concentration up to a value of 2500 mg/L. Batch anaerobic digester results indicated that the increased soluble substrate improved anaerobic biodegradability concurrently, again with the maximal improvement observed for 15 min of sonication. Results from semi-continuous reactors indicated that at SRT of 15 days and OLR of 0.5 kg/m(3) d, ultrasonic pretreatment improved the daily biogas production, methane production and volatile solids reduction significantly when compared to control system. During the operation of reactors at 7.5 days of SRT, pretreatment helped to keep the reactors working. A simple economical analysis of the system was performed using the data obtained during the laboratory study. (C) 2010 Elsevier Ltd. All rights reserved.