NaOH was used to disintegrate a mixture composed mainly of primary sludge with biofilm sludge before anaerobic digestion in batch experiments. NaOH pretreatment dissolved some organic substances, and the optimum dose was 0.1 mol/L. After the alkali-treated sludge was fed into the digesters, the higher pH delayed the start of digestion and reduced the biogas production during the initial stage, although the system recovered after a lag phase when the dose was lower than 0.04 mol/L. Acid conditioning was necessary, but the increased salinity also impacted on the digestion efficiency. For sludge pretreatment, the optimum NaOH dose was 0.1 mol/L, and the initial pH of the batch digesters needs to be controlled at less than eight. Under optimized conditions, the organic degradation rate was 38.3% and the biogas yield was 0.65 L/g volatile suspended solid (VSS), whereas these values for the control were 30.3% and 0.64 L/g VSS, respectively. (C) 2012 Elsevier Ltd. All rights reserved.