Hydrolysis, acidification and dewaterability of waste activated sludge (WAS) were investigated at pH 10 controlled by the addition of NaOH, Ca(OH)(2) or their mixtures at various ratios. Similar efficiency of WAS solublisation was observed in all cases, at 38-40%. High volatile fatty acid (VFA) production and good sludge dewaterability could not be achieved simultaneously by adding NaOH or Ca(OH)(2) alone, but could be achieved by adding mixtures of NaOH and Ca(OH)(2). VFA production in the case with the addition of Ca(OH)(2) only (1201 mg(COD)/L) was lower than in the cases with the addition of NaOH or its mixtures with Ca(OH)(2) (1813-1868 mg(COD)/L), and the lower VFA production with Ca(OH)(2) addition alone could be related to the fact that a higher concentration of Ca2+ was released into the fermentation liquid, which likely inhibited the hydrolysis process of protein. Furthermore, adding mixtures of NaOH and Ca(OH)(2) was more economical for VFA production. (C) 2013 Elsevier Ltd. All rights reserved.