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.