The　effects　of　two　alkali　agents,　NaOH　and　Ca(OH)(2),　on　enhancing　waste　activated　sludge　(WAS)　fermentation　and　short　chain　fatty　acids　(SCFAs)　accumulation　were　studied　in　semi-continuous　stirred　tank　reactors　(semi-CSTR)　at　different　sludge　retention　time　(SRT)　(2-10　d).　The　optimum　SRT　for　SCFAs　accumulation　of　NaOH　and　Ca(OH)(2)　adding　system　was　8　d　and　10　d,　respectively.　Results　showed　that　the　average　organics　yields　including　soluble　chemical　oxygen　demand　(SCOD),　protein,　and　carbohydrate　in　the　NaOH　system　were　as　almost　twice　as　that　in　the　Ca(OH)(2)　system.　For　Ca(OH)(2)　system,　sludge　hydrolysis　and　protein　acidification　efficiencies　were　negatively　affected　by　Ca2+　precipitation,　which　was　revealed　by　the　decrease　of　Ca2+　concentration,　the　rise　of　zeta　potential　and　better　sludge　dewaterability　in　Ca(OH)(2)　system.　In　addition,　Firmicutes,　Proteobacteria　and　Actinobacteria　were　the　main　microbial　functional　groups　in　both　types　of　alkali　systems.　NaOH　system　obtained　higher　microbial　quantities　which　led　to　better　acidification.　For　application,　however,　Ca(OH)(2)　was　more　economically　feasible　owning　to　its　lower　price　and　better　dewaterability　of　residual　sludge.　(C)　2017　Elsevier　Ltd.　All　rights　reserved.