The　effect　of　four　alkali　reagents　(NaOH,　KOH,　Ca(OH)(2),　mixed　alkali)　on　waste　activated　sludge　(WAS)　hydrolytic　acidification　and　microbial　community　was　studied　in　semi-continuous　fermentation　systems　at　low　temperature　(15　degrees　C)　over　long　term　operational　time　(65　day).　The　results　showed　that　protein　and　polysaccharide　of　NaOH　(124.26,　11.92)　was　similar　to　that　of　KOH　(109.53,　11.30),　both　were　higher　than　Ca(OH)(2)　(70.66,　3.74)　and　mixed　alkali　(90.66,　8.71).　The　short　chain　fatty　acids　(SCFAs)　of　NaOH　(231.62)　was　higher　than　KOH　(220.62　mg　chemical　oxygen　demand　(COD)/g　VSS).　Although　Ca(OH)(2)　system　had　strong　acidification　capacity,　the　shortage　of　SCFAs　occurred　due　to　the　low　activity　of　hydrolase.　Illumina　MiSeq　sequencing　revealed　that　Tissierella　and　Erysipelothrix　were　enriched　in　the　NaOH　and　Ca(OH)(2)　systems,　where　Peptostreptococcaceae　incertae_sedis　was　enriched　in　the　NaOH　and　KOH　systems,　less　Anaerolinea　was　involved　in　Ca(OH)(2)　condition.　(C)　2015　Published　by　Elsevier　Ltd.