Adding　nitrite　(NO2-)　to　waste　activated　sludge　(WAS)　fermentation　systems　is　an　efficient　approach　to　integrate　fermentation　with　denitrification,　and　utilize　volatile　fatty　acids　(VFAs)　to　achieve　a　high　nitrogen　removal　rate　even　at　low　C/N　ratios.　In　this　study,　the　effect　of　nitrite　on　the　integrated　WAS　fermentation　and　denitrification　(termed　as　WASFD)　was　investigated　under　acidic　and　alkaline　conditions.　The　results　indicated　that　adding　nitrite　achieved　a　most　reduction　of　nitrite　to　N-2　and　improved　the　acidification　of　WAS　with　high　VFAs　production.　Under　acidic　condition　(pH　=　5),　the　maximum　VFAs　produced　with　nitrite　addition　was　3.3　times　that　without　nitrite　addition.　Higher　concentration　of　free　nitrous　acid　(FNA)　at　the　pH　of　5　increased　WAS　particulates,　improved　the　hydrolysis　of　organic　substrates,　and　finally　promoted　VFAs　yields.　Under　alkaline　condition　(pH　=　9),　adding　nitrite　only　increased　the　VFAs　production　by　1.5　times,　indicating　that　acidic　condition　was　preferable　for　acidification　than　alkaline　condition.　(C)　2014　Published　by　Elsevier　Ltd.