Achieving　nitrite　accumulation　still　remains　challenging　for　efficient　short-cut　biological　nitrogen　removal　in　municipal　wastewater　treatment.　To　tackle　the　problem　of　insufficient　carbon　in　incoming　wastewater　for　biological　nutrient　removal,　a　return　activated　sludge　(RAS)　fermentation　method　has　been　proposed　and　demonstrated　to　enable　producing　supplemental　volatile　fatty　acids　(VFAs)　and　enhance　biological　phosphorus　removal　via　sludge　cycling　between　mainstream　and　a　sidestream　anaerobic　reactor.　However,　the　impacts　of　long　anaerobic　exposure　with　acetate　on　nitrifying　bacteria,　known　as　the　aerobic　chemoautotrophic　microorganisms,　remains　unexplored.　In　this　study,　the　activated　sludge　underwent　a　cyclic　anaerobic　treatment　with　the　addition　of　acetate　(Ac),　the　effects　on　nitrification　rate,　abundance　and　microdiversity　of　nitrifying　communities　were　comprehensively　assessed.　Firstly,　batch　activity　tests　proved　the　direct　addition　of　high　acetate　(above　1000　mg/L)　could　cause　inhibition　on　the　nitrification　rate,　moreover,　the　inhibitory　effect　was　stronger　on　nitrite-oxidizing　bacteria　(NOB)　activity　than　that　of　ammonia-oxidizing　bacteria　(AOB).　Then,　a　sequencing　batch　reactor　(SBR)　was　applied　to　test　the　nitrogen　conversion　performance　for　low-strength　ammonium　wastewater.　Nitrite　accumulation　could　be　achieved　via　the　cyclic　anaerobic　exposure　with　1000-5000　mg　Ac/L.　The　maximum　effluent　concentration　of　nitrite　was　40.8　+/-　3.5　mg　N/L　with　nitrite　accumulation　ratio　(NAR)　of　67.6　+/-　3.5%.　The　decrease　in　NOB　activity　(72.7%)　was　greater　than　AOB　of　42.4%,　promoting　nitrite　accumulation　via　nitritation　process.　Furthermore,　the　cyclic　anaerobic　exposure　with　acetate　can　largely　reshape　the　nitrifying　communities.　As　the　dominant　AOB　and　NOB,　the　abundance　of　Nitrosomonas　and　Nitrospira　were　both　decreased　with　species-level　microdiversity　in　the　nitrifying　communities.　However,　the　heterotrophic　microorganism,　Thauera,　were　found　to　be　highly　enriched　(from　0　to　17.3%),　which　may　act　as　the　potential　nitrite　producer　as　proved　by　the　increased　nitrate　reduction　gene　abundance.　This　study　can　provide　new　insights　into　achieving　mainstream　nitrite　accumulation　by　involving　sidestream　RAS　fermentation　towards　efficient　wastewater　treatment　management.