Achieving　mainstream　short-cut　nitrogen　removal　via　nitrite　has　become　a　carbon　and　energy　efficient　way,　but　still　remains　challenging　for　low-strength　municipal　wastewaters.　This　study　integrated　sidestream　enhanced　biological　phosphorus　removal　system　in　a　pilot-scale　adsorption/bio-oxidation　(A-B)　process　(named　A-BS2EBPR　system)　and　nitrite　accumulation　was　successfully　achieved　for　treating　the　municipal　wastewater.　Nitrite　could　accumulate　to　5.5　f　0.3　mg　N/L　in　the　intermittently　aerated　tanks　of　B-stage　with　the　nitrite　accumulation　ratio　(NAR)　of　79.1　f　6.5　%.　The　final　effluent　concentration　and　removal　efficiency　of　total　inorganic　nitrogen　(TIN)　were　4.6　f　1.8　mg　N/L　and　84.9　f　5.6　%,　respectively.　In-situ　process　performance　of　nitrogen　conversions,　routine　batch　nitrification/denitrification　activity　tests　and　functional　gene　abundance　of　nitrifiers　collectively　suggested　that　the　nitrite　accumulation　was　mainly　caused　by　partial　denitrification　rather　than　out-selection　of　nitrite　oxidizing　bacteria　(NOB).　Moreover,　the　single-cell　Raman　spectroscopy　analysis　first　demonstrated　that　there　was　a　specific　microbial　population　that　could　utilize　polyhydroxyalkanoates　(PHA)　as　the　potential　internal　carbon　source　during　the　partial　denitrification　process.　The　integration　of　S2EBPR　brings　unique　features　to　the　conventional　A-B　process,　such　as　extended　anaerobic　retention　time,　lower　oxidation-reduction　potential　(ORP),　much　higher　and　complex　volatile　fatty　acids　(VFAs)　etc.,　which　can　largely　reshape　the　microbial　communities.　The　dominant　genera　were　Acinetobacter　and　Comamonadaceae,　which　accounted　for　(17.8　f　15.5)%　and　(6.7　f　3.4)%,　respectively,　while　the　relative　abundance　of　conventional　nitrifiers　was　less　than　0.2%.　This　study　provides　insights　into　phylogenetic　and　phenotypic　shifts　of　microbial　communities　when　incorporating　S2EBPR　into　the　sustainable　A-B　process　to　achieve　mainstream　short-cut　nitrogen　removal.