A　two-sludge　system　consisting　of　anaerobic　anoxic　oxic　biological　contact　oxidation　(A(2)/O　BCO)　was　developed　to　treat　domestic　wastewater　with　a　low　carbon/nitrogen　(COD/TN)　ratio　(around　3.21)　by　shortening　sludge　retention　time　(SRT)　for　phosphorus　accumulating　organisms　(PAOs)　in　the　A(2)/O　reactor　and　prolonging　SRT　for　nitrifiers　in　the　BCO　reactor.　Specifically,　the　BCO　reactor　was　composed　of　three　stages　in　series　(N-1,　N-2　and　N-3),　so　that　simultaneous　nitrogen　and　phosphorus　removals　by　denitrifying　PAOs　(DNPAOs)　was　achieved　in　the　A(2)/O　reactor　with　NO.　N　as　the　electron　acceptor　from　the　BCO　reactor.　Long　term　operational　tests　(600　days)　were　conducted　with　various　operational　parameters　[e.g.,　hydraulic　retention　time　(HRTs),　nitrate　recycling　ratio　(Rs),　volume　ratio　(Vs)]　to　examine　the　denitrifying　phosphorus　removal　performance.　The　system　exhibited　the　highest　removal　of　TN　and　PO43--P　at　the　HRTs　of　8　h,　Rs　of　300%　and　Vs　of　2:4:1.　The　optimal　IN　and　PO43--P　removals　were　80.30%　and　96.61%　at　low　COD/TN　of　3.21.　The　species　diversity　and　microbial　community　examined　by　the　Illumina　MiSeq　method　demonstrated　the　fact　of　two　sludge　system,　and　the　improved　community　structure　by　long-term　optimization　was　prominent　comparing　with　the　seed　sludge.　Additionally,　Accumulibacter　and　Dechloromonas　were　the　dominant　functional　PAOs　with　25.74%　in　the　A(2)/O　reactor,　while　nitrifiers　(including　Nitrosomonas　and　Nitrospira)　were　gradually　enriched　with　13.10%,　21.33%,　and　31.10%　in　the　three　stages　of　the　BCO　reactor.　(C)　2016,　The　Society　for　Biotechnology,　Japan.　All　rights　reserved.