A　novel　two-sludge　system　of　anaerobic　anoxic　oxic-biological　contact　oxidation　(A(2)/O-BCO)　was　developed　for　high-efficiency　utilization　of　influent　carbon　sources　treating　wastewater　with　low　carbon/nitrogen　(COD/N)　ratio.　Unlike　traditional　A(2)/O　process,　the　oxidation　of　ammonium　(NH4+-N)　in　this　system　mainly　took　place　in　the　BCO　reactor　through　nitrate　(NO3--N)　recycling　to　provide　electron　acceptors　for　denitrifying　phosphorus　removal.　The　results　showed　that　the　A(2)/O　reactor　achieved　the　efficient　utilization　of　carbon　sources,　with　75%　chemical　oxygen　demand　(COD)　in　the　anaerobic　zone　being　consumed　to　synthesize　poly-beta-hydroxyalkanoates　(PHA).　The　PHA　utilization　rate　in　the　anoxic　zone　was　more　than　70%　with　the　denitrifying　phosphorus　removal　of　97.83%.　The　batch　tests　of　phosphorus　accumulating　organisms　(PAOs)　and　denitrifying　PAOs　(DNPAOs)　activities　showed　that　the　DNPAOs　accounted　for　73.80%　of　the　PAOs　in　the　system.　The　A(2)/O　sludge　of　lower　extracellular　polymeric　substances　(EPS)　production　exhibited　a　better　settling　ability　than　conventional　denitrification　and　phosphorus　removal　processes.　The　denitrifying　phosphorus　removal　process　of　higher　phosphorus　percentage　promoted　the　formation　of　granular　sludge,　which　was　confirmed　by　scanning　electron　microscopy　(SEM)　and　energy　dispersive　spectrometry　(EDS).　The　fluorescence　in　situ　hybridization　(FISH)　results　showed　that　PAOs　became　dominant　with　low　amount　of　nitrifiers　in　the　A(2)/O　reactor,　and　nitrifiers　were　enriched　in　the　BCO　reactor.　(C)　2015　Elsevier　B.V.　All　rights　reserved.