Removal　behaviors　of　biological　phosphorus　(P)　and　nitrogen　(N)　in　real　domestic　sewage　with　low　C/N　ratio　were　studied　in　a　laboratory-scaled　anaerobic/anoxic/oxic　(A2/O)-biological　aerated　filter　(BAF)　combined　system.　In　order　to　improve　removal　efficiency　of　the　combined　system,　short　sludge　retention　time　(SRT)　was　applied　in　A2/O,　which　was　favorable　for　P　removal　and　denitrification,　while　longer　SRT　used　in　biological　aerated　filter　(BAF),　which　was　favorable　for　polyphosphate-accumulating　organisms　(PAOs)　and　for　nitrification　to　remove　the　ammonium　nitrogen.　The　recycled　liquid　involving　nitrate　from　BAF　provided　electron　acceptor　enough　for　the　anoxic-zones　of　A2/O　and　necessary　condition　for　de-phosphorus　by　denitrifying.　Experimental　results　showed　that　with　these　operations　in　this　combined　system　COD,　N　and　P　could　be　removed　simultaneously.　At　conditions　of　HRT　7.5　h,　SRT　15　d,　the　recycling　ratio　of　liquid　400%,　the　concentrations　of　COD,　ammonium　nitrogen,　total　nitrogen　(TN)　and　total　phosphorus　(TP)　could　be　reduced　from　at　average　values　of　376,　67.9,　72.9　and　6.1　mg·L-1in　the　raw　waste　water　to　42,　0.86,　9.6　and　0.1　mg·L-1　in　the　effluent,　respectively.　The　rate　of　phosphorus　removal　by　denitrifying　were　1.49　and　1.64　g　TP·(g　MLVSS)-1·d-1　at　recycling　ratios　of　200%　and　400%,　respectively.　The　results　of　batch　tests　showed　that　the　population　of　DPAOs　(denitrifying　PAOs)　was　up　to　68.2%　of　the　total　PAOs.　The　removal　efficiency　of　TP　decreased　with　increasing　SRT.　©　All　Rights　Reserved.