Using　a　A2O-BAF　system　to　treat　sewage　with　a　low　carbon　and　nitrogen　ratio,　the　best　volume　ratio　of　anaerobic　zone,　anoxic　zone,　and　aerobic　zone　of　A2O　and　nitrite　return　ratio　are　investigated,　and　how　to　strengthen　denitrifying　dephosphatation　is　also　researched.　Results　show　that　under　the　conditions　of　hydraulic　retention　time　(HRT)　5.6　h,　sludge　retention　time　(SRT)　9　d,　sludge　return　ratio　100%,　nitrate　recycle　ratio　200%　of　A2O,　HRT　30　min,　and　effluent　ρ(DO)　6-8　mg/L　of　BAF,　there　exists　dephosphatation　in　the　system.　When　the　volume　ratio　of　anaerobic/anoxic/aerobic　zone　is　3:4:2,　which　not　only　satisfies　the　development　of　PAOs,　but　also　suppresses　reproduction　of　nitrifying　bacteria,　67.4%　of　TN　and　98.6%　of　TP　are　removed.　With　the　same　volume　ratio,　when　the　nitrate　return　ratio　is　250%,　the　dephosphatation　efficiency　is　the　best.　Most　of　PAOs　have　the　denitrifying　dephosphatation　ability,　and　the　effluent　nitrate　nitrogen　and　TP　are　nearly　0.　The　two-sludge　system　can　fully　exert　advantages　of　the　activated　sludge　process　and　biological　membrane　technology.　It　is　a　suitable　system　for　treating　low　carbon　nitrogen　ratio　sewage　and　can　achieve　advanced　phosphorus　and　nitrogen　removal.