Abstract: 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.