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-1 in 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·（