To overcome the shortcoming of biological aerated filter(BAF) on phosphorus removal,poly aluminum chloride(PAC) was dosed into the anoxic tank of anoxic-biological aerated filter(A-BAF).The effects of PAC on the phosphorus,organic matter and nitrogen removal by A-BAF and the head-loss development of BAF were investigated through a lab-scale experiment.The results showed that the addition of PAC had a positive effect on phosphorus removal by A-BAF,but had no effect on the removal of organic matter and nitrogen.With the PAC dosage of 100 mg/L,the TP concentration in effluent was reduced from 2.57 mg/L to 0.34 mg/L,when the influent TP concentration was 3.5 mg/L.The COD,NH+4-N and TN concentration in effluent were 30,1 and 11 mg/L separately,which were similar to those in the effluent without PAC.Meanwhile,the backwashing cycle of BAF was increased from 4 d to 10 d and the specific resistance to filtration of the excess sludge was decreased from 210×1012m/kg to 125×1012m/kg after PAC dosing.