The performance of the hydrolyzation film bed and biological aerated filter (HFB-BAF) combined system in pilot scale (with a daily treatment quantity of 600-1300 m(3) d(-1)), operated for 234 days, for low-strength domestic sewage was assessed using different amounts of aeration, reflux ratios and hydraulic loading rates (HLR). In steady state it was found that the average removal efficiency of chemical oxygen demand (COD) and biological oxygen demand at 5 days (130135) were 82.0% and 82.2% and the average effluent concentrations were 15.8 mg L-1 and 9.4 mg L-1 respectively as the HFB was running at an HLR of 1.25-1.77 m(3) m(-2) h(-1) and the BAF was running at an HLR of 1.56-2.21 m(3) m(-2) h(-1). In general, the removal efficiency of total nitrogen (TN) fluctuated with the HLR, gas-water ratio and reflux ratio, so the ratio of gas to water should be controlled from 2:1 to 3:1 and the reflux ratio should be as high as possible. The effluent concentration of TN was 10.4 mg L-1 and the TN removal averaged 34.3% when the gas-water ratio was greater than 3:1 and the reflux ratio was 0.5. The effluent concentration and removal efficiency of NH4+-N averaged respectively 2.3 mg L-1 and 78.5%. The overall reduction of total phosphorus (TP) was 30% and the average effluent concentration was 0.95 mg L-1. The removal efficiency of linear alkylbenzene sulfonates (LAS) reached 83.8% and the average effluent concentration was almost 0.9 mg L-1. The effluent concentration and removal efficiency of polychlorinated biphenyls (PCBs) were 0.0654 mu g L-1 and 37.05% respectively when the influent concentration was 0.1039 mu g L-1. The excess sludge containing water (volume 15 m(3)) was discharged once every 3 months. The power consumption of aeration was 0.06-0.09 kWh of sewage treated. The results show that the HFB-BAF combined technology is suitable for the treatment of low-concentration municipal sewage in south China. (c) 2005 Society of Chemical Industry.