An investigation on the characteristics of biologically aerated filter (BAF) backwashed sludge and its biological flocculation performance were carried out by a bench-scale experiment. The bio-flocculation ability of backwashed sludge, the combined coagulation efficiency of chemical coagulant FeCl3 Al-2(SO4), and biological sludge, and the properties of the settled sludge produced in the pretreatment process were investigated, respectively. The results showed that the backwashed sludge had preferable biological flocculation performance, biological activity and a complex biota. It was observed that the treatment efficiency would be improved greatly whether by using the backwashed sludge or combined using chemical coagulant and biological sludge in the enhanced domestic wastewater pretreatment process. The removal efficiency of COD, SS and TP were improved about 40%, 40% and 75% than that of the comparisons, respectively, when backwashed sludge dosage was 76 mg and FeCl3 dosage was 30 mg/L domestic wastewater. A promising property of settling and dewatering of the secondary sludge produced in the bio-chemically enhanced coagulation process was also revealed.