Recent advance on the application of the composite coagulant in the coagulation-flocculation process has attracted a widespread interest. Previous research has demonstrated that iron-based composite coagulant could enhance the performance of the coagulation-flocculation. In this research, results of an experimental study using polymeric aluminum ferric sulfate (PAFS) as a modified coagulation reagent for treating wastewater was presented. The structure and morphology of PAFS were investigated using some conventional methods, and response surface method (RSM) was employed to optimized the coagulation-flocculation process while the evaluation of treatment efficiency was determined by measuring both the reduction of turbidity and chemical oxygen demand (COD). In addition, parameters affecting coagulation-flocculation behavior such as coagulant dosage, wastewater initial pH, were also examined. The results showed the structure of PAFS as a non-stoichiometric basic iron sulfate salt taking the shape of a compact network and exhibited better coagulation-flocculation performance when the species distribution of (Fe-Al)(a), (Fe-Al)(b) and (Fe-Al)(c) in PAFS were 37.4%, 3.79% and 59.08%, respectively. Compared with PFS. PAFS showed a superior flocculation performance with the maximum COD removal efficiency of 83.6% and the lowest residual turbidity of 0.96 NTU at the coagulant dosage of 45 mg/L. In the coagulation-flocculation process, the adsorption-bridging and charge neutralization mechanisms played an important role in colloidal destabilization and aggregation. (C) 2011 Elsevier B.V. All rights reserved.