BACKGROUND: A novel composite coagulant was prepared from polyferric chloride (PFC) and polydimethyldiallylammonium chloride (PDM) using two different methods: co-polymerization and composite polymerization. In this investigation, effects of preparation parameters such as PDM/Fe weight ratio (P) and PDM molecular weight on zeta potential and Fe (III) species distribution were also studied. In addition, the effects of those parameters on coagulation efficiency of prepared coagulants in treating drilling wastewater was evaluated and the synthetic coagulants were characterized in terms of typical properties and morphological analysis. RESULTS: The improvements in preparation parameters were better charge neutralization capacity and the highest content of active Fe (III) coagulating species due to the presence of cationic polymer. The optimal coagulation efficiency was obtained at a PDM/PFC weight ratio of 0.3 and PDM molecular weight of 200-350 kDa. PFC-PDM turned out to be a physical mixture according to Fourier transform infrared (FTIR) analysis, although the coagulation behavior was changed after the PFC/PDM composite was formed. Morphology analysis displayed an irregular surface and multi-branched structure for PFC-PDM. CONCLUSION: The novel composite coagulant PFC-PDM exhibited a superior flocculation performance and obtained favorable coagulation behavior with more compact flocs and a much better sludge compressibility compared with PFC alone. (C) 2016 Society of Chemical Industry