Abstract The optimal dosage of four acrylamide polymer coagulants commonly used in industry, namely anionic polyacrylamide P(AA-co-AM), cationic polyacrylamide P(DMB-co-AM), nonionic polyacrylamide P(AM), and hydrophobic polyacrylamide P(OA-co-AM), is explored using coagulation–flocculation jar tests in laboratory. Experimental results show that P(DMB-co-AM) generates a maximum transmittance of 90.8% at 20mgL−1, and therefore has the best coagulation performance. In the second part of this study, the impact of polyaluminum chloride (PACl) and P(DMB-co-AM) concentrations on coagulation–flocculation properties is evaluated using a pilot scale test in a petrochemical plant in Jiangsu, China. Results shows that the running time can reach 23h when the dosages of P(DMB-AM) and PACl are 20mgL−1 and 15mgL−1. The mean diameter of particles (by number) in the feed water, in the water after coagulation, and in the water after the multimedia filter (MMF) was 0.71μm, 0.52μm, and 0.41μm, respectively. Other parameters, including chemical oxygen demand (COD), total dissolved solids (TDS), total Fe, and turbidity, also decreased following the coagulation–flocculation process and MMF. Results of the above tests comprehensively suggest that PACl and P(DMB-AM) can be used successfully in wastewater pre-treatment for the reverse osmosis (RO) process.