In this study coagulation–flocculation method was evaluated to treat municipal wastewater using aluminum sulphate and calcium hydroxide as coagulants, and polyacrylamide as flocculant by varying pH from 5 to 7 and coagulant dosage from 100 to 300 mg L–1 at constant flocculant dosage (10 mg L–1), time (1 min for rapid mixing followed by 30 min for slow mixing), temperature (30°C ± 2°C), agitation speed (150 rpm for rapid mixing followed by 50 rpm for slow mixing) and settling time (2 h) by jar test method. The responses were percentages reduction in turbidity, chemical oxygen demand (COD) and biochemical oxygen demand (BOD). The sample was real-time wastewater from sewage treatment plant of Salalah Sanitary Drainage Services (SSDS) Company, Salalah, Sultanate of Oman. The trials were performed with single and mixed or combined coagulants. Response surface methodology (RSM) based central composite design (CCD) was used for the optimization of pH and coagulant dosage to maximize percentage reduction in turbidity, COD and BOD. The maximum of 99.7%, 98%, and 95.5% were achieved for turbidity removal using aluminum sulphate, calcium hydroxide and combined coagulants respectively. However, maximum of 60%, 58.1%, and 42.5% were achieved for COD removal using calcium hydroxide, combined and aluminum sulphate respectively. Moreover, maximum of 79.5%, 78.5%, and 54.5% were achieved for BOD removal using aluminum sulphate, calcium hydroxide and combined coagulants respectively. The obtained results indicate that treatment using combined coagulant is effective for removal of turbidity and COD, and could be used for the treatment of municipal wastewater. © 2019 Desalination Publications. All rights reserved.