Cooling tower effluent in petrochemical industries contains significant amounts of iron, phosphate, and suspended particles, which causes the formation of highly turbid effluent at the bottom of the cooling tower basin. Direct discharge of this effluent to the environment or its recycling in the cooling tower cycle without proper treatment creates numerous operational and environmental problems. In this study, the chemical treatment of cooling tower effluent from Kangan Petrorefining Company is investigated using three common coagulants including ferric chloride, alum, and polyaluminium chloride (PAC). The effect of pH variation at three levels of 5, 7, and 9, coagulant concentration at five levels between 10 to 300 ppm, and retention time at three levels of 5, 10, and 25 minutes on turbidity reduction is analyzed using the D-Optimal experimental design method in Design-Expert software version 11. A total of 135 jar test were conducted. The results show that PAC has the best performance in the acidic and neutral pH. The greatest turbidity reduction is achieved in the optimal conditions of pH equals to 5, concentration of 55.5 ppm and the retention time of 25 minutes. Alum also performs well in the optimal conditions of pH equals to 5, concentration of 63.5 ppm and retention time of 25 minutes. In contrast, ferric chloride does not show the required efficiency within a retention time of 25 minutes and in some cases even increases the turbidity. Finally, PAC is suggested as the optimal coagulant in terms of coagulant volume consumption, turbidity removal efficiency, and also the compatibility with environments with different pH values. This research can serve as a practical basis for designing wastewater treatment units in petrochemical industries.