In this study, the treatment of oily wastewater was investigated using a combined coagulation and polyethersulfone (PES) ultrafiltration membrane process. The PES membrane was fabricated via the phase inversion method, and its morphology was examined using scanning electron microscopy (SEM). In the coagulation–ultrafiltration process, ferric chloride (FeCl₃) and polyaluminum chloride (PAC) were applied as coagulants at different concentrations and at pH levels of 3, 5, 7, and 10. These conditions were tested both individually and in combination, in the presence and absence of ultraviolet (UV) irradiation, over a period of 30 minutes, to determine the optimal conditions for chemical oxygen demand (COD) removal. The results indicated that in the coagulation–ultrafiltration process, increasing the coagulant concentration and adjusting the pH significantly improved permeate flux and reduced COD levels. UV irradiation was found to enhance the process efficiency by partially decomposing organic compounds and influencing floc formation, thereby improving both permeate flux and COD removal performance. Overall, the integrated coagulation–PES membrane process, whether applied with or without UV irradiation, led to increased flux rates and enhanced COD removal efficiency. The COD removal efficiencies achieved were 86% for the coagulation process alone and 96% for the combined coagulation–ultrafiltration process.