A large quantity of water is used at oil refineries; Consequently, high amounts of wastewater are produced. The main aim of this study is to evaluate COD removal rate using an electrocoagulation process as well as a photocatalytic process with ZnO nanoparticles. Subsequently, the combined treatment was also implemented to reach energy savings and higher performance. At the optimum condition of the EC process (COD concentration of 900 m/L, current density of 20 mA/cm(2), pH of 8.5 and NaCl concentration of 0.5 g/L), COD removal rate was 94% after 60 min. For the photocatalytic process at optimum conditions (COD concentration of 600 mg/L, ZnO concentration of 80 g/m(2), pH of 5 and irradiation power of 32 W), 76% of COD removal efficiency was obtained, after 300 min. Thereafter, the combined system was implemented, with initial COD concentration of 1000 mg/L at the optimum conditions; First, the COD removal efficiency was achieved to be 47% after 8.5 min using the EC process; after that, the effluent entered the concrete photoreactor for 120 min, which lead to 85% of COD reduction and final COD concentration reached to 75 mg/L. The GC-Mass analysis was also performed which approved the removal of oil compounds from the wastewater. (C) 2019 Published by Elsevier B.V. on behalf of Institution of Chemical Engineers.