This present work aims to reduce the COD of petroleum refinery wastewater using individual and combined processes based on the electrocoagulation and photocatalytic process with immobilized nano-zinc oxide. Also, energy savings alongside performance improvements were evaluated within this conglomerated system. Results showed that the removal efficiency of COD was 72% after 120 min when using electrocoagulation process at conditions (15 mA/cm2, pH of 7, and flow rate of 0.5 L/min). Regarding the process involving photocatalysis, the removal efficiency of COD was 76% after 120 min at conditions (ZnO concentration of 80 g/m2, pH of 7, power of irradiation equal to 65 W, and flow rate of 0.5 L/min). Several combined sequential and simultaneous systems were tested. Results confirmed that the simultaneous photo-EC system operated at 30 min is the best one and has the ability to achieve COD removal of 82% at conditions (15 mA/cm2, pH of 7, and flow rate of 0.5 L/min, ZnO concentration of 80 g/m2, and 65 W). Furthermore, the further notable features of the combined simultaneous photo-EC system were operating at shorter operation time and lowering dissolution rate of anode electrode (0.2 gram) that makes the system to be the most economic process with an energy consumption of 28.44kWh/kg COD.