Petroleum effluents pollution is an important challenge for many countries. The aim of this study was to investigate the efficiency of Fenton, Photo-Fenton (PF), Electro-Fenton (EF) and Photo-Electro-Fenton (PEF) processes in reducing chemical oxygen demand (COD) of desalination unit effluent using steel slag as catalyst, cathode and anode. For achieving this goal the experimental design by Taguchi method using Mintab18 software was used. The desired variables include pH (3, 5 and 7), hydrogen peroxide (5, 10 and 15 mmol/l), contact time (30, 60 and 90 minutes), catalyst (5, 7.5 and 10 g/l), radiant intensity (16, 24 and 32 watts) and voltage (5, 15 and 30 volts). In order to perform the experiments, the pilot was made using UV-C lamps (wavelengths of 200-280 nm) each with a power of 8 watts and an AC power supply of 0-30 volts in the range of 0-2 amps. According to the results, PEF process under pH = 7, 10 mmol/l hydrogen peroxide, catalyst content of 5 g/l, radiant intensity of 24 watts and voltage of 5 volts for 90 minutes, provides the highest COD removal rate (98.7%). The present study showed that following the combined process of PEF, EF, PF and Fenton processes have a high potential in reducing COD using steel slag.