The study of the possibility of removing organic compounds from wastewater originating from the biodiesel purification stage by two catalytic processes, HSO₅^-/transition metal and Fenton method has been presented. The source of the ion HSO₅^- is potassium monopersulphate (2KHSO₅·KHSO₄·K₂SO₄) (Oxone) that may be decomposed into radicals (OH^., SO₄^-., SO₅^-.) by means of transition metal as Co(II). Different concentrations were used for both compounds and the combination ([Co²⁺] = 1.00μM/[HSO₅^-] = 5.00·10^-2 M) achieved the highest COD removal (60%) and complete decomposition of the oxidant was verified for contact times of 45 min. This process has some advantages comparing to the conventional Fenton method such as the absence of the costly pH adjustment and the Fe(III) hydroxide sludge which characterize this treatment process. The Fenton process showed that the combination of [H₂O₂] = 2.00M/[Fe²⁺] = 0.70 M was the best and archived COD removal of 80%. The treatments studied in this research have achieved high COD removal, but the wastewater from the biodiesel purification stage presents very high parametric values of Chemical Oxygen Demand (667,000 mgO₂/L), so the final COD concentration reached is still above the emission limit of discharge in surface water, according the Portuguese Law (Decree-Law 236/98). However, both treatments have proved to be feasible techniques for the pre-oxidation of the wastewater under study and can be considered as a suitable pre-treatment for this type of wastewaters. A rough economic analysis of both processes was, also, made.