In the process for decreasing the COD-content of effluent by treating with hydrogen peroxide in the presence of a transition metal compound, the improvement which comprises adding to the effluent about 5 to 40% of the calculated quantity of H.sub.2 O.sub.2 required for the total oxidation of the total COD-content of the effluent, the transition metal compound being dissolved in the effluent in an amount such that the molar ratio of H.sub.2 O.sub.2 to transition metal is from about 30:1 to 3:1, bringing the pH initially to about 2 to 9, maintaining the effluent at about 5.degree. to 100.degree. C., and separating from the effluent whatever material has flocculated. Thereafter the effluent can be subjected to conventional biological degradation such as by the activated sludge process.