PURPOSE: To efficiently remove COD in a waste liquid under physically mild conditions by utilizing a hydroxyl free radical by allowing a transition metal catalyst to coexist in the waste liquid and mixing the waste liquid with a hydrogen peroxide soln. and/or ozone water in a pressure system held to specific conditions.

CONSTITUTION: A transition metal catalyst is allowed to coexist in a waste liquid such as an industrial waste liquid and the waste liquid is mixed with a hydrogen peroxide soln. and/or ozone water in a pressure system of 30kg/cm-2 or more to be treated. By this constitution, reaction is advanced by the generation of a (chemically radical) hydroxyl free radical having strong oxidizing capacity. Therefore, COD in the waste liquid is decomposed into nitrogen, carbon dioxide and water under physically mild conditions to be efficiently removed.