It is one of the advanced oxida­tion processes (AOP processes) used in the treat­ment of heav­ily cont­a­m­i­nated indus­trial waste­water. The basic prin­ci­ple was discov­ered way back by Henry John Horstman Fenton at the end of the 19th century. This process involves hydro­gen perox­ide being acti­vated by iron salts to increase the oxida­tion effect. So-called hydroxyl radi­cals with a high oxida­tion poten­tial are formed. These break up large mole­cules that are diffi­cult to degrade. This pretreat­ment then enables waste­water to be biotreated in a waste­water treat­ment plant.