Advanced oxida­tion processes (AOP) are used to treat heav­ily cont­a­m­i­nated waste­water, orig­i­nat­ing, for exam­ple, from the chem­i­cal or phar­ma­ceu­ti­cal indus­tries. These processes involve hydro­gen perox­ide being acti­vated by adding compo­nents, such as ultra­vi­o­let light, ozone or ‒ as in the Fenton reac­tion ‒ iron salts, to increase the oxida­tion effect. During acti­va­tion, hydroxyl radi­cals are formed that have a very high oxida­tion poten­tial. These break the complex, diffi­cult-to-degrade mole­cules down into small frag­ments, which are then biodegradable.