Oxidation Processes” for the first time in 1987. AOPs still have not been put into commercial use on a large scale (especially in developing countries) even up to today mostly because of relatively high associated costs. Nevertheless, its high oxidative capability and efficiency make AOPs a popular technique in tertiary treatment in which the most recalcitrant organic and inorganic contaminants are to be eliminated. The increasing interest in water reuse and more stringent regulations regarding water pollution are currently accelerating the implementation of AOPs at full-scale. There are roughly 500 commercialized AOP installations around the world at present, mostly in Europe and the United States. Other countries like China are showing increasing interests in AOPs. Chemical principles Generally speaking, chemistry in AOPs could be essentially divided into three parts: Formation of ·OH; Initial attacks on target molecules by ·OH and their breakdown to fragments; Subsequent attacks by ·OH until ultimate mineralization. The mechanism of ·OH production (Part 1) highly depends on the sort of AOP technique that is used. For example, ozonation, UV/H2O2, photocatalytic oxidation and Fenton's oxidation rely on different mechanisms of ·OH generation: UV/H2O2: H2O2 + UV → 2·OH (homolytic bond cleavage of the O-O bond of H2O2 leads to formation of 2·OH radicals) UV/HOCl: HOCl + UV → ·OH + Cl· Ozone based AOP: O3 + HO− → HO2− + O2 (reaction between O3 and a hydroxyl ion leads to the formation of H2O2 (in charged form)) O3 + HO2− → HO2· + O3−· (a second O3 molecule reacts with the HO2− to produce the ozonide radical) O3−· + H+ → HO3· (this radical gives to ·OH upon protonation) HO3· → ·OH + O2 the reaction steps presented here are just a part of the reaction sequence, see reference for more details Fenton based AOP: Fe2+ + H2O2 → Fe3++ HO· + OH− (initiation of Fenton's reagent) Fe3+ + H2O2 → Fe2++ HOO· + H+ (regeneration of Fe2+ catalyst) H2O2 → HO· + HOO· + H2O (Self scavenging and decomposition of H2O2) the reaction steps presented here are just a part of the reaction sequence, see reference for more details Photocatalytic oxidation with TiO2: TiO2 + UV → e− + h+ (irradiation of the photocatalytic surface leads to an excited electron (e−) and electron gap (h+)) Ti(IV) + H2O Ti(IV)-H2O (water adsorbs onto the catalyst surface) Ti(IV)-H2O + h+ Ti(IV)-·OH + H+ the highly reactive electron gap