## 2 Advanced Oxidation Processes  
Advanced Oxidation Processes are technologies that can generate hydroxyl radicals that interact with the organic molecules and oxidize them into simpler substances [40]. This oxidation converts the pollutants into intermediate compounds or CO\({}_{2}\) and H\({}_{2}\)O when mineralization occurs [40].  
The degradation mechanisms of AOPs can be hydrogen abstraction, electron transfer, or radical addition, and these technologies can eliminate complex compounds that cannot be removed by conventional approaches [40, 41]. In WWTPs, AOPs can be used as preliminary processes in order to transform persistent and recalcitrant compounds into compounds that biological treatments can remove, or they can be applied as a final deputation step before the discharge of the effluent into water sources [42].  
AOPs present several advantages, such as elimination of complex and persistent compounds [41], treatment of different effluents [42], toxicity reduction [43], increasing the effluent biodegradability [43], elimination of pathogenic agents, or achieve pollutants complete oxidation into CO\({}_{2}\) and H\({}_{2}\)O [44]. In addition, some AOP treatments can be important to decrease COD (chemical oxygen demand), BOD (biochemical oxygen demand), coloration, and turbidity, which are important parameters regarding effluent discharge regulations. The main disadvantage of the use of these technologies is their expensive costs since they demand a significant amount of energy or reagents [41]. Moreover, in general, when real wastewater is considered, the AOP treatments can present efficiency problems, since the presence of carbonate or bicarbonate ions (or other scavenging compounds) can act as scavengers of hydroxyl radicals, which can negatively affect their reaction mechanism and decrease the pollutants removal [42; 45].  
Hydroxyl radical is the most common radical produced by AOPs and presents a high oxidative power and non-selective behavior. Furthermore, the generation of other oxidant species can occur during the use of AOPs, due to the existence of specific operational conditions. Examples of oxidizing radicals can be seen in Table 1[42; 46].  
Other radicals such as ozonide (O\({}^{3-}\)), superoxide (O\({}^{2-}\)), or hydroperoxyl radical (HO\({}^{2-}\)) can also appear [40; 47]. The radical's generation occurs in situ and can occur due to the existence of reagents, catalysts, or radiation [41].  
The group of AOPs is composed of several technologies, such as ozonation, photocatalysis, Fenton, wet air oxidation, radiation, oxidation by reagents, electrochemical oxidation, anodic oxidation, sonochemical methods, and others [41; 46]. The combination and integration of several single techniques is also part of AOP, like photo-Fenton, photocatalytic ozonation, sono-Fenton, electro-Fenton, sono-photocatalysis, photo-ozonation, catalytic ozonation, and peroxone, among others [41; 42; 43]. Figure 1 presents a scheme summarizing some of the AOPs technologies reported in the literature.  
Several studies are being carried out with the objective of lowering the operational costs, using new reactors [41], catalysts, reagents, and even radiation, particularly the use of solar radiation for being free, infinite, and considered as a \"green-energy\" [44].