## 3 Applications and Limits of ATPs in Water and Wastewater Treatment
may include both organic (e.g., humic and fulvic acids, natural organic matter--NOM, aminoacids, proteins, carbohydrates) and inorganic species (e.g., sulfide, bromide). As a consequence of the introduction of ATPs in water and wastewater treatment, the generation of by-products from CECs transformation, water or wastewater original matrix oxidation, or reactions with disinfecting chemicals may be observed. Some of these by-products may be proven or potential carcinogenic, or carcinogenic precursor compounds, or induce otherwise undesired contamination if released into the environment. This may occur when insufficient applied oxidation or reductants does do not result in complete mineralization of the target pollutants. For example, UV irradiation alone is considered the least effective AOP for organic contaminants' removal. An indication that this process cannot be considered as a feasible standalone process was shown in a study proving that, while it could remove 50-80% of targeted antibiotics, it required doses 100 times greater than those required for disinfection alone to achieve this target. This would be highly expensive, and such high doses unpractical to deliver [58]. Preference should therefore be given to processes with a high radical yield and reaction rates, with reduced process contact time, and in which doses could be rapidly adjusted, when needed. In addition to the greater radical generation capacity, the achievable EB dose-rates are about 100 times higher than a gamma's source, requiring very short irradiation exposure. The literature on radical reactions in water solutions is established [34], with works on the kinetics of decomposition of various types of environmental pollutants. However, studies have mostly been carried out on single-component synthetic solutions. Prediction (modeling) of specific pollutant's degradation in pure monocomponent solutions can be achieved in most cases by using reaction rate constants as a function of the solute concentration, pH, and operational conditions, but the influence of the actual solution matrix on the process can be determined, to date, only experimentally. Hence, ATP application cannot be based solely on reaction yield calculations but must be empirically tested in real conditions. While most AOPs and ARPs require the addition of chemical substances and/or catalyzers in order to increase the intensity of the reactions, in radiolytic processes (especially with EB technology), this is not strictly necessary. However, in the presence of external additives, which interact with radiolytic reaction products, increased radical generation may occur. For example, N2O added to an irradiated solution exhibits fast reactivity with both e\({}^{-}\)aq and -H, converting the former to -OH. Studies showed that oxidants, such as hypochlorite (NaOCI) [59] or ozone [60], added to irradiated solutions could significantly enhance process yields. When irradiation is carried out in air-saturated solutions, -O2 and -HO2 radicals are present, as a result of the oxygen reaction with e\({}^{-}\)aq and -H. H2O2's presence, which reacts with those radicals, could be an additional source of hydroxyl radicals, although excessive peroxide concentrations may act as scavengers. Hydrated electrons predominate when irradiated neutral solutions are saturated with N2 or Ar gas, while at pH < 2, -H radicals predominate. Due to technological reasons, the most practical process modification to increase the concentration of hydroxyl radicals in irradiated solutions is the moderate addition of O3 or H2O2.