## 2 Centralized Treatment Plants
### Urban Wastewater Treatments
#### 2.1.1 UWW Disinfection  
Given the problems connected with the chlorine disinfection by-products, alternative compounds are under study. The performance of peracetic acid (PAA) was evaluated at pilot scale for six months on a real WWTP secondary effluent with a flow higher than 50,000 m\({}^{3}\)/day, including preliminary, primary and secondary treatment with conventional activated sludge and nitrogen removal. The effluent water quality was stated according to the EU's newest legislation on wastewater reuse [43]. Three PAA doses (5, 10 and 15 mg/L)and three contact times (5, 10 and 15 min) were studied. The results suggest that PAA can be used as a wastewater disinfect, ensuring the achievement of legal requirements to reuse wastewater for irrigation. However, only when 15 mg/L or 10 mg/L-15 min contact times were used did the reclaimed water achieve the second highest water quality class (class B) [44]. Conventional wastewater disinfection processes, behind the disinfection by-product formation, are characterized by some drawbacks, such as biofouling problems [45] and/or poor removal of antibiotic resistance genes (ARGs) [46]. Moreover, contaminants of emerging concern (CECs) are rarely removed. When operating at pilot scale, different advanced oxidation processes (AOPs) were tested as tertiary treatments of municipal wastewater and evaluated concerning the energy efficiency, the disinfection by-product formation and the pathogen inactivation. In one such study, the investigated AOPs included UV/H\({}_{2}\)O\({}_{2}\), UV/Cl\({}_{2}\), O\({}_{3}\), O\({}_{3}\)/UV, H\({}_{2}\)O\({}_{2}\)/O\({}_{3}\)/UV and Cl\({}_{2}\)/O\({}_{3}\)/UV and were operated using various ozone doses (1.5-9 mg \(\times\) L\({}^{-1}\)) and UV irradiation power (191-981 m) \(\times\) cm\({}^{-2}\)). Ozonation resulted in the most energy-efficient technology for CEC degradation. However, the needed high ozone doses can lead to high concentration of disinfection by-products such as bromate or N-nitrosodimethylamine (NDMA). UV-based AOPs were found to have a high energy cost for micropollutant oxidation, whereas AOPs combining ozone and UV were able to reduce the energy costs, show good disinfection capabilities and reduce the risk of NDMA formation. Finally, the H\({}_{2}\)O\({}_{2}\)/O\({}_{3}\)/UV treatment hindered bromate formation, thus representing a very promising technology for wastewater reclamation for different purposes [47]. Also, the comparison among the performance of conventional UVC/H\({}_{2}\)O\({}_{2}\), UVC photo-Fenton with two chelating agents and ozonation was carried out at pilot scale in real municipal wastewater. The degradation of three pharmaceuticals (PhCs) and the _E. coli_ and _Enterococcus_ inactivation were the test's parameters. Conventional UVC/H\({}_{2}\)O\({}_{2}\) 50 mg/L was the faster option, being able to degrade 80% of the sum of PhCs after 2.5 min and to reach the detection limit (DL) for _E. coli_ and _Enterococcus_ within 5 and 10 min, respectively, by using only 15 mg/L H\({}_{2}\)O\({}_{2}\). Photo-Fenton achieved 60% of PhC removal after 2.5 min, and O\({}_{3}\) treatment obtained the same result well beyond 75 min of treatment at the highest dose (0.45 g O\({}_{3}\)/L h). O\({}_{3}\)-treated WW showed _E. coli_ presence under the DL of 1 CFU/100 mL after 120 min, while _Enterococcus_ was still present at 10 CFU/100 mL after 120 min. The authors suggested that UVC/H\({}_{2}\)O\({}_{2}\) 15 mg/L would be an effective solution for simultaneous PhC removal and bacteria inactivation. For full-scale application, a lower H\({}_{2}\)O\({}_{2}\) concentration, selected according to PhC initial concentration and their refractoriness to degradation, can be recommended, since a lower amount of residual oxidant will not be required for post-treatment [48].