Disinfection of Secondary Urban Wastewater Using Hydrogen Peroxide Combined with UV/Visible Radiation: Effect of Operating Conditions and Assessment of Microorganism Competition

The growing and unprecedented water crisis leads to the need to find alternative water resources, and the reuse of treated urban wastewater is an excellent approach. Accordingly, in this work, the disinfection of a secondary effluent (W) discharged from a wastewater treatment plant (WWTP) by hydrogen peroxide combined with radiation (H2O2+UV/visible) was studied with the aim of obtaining treated water that can be reused. Firstly, the effect of hydrogen peroxide alone, radiation per se and the combined H2O2+UV/Visible process in the inactivation of enterobacteria were assessed. It was found that the oxidant alone is not efficient; the maximum inactivation is achieved when the oxidant and radiation are used simultaneously. For the first time, the effect of some operational parameters, namely the hydrogen peroxide concentration (between 50 and 125 mg/L), initial pH (from 5.0 to 7.0), temperature (between 15 and 25 °C), and radiation intensity (100 to 500 W/m2), on the efficiency of the disinfection process was assessed. When the process was carried out under the best operating conditions found ([H2O2] = 75 mg/L, pH = 5.0, T = 25 °C, and UV/visible light with I = 500 W/m2), total enterobacteria and total heterotrophs were inactivated and the abundance of the 16S rRNA, blaTEM, qnrS, and intl1 genes was reduced. The cultivable microorganisms grew again after 3 days of storing the treated wastewater (TW), making it impossible to reuse such effluent after storage. Therefore, the potential capacity of a diverse bacterial community present in river water to inhibit the regrowth of potentially harmful bacteria present in the urban secondary wastewater after the application of the treatment process was also evaluated. To the authors’ knowledge, this has never been studied before. For this purpose, the TW was diluted with river water (R) at a volumetric percentage of 50/50—sample R+TW. It was found that, after storage, only the total heterotrophs grew, while the abundance of the targeted genes remained practically constant. The R+TW sample after storage met the legal limits for reuse in urban and agricultural applications. The results of this study suggest that the combination of the H2O2+UV/visible radiation treatment with dilution of the final treated effluent with natural surface water can contribute to reducing the burden of water scarcity.