| Summary: | This work presents the comparison of four advanced oxidation processes driven by UVC-LED radiation (278 nm—2 W/m<sup>2</sup>) for simultaneous bacteria inactivation (<i>Escherichia coli</i>—10<sup>6</sup> CFU/mL) and microcontaminant removal (imidacloprid—50 µg/L) in simulated wastewater secondary effluent. To this end, the activation of H<sub>2</sub>O<sub>2</sub> and S<sub>2</sub>O<sub>8</sub><sup>2−</sup> as precursors of HO<sup>•</sup> and SO<sub>4</sub><sup>•−</sup>, respectively, by UVC-LED and UVC-LED/Fe<sup>3+</sup>–NTA (ferric nitrilotriacetate at 0.1 mM) has been studied at different oxidant concentrations. For the purpose of comparison, conventional chlorination was used as the baseline along with bacterial regrowth 24 h after treatment. Disinfection was achieved within the first 30 min in all of the processes, mainly due to the bactericidal effect of UVC-LED radiation. UVC-LED/H<sub>2</sub>O<sub>2</sub> did not substantially affect imidacloprid removal due to the low HO<sup>•</sup> generation by UVC irradiation at 278 nm, while more than 80% imidacloprid removal was achieved by the UVC-LED/S<sub>2</sub>O<sub>8</sub><sup>2−</sup>, UVC-LED/Fe<sup>3+</sup>–NTA/S<sub>2</sub>O<sub>8</sub><sup>2−</sup>, and UVC-LED/Fe<sup>3+</sup>–NTA/H<sub>2</sub>O<sub>2</sub> processes. The most efficient concentration of both oxidants for the simultaneous disinfection and microcontaminant removal was 1.47 mM. Chlorination was the most effective treatment for bacterial inactivation without imidacloprid removal. These findings are relevant for scaling up UVC-LED photoreactors for tertiary wastewater treatment aimed at removing bacteria and microcontaminants.
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