Effective mitigation of chlorinated byproducts and biotoxicity in municipal wastewater by VUV/UV/NH2Cl: Molecular-level comparison with UV/Cl2, VUV/UV/Cl2, and UV/NH2Cl

UV-based advanced oxidation processes (UV-AOPs) show great potential for municipal wastewater advanced treatment. Although chlor(am)ine-based UV-AOPs have been widely investigated, discrepancies in chlorinated byproduct (Cl-BP) formation and biotoxicity risks among UV/chlorine (UV/Cl 2 ), UV/chloramine (UV/NH 2 Cl), vacuum ultraviolet/ultraviolet/chlorine (VUV/UV/Cl 2 ), and VUV/UV/chloramine (VUV/UV/NH 2 Cl) processes for municipal secondary effluent treatment remain poorly understood. Herein, gas chromatography, Fourier transform ion cyclotron resonance mass spectrometry, and Vibrio fischeri were employed to reveal the formation characteristics of Cl-BPs and acute toxicity variations during these treatments. The results indicated that UV/Cl 2 process produced the highest levels of total organic chlorine (TOCl), known Cl-BPs, calculated cytotoxicity, and acute toxicity. In contrast, VUV/UV/NH 2 Cl treatment exhibited the lowest values for these parameters, with reductions of 61.3%, 96.3%, 99.7%, and 28.0% relative to UV/Cl 2 system. UV/Cl 2 treatment yielded the largest number of unknown Cl-BPs with high saturation, while VUV/UV/NH 2 Cl process generated the fewest Cl-BPs, consisting of more unsaturated CHONCl compounds. Mass difference analysis revealed that Cl 2 -based AOPs had the most precursors and precursor-product pairs, with oxygenation, chlorine substitution and addition as the principal reactions, whereas NH 2 Cl-based ones were dominated by nitrogen-related reactions. Considering Cl-BP and biotoxicity mitigation, VUV/UV/NH 2 Cl process emerges as the most promising technology for advanced treatment of municipal secondary effluent. Overall, this study provides fundamental insights and technological guidance for applying (VUV/)UV-AOPs to real municipal wastewater.