The tapping of municipal wastewater for potable reuse significantly enhances drinking water supply in drought-stricken regions worldwide. Membrane-based potable reuse treatment trains commonly employ ultraviolet-based advanced oxidation processes (UV-AOPs) to degrade trace organic contaminants in water to produce high-quality recycled water. Hydrogen peroxide (H₂O₂) is used as the default photo-oxidant. Meanwhile, chloramines, which are added to prevent biofouling, pass through the membranes and impact the treatment efficiency of UV-AOP. Water reuse facilities therefore face the dilemma of optimizing H₂O₂ (an added photo-oxidant) and chloramines (a carry-over photo-oxidant) doses. Utilizing a uniquely designed pilot-scale reactor and real-time recycled water, we evaluated treatment efficiencies of UV-AOP on six important indicator contaminants, with monochloramine (NH₂Cl) and H₂O₂ as photo-oxidants. Hydroxyl radical (HO^•) and reactive chlorine species, such as the chlorine atom (Cl^•) and chlorine dimer (Cl₂^•-), were the major reactive species. Overall, radicals generated from photolysis of NH₂Cl alone achieved removal of indicator compounds, which can be further improved by optimizing UV fluence, i.e., the UV dose. Furthermore, the addition of H₂O₂ enhanced HO^• formation and improved contaminant removal. However, the addition of H₂O₂, when the background NH₂Cl level was above 2 mg L^-1 (as Cl₂), provided limited improvement in treatment efficiency. These trade-offs between chloramine and H₂O₂ as oxidants, and the recommended optimization of the associated effective UV fluence, are critical for energy-efficient and cost-effective potable reuse to address the challenges of global water scarcity.

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