Reducing or elevating? Divergent effects of UV-based AOPs on DOM removal and DBP formation for stormwater reuse safety

Abstract

Stormwater runoff contains high levels of dissolved organic matter (DOM) that serve as precursors for disinfection byproducts (DBPs) during chlorination. The effect of advanced oxidation processes (AOPs) pretreatment on subsequent DBP formation remains uncertain, particularly under varying water matrices. Stormwater runoff from four functional zones (residential, commercial, transportation, and educational) was treated with three UV-based AOPs (UV/H₂O₂, UV/persulfate, and UV/ferrate), followed by chlorination. DOM transformation was characterized by dissolved organic carbon (DOC) removal, molecular weight (MW) distribution, hydrophobicity, and fluorescence indices. All three AOPs achieved > 87% DOC removal, near-zero aromaticity, and a shift of MW to < 1 kDa. However, subsequent chlorination revealed divergent DBP formation patterns governed by ammonia-nitrogen (NH₄⁺-N) concentration. In high-NH₄⁺-N runoff (residential and commercial, 15 and 6 mg/L), the ammonia-induced chlorine demand largely depleted free chlorine during post-chlorination, forming monochloramine as the dominant chlorinating agent. AOP converted DOM into low-MW intermediates that exhibited enhanced reactivity toward monochloramine, increasing both trihalomethanes (THMs, up to 6-fold) and haloacetonitriles (HANs, up to 10-fold). In low-NH₄⁺-N runoff (transportation and educational, 2–3 mg/L), free chlorine dominated and AOPs directly degraded DBP precursors, reducing both THMs (up to 65%) and HANs (up to 67%). Thus, high NH₄⁺-N drives a risk shift toward increased DBPs, whereas low NH₄⁺-N enables effective DBP control. These findings suggest that for high-NH₄⁺-N stormwater, ammonia pre-removal is advisable before AOP + chlorine, while for low-NH₄⁺-N runoff, AOPs alone can serve as an effective DBP control strategy.

Graphical abstract

Keywords

Disinfection byproducts / UV-based AOPs / Dissolved organic matter / Stormwater runoff / Risk shift

Highlight

	● UV-based AOPs remove > 87% DOC and nearly eliminate DOM aromaticity.
	● AOPs shift DOM molecular weight to < 1 kDa (> 80% fraction).
	● Hydrophobic components resist mineralization, enriching in residual DOM.
	● AOPs enrich hydrophilic microbial byproduct-like fluorescent components.
	● NH4+-N governs whether AOP + chlorine reduces or elevates DBPs.

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