Unregulated Disinfection Byproducts from de facto Water Reuse: Molecular Evidence for EfOM-Induced Redirection of Chlorination Pathways

Unplanned potable reuse introduces effluent-derived organic matter (EfOM) into drinking water sources. While conventional coagulation partially removes EfOM, the residual fraction undergoes chlorination and may form toxic disinfection byproducts (DBPs) that escape current monitoring. However, the molecular mechanisms by which EfOM alters DBP formation pathways remain poorly understood. Here, we combine ultrahigh-resolution mass spectrometry with reaction network analysis to reveal that EfOM significantly redirects chlorine pathways. Compared to EfOM-free surface water, the presence of 20% EfOM decreases direct halogenation pathways (addition and substitution) from 4.2% to 1.8% of total transformation events. This shift occurs because EfOM introduces reduced, high-molecular-weight, heteroatom-rich molecules that cannot be directly halogenated; they require HOCl-driven oxidative fragmentation to generate halogenation-susceptible intermediates. Consequently, chlorinated finished water accumulates sulfur-containing DBPs (CHOS compounds, with relative intensity increasing from 35% to 60%) and complex “orphan” DBPs lacking identifiable precursors (with molecular features linked to elevated toxicity). These findings highlight an urgent need to update DBP monitoring frameworks to include unregulated sulfur-containing and orphan byproducts in de facto reuse scenarios.