Implementation of Ozonation and Biological Activated Carbon (O 3 –BAC) Process in China Mitigates Disinfection Byproduct Formation Trends in Finished Drinking Water

Source water quality and precursor removal efficiency critically influence toxic disinfection byproduct (DBP) formation in drinking water treatments. Riverine pollutant accumulation could exacerbate DBP precursor enrichment in downstream urban water sources. This study investigates longitudinal DBP precursor profiles across 19 cities in the Yangtze River Basin, revealing how advanced treatment processes can counteract source water degradation. Through (non-) target analysis and cytotoxicity evaluations, we demonstrate that the ozonation and biological activated carbon (O3-BAC) processes widely deployed in 72% of downstream cities selectively remove unsaturated DBP precursors, as evidenced by a 25% reduction in dissolved organic matter unsaturation in O3-BAC-treated tap water relative to conventionally treated samples. This molecular mechanism explains the paradoxical 45% decrease in known DBP formation (median: 47.9 μg/L downstream vs 87.4 μg/L midstream) despite downstream source water containing nearly 2-fold higher precursors. Consequently, downstream tap water did not exhibit elevated DBP-related cytotoxicity. Our findings elucidate how O3-BAC mitigates DBPs despite deteriorating source water quality, offering a replicable strategy to enhance water infrastructure resilience in alignment with United Nations Sustainable Development Goals.