Molecular-level insights into the disinfection by-product precursor removal by biological activated carbon process...

Biological activated carbon (BAC) filters are extensively employed in drinking water treatment plants (DWTPs) as a post-ozonation step to enhance water quality. However, the efficacy of BAC in removing ozonation transformation products (OTPs) remains inadequately characterized. This study comprehensively evaluated BAC performance by investigating dissolved organic matter (DOM) transformation and disinfection by-product formation potential (DBP FP) in samples collected from a full-scale DWTP utilizing the O₃-BAC process. Results showed limited overall DOM removal by O₃-BAC: 7.7 % for dissolved organic carbon and 15.0 % for fluorescent fractions. Molecular transformation analysis demonstrated that BAC filtration selectively targeted ozonation-derived saturated and oxidized molecules (characterized by H/C_w = 1.14 and O/C_w = 0.36) while allowing more unsaturated and aromatic OTPs to persist, confirming BAC's crucial role in determining the fate of OTPs and subsequent DBP FP. The BAC filtration achieved significant removal: 25.2 % for trihalomethanes (THMs), 47.4 % for haloaldehydes, 4.6 % for haloacetonitriles (HANs) and 47.0 % for halonitromethanes relative to O₃ effluent levels. Spearman analysis revealed the characteristics of molecular formulas strongly associated with DBP FP. Specifically, haloketones and HAN FP-correlated formulas shared more reduced and less oxygenated structural characteristics compared to those associated with other DBP classes. The molecular mechanism underlying O₃-BAC efficacy was elucidated: OTPs serve as key DBP precursors; BAC preferentially removes OTP-derived precursors (e.g., 89.1 % for THMs versus 5.8 % for original precursors); this selective removal counteracts ozonation's precursor generation effect. This synergy provides the molecular basis for DBP-FP reduction, clarifying BAC's critical role in controlling ozonation-derived risks.