Effect of media age on cytotoxicity of drinking water treated by biological activated carbon filters and chlorine disinfection.

Biological activated carbon (BAC) filters are widely employed for controlling the formation of harmful disinfection byproducts (DBPs) prior to chlorine disinfection. Aged BAC filters are commonly practiced at drinking water treatment plants due to the cost of replacement. The potential mitigation of cytotoxicity of chlorinated waters by young and aged BAC filters was assessed in this study. Increasing BAC filter media age from 3.5 to 10 years decreased the cytotoxicity reduction from >30 % to <10 %. Young BAC filters mitigated the water cytotoxicity via the abatement of precursors for nonvolatile DBPs, contributing to >86 % of chlorinated water cytotoxicity. High-resolution mass spectrometry analyses revealed that young BAC filters could reduce the numbers of halogenated formulas in the nonvolatile fractions more effectively than aged ones (43 % vs. 10 %). Lignin/carboxylic-rich aromatic moieties and aromatic structures were important precursors for halogenated DBPs. Thirty-eight DBPs in nonvolatile fractions were tentatively identified via suspect screening and machine learning prioritization, among which aromatic DBPs were predominant. Young BAC filters with active aerobic metabolism abated the precursors for aromatic DBPs via biosorption and biodegradation. Pre-ozonation increased the biodegradability of these precursors, facilitating their biodegradation in young BAC filters. The deteriorated abatement of precursors for cytotoxic DBPs in aged BAC filters could be ascribed to both the saturated biosorption capacity and decreased microbiological activity. This study provides insights into the evaluation of drinking water cytotoxicity mitigation using aged BAC filters.