Biofiltration of Known and Unknown Disinfection Byproducts across Chlorinated, Chlorine-to-Chloramine Transition, and Chloraminated Drinking Water

Disinfection byproducts (DBPs) in drinking water are a major health concern, and recent evidence suggests that unknown DBPs dominate cytotoxicity. However, current treatment strategies primarily target regulated DBPs, and strategies for removing unknown DBPs remain limited. This study evaluates biofiltration as a treatment barrier for known and unknown DBPs in chlorinated and chloraminated drinking water. Laboratory biofilters packed with sand, anthracite, and two granular activated carbons (GAC) were operated for 540 days under chlorinated water and chlorine-to-chloramine transition conditions to assess removal of total organic halogen (TOX), regulated and emerging DBPs, unknown TOX (UTOX), and calculated additive cytotoxicity (CAT). Trihalomethanes (THMs) removal remained below 15% under disinfectant regimes, whereas haloacetic acids (HAAs) were removed by more than 93%. Substantial UTOX reduction was observed, reaching 62.5% under chlorination and 47.9% under chloramination in biologically active carbon systems. Because the unknown fraction accounts for more than 50% of TOX, its preferential reduction indicates selective dehalogenation of the pool associated with mixture toxicity. The disinfectant regime influenced biodegradability, with chloramination producing a more recalcitrant unknown fraction than chlorination. The chlorine–chloramine transition caused temporary leaching of THMs from GAC biofilters, indicating that THM levels should be monitored during disinfectant switching. CAT decreased by 60–78% during biofiltration, demonstrating a shift toward less cytotoxic DBP mixtures. These findings suggest that biological filtration can selectively target toxicity-dominant DBPs and improve the toxicological profile of drinking water.