Tracing disinfection byproducts during shock chlorination of a seawater reverse osmosis (RO) desalination pilot and the blended and chlorinated potable water produced

Shock chlorination is an increasingly recognized strategy applied in seawater reverse osmosis (RO) desalination plants to mitigate membrane fouling. The obtained desalinated seawater is often blended with treated drinking water and subsequently chlorinated for domestic use. This study traced the formation of disinfection byproducts (DBPs) and the associated cytotoxicity during shock chlorination of a pilot seawater RO desalination system and the subsequent chlorination of the blended water at different blending ratios. Brominated DBPs (Br-DBPs) including tribromomethane, tribromoacetic acid, and dibromoacetic acid dominated right after shock chlorination. The dissolved air flotation combined with the sand filtration partially removed DBPs, possibly through volatilization, adsorption, biodegradation, and transformation of DBPs. The rejection rate of Br-DBPs was significantly higher than that of chlorinated DBPs, leading to the overall high RO rejection rate of trihalomethanes (THMs) (98.92 %), haloacetic acids (HAAs) (99.01 %), and haloacetonitriles (HANs) (94.47 %). In the chlorinated blended water, the total concentration of DBPs decreased while the fraction of Br-DBPs increased with increasing ratio of desalinated seawater, peaking at a blending ratio of 20 % of desalinated seawater. This is due to the dilution of DBPs and DBP precursors (i.e., natural organic matter) originated in treated drinking water by the desalinated seawater, which however increased Br⁻ concentrations in the blended water. Brominated HANs and brominated HAAs were the main contributors to DBP-associated cytotoxicity. The blending ratio of 60 % or higher of desalinated seawater is suggested to minimize DBP concentration and cytotoxicity in the chlorinated blended water. The findings of this study enhance our understanding of the behavior of DBPs and their associated cytotoxicity during shock chlorination in seawater RO desalination plants and in chlorinated blended water, providing a theoretical basis for real-world application.