Cytotoxicity Comparison between Drinking Water Treated by Chlorination with Postchloramination versus Granular...

Granular activated carbon treatment with postchlorination (GAC/Cl₂) and chlorination followed by chloramination (Cl₂/NH₂Cl) represent two options for utilities to reduce DBP formation in drinking water. To compare the total cytotoxicity of waters treated by a pilot-scale GAC treatment system with postchlorination (and in some instances with prechlorination upstream of GAC (i.e., (Cl₂)/GAC/Cl₂)) and chlorination/chloramination (Cl₂/NH₂Cl) at ambient and elevated Br^- and I^- levels and at three different GAC ages, we applied the Chinese hamster ovary (CHO) cell cytotoxicity assay to whole-water extracts in conjunction with calculations of the cytotoxicity contributed by the 33 (semi)volatile DBPs lost during extractions. At both ambient and elevated Br^- and I^- levels, GAC/Cl₂ and Cl₂/NH₂Cl achieved comparable reductions in the formation of regulated trihalomethanes (THMs) and haloacetic acids (HAAs). Nonetheless, GAC/Cl₂ always resulted in lower total cytotoxicity than Cl₂/NH₂Cl, even at up to 65% total organic carbon breakthrough. Prechlorination formed (semi)volatile DBPs that were removed by the GAC, yet there was no substantial difference in total cytotoxicity between Cl₂/GAC/Cl₂ and GAC/Cl₂. The poorly characterized fraction of DBPs captured by the bioassay dominated the total cytotoxicity when the source water contained ambient levels of Br^- and I^-. When the water was spiked with Br^- and I^-, the known, unregulated (semi)volatile DBPs and the uncharacterized fraction of DBPs were comparable contributors to total cytotoxicity; the contributions of regulated THMs and HAAs were comparatively minor.