Cytotoxicity Comparison between Drinking Water Treated by Chlorination with Postchloramination versus Granular Activated Carbon (GAC) with Postchlorination.

Granular activated carbon treatment with postchlorination (GAC/Cl) and chlorination followed by chloramination (Cl/NHCl) 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., (Cl)/GAC/Cl)) and chlorination/chloramination (Cl/NHCl) 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/Cl and Cl/NHCl achieved comparable reductions in the formation of regulated trihalomethanes (THMs) and haloacetic acids (HAAs). Nonetheless, GAC/Cl always resulted in lower total cytotoxicity than Cl/NHCl, 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 Cl/GAC/Cl and GAC/Cl. 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.