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-2) and chlorination followed by chloramination (Cl-2/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., (Cl-2)/GAC/Cl-2)) and chlorination/chloramination (Cl-2/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/Cl-2 and Cl-2/NH2Cl achieved comparable reductions in the formation of regulated trihalomethanes (THMs) and haloacetic acids (HAAs). Nonetheless, GAC/Cl-2 always resulted in lower total cytotoxicity than Cl-2/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 Cl-2/GAC/Cl-2 and GAC/Cl-2. 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.

Automated summary

This study compared the effects of granular activated carbon treatment with postchlorination and chlorination/chloramination on the total cytotoxicity of water. The results showed that GAC/Cl-2 resulted in lower total cytotoxicity than Cl-2/NH2Cl, even at high levels of organic carbon breakthrough.