Regulated 1-2 Carbon Disinfection Byproducts do not Correlate with Cytotoxicity with Increasing Disinfectant Contact Time During Chlorination, Chlorination Followed by Chloramination or Granular Activated Carbon Followed by Chlorination

Regulations typically use four trihalomethanes (THM4) and five haloacetic acids (HAA5) as metrics of consumer exposure to disinfection byproducts (DBPs) and their chronic health risks. Their use as exposure metrics assumes that their concentrations correlate with DBP-associated toxicity. For a chlorine-disinfected surface water, this study demonstrates that increasing chlorine contact time from 1 to 7 days was associated with a 62-76% increase in THM4 and HAA5 but a 40-47% decrease in total cytotoxicity. Thus, the use of THM4 and HAA5 may divert regulatory attention away from the low water age sections of distribution systems near treatment facilities that may feature the highest cytotoxicity but lowest THM4/HAA5 concentrations. Among common options to reduce THM4/HAA5, this study also shows that chlorine disinfection followed by chloramines for maintaining a distribution system residual did not substantially reduce cytotoxicity. Granular activated carbon followed by chlorine reduced cytotoxicity by 28-80%, even at the lowest water ages where cytotoxicity was maximized. These findings highlight the need to identify DBPs that better correlate with toxicity than THM4/HAA5 to serve as metrics of exposure. These metrics could help identify distribution system locations exhibiting higher consumer risk and develop modifications to disinfection systems that effectively reduce consumer risk.