Regulated\n1–2\nCarbon Disinfection Byproducts\ndo not Correlate with Cytotoxicity with Increasing Disinfectant Contact\nTime During Chlorination, Chlorination Followed by Chloramination\nor Granular Activated Carbon Followed by Chlorination

Regulations typically use four trihalomethanes\n(THM4)\nand five\nhaloacetic acids (HAA5) as metrics of consumer exposure to disinfection\nbyproducts (DBPs) and their chronic health risks. Their use as exposure\nmetrics assumes that their concentrations correlate with DBP-associated\ntoxicity. For a chlorine-disinfected surface water, this study demonstrates\nthat increasing chlorine contact time from 1 to 7 days was associated\nwith a 62–76% increase in THM4 and HAA5 but a 40–47%\ndecrease in total cytotoxicity. Thus, the use of THM4 and HAA5 may\ndivert regulatory attention away from the low water age sections of\ndistribution systems near treatment facilities that may feature the\nhighest cytotoxicity but lowest THM4/HAA5 concentrations. Among common\noptions to reduce THM4/HAA5, this study also shows that chlorine disinfection\nfollowed by chloramines for maintaining a distribution system residual\ndid not substantially reduce cytotoxicity. Granular activated carbon\nfollowed by chlorine reduced cytotoxicity by 28–80%, even at\nthe lowest water ages where cytotoxicity was maximized. These findings\nhighlight the need to identify DBPs that better correlate with toxicity\nthan THM4/HAA5 to serve as metrics of exposure. These metrics could\nhelp identify distribution system locations exhibiting higher consumer\nrisk and develop modifications to disinfection systems that effectively\nreduce consumer risk.