Chasing disinfection byproducts through the pipes: How 66 DBPs change over time in chlorinated vs. chloraminated distribution systems

While disinfection byproducts (DBPs) are typically measured at drinking water treatment plants, levels can change dramatically within the distribution system before reaching the consumer. In this study, the spatio-temporal trends of 66 DBPs across 9 different classes were examined in two drinking water distribution systems with similar source waters, but different pretreatments and residual disinfectants. One system uses residual chlorine in the distribution system, and the other uses chloramine, allowing for examination of how DBP concentrations change over time in distribution systems with different residual disinfectants. Four routes were sampled for each system with six time points over three days travel time, collected at increasing distance from the treatment plant to "chase" the same packet of water. On average, total DBP levels increased 32% over time in the chlorine system, but decreased 23% in the chloramine system, indicating that DBP formation in the pipes outpaced DBP degradation in the chlorine system and vice-versa in the chloramine system. Among emerging DBP classes, iodo-DBPs decreased overall by 64% (but dichloroiodomethane maintained steady levels) in the chlorine distribution system, while haloacetic acids and haloacetaldehydes reached maximum concentrations in the middle of the chloramine distribution system before decreasing by an average of 29% and 54%, respectively, by the end of the system. Comparatively, I-DBPs degrade slightly over time in the chloramine DSs, but 0.5-1.9 µg/L total levels were still present in the tap water after 72 h. Haloacetonitriles were detected at similar levels in both distribution systems and followed similar trends, with an initial maximum in the finished water or at 2 h, then a decrease in concentration at medium detention times, followed by an increase to a second maximum at 48 h or 72 h. Haloacetamides decreased by an average of 26% from finished water to 72 h in the chloramine DS and were detected in only one route of the chlorine DS2 at low levels, up to 8.6 µg/L total. Calculated cytotoxicity based on measured DBPs decreased by 41% on average in the chlorine system and by 33% in the chloramine system from finished water to the end of the distribution system. The decrease in calculated cytotoxicity in the chloramine system corresponds with a universal decrease of all measured DBP classes from finished water to 72 h, meanwhile the decrease in calculated cytotoxicity in the chlorine system was driven by a shift towards more chlorinated DBPs vs. brominated DBPs.