Abstract
The objective of this study was to determine and evaluate some of the factors that affect the formation and decay of disinfection by-products (DBPs), specifically trihalomethanes (THMs) and haloacetic acids (HAAs), in full-scale drinking water distribution systems. Five utilities throughout the United States were sampled 6–8 times over a period of 15 months. Utilities were chosen based on source water characteristics, seasonal temperature variations, geographic location, type of secondary disinfectant utilized, and several characteristics specific to each distribution system. In addition, each utility had a hydraulic model of their distribution system that was used to determine residence times at sampling locations in their system. This paper describes the results for one of the five utilities chosen to participate in the study. The selected utility employed combined chlorine as their secondary disinfectant and operated the distribution system using several different hydraulic scenarios. Sampling locations were chosen on the basis of residence time, pipe material and pipe diameter, and other factors specific to the distribution system. Twelve to sixteen sampling locations were chosen to represent a range of conditions. Samples collected were analyzed for pH, temperature, secondary disinfectant residual, assimilable organic carbon (AOC), total organic carbon (TOC), ultraviolet (UV) absorbance, heterotrophic plate counts (HPCs), THMs, HAAs, bromide, and ammonia, nitrite and nitrate. Results obtained for the subject utility demonstrated that, for some hydraulic scenarios, the total chlorine residual remained constant throughout the distribution system, while for other scenarios, a significant decrease in residual chlorine was observed at several of the sampling locations. THM and HAA concentrations were uniform throughout the system when the disinfectant residual remained constant, while a decrease in HAA concentrations was seen at those sampling locations that exhibited a decrease in total chlorine residual to values less than 0.6 mg/L. Elevated HPCs and evidence of nitrification were also observed at these same locations .
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