The Formation and Decay of Disinfection By-Products in the Distribution System

Most DBP studies conducted to date have focused on the formation and control of DBPs in treatment plants. They were conducted in bottles in the laboratory under controlled conditions. Little is known about the changes in DBPs in full scale distribution systems. Also, information to date suggests that batch incubations in bottles may not accurately represent the DBPs in full-scale systems because of their inability to consider pipe walls and biofilms, spatial and temporal DBP variability within a system, and the dynamic nature of the system. Utilities were selected based on finished water characteristics, seasonal temperature changes, geographic location, type of final disinfectant, DBP concentrations, distribution system hydraulic model, and others. Two utilities used free chlorine as residual disinfectant, two utilities used chloramines, and one utility used chloramines with a yearly one-month switch to free chlorine. Within each system, sampling locations were selected based on water age, pipe material and diameter, and others. One system examined the effects of a storage reservoir and a booster chlorination station. Samples were collected six to eight times over a one-year period. In the system that periodically switches to free chlorine, two samplings were conducted before the switch, two samplings during the free chlorination period, and two samplings following the return to chloramines.