Changes in chlorine and DOX Concentrations in Distribution Systems

Abstract

The fate of disinfectants and disinfection by‐products (DBPs) has been studied in laboratories under a variety of controlled, constant conditions, but limited information is available about their fate in full‐scale distribution systems. A study of the City of Laval (Québec) distribution system examined changes in the concentrations of chlorine and dissolved organic halogens (DOX) as the water flows through the distribution system. Water was collected in small‐ and large‐diameter pipes with respect to the residence time (RT) of each sampling point. Free and total chlorine concentrations decreased with increasing RT in the distribution system. The influence of pipes on chlorine decay was particularly important in warm water; at similar RTs, chlorine concentrations were lowest in the small‐diameter pipes, greater in the main pipes, and greatest in the batch incubations. The presence of corrosion by‐products in pipes appeared to be the major factor influencing chlorine decay in the distribution system. DOX concentrations generally increased with increasing RT in the presence of free chlorine and decreased when free chlorine concentrations were below the method detection limit. Results point to significant pipe influence on DOX concentrations. DOX decay seemed to be attributable to the chemical instability of some DBPs and the presence of microbial biomass.