Changes in chlorine and DOX concentrations in distribution systems

The fate of disinfectants and disinfection by-products (DBPs) has been stud ied in laboratories under a variety of controlled, constant conditions, but limited information is available about their fate in full-scale distributi on systems. A study of the City of Laval (Quebec) distribution system exami ned changes in the concentrations of chlorine and dissolved organic halogen s (DOX) as the water flows through the distribution system. Water was colle cted in small- and large-diameter pipes with respect to the residence time (RT) of each sampling point. Free and total chlorine concentrations decreas ed with increasing RT in the distribution system. The influence of pipes on chlorine decay was particularly important in warm water; at similar RTs, c hlorine concentrations were lowest in the small-diameter pipes, greater in the main pipes, and greatest in the batch incubations. The presence of corr osion by-products in pipes appeared to be the major factor influencing chlo rine decay in the distribution system. DOX concentrations generally increas ed with increasing RT in the presence of free chlorine and decreased when f ree 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 presen ce of microbial biomass.