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.