Chlorite and Chlorate ion variability IN DISTRIBUTION SYSTEMS

This study tracked changes in chlorite ion (ClO2–) and chlorate ion (ClO3–) concentrations in the distribution system of the City of Laval, Québec, following use of chlorine dioxide (ClO2) or free chlorine. Water was collected in small‐ and large‐diameter pipes with respect to the residence time of each sampling point. In the network fed by a treatment plant using ClO2 in postdisinfection, ClO2– concentrations decreased with increasing water residence time. In the networks fed by treatment plants using free chlorine in postdisinfection, ClO2– levels remained below the method detection limit (MDL). ClO3– concentrations measured in the network following ClO2 postdisinfection were found to be either below the MDL or stable at a low level (0.09 mg/L) in cold water; ClO3– levels decreased slightly in warm water with increasing residence time. In the networks using free chlorine as a postdisinfectant, ClO3– levels showed significant variations in their overall concentrations during different samplings or with specific changes in their concentration related to residence time.