Chlorite and Chlorate ion variability IN DISTRIBUTION SYSTEMS

Abstract

This study tracked changes in chlorite ion (ClO 2 – ) and chlorate ion (ClO 3 – ) concentrations in the distribution system of the City of Laval, Québec, following use of chlorine dioxide (ClO 2 ) 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 ClO 2 in postdisinfection, ClO 2 – concentrations decreased with increasing water residence time. In the networks fed by treatment plants using free chlorine in postdisinfection, ClO 2 – levels remained below the method detection limit (MDL). ClO 3 – concentrations measured in the network following ClO 2 postdisinfection were found to be either below the MDL or stable at a low level (0.09 mg/L) in cold water; ClO 3 – levels decreased slightly in warm water with increasing residence time. In the networks using free chlorine as a postdisinfectant, ClO 3 – levels showed significant variations in their overall concentrations during different samplings or with specific changes in their concentration related to residence time.