ClO2 and by‐product persistence in a Drinking Water System

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This research evaluated a chlorine gas–solid sodium chlorite chlorine dioxide (ClO2) generation system at the Roanoke County (Va.) Spring Hollow Water Treatment Facility and monitored concentrations of the oxidant and its by‐products within the facility and distribution system. Project objectives were to (1) document changes in ClO2, chlorite ion (ClO2–), and chlorate ion (ClO3–) concentrations throughout the facility and distribution system following raw water ClO2 pretreatment; (2) evaluate ClO2– removal by the postfilter granular activated carbon (GAC) contactors; and (3) determine distribution system ClO2 concentrations associated with odor complaints. The average raw water ClO2 dose was 0.4 mg/L, and overall ClO2– removals by GAC averaged 63% but declined markedly with increasing throughput volume. Distribution system ClO2– concentrations were generally <0.2 mg/L and decreased with increasing distance from the treatment facility; ClO3– never exceeded 0.1 mg/L. ClO2 was measured at low concentrations (0.017–0.17 mg/L) in the distribution system, and a measured concentration of 0.03 mg/L was found at the home of one customer who complained of odors.