WATER QUALITY IMPACTS OF PURE CHLORINE DIOXIDE PRETREATMENT AT THE ROANOKE COUNTY (VIRGINIA) WATER TREATMENT PLANT

Chlorine dioxide (ClO2) was included in the Spring Hollow Water Treatment Plant (Roanoke County, Virginia) to oxidize manganese and iron, prevent tastes and odors, and avoid the formation of excessive halogenated disinfection by-products. A state-of-the-art, gas:solid ClO2 generation system manufactured by CDG Technology, Inc. was installed at the plant and is the first full-scale use of this technology in the world. The ClO2 generator produces a feed stream free of chlorine, chlorite ion (ClO2-), and chlorate ion (ClO3-), resulting in lower by-product concentrations in the treatment system The objectives of this project were to study ClO2 persistence and by-product concentrations throughout the treatment plant and distribution system and to evaluate granular activated carbon (GAC) columns for removing ClO2- from the finished water. The ClO2 dosages applied during this study were relatively low (<0.75 mg/L), and, as a result, ClO2- concentrations never approached the maximum contaminant level (MCL) (1.0 mg/L). Likewise, the plant effluent ClO2 concentration never approached the maximum residual disinfectant level (MRDL) (0.80 mg/L), but concentrations as high as 0.15 mg/L reformed in the distribution system by ClO2- reaction with chlorine