AWWA ACE91058

In Evansville, Indiana, for the past eight years, the use of chlorine dioxide has kept total trihalomethane levels between 0.05 and 0.08 mg/L. However, in light of impending regulations on chlorite and chlorate disinfection by-products, the Evansville utility initiated a research effort to fully evaluate and optimize disinfection by-product reduction using chlorine dioxide. The research focused on: (1) parallel pilot-plant evaluation of disinfection using chlorine dioxide and ozone, and (2) reducing both chlorine dioxide residual and the formation of the inorganic byproduct ions, chlorite and chlorate. Trihalomethanes, haloacetonitriles, haloacetic acids, chloral hydrate, chloropicrin, chloropropanone, total organic halide, total organic carbon, aldehydes and ketones are among a select group of byproduct compounds and surrogates evaluated in this project. Applying ferrous iron to drinking water treated with chlorine dioxide appears to be an effective and economic method of removing residual chlorine dioxide and chlorite ion. Both oxidants were eliminated from the water supply in minutes at pH 6.0-7.0, and results indicate that excess reductant is easily controlled by prefilter chlorination. Coupled with control of chlorate ion formation by optimized chlorine dioxide generation and application, the use of this reducing agent greatly minimizes finished water residual oxidant levels.