AWWA JAW57844

Chlorine dioxide (ClO2) has become increasingly popular for drinking water treatment, partly because it oxidizes rather than chlorinates organic matter and therefore does not form trihalomethanes or other chlorination byproducts. Its reaction with organic matter, however, produces chlorite ion (ClO2-) and chlorate ion (ClO3-). This study, conducted at the Spring Hollow Water Treatment Facility (SHWTF) in Virginia, was the first full-scale evaluation of a chlorine gas-solid sodium chlorite ClO2 generation system. In addition to evaluating the system, the utility wanted to determine the efficacy of its deep-bed granular activated carbon (GAC) filters following conventional filtration for removing ClO2-. Finally, SHWTF wanted to determine the relationship between ClO2- concentrations leaving the treatment plant and the levels of ClO2 that reformed following final chlorination and led to complaints from customers of odors resembling kerosene and cat urine. Utilities facing similar customer complaints may try reducing the ClO2 dosage, but complaints will not be completely eliminated until ClO2- concentrations in water leaving the treatment plant are <0.4 mg/L. GAC contactors afford limited protection against high ClO2- concentrations entering the distribution system if the ClO2 dosage remains low, but the effectiveness of GAC declines rapidly. To keep ClO2 dosages at an effective treatment level without triggering odor complaints, water providers can either remove ClO2- at the treatment plant or substitute chloramines for free chlorine in the distribution system. Includes 36 references, figures.