Reduction of odors associated with chlorine dioxide applications to drinking water
Orr, Margaret Prehn
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Chlorine dioxide has been used in drinking water treatment for the reduction of trihalomethanes and tastes-and-odors. Recently, chlorine dioxide was implicated as the source of offensive "cat-urine-like" and "kerosene-like" odors in drinking water. The purpose of this project was to determine the cause of odors observed in customers homes at times when chlorine dioxide was being applied to drinking water. Data were obtained through a survey and field sampling at utilities experiencing odor events. Once the cause was determined, potential water-treatment procedures were evaluated in laboratory-scale studies. Results from a survey sent to water utilities throughout the United States showed that customers who recently had installed new carpeting complained of odors more than those who had not. The use of chlorine dioxide caused responses to intensity of certain tastes-and-odors to increase, two of these odors were cat urine and kerosene. Field sampling at two water companies during odor events resulted in the discovery of low-levels of residual chlorine dioxide (0.2-0.7 mg/L) at the tap's of customers. Both the cat-urine- and kerosene-like odors were detected by the research team. To verify a proposed mechanism of odor generation, chlorine dioxide was volatilized in a room with new carpeting. Both odors were produced in amounts great enough to be easily detected by the entire panel. The removal of residual chlorite from drinking water would block a mechanism by which chlorine dioxide may be reformed from reactions between residual chlorite and free chlorine added for distribution system residual maintenance. Chlorite was removed by powdered activated carbon (PAC) enmeshed in alum floc in laboratory-constituted water and Po River water. A high level of PAC (50 mg/L) was required to effect good removal. A simulated floc-blanket clarifier effectively removed high levels of chlorite (>5 mg/L) from drinking water for long periods (12 hours) when PAC in high concentration was enmeshed in the floc. Chlorine dioxide reformed in the distribution system of two water companies. When customers opened water faucets, chlorine dioxide volatilized into the room and reacted with organic compounds present in the household air (e.g. from new carpeting) producing the offensive cat-urine-like and kerosene-like odors. The reformation of chlorine dioxide could be effectively eliminated if residual chlorite could be removed by contact with high levels of PAC enmeshed in an alum floc such as one could produce in a floc-blanket clarifier.
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