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CIO2 to the Rescue

In the upper atmosphere, ozone blocks ultraviolet radiation while in the lower layers, it contributes to smog and greenhouse gases. Closer to earth, this versatile gas is now being widely used to purify drinking water as a substitute for chlorine and chloramines (see "Chlorinated Clues", p.3). Ozone is the primary disinfectant at the Contra Costa Water District's Bollman Water Treatment Plant near Oakley. But there is a catch: when applied to Delta water, which contains bromine, ozone produces bromate, an unwanted byproduct with a U.S. EPA allowable contaminant level of 10 parts per billion (ppb).

While investigating treatment alternatives with funding from the American Water Works Association Research Foundation and the California Energy Commission, the District hit on a new approach: adding chlorine dioxide before ozone. "We found that when we added chlorine dioxide ahead of ozone, we needed much less ozone, and bromate production dropped a lot," the District's Tom Linville explains. Bromate levels dropped from 5 ppb to below the detection threshold of 2 ppb, and the reduced use of ozone cut energy requirements by 30%. Chlorine dioxide forms fewer regulated byproducts than pure chlorine and is shorter-lived than chlorine or chloramine.

Linville recalls that the key insight came from Dale Newkirk, then the District's manager of water quality. "He looked at the numbers and said 'Aha, there's something going on here.' He was able to spend more time and isolate it out."

Working with the funding agencies and Black & Veatch, an engineering firm also involved in the research, the District obtained a patent for the chlorine dioxide treatment that keeps it in the public domain. "It made sense for us because of the way it was funded," Linville says. "A lot of for-profit individuals would patent a process-using ultraviolet light as a disinfectant, for example-and collect royalties for its use." This way, other water treatment operations can use the process free of charge. Final publication of the patent is expected early this year.

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