Major California Water Supplier Switches From Chlorine To Ozone For Treatment

The Metropolitan Water District of Southern California has switched its primary disinfectant at its largest treatment plant from chlorine to ozone. According to Metropolitan, the switch, which began July 1, will enhance the quality, taste and aroma of the tap water received by millions of Southland residents. The largest of Metropolitan's five water treatment plants throughout the region, treating up to 750 million gallons per day, it is also the largest water treatment plant in the United States to employ ozone disinfection.

The Joseph P. Jensen Water Treatment Plant in Granada Hills, which serves water agencies in Ventura, Los Angeles and Orange counties, has switched its primary disinfectant from chlorine to ozone.

"Ozone is a safe, colorless gas that is an excellent disinfectant -- and, in fact, has been used as a water disinfectant in Europe since the 1800s," said Dr. Mic Stewart, Metropolitan's water quality manager.

Ozone destroys a wider range of organisms in drinking water, removes most objectionable tastes and odors, and produces fewer potentially harmful byproducts than the traditional chlorine treatment, Stewart said.

"Installing the ozone treatment process at the Jensen plant is a milestone in Metropolitan's $856.4 million program to install ozone disinfection at all five of our plants," said Metropolitan board Chairman Wes Bannister. "Now that Jensen and our Mills plant at Riverside are retrofitted, we move on to the Skinner treatment plant in southern Riverside County."

The Jensen plant treats water delivered by the state Department of Water Resources from northern California, via the State Water Project. That water contains dissolved organic materials, which cause potentially carcinogenic byproducts when the water is disinfected with chlorine, Metropolitan officials said.

Water from Jensen and Metropolitan's other treatment plants is piped to Metropolitan's member public water agencies, which, in turn, send it to retail water agencies or directly to homes and businesses. In most cases, local water agencies mix Metropolitan's water with supplies from local wells before sending it to consumers.

Retrofit of the Jensen plant required an extensive array of new buildings and equipment. Ozone is formed when oxygen gas is passed through an electrical field in a specially designed generator; a small portion of the oxygen -- less than 10 percent -- is converted into ozone. The ozone gas is immediately bubbled through the untreated water as it flows through a serpentine series of chambers.

Finally, the treated water is given a dose of chloramines -- a combination of chlorine and ammonia -- to maintain quality for the sometimes lengthy period between leaving the plant and flowing to a homeowner's faucet.

More information about the Metropolitan Water District of Southern California can be found at

This article originally appeared in the 07/01/2005 issue of Environmental Protection.

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