First Arizona Water Treatment Plant Using Ozone Now On-Line

Arizona got its only fully on-line ozone water treatment plant when Peoria's newly constructed facility began operating on June 10. The event raises questions about the use of ozone to treat drinking water in the state: Why at this point in time is ozone being used in Arizona? Will other Arizona municipalities follow Peoria's example? And, in the larger context, does Peoria's decision reflect a general acceptance of ozone for meeting new water quality standards?

The use of ozone is relatively new to Arizona, compared to other parts of the country. In January 2000, about 330 U.S. municipal ozone installations were known to be operating. At the same time, however, the United States lags significantly behind European countries in the adoption of ozone. While European countries mainly rely on ozone for treating drinking water, the water treatment method of general choice in the United States is chlorine.

Tucson deserves qualified recognition as the first municipality in the state to adopt ozone for water treatment. With Central Arizona Project waters on the way, the city spent more than $85 million to construct the Hayden-Udall Water Treatment Plant. The city decided that using ozone as a primary disinfectant and chloramine as a residual disinfectant would ensure the highest water quality. The plant, with a 150 million gallons per day (mgd) capacity, came online in 1992.

Unfortunately circumstances beyond the design of the plant complicated Tucson's plans. CAP water with its high mineral content was being delivered through old steel pipes previously used only for groundwater. The situation was further aggravated when the pH level of the released water was not properly adjusted. Experts say releasing water with a pH level below 7 was asking for trouble, and trouble occurred. Discolored water and damaged pipes resulted, and a political debacle arose. Politics drove water policy, and the treatment plant was shut down in 1994.

The circumstances that led Peoria to choose ozone were similar to those that confronted Tucson and are familiar to other Arizona communities as well. Peoria was reducing its groundwater consumption by turning to surface water. Purchased from the Salt River Project, the new water supply raised various water quality concerns. Compared to the relatively pristine groundwater, surface water, derived from free-flowing rivers and likely stored in reservoirs before delivery via an open canal, differs in its overall chemical, physical and aesthetic makeup. Communities changing from groundwater to surface water confront water treatment choices.

Once, chlorine would have been the obvious choice. Still the most frequently used method to treat water in the United States, chlorine is now viewed as possibly posing health hazards. When used to treat water, chlorine can react with organic substances to produce chlorinated compounds, shown to cause cancer in laboratory animals. These include trihalomethanes (THMs), haloacetic acids (HAAs) and chlorite. EPA has established limits for these byproducts.

In adopting ozone, Peoria was mainly concerned that its customers not experience any differences in the aesthetic quality between surface water and groundwater. Along with their concern about taste and odor, officials looked to ozone to ensure that regulated byproducts were not an issue when surface water was treated. The increased expense in using ozone was offset by the city's use of surface water that costs less than pumping groundwater.

Ozone is lauded for its effectiveness in destroying hazardous pathogens, including Cryptosporidium, but it lacks a long-lasting residual to control biological contaminants within the distribution system. In compliance with the law requiring post residual disinfection with a chlorine compound, Peoria's final step in its water treatment process after ozonation, coagulation, sedimentation and filtration is chlorination to ensure a residual disinfectant.

Gilbert is another Arizona city now using ozone. When increasing its treatment plant capacity from 15 to 30 mgd, Gilbert switched to ozone to treat its drinking water. The original plant came online in 1997, at the time the city began using SRP water. Prior to that time Gilbert relied mostly on groundwater. Concern about the need to meet future water quality standards convinced the city to adopt ozone treatment. The city had previously been using chlorine to treat its water.

Along with ozonation, the Gilbert treatment process includes sedimentation and filtration, with chlorine added as a residual. The ozone component is currently not fully operating, still in its testing and debugging phase.

The City of Phoenix is looking at ozone along with other treatment strategies as part of its water quality master plan. An ongoing project, the plan involves examining various methods for meeting future water quality standards. At this point, Phoenix does not use ozone to treat drinking water.

Ozone obviously holds promise for communities who are using or planning to use surface water. Las Vegas provides a recent example with its construction of two ozone treatment plants to treat Colorado River water. Scheduled for completion in 2003, the plants, one a 600 mgd plant and the other a 150 mgd plant, will be among the largest such plants in the world. Las Vegas is investing heavily in ozone for disinfection since crypto outbreaks have occurred in the area.

The use of ozone, however, is not without its disadvantages. It was once the up-and-coming water treatment of choice, but in the mid-1980s researchers found that hazardous by-products were also associated with its use. When water containing bromide was treated with ozone, bromate was formed. Research linked bromate to kidney cancer in laboratory animals.

This development raised concerns, and in 1999 the EPA included bromate in its list of disinfection byproducts to be regulated. Large utilities had to comply with a 10 ppb limit by January 2002, and small systems have until January 2004 to comply with the new law.

It was the bromate issue that contributed to the City of Scottsdale's decision to go with membrane filtration rather than ozone. Ozone was considered for the city's first water treatment plant on the SRP system. The canal, however, will be delivering an increased ratio of CAP water to SRP water. CAP water has a higher concentration of bromide than SRP water, and the city determined that the ozone dosage needed to treat the water would result in a bromate problem. The 30 mgd plant, which will be the largest membrane filtration plant in the state, is scheduled to go online 2005.

Choosing an appropriate water treatment method is a balancing act, as efforts are made to take advantage of the benefits of a particular method without experiencing its limitation. Various strategies come into play, with the chosen method depending upon treatment goals. In fact, the Safe Drinking Water Act promotes this approach to ensure multiple barriers to protect public health.

For example, a utility may have a water source from a protected watershed. Treating for crypto therefore may not be a priority. Using chlorine, however, might result in an excess number of byproducts. An effective treatment method then might be to use ozone to preoxidize, thereby reducing organics, which are the precursors of THMs. The next steps would be coagulation and filtration, with chloramine applied for final disinfection and distribution. The various treatment stages work together synergistically.

The wave of the future in water treatment will likely be a combination of the various treatment strategies to produce the highest water quality.

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