In the News

Drinking Water From Lake Tahoe is Treated
date posted: December 22 2005

Lake Tahoe Report #97
Dec. 20, 2004
By John Cobourn and Heather Segale


The Incline Village Water Disinfection Plant on the north shore of Lake Tahoe is one of a handful of surface-water treatment plants across the nation that meets stringent national drinking water quality standards solely through watershed management, water quality monitoring, and disinfection with ozone. The Environmental Protection Agency and the Nevada Bureau of Health Protection Services have granted the Incline Village General Improvement District (IVGID) an exemption from the Safe Drinking Water Act’s filtration requirement, largely because Lake Tahoe is such a high quality water source.

The lake has been the focus of extraordinary protective measures for decades. For example, all wastewater is collected, treated, and discharged outside of the basin to prevent microorganism and nutrient contamination in the lake. Other water quality improvement projects also have been implemented to protect lake clarity. These programs have benefited our drinking water as well.

To continue providing clean and high-quality drinking water without needing filtration, active source-water protection programs are necessary. Source-water protection programs are efforts to involve citizens, businesses, and jurisdictions in preventing pollution in local watersheds. In the case of Lake Tahoe, we protect the source of our drinking water by teaching people to prevent spills, dumping or misuse of hazardous chemicals or other pollutants anywhere in the Tahoe Basin.

Three other Nevada communities at Lake Tahoe also use ozonization as their primary disinfection method for drinking water. The Kingsbury General Improvement District, Zephyr Water Utility District and Glenbrook Water Company also are exempt from the filtration plant requirement. They have joined with IVGID to form the Nevada Tahoe Water Supplier Association. Through this association, these entities cooperate to implement source-water protection programs at the lake.

The IVGID Plant draws its source water directly from Lake Tahoe. One reason that IVGID chose to avoid filtration was the significant cost savings. Dan St. John, director of engineering for IVGID, says, “Since IVGID didn’t have to build a costly filtration plant, savings were substantial for construction, operations and maintenance. The disinfection plant costs less than half as much to build, maintain and operate as a filtration plant. Based on an expected 30-year plant design life, our total extra costs for filtration would have topped $10 million. Those additional costs would have been passed on to our water customers as higher rates.”

The new disinfection plant was designed by Kennedy/Jenks Consultants. Ozone was chosen for the plant because it is a powerful disinfectant that does not alter the mineral quality or taste of the water. Ozone is effective in deactivating Giardia and Cryptosporidium cysts.

Ozone is generated on site from liquid oxygen stored in two 3,000-gallon tanks. Three ozone “eductors” add 10 pounds to 20 pounds per day of the ozone gas mixture to 3.0 million gallons per day of lake water. Lake water mixes with ozone gas in the eductor vault, then flows through the 250-foot long, 5-foot diameter “contactor” pipeline, where ozone has time to destroy any pathogens that might be present.

Emergency power is provided by a backup engine-generator, capable of powering the entire plant, albeit not at full capacity. Future treatments will be designed to meet enhanced surface water treatment rules addressing Cryptosporidium deactivation requirements. After disinfection with ozone, the water is “quenched” to remove any residual ozone that could be corrosive in the water system and building plumbing. Chlorine “bleach” is added to assure a disinfection residual throughout the distribution system.

“IVGID is extremely pleased with the performance of its water disinfection plant,” says St. John. “The ozone treatment easily provides the 99.9 percent disinfection of Giardia cysts and 99.99 percent disinfection of enteric viruses required by the Surface Water Treatment Rule. This compact, efficient plant complies with water quality regulations and delivers a high quality, good-tasting drinking water for much less cost than a conventional treatment plant.”

The North Tahoe Public Utility District also has an innovative treatment system at its main plant on the north shore. This plant treats lake water first with chlorination, then with a 25-micron screen to remove debris, and finally with ultraviolet radiation to disinfect the water for drinking purposes. This treated lake water also can be blended with well water that is treated with chlorine.