Tolt Water Treatment

The Tolt Water Treatment Facility started operations in 2001 treating water from the Tolt Reservoir located on the South Fork of the Tolt River. It provides about 30 to 40 percent of the water for Seattle and its regional customers.

While we start with a great source of water, the Tolt Treatment Facility makes that water safe to drink, protecting the health of consumers, meeting regulatory requirements, and improving the taste of the water.

Located in the foothills of the Cascade Mountains, the Tolt Watershed is well protected. The land is undeveloped, with no housing, no industry, no agriculture, so the water is not exposed to contaminants from these activities. Untreated water from the South Fork Tolt River is gravity fed from the Tolt Reservoir into a regulating basin and then on to the treatment facility.

Aerial photo of the Tolt Watershed and Water Facility regulating basin

The Water Treatment Processes

Water continuously flows through a series of treatment processes to make it safe to drink.

The Tolt River water supply is fed from the reservoir into a regulating basin. From there, it experiences:

  • Disinfection with ozone
  • Water conditioning and filtration
  • pH and alkalinity adjustment
  • Chlorination and fluoridation

From there, it goes to a clearwell for water storage before distribution to the Seattle water system.

Photo of the Tolt Treatment Facility, showing waterfalls on each side of photo

Photo of the water treatment process


Ozone Generation and Injection

Ozone is a disinfectant and the first process in making the water safe to drink. Ozone is also great at improving the taste of the water.

At the ozone facility, oxygen gas passes through an ozone generator and a portion of that oxygen is converted to ozone. The ozone is transferred to the water by diffusing the gas into the water flow within concrete injection chambers. The water stays in contact with the ozone for 25 minutes or more to complete the disinfection and oxidation processes, disinfecting pathogens and improving the taste and odor of the water.

Photo of liquid oxygen stored in tanks

Photo of ozone passed through diffuser stones

Water Conditioning and Filtration

Water is filtered through a 6-foot deep bed of granular media. To trap contaminants in the filter, trace amounts of conditioners are added first, and the water is slowly mixed upstream of the filters. Operators use instruments to continuously monitor the quality of water from the filters ensuring that stringent performance requirements are met. After a day or two of operation, filters are cleaned through a backwashing process.


pH and Alkalinity Adjustment

pH and alkalinity adjustment are used to make the water less corrosive to building plumbing, especially lead and copper plumbing. This is called corrosion control. Alkalinity is increased with the addition of lime (the mineral, not the fruit). pH is then adjusted by the addition of a small amount of carbon dioxide (but not enough to make it soda water). Water leaves the treatment plant with a pH of 8.2 and an alkalinity of 19 mg/L.


Chlorination and Fluoridation

Chlorine disinfects the water at the treatment plant and then remains in the water over time. The goal is to maintain a small amount of chlorine in the water to protect it all the way to your faucet. Chlorine dissipates over time, so people who receive their water closer to the treatment plants will have a little more chlorine in their water than people who live farther away. Chlorine is also added at some of our downstream water storage facilities to help maintain that residual as water travels farther away from the treatment plant.

You might hear about different forms of chlorine. Seattle's water system uses "free chlorine," not chloramines. Learn more about chlorine in drinking water.

Fluoride is added at the Tolt Treatment Facility, and to all of Seattle's water sources, for dental health. This is based on a vote of the public. Read more about the fluoridation of Seattle's water supply.

Before leaving the treatment facility, chlorinated and fluoridated water passes into a storage reservoir or "clearwell." This reservoir allows time for the chlorine disinfection process and provides a buffer to changes in water flow. People use water at varying rates during the day, with more use in the morning and evenings and less use at night. The storage clearwell, and other water storage facilities farther downstream, help balance these fluctuations in water usage.

The Tolt Facility's Design and Construction

The Tolt Water Treatment Facility was created through a public-private partnership using a contracting approach known as design-build-operate (DBO). Qualified companies each submitted one bid covering the design, construction, and operation of the facility for a minimum of 15 years and extending up to 25 years. The City owns the facility and City engineers oversee the operation. The DBO approach was used to better align the design engineers, the contractor/builders, and the operations experts, saving the City tens of millions of dollars in capital investment.

Photo of the Tolt facility exterior

Public Utilities

Andrew Lee, General Manager and CEO
Address: 700 5th Avenue, Suite 4900, Seattle, WA, 98104
Mailing Address: PO Box 34018, Seattle, WA, 98124-5177
Phone: (206) 684-3000

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Seattle Public Utilities (SPU) is comprised of three major direct-service providing utilities: the Water Utility, the Drainage and Wastewater Utility, and the Solid Waste Utility.