Tolt Treatment Facility

The Tolt Water Treatment Facility started operations in 2001 and treats up to 120 million gallons of water per day from the South Fork of the Tolt River. It provides about one-third of the water for Seattle and its regional customers.


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. The facility contract is with American Water CDM, which joined the engineering firm CDM, Dillingham Construction, and the utility service provider American Water.

  • Reflecting a Northwest style, the Tolt facility is constructed of heavy timbers with textured concrete exterior walls. The roof is zinc-coated metal.
  • The water treatment process uses a variety of compounds to achieve optimal water quality and includes filtration, ozonation, chlorination, fluoridation and the addition of minerals for corrosion control.
  • 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.
  • Liquid oxygen stored in tanks is vaporized and fed in as a gas. Inside an ozone generator, oxygen is passed through an electric field and a percentage of the oxygen is converted to ozone.
  • Ozone is a strong oxidant and serves to disinfect the water as well as eliminate any unwanted flavors in the water. Ozone is passed through diffuser stones in water, and the ozone bubbles dissolve in the water.
  • Coagulation chemicals are added to attach to tiny particles in the water. As the water travels along and mixes, the particles join together into larger particles called floc. These floc are easily removed by filters.
  • High-rate filters remove particles from the water. The quantity of anthracite coal placed in the filter media beds can fill 15 million home water purifier cartridges.
  • Water flows through six feet of anthracite coal filter media. The media depth allows more water to pass through each square foot of filter, so the filters have a much smaller footprint than conventional designs.
  • The filters are cleaned regularly by reversing the water flow (backwashing).
  • After filtration, the water is further disinfected with chlorine. Fluoride is added for dental health. In order to make the water less corrosive to plumbing, the pH and alkalinity is adjusted using lime and carbon dioxide.
  • A 7.4 million gallon buried clearwell reservoir (shown under construction) is used to store treated water and help balance the fluctuations in water use.
  • Water is released from the clearwell to the transmission piping system and customers downstream. Archive photo shows transmission main construction circa 1962.
  • Controls systems automate many process operations. Water quality and supply, equipment status, and treatment performance are continuously monitored.
  • Washwater from filter backwashing flows downhill to shallow recovery basins where particulate matter settles to the bottom. Clarified water is returned to the facility inlet, and basin solids are dried in the summer prior to disposal. No liquid waste comes from the treatment facility.