The Tolt Treatment Facility processes water from the South Fork Tolt River, one of the two large, unfiltered surface water sources that supply 1.3 million people in and around Seattle with drinking water. The other source is the Cedar River. The Tolt River source was developed in the early 1960's and is comprised of an isolated reservoir in the Cascade Mountains surrounded by an uninhabited, undeveloped 13,000 acre (5,300 hectare) watershed. Opened at the end of 2000, the facility allows long range conformity with anticipated regulations, increases system yield, and improves the water supply system by permitting continuous operation of Seattle's Tolt source through periods of high turbidity.

Historically, the Tolt has provided very high quality water that meets all current drinking water regulations and requires minimal treatment -- disinfection by free chlorine, pH and alkalinity adjustment with lime and soda ash, and the addition of fluoride for dental benefits.

However, turbidity, or sediment in the water, can be a problem during certain conditions such as low reservoir levels, high winds, and heavy rain events. The normal turbity range for the Tolt is between about 0.5 neptha turbidity units (NTU) to 4.0 NTU. Spot tests during these conditions have been as high as 15 NTU, well above the 5 NTU limit for well protected watersheds on unfiltered water supplies such as the Tolt.

The Tolt Treatment Facility opened in 2000 to improve and increase the water supply. The facility was designed, built and is operated under a unique contract for $101million (1998 dollars).

The facility is equipped for both filtration and ozonation. Filtration increases the reliability and flexibility of the water supply system by allowing the Tolt supply to be operated over a much wider range in weather and reservoir levels. These two processes improve water quality to meet Seattle's public health objectives as well as satisfying future stricter surface water treatment and disinfection by-product regulations. The facility also has an increased yield of approximately 9 million gallons per day (MGD), or 34,000 cubic meters per day (CMD), for a total supply of 120 MGD from the South Fork Tolt River.