Cedar Water Treatment

The Cedar Water Treatment Facility started operation in 2004 and treats up to 180 million gallons of water per day from the Cedar River and Lake Youngs. It provides about two-thirds of the water for Seattle and its regional customers. The facility relies on both time-tested and innovative techniques to supply high-quality drinking water. The time-tested techniques include watershed protection and chlorination, and the innovative techniques include ozonation and ultraviolet light disinfection.

Watershed Protection and Unfiltered Supply

The production of high-quality water starts at the source: the Cedar River Watershed. The watershed encompasses 90,500 acres of forest land. The land is undeveloped, with no housing, no industry, no agriculture, so the water is not exposed to contaminants from these activities. With such protection, the Cedar River Watershed is special. It is one of only four major drinking-water systems in the country that do not require filtration as part of treating the water.

Photo of Cedar River Watershed
The highly protected watershed reduces the level of water treatment required.

Lake Youngs and Raw Water Intake System

Lake Youngs serves as a reservoir for Cedar River water prior to its treatment. Like the watershed, access to Lake Youngs is restricted, and only water from the Cedar River enters Lake Youngs, minimizing the opportunity for contaminants to enter the water.

Water quality within the lake can vary due to seasonal changes, such as algae growth and temperature stratification. SPU scientists routinely monitor the lake water quality to understand ecological changes and implications for the treatment systems.

To ensure that clear water is drawn from the lake, the raw water intake and pump station is located 400 feet offshore in water that is approximately 50 feet deep. Submersible pumps deliver the water to the first treatment step.

Photo of intake pipes
Intake pipes draw water from Lake Youngs.

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. Naturally occurring algae can add unpleasant tastes and odors to the source water. Ozone eliminates those tastes and odors.

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 about 10 to 20 minutes to complete the disinfection and oxidation processes, disinfecting pathogens and improving the taste and odor of the water.

Rather than build a new ozone contact chamber, the facility uses two 78-inch-diameter pipelines for ozone contact. Ozonation occurs as the water travels from the ozone injection facility to the UV disinfection facility. The ozone decays rapidly along the way. Any ozone remaining is neutralized before reaching the UV treatment step.

Photo of ozone generator
Ozone generator disinfects water and improves taste.

Ultraviolet Light Disinfection

The Cedar Water Treatment Facility was among the first and is still one of the largest facilities in the United States to use ultraviolet (UV) technology to disinfect drinking water. The UV disinfection facility exposes water to high intensity light to inactivate the pathogens. UV light is the primary disinfectant barrier for Cryptosporidium, a pathogen that is resistant to chlorine. UV light is also highly effective against Giardia. UV light has other benefits as well: it limits the amount of disinfectant chemicals and it is not known to produce any byproducts.

Photo of UV reactors
UV reactors kill Cryptosporidium, Giardia, and other pathogens in water.

pH Adjustment

pH adjustment is used to make the water less corrosive to building plumbing, especially lead and copper plumbing. This is called corrosion control. To achieve this, lime addition raises the pH at the Cedar Treatment Facility to a target of about 8.2. Learn more about lead and drinking water.



Fluoride is added to the Cedar water supply, and to all of Seattle's water sources, for dental health. This is based on a vote of the public. For the Cedar supply, fluoride is added in the pipelines upstream of Lake Youngs. Read more about the fluoridation of Seattle's water supply.


Chlorination and Clearwell Water Storage

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.

Before leaving the treatment facility, chlorinated water passes into one of two large storage reservoirs or "clearwells." These reservoirs allow time for the chlorine disinfection process and they provide 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 clearwells, and other water storage facilities farther downstream, help balance these fluctuations in water usage.

Photo of Clearwell reservoirs
Clearwell reservoirs hold treated water.

The Cedar Facility's Design and Construction

The Cedar 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. In addition, the operations building at the treatment facility achieved a LEED gold rating. The design was low-impact and sustainable, with approximately 85 percent of the waste generated during construction recycled.

Photo of Cedar operations building
The Cedar operations building achieved a LEED Gold rating.

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.