Water Treatment Process

Durham Water Resource Recovery Facility

Durham Water Resource Recovery Facility, located in Tigard near Cook Park and Tigard High School, is a nationally acclaimed, state-of-the-art facility, serving Washington County residents in the cities of Beaverton, Durham, King City, Sherwood, Tigard, Tualatin, and small portions of southwest Portland and Lake Oswego.

Today, the facility cleans an average of 26 million gallons of used water each day to among the highest safety and quality standards in the nation. Through advanced technology and processes, the Durham facility treats and removes valuable resources from water collected from homes and businesses. This water flows through a strategic process of liquids and solids recovery. The water is then returned to Washington County’s only river — the Tualatin. The returned water is so clean, it improves the river’s water quality.

Cleaned water is also used for local irrigation, and natural byproducts of the treatment process are converted to electricity and heat and used as soil amendments. Captured methane gas, a byproduct of anaerobic digestion, supplies electrical power for the facility. In 2009, Durham installed the first commercial nutrient recovery facility in the nation in partnership with Ostara Nutrient Recovery Technologies in Canada. The facility captures 80% of the phosphorus from the wastewater recycle stream and converts it into a premium, slow-release fertilizer used in agriculture and nurseries.

The Durham facility began operations in 1976 to reverse decades of severe water pollution in the Tualatin River and its tributaries, and to meet the demands of a growing population. This facility centralized a scattered system of inefficient wastewater treatment plants, creating one of the most efficient and advanced facilities in the world.

Durham water resource recovery facility Aerial view on a sunny day.

Durham Water Resource Recovery Facility Process

Used water flows through a series of processes (PDF): preliminary, primary, secondary, tertiary, disinfection, and effluent discharge.

Cogeneration: Waste to Watts

In 2016, Clean Water Services, Energy Trust of Oregon, and the Oregon Department of Energy dedicated a new cogeneration system that converts wastewater and food grease into clean, renewable energy. With this innovative system, the Durham Treatment Facility is the third water resource recovery plant in Oregon to co-digest fats, oils, and grease.

The cogeneration system triples Durham’s renewable energy generation, producing 60 percent of the electricity needed to run the water resource recovery facility when coupled with its existing 403-kilowatt solar electric system. Renewable electricity and heat produced are used onsite, reducing CWS’s energy costs by nearly $800,000 annually, ensuring value for ratepayers. Generating clean, renewable energy from biogas reduces greenhouse gas emissions and helps Oregon meet its carbon reduction goals.

Durham had operated a 500-kilowatt cogeneration system since 1993 using biogas from treatment of the communities’ wastewater to offset its own energy usage. By replacing this smaller engine with two new engines, Durham now has a 1.7-megawatt cogeneration system fueled by biogas produced from the anaerobic digestion of municipal wastewater solids as well as fats, oils, and grease (FOG) collected from Washington County restaurants and others. FOG, also known as “brown grease,” is pumped out of restaurant grease traps and interceptors at regular intervals.

This is just one project where CWS and Energy Trust have teamed up to invest in projects that save and generate energy. Since 2004, CWS has worked with Energy Trust on more than 100 energy-saving improvements throughout its facilities — everything from large-scale capital improvements to new energy-efficient lighting, pumps, and drives and operations and maintenance improvements. This has resulted in more than 9 million kilowatt hours of electricity saved per year for Clean Water Services, and lower utility bills and operating costs translates to saving for ratepayers.

Size and Scope

  • 1.7-megawatt cogeneration system: two Jenbacher 848-kilowatt cogeneration reciprocating engines fueled by biogas, not fossil fuel
  • Combined with the 403-kilowatt solar electric system, generates more than 12,800 megawatt-hours annually — enough electricity to power 1,100 homes for a year and to help avoid producing 6,000 tons of carbon dioxide
  • Average gallons of fats, oils and grease (FOG) co-digested per week: more than 150,000
  • Cost: $16.8 million
  • Energy Trust of Oregon incentive: $3 million
  • Oregon Department of Energy Tax Credit for combined heat and power: $2.8 million


  • Cuts Clean Water Services operating costs saving money for ratepayers.
  • Keeps fats, oils, and greases out of pipes and treatment plant, saving operating costs, and preventing sewer backups
  • Reduces the level of greenhouse gases released into the environment
  • Recovers waste that would be otherwise be disposed of or landfilled
  • Advances sustainability goals for Oregon

Durham Facility Facts

  • Provides a higher level of treatment than 98% of facilities in the nation
  • Meets over 1,000 permit conditions, including monthly, weekly, and daily limits established to protect the Tualatin River
  • Operates 24-hours a day, 365 days a year
  • Serves a growing population of approximately 250,000
  • Cleans an average of 26 million gallons of wastewater per day
  • Recycles more than 50 million gallons of water a year for local irrigation
  • Recycles more than 12 dry tons of biosolids daily for use as a soil amendment
  • Produces approximately 400 tons of Crystal Green®, a commercial, high-value fertilizer
  • Meets 60% of facility electrical needs through the self-generation of energy
  • U.S. EPA National Clean Water Act Recognition Award (2007) for the best operated and maintained large, advanced treatment facility in the nation
  • National Association of Clean Water Agencies (NACWA) Platinum Award for 100% permit compliance achieved over multiple years