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King County Regional Wastewater Services Plan

King County Regional Wastewater Services Plan

Since 1958, King County has protected water quality in the Puget Sound region by providing wastewater treatment services to King, Pierce, and Snohomish counties.

September 2021

Many of the major projects outlined in the RWSP have been completed, are underway, or are planned for the future. Information related to RWSP major components, elements, costs, and programs is provided on WTD’s website, through regular briefings to the King County Council’s Regional Water Quality Committee, and through the King County budget process. In addition, in 2018, WTD is initiating a wastewater systemwide planning effort that is expected to result in a new plan that sets direction for WTD beyond 2030. Changes in WTD projects and programs before 2030 will also be considered in the systemwide planning process. For these reasons, the RWSP 2017 Annual Report  is the final RWSP annual report.

In the 1990s, flow estimates based on projected population growth estimates in King County's wastewater service area indicated that King County's regional wastewater treatment system would run out of capacity by 2010. To ensure the continuation of high quality wastewater treatment services in the future, King County carried out an intensive planning effort, involving numerous elected officials, representatives from local sewer agencies, organizations and individuals from around the region. The Regional Wastewater Services Plan (RWSP) resulted from these efforts, which was adopted by the King County Council in November 1999 via Ordinance 13680.

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RWSP major components

The RWSP called for building a new treatment plant by 2010, known as "Brightwater," to accommodate growth in the northern portion of our wastewater service area. The Brightwater System includes a 36 million gallons per day (mgd) treatment plant, conveyance (pipes and pumps that take the wastewater to and from the plant), and a marine outfall that discharges effluent (treated wastewater) from the Brightwater Treatment Plant into Puget Sound. The Brightwater conveyance system consists of approximately 14 miles of pipeline built in underground tunnels.

King County's regional wastewater conveyance system consists of more than 390 miles of pipes and 47 pump stations that move wastewater from local communities to the county's three regional wastewater treatment plants. The RWSP calls for improvements to our conveyance system to meet the twenty-year design storm and accommodate increased flows where needed.

The RWSP calls for improvement to reduce existing and future levels of infiltration and inflow (I/I) into local collection systems. I/I is clean stormwater and groundwater that enter the sewer system through cracked pipes, leaky manholes, or improperly connected storm drains, down spouts, and sump pumps. Most inflow comes from stormwater and most infiltration comes from groundwater. I/I affects the size of King County conveyance and treatment systems and, ultimately, the monthly rates that businesses and residents pay to operate and maintain them.

The RWSP calls for the control of all county combined sewer overflows (CSOs) by 2030 . Combined sewer overflows (CSOs) are events where untreated wastewater and stormwater from combined sewers discharge directly from outfall pipes into water bodies during heavy rainstorms when sewers are full.

Combined sewers, which carry both wastewater and clean stormwater, exist in many parts of older cities across the nation, including Seattle. To protect treatment plants and avoid sewer backups into homes, businesses, and streets, combined sewers in Seattle sometimes overflow at specific locations (CSOs) into Puget Sound, the Duwamish Waterway, Elliott Bay, Lake Union, the Lake Washington Ship Canal, and Lake Washington. Although the wastewater in CSOs is greatly diluted by stormwater, CSOs may be harmful to public health and aquatic life because they can carry chemicals and disease-causing pathogens.

Efforts to control CSOs began in the late 1970s. Since 1988, when monitoring and measuring of CSO flows began, these control efforts have reduced CSO volumes by nearly 60 percent. Visit the CSO Control Program web site for more information.

The RWSP CSO control policies also call for development of a long-range sediment management strategy to prioritize cleanup of contaminated sediments at specific CSO locations.

The RWSP includes policy guidance to achieve King County's odor control goal to prevent and control nuisance odor occurrences at all treatment plants and associated conveyance facilities and to carry out an odor prevention program that goes beyond traditional odor control.

The RWSP policies guide the county to continue to produce and market Class B biosolids and to evaluate alternative technologies to produce the highest quality marketable biosolids. Loop biosolids are the nutrient-rich organic material produced by treating wastewater solids. After processing and treatment, they can be beneficially recycled as a fertilizer and soil amendment.

The RWSP calls for the county to pursue and explore opportunities for expanded water reuse at the county's existing treatment facilities. The policies also call for the county to explore water reuse opportunities at all new treatment facilities. 

Library and resources

The RWSP annual report, due in September, describes the progress made by King County in implementing the major elements of the RWSP for the previous year. The report is presented in response to the RWSP reporting policies (for the annual report) outlined in Ordinance 15384 and King County Code 28.86.165.

2017 annual report, September 2018; Final RWSP report

2016 annual report, September 2017

2015 annual report, September 2016

2014 annual report, September 2015

2013 annual report – refer to the 2013 RWSP comprehensive review report

2012 annual report, September 2013

2011 annual report, September 2012

2010 annual report, September 2011

2009 annual report, September 2010

2008 annual report, September 2009

2007 annual report, September 2008

2006 annual report – refer to the 2006 comprehensive review and annual report

2005 annual report, September 2006 

Note: prior to 2005, progress reports were issued every six months describing RWSP implementation activities for the current year. The RWSP Semi-annual Review, due in June, provided an overview of RWSP implementation during the first half of the year. The RWSP annual report, due in December, described activities for the entire year.






The Regional Wastewater Services Plan (RWSP) outlines important projects, programs, and policies for King County to implement through 2030 to continue to protect public health and water quality and ensure sufficient wastewater capacity to meet future growth. In adopting the RWSP in 1999, the Metropolitan King County Council recognized the importance of reviewing implementation of the RWSP and adopted specific RWSP reporting policies that call for regular reviews and reports.

2013 Comprehensive Review, June 2014

The 2013 Comprehensive Review presents a comprehensive review of RWSP policy implementation from 2007 through 2013. This is the third comprehensive review report since adoption of the RWSP.

  • Transmittal letter
  • Report and appendices (3.5MB)
  • The report and transmittal letter are also available on Legisearch
  • RWSP 2013 cost estimates 
  • Updated planning assumptions for wastewater flow forecasting, July 2014
    This report documents the methodology and results of the process to review and update the planning assumptions that the King County Wastewater Treatment Division (WTD) uses to forecast future wastewater flows. The planning assumptions were developed and applied during preparation of the Regional Wastewater Services Plan (RWSP). They are reviewed and updated as part of RWSP comprehensive reviews (“updates”), Conveyance System Improvement (CSI) Program plan updates, and development of the Regional Infiltration and Inflow Control Program.
  • Treatment plant flow and wasteload projections, 2010 - 2060, November 2014
    This report documents the methodology and results of the 2014 flow and wasteload projections for King County’s three regional treatment plants: West Point, South, and Brightwater plants. It then compares the projected flows and loads with design capacities at the plants to determine if additional capacity will be needed in the next 50 years.

2006 Comprehensive Review and Annual Report, September 2007

The 2006 Comprehensive Review and Annual Report presents a comprehensive review of RWSP policy implementation from 2004 through 2006, and includes all elements of the RWSP annual report for the year 2006. This is the second comprehensive review report since adoption of the RWSP.

2004 update, April 2004

The first comprehensive review, RWSP 2004 Update, focused on RWSP policy implementation from 1999 through 2003.

The purpose of the water quality report is to ensure that the Regional Wastewater Services Plan reflects current environmental conditions. It is also to track the progress of the Executive's comprehensive water quality monitoring program of streams and water bodies that are or could be impacted by wastewater influent, effluent, sanitary system overflows, or CSOs.

Note: prior to 2006, the water quality report was submitted in March of each year. This report is now included in the RWSP Progress (annual) report per RWSP reporting policies.

  • 2005, Appendix D to 2005 annual report, issued Sept. 2006 (971KB)
  • 2004, issued March 2005 (1.5MB)
  • 2003, Appendix D to RWSP 2004 update, issued March 2004 (1.5MB)
  • 2002, issued April 2003 (1.8MB)
  • 2001, issued March 2002 (1.7MB)
  • 2000, issued March 2001 (354KB)

The Operational Master Plan, or OMP, explains how King County will implement the Regional Wastewater Services Plan as required by Ordinance 13680, which adopts changes to King County's Comprehensive Water Pollution Abatement Plan. Whereas the ordinance focuses on the policies that drive the RWSP, the OMP focuses on defining the performances measures, needed resources, and projected workload necessary to implement the RWSP.

Ordinance 13680 was codified in the King County Code as Chapter 28.86. Amendments to Ordinance 13680 and the King County Code have been made since the adoption of the RWSP and are included in the King County Code, Chapter 28.86.010 through Chapter 28.86.180. Amendments have included updates to the RWSP financial policies, conveyance policies, new odor control policies for the county's existing treatment plants and conveyance facilities, and a new section on reporting policies.

  • King County code (refer to Chapter 28.86.010 through 28.86.180)
  • Ordinance 17587 (enacted 05/31/2013), amending combined sewer overflow control policies
  • Ordinance 17480 (enacted 12/13/2012), amending planning and review policies relating to the Regional Wastewater Services Plan
  • Ordinance 17492 (enacted 12/21/2012), revising a financial policy of the Regional Wastewater Services Plan addressing debt financing and borrowing
  • Ordinance 16033 (enacted 3/20/2008), approving amendments to RWSP conveyance policies
  • Ordinance 15602 (enacted 10/5/2006), adopting RWSP technical and policy amendments
  • Ordinance 15384 (enacted 3/22/2006), adopting RWSP reporting policies
  • Ordinance 14712 (enacted 7/25/2003), adopting policies relating to odor control at existing treatment plants and conveyance facilities
  • Ordinance 14219 (enacted 10/11/2001), updating RWSP financial policies

RWSP Executive's Preferred Plan

The RWSP Executive's Preferred Plan was transmitted to the King County Council in April 1998. The plan includes details on the proposed major projects and programs, and information on costs, rates, and the schedule for implementing the RWSP.

Letter from King County Executive Ron Sims, April 1998

Dear friends,

Forty years ago, concerned residents rallied and formed a grassroots movement to clean up Lake Washington. The water was so polluted with untreated wastewater that their families could not swim or fish in it. These residents accomplished their goal, but they did so much more. Their efforts resulted in the first-rate wastewater collection and treatment system that we enjoy today.

Now we have the opportunity to do the same for ourselves and for future generations. The big difference between 40 years ago and today, however, is that we can rally before our water resources degrade to the point where we can no longer enjoy them. But we must act quickly: the recent proposal to list Puget Sound salmon under the federal Endangered Species Act (ESA) requires us to act quickly to protect water quality, and growing population in King and Snohomish Counties threatens to exhaust the capacity in our existing wastewater system by the year 2010.

For the past several years, we have been developing possible ways to meet the increasing demands of population growth while keeping our system flexible enough to respond to new circumstances like the ESA. We asked the public to help us decide on the best approach to take. Now I am pleased to present my recommendations for managing the wastewater needs of this region for another 40 years or more:

  • Build a new north treatment plant, associated conveyance improvements, and a new outfall system that discharges treated effluent to Puget Sound
  • Accelerate the program to control combined sewer overflows
  • Implement a new incentive-based program to reduce inflow and infiltration
  • Continue recycling biosolids and evaluate new biosolids technologies
  • Investigate new ways to recycle and reuse water from new and existing treatment plants to help meet other water resource needs in the region

I urge you to become involved in developing a final Regional Wastewater Services Plan. Please review this document for more detail on these recommendations and provide us with your comments, questions, and concerns during the upcoming Council adoption process. Together, we can protect and preserve our important water resources.

Ron Sims
King County Executive

The RWSP final EIS, published in April 1998, compared and addressed the probable environmental impacts of four service strategies developed as part of the Regional Wastewater Services Plan planning process. This programmatic-level EIS provided information necessary to decide on an overall plan of action for the RWSP.

Complete document (4.1MB)

Table of contents

Fact sheet

Part I – Executive's Preferred Plan and changes in service strategies due to revised population and flow projections

Part II – Full text of draft EIS, revised in response to comments