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King County Regional Wastewater Services Plan

Since 1999, the Regional Wastewater Services Plan (RWSP) has guided the programs and policies of King County’s wastewater system. The RWSP must be updated to address the next 30 years of our clean water services.

In the 1990s, flow estimates based on projected population growth in King County's wastewater service area indicated that our regional wastewater treatment system would run out of capacity by 2010. To ensure essential wastewater treatment services continued into the future, King County carried out an intensive planning effort that involved numerous elected officials, representatives from local sewer agencies, and organizations and individuals from around the region.

These efforts resulted in the Regional Wastewater Services Plan, which was adopted by the King County Council in November 1999 via Ordinance 13680. The RWSP identifies projects and programs needed to provide wastewater capacity for homes and businesses in King County’s wastewater service area through 2030. It also provides policy direction for the operation and continued development of the wastewater system.

Current status

Many of the major projects outlined in the RWSP have been completed or are underway. The process to update the RWSP started in 2019 as the Clean Water Plan, which was paused at the end of 2021 to fully consider and address feedback we received. The pause also provided an opportunity for more regulatory certainty regarding our combined sewer overflow (CSO) and nutrient reduction obligations.

The planning process restarted in 2024 as the update to the RWSP. The process to update the RWSP contains important adjustments intended to address feedback received during the Clean Water Plan process, including the need for a long-range strategic vision to serve as a guide for the decisions and policies we will make during this process.

A vision for protecting water quality

King County is planning for the future based on the values and priorities of the people who live here. A key part of this process is hearing from communities about what they care about to create a Vision for Clean Water that broadly reflects our region.

The Vision for Clean Water will serve as a guiding light for where we want our regional wastewater system to be by the year 2100, helping to shape the future of clean water in our region. Our region has big, significant choices to make about how to invest in our regional wastewater system, and the Vision will guide these decisions.

Everyone can be part of imagining the future of clean water in King County. In 2024 and 2025, we will gather input through surveys and at community meetings and events. Join us to create a more sustainable future with the waste we all produce.

Major components of the RWSP

The following sections describe the major components in the 1999 RWSP, many of which have been completed or are underway.

Brightwater treatment system

The RWSP called for building a third regional 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 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 Brightwater Treatment Plant into Puget Sound. The Brightwater conveyance system consists of approximately 14 miles of pipeline built in underground tunnels.

Conveyance system improvements

King County's regional wastewater conveyance system consists of more than 390 miles of pipes and 48 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 20-year design storm and accommodate increased flows where needed.

Regional infiltration and inflow control

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 maintenance holes, 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's conveyance and treatment systems and, ultimately, the monthly rates that businesses and residents pay to operate and maintain them.

Combined Sewer Overflow control

The RWSP calls for the control of all King County combined sewer overflows (CSOs) by 2030. 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%. Visit the CSO Control Program website 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.

Odor control

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.

Loop biosolids recycling

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.

Reclaimed water

The RWSP calls for King 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.

Documents and resources

The RWSP annual report 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.

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.

  • 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.

Executive's Preferred Plan - RWSP (1.8MB)

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.

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