Achievements in Reducing Combined Sewer Overflows (CSO)
Reducing pollution in our waters since 2008
Seattle’s combined sewer system carries both stormwater (rain) and wastewater (which includes sewage). During heavy rains, it overflows into our local waters; these are called Combined Sewer Overflows or CSOs.
In the late 20th century, City-led and County-led infrastructure programs worked to address the problems. Despite this work, combined sewer overflows remained a significant source of pollution to our local waters in this century.
Consent Decree
In 2008, the U.S. Environmental Protection Agency (US EPA) found that Seattle needed make operational and infrastructure changes to reduce combined sewer overflows. In 2013, the City of Seattle, US EPA, U.S. Department of Justice (US DOJ), and the Washington Department of Ecology signed a wastewater consent decree that detailed specific actions for the City to take to reduce combined sewer overflows. King County Wastewater treatment division also signed a wastewater consent decree to address their combined sewer overflows.
Seattle Public Utilities’ (SPU) has invested almost $1 billion in Seattle’s sewer system to reduce combined sewer overflows since 2008.
Learn more about how SPU has invested your dollars in infrastructure that reduces pollution, and how we will continue this work in decades to come.
A New Era of Investments
2010-2024
We have significantly reduced CSOs when measured by both frequency and volume.
Here is how...
We built four large storage tanks along Lake Washington from 2010 through 2017.
- Windermere storage tank, near Magnuson Park, was completed in 2015. It can store more than 2 million gallons of combined sewage per storm.
- The Genesee storage tanks, on Lake Washington Boulevard S at 49th Avenue S and at 53rd Avenue S, were completed in 2015. They can store about half a million gallons of combined sewage per storm.
- North Henderson storage tank , in Seward Park, was completed in 2017. It can store more than 2 million gallons of combined sewage per storm.
In 2019, we began construction on the Ship Canal Water Quality Project in partnership with King County Wastewater Treatment Division.
The Ship Canal Water Quality Project is a large underground storage tunnel that will reduce the polluted water flows into the Lake Washington Ship Canal, Salmon Bay and Lake Union. Project construction spans five neighborhoods. It will be operational before 2030.
We improved our response to sewer overflows.
Broken or blocked sewers cause backups into homes or onto the street. Our response to these issues focuses on public health and safety, and we take steps to protect the environment. Our First Response Crews works hard to stop and fix sewer backups. They partner with the Spill Response Team to contain, cleanup and notify the public when sewage is released into the environment—including our waters.
We increased preventative cleaning and maintenance.
Maintenance planners inspect every sewer pipe in the city—more than 1400 miles of pipe—about every 10 years using close-circuit television cameras (CCTV). These inspections identify problems early and ensure cleaning. Over the last 5 years we have spent approximately $30M/year fixing broken pipes and we plan to continue this rate of investment in the years to come.
We completed 50+ sewer system improvement projects.
This includes projects that modernized our wastewater pump stations and made them more efficient. Here are three examples:
- The Magnolia Pump Station 22 project increased the station’s pumping capacity from 0.86 to 4.0 million gallons per day.
- The East Montlake Pump Station and Force Main project increased the pump station’s capacity from 0.9 to 2.8 million gallons per day.
- Portage Bay Retrofit project increased the pumping capacity of Pump Station 20 from 1.1 to 1.5 million gallons per day.
We built green stormwater projects.
Green Stormwater projects —like roadside bioretention— can slow and treat stormwater before it enters the drainage system. In combined sewer areas, directing stormwater to green stormwater projects helps reduce the amount of stormwater that may cause an overflow.
Since 2013, we have built the Ballard Natural Drainage System and the Delridge Natural Drainage System . Together, these projects manage and treat stormwater from nearly 13 acres of impervious area in combined sewer overflow basins.
Continued Work
2024 and beyond
Addressing additional combined sewer overflows is still a priority. We will complete construction of the Ship Canal Water Quality Project and will construct the South Park Water Quality Facility .
We will also develop an updated plan to address our remaining CSOs. This plan will include integrated, community-centered solutions to control remaining overflows and maintain control into the future.
Our infrastructure investments support communities, the city, and the environment, in fulfillment of our Strategic Business Plan .
Our future investments will also reflect the priorities in our Community Vision , which includes increasing resilient and equitable multi-benefit investments and community-centered partnerships.
Find out more our how we are planning our next 50 years of drainage and wastewater systems and the specific types of solutions we are considering at our Shape Our Water project .
We are proud of our work reducing combined sewer overflows into Seattle’s waters for Seattle’s communities, and excited about our next decades of work. Thank you to our ratepayers for their sustained commitment to public health and the environment.
