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Combined Sewer Overflow Reduction Plan
Combined Sewer Overflow Reduction Plan
Seattle’s CSO Program
The City of Seattle has been addressing water quality issues for decades and through Seattle Public Utilities, the City is currently implementing a Combined Sewer Overflow Reduction Program with a focus on reducing overflow events to improve water quality and achieve State and Federal requirements for CSO control.
What’s a CSO?
Combined Sewer Systems (CSS) exist in some areas of Seattle and unlike new separated systems that keep stormwater (rain runoff) and wastewater apart, the old combined system joins sewage from homes and businesses and stormwater together in a single pipe to the wastewater treatment plant. Combined systems were built until the first part of the 20th century — these combined systems relying on overflows were considered an acceptable way to manage excess water. . Seattle manages about 90 CSO locations (PDF).
Combined Sewer Overflows happen when the CSS is pushed over capacity during heavy rain events — Combined Sewer Overflow (CSO) locations are relief points for the excess flow to keep sewer backups, surface flooding, or operational issues at the regional wastewater treatment facilities where both stormwater (about 90% of the CSS volume) and sewage are treated most of the time
What are we doing about CSO’s?
SPU’s CSO Program will use many different options to improve water quality and reduce overflows.
Each option is guided by scientific data, environmental goals and regulations, and public health concerns. Control strategies must include working on all sources of pollution affecting water quality. Thorough integration of programs, projects and activities in each area of the city will help water quality and reducing the impacts of sewer overflows.
The CSO Program represents an investment in human health, water quality and the environment while helping achieve compliance with regulatory standards. Since 2004, the Restore Our Waters initiative has steadily guided the City’s water quality improvement work — the initiative shapes the framework for SPU’s CSO Program and will help protect Seattle’s waters for people, salmon, and future generations.
Addressing CSO’s is critical to restoring and preserving Seattle’s waters because overflows discharge into nearby lakes, streams or Puget Sound. Reducing CSOs will help to improve the water quality in the region, but other efforts such as pollution prevention, stormwater controls, natural drainage and green stormwater systems, and habitat improvements will also play a significant role in improving water quality
The City is developing a CSO Long Term Control Plan (LTCP) using detailed information about stormwater in Seattle — how much it rains, where that rain collects, and where there are most likely to be CSOs to help identify where and how CSOs can most effectively be controlled. LTCP options will then be further analyzed, refined, and narrowed into “preferred alternatives” that will serve as the basis to implement specific CSO projects.
Both SPU and King County have been working since the 1960's to control CSOs in the City of Seattle —48 of the 90 CSO locations (pdf available online) managed by the City of Seattle are currently uncontrolled. In the early 1960's, the City and County were discharging approximately 20 to 30 billion gallons of CSO to the region’s receiving waters. By 1988 the City’s CSO Control work reduced discharge to 470 million gallons for the City and 2.4 billion gallons for the County. For the past 5-year period, the average annual City discharge has been 537 million gallon with a range of 24 million gallons (low) in 2008 to a high of 1500 million gallons in 2006.
Our real measure of success is the reduction in overflow frequency and improved water quality. Overflow frequency has declined (since the 1980's) from the baseline condition of about 2,800 events per year to a recent annual average of 321 and a range of 187 to 556. This estimated 89 percent reduction, while substantial, does not achieve the SPU Level of Service and NPDES permit target of one event per year per outfall.
SPU’s CSO Control Program has two major foundations:
1. Implementation of “Nine Minimum Controls” or shorter-term operations and maintenance (O&M) related activities and best management practices (BMPs) to reduce CSOs and
2. development and implementation of a “Long-Term Control Plan”, which consists of longer-term more capital intensive projects. SPU has received guidance from an EPA Administrative Order on how to implement the Nine Minimum Controls, or shorter-term O&M activities and BMPs.
CSOs are regulated by both federal and state laws. The City of Seattle possesses a National Pollutant Discharge Elimination System (NPDES) permit which is issued by the Washington State Department of Ecology (Ecology) which allows it to discharge CSOs during wet weather events. In accordance with both state and federal law and its NPDES permit, the City is required to reduce CSOs through both shorter-term best management practices (BMPs), proper operations and maintenance programs, and longer-term more capital intensive projects.
The primary instrument for the State Department of Ecology to regulate the City’s CSO Program is through the NPDES permit. In some cases, the State or the Federal government (Environmental Protection Agency and/or Department of Justice) can issue an Administrative Order or Consent Decree as a supplement to the NPDES permit to include additional requirements for the City to comply with state and federal guidelines concerning CSO control.
Protecting Yourself from Overflows
To protect yourself and family members from health concerns associated with sewer overflows, it’s best to avoid contact with water near discharges both during and following rain events for at least 72 hours. This includes activities such as boating, wading, fishing and swimming. Before you recreate near local waterways, be sure to check alerts from the health department.
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