How sewer overflows occur

Older parts of Seattle, including Ballard, Fremont, Wallingford, and Queen Anne are served by a combined sewer system. This system was designed in the early 20th Century to carry sewage from buildings as well as stormwater runoff from streets, rooftops, and parking lots in a single pipe to a wastewater treatment plant. The system’s purpose was to keep polluted water and sewage away from people in order to protect public health. During dry weather, polluted water flows to a wastewater treatment plant. When it rains, the pipes can become overloaded. A mixture of stormwater (about 90 percent) and raw sewage may overflow into lakes, streams, and the Puget Sound, which can harm people, fish, wildlife, and the environment.

Why overflows are a problem

The mixture of stormwater and sewage may harm people, fish, and aquatic life in the areas where overflows occur. Because of these impacts to water quality, the federal Clean Water Act and state regulations require that we take action and reduce overflows to an average of no more than one overflow per outfall per year. The City of Seattle and King County are also legally required to address all combined sewer overflows throughout the city under a Federal and State Consent Decree.

History of Seattle’s sewer system

Seattle has been working to improve its sewer system to protect the health of people and the environment for over 150 years. Before the 1909 Alaska-Yukon-Pacific Exposition, the city made a number of improvements to its sewer system, building much of the combined sewer system that is still in place in older parts of Seattle.

Since the passage of the Clean Water Act in 1972, Seattle has made many improvements to the drainage and wastewater system.

Project history

SPU’s Plan to Protect Seattle’s Waterways is a long-range plan that creates a comprehensive strategy to protect Seattle’s waterways. The plan:

  • Identifies areas of Seattle where CSO reduction projects are required
  • Evaluates solutions for reducing CSOs in affected areas
  • Selects a preferred alternative
  • Recommends a schedule for designing and constructing projects (2016-2030)

The plan also describes how SPU will meet legal and regulatory requirements. In July 2013, Seattle entered into a historic agreement – a Consent Decree  – with the Environmental Protection Agency, Department of Justice, and the Washington State Department of Ecology to reduce sanitary sewer overflows and CSOs into Seattle’s waterways.

Interested in the history of the Ship Canal? Visit King county archives to learn more.