Why do we have CSOs?
Up until the 1940s, the standard engineering practice in the U.S. was to provide combined sewers (sewers that carry wastewater and stormwater runoff in a single pipe) to convey wastewater and runoff to the nearest water body where these were discharge occurred with little or no treatment.

Beginning in about the 1950s, separate sewer systems were built for wastewater and stormwater, and in the late 1950s, treating wastewater became the standard practice. Interceptor pipes were built to transport all wastewater (from either combined or separate systems) to wastewater treatment plants. Many of the older, combined sewer pipelines remain in operation throughout the country, including some in Port Angeles.

CSOs occur because these combined sewer interceptors and pipelines have a limited capacity, and intense rain storms cause flows that exceed pipe capacities. Under such conditions, the flow could damage the city's wastewater treatment plant and/or back up into homes and streets. To prevent system damage and sewer backups, specific structures have been built to allow the combined sewer pipes to discharge and thereby relieve the pressure.

Show All Answers

1. What are combined sewer overflows?
2. Where are the combined sewer overflows?
3. Why do we have CSOs?
4. Are CSOs legal?