Combined Sewers & Reducing Overflows

At the City of Spokane, we've committed to reduce the amount of untreated overflows from combined sanitary and stormwater that reach the Spokane River. This work is part of our Integrated Clean Water Plan. We think it's important for our citizens to have a greater understanding of these sewers and why this work is important.

What is a Combined Sewer?

Combined Sewers include both sewage and stormwater. When City of Spokane residents dispose of household wastewater, the flow generated travels through sanitary sewer pipes that carry it to the City's Riverside Park Water Reclamation Facility. In some older parts of the City, stormwater runoff from roofs, parking lots, and streets empty into the same system that carries sanitary wastes to the Water Reclamation Facility. These systems are referred to as combined sewers.

These older combined systems are a legacy inherited from the past. The City's original wastewater collection system was built to carry all flows directly to the Spokane River and Latah Creek without treatment. In the late 1950s, the City built the first primary treatment plant and interceptor systems to provide treatment prior to discharge to the rivers. Today, more than 400 miles of combined sewers still exist, predominantly on Spokane's South Side.

What are Combined Sewer Overflows?

During heavy rainstorms and rapid snowmelt, extra flow from stormwater runoff into these combined sewers is greater than the interceptor pipes and treatment plant can accommodate. At these times, the combined wastewater surpasses the capacity of the pipes, resulting in an overflow to the Spokane River. These Combined Sewer Overflows are referred to as CSOs.

When CSOs occur, they discharge untreated sanitary wastewater and runoff from rainfall and snowmelt to the Spokane River. The combination of raw sewage and stormwater can carry a variety of human bacteria and viruses. In addition, combined sewer overflows contain a variety of chemicals, oils and other wastes. Although the untreated overflow is typically diluted by rain and river water, it still poses a potential health and environmental hazard. Those most likely to be affected by these overflows include people involved in water contact sports (i.e., boaters, swimmers, people who fish, etc.).


City of Spokane CSO Reduction Efforts

In the 1980s, the City of Spokane spent approximately $50 million dollars to construct separate stormwater systems and eliminate the combined sewers in most of north Spokane. This effort eliminated approximately 85 percent of the volume of combined sewer overflows to the Spokane River. The remaining combined sewers are in areas that are not easily separated.

The City currently is updating its plans to manage overflows to comply with state and federal regulatory requirements that limit overflows to one per year per location. The City is amending its CSO Reduction Plan and is creating an Integrated Clean Water Plan that considers all flows that go into the Spokane River.

One strategy includes building underground concrete tanks to hold the combined wastewater during a storm and then meter it back to the wastewater treatment plant when the storm surge subsides. A number of these tanks already have been built and others are under construction, including a 1 million gallon tank at 21st & Ray. Construction on a 1.5 million-gallon tank at Underhill Park begins in March 2014.

The City also will reduce stormwater runoff at the source, retrofit the sewer system to limit overflows; improve water quality of the overflows; and increase sewer line size and treatment plant capacity.

The City submitted an amendment (PDF 5.0 MB) to its CSO Reduction Plan this year that will deliver the results needed for a significantly lower cost.

View the 2005 CSO Reduction Plan (PDF 7.1 MB)

Where are the CSO Outfalls?

The City of Spokane has posted signs, like the one illustrated here, at the combined sewer overflow locations along the Spokane River. A total of 20 signs are installed, each with a specific number that references its associated overflow location along the river. The signs provide necessary information for the City's Wastewater Management Department to respond quickly if an overflow is detected during dry weather conditions.

What Should Spokane Residents do during CSO Events?

City of Spokane residents should avoid water contact sports and activities (i.e., swimming, boating, fishing) during rainfall and snowmelt conditions and when any discharge is observed from a combined sewer overflow pipe. Also, residents can help by calling 509.625.7900 whenever they observe a discharge from the CSO pipe to the river (especially during the summer months). Just make sure to note the CSO number on the sign so City staff will know exactly where the overflow occurred. The City will then confirm whether the discharge is a combined flow or stormwater only.

Real Time Combined Sewer Overflow Monitoring

During large rain storms or periods of snow melt, combined wastewater and stormwater sewers can overflow to the Spokane River. The City has 25 monitoring stations at the locations where these combined sewer overflows (CSOs) can reach the river. This listing provides real-time information when overflows occur. If a monitoring site shows a warning icon, that indicates an overflow event is occurring or has occurred in the last 48 hours. If a date and time are listed, this represents the start time of the overflow event. This information will clear after 48 hours. The City provides this information as a service to citizens who recreate along the river and others interested in this information.

CSO Stations


CSO Information

Archived Monthly CSO Reports


Annual Reports

Older Monthly and Annual CSO Reports available upon request.