The intent of the CSO Reduction program is to ensure that untreated overflows to the Spokane River from the combined sewer system (which carry sanitary and stormwater runoff in a single pipe) are reduced. This is being accomplished through implementation of capital projects which are directed at reducing the number of overflows to once per year and meeting the State of Washington regulated water quality standards.

Combined Sewer Overflow Update

The City of Spokane has posted warning signs at the CSO locations along the Spokane River and maintains a CSO information line (509) 625-7900 and web site notification of recent CSO events. To review CSO reports click here.

What is a Combined Sewer?

When City of Spokane residents dispose of household wastewater, the flow generated travels through sanitary sewers into an interceptor pipe which carries the flow to the City's regional wastewater treatment plant. In some older parts of the City stormwater runoff from roofs, parking lots, and street runoff empty into the same system that carries sanitary wastes to the wastewater treatment plant. 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 (combined household wastes and stormwater) directly to the Spokane River and Latah Creek without treatment. In the late 1950's, the City built the first primary treatment plant and interceptor systems in order to provide treatment prior to discharge to the rivers. Today over 50% of the City's old wastewater collection system has been separated into sanitary only and storm only facilities. However, over 400 miles of combined sewers still exist, predominantly on the 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 (including the stormwater runoff) overtop flow regulator structures, resulting in combined sewer overflows to the Spokane River. These Combined Sewer Overflows are often 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% of the volume of combined sewer overflows to the Spokane River. The remaining combined sewers are in areas that are not as easily separated.

The City is currently in the process of evaluating the remaining combined sewer basins and associated overflows to comply with state and federal regulatory requirements and develop plans for reducing the numbers of overflows to one per year per location. This expensive 20-year program will likely involve alternate types of controls and construction techniques to reduce CSO discharges, such as: reducing stormwater runoff at the source; retrofitting the sewer system to limit overflows; improving water quality of the overflows; and increasing sewer line size and treatment plant capacity.

For information on the current and upcoming construction projects to reduce CSO events click here.

To view the 2005 CSO Reduction Plan click here. (PDF 6,685K)

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 24 signs are installed, each with a specific number that references its associated overflow location along the river. In addition and with your help, the signs provide necessary information for the City's Wastewater Management Department to respond quickly if an overflow is detected during dry weather conditions.

You can view a map that shows the locations of the CSO outfalls along the Spokane River by clicking here (PDF, 824K)

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.

Wastewater Management, 909 E. Sprague. Spokane, WA. (509) 625-7900. A City of Spokane organization.
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