Combined Sewer Overflow Reduction
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
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
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 Water Reclamation Facility. 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 Water Reclamation Facility. These systems are referred to as combined
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.). Wastewater Management teamed up with the City's Cable 5
department to develop a video about CSOs. To view the video Click Here.
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 construction projects to
reduce CSO events, start on page 39 of this document..
To view the 2005 CSO Reduction Plan
click here. (PDF 7,782K)
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.
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.
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.