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Combined Sewer Overflows – why we should care

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On Tuesday, the Seattle Public Utilities and Neighborhoods Committee passed resolution 31201 endorsing the Seattle Public Utilities Combined Sewer Overflow (CSO) Reduction Plan Amendment.  Exciting, right?

Some of the wonky background — Part of the City of Seattle’s drainage and wastewater system (about one third of the city) is a combined sewer system, which runs sewer and stormwater runoff through a single pipe.  During heavy rains, the stormwater overwhelms the combined sewer system which overflows and contributes pollutants directly into lakes, streams and marine waters.  The City of Seattle has 90 CSO outfall locations in addition to King County owned and operated outfalls inside the City of Seattle.

The overflows are regulated by the Environmental Protection Agency (EPA) and the State Department of Ecology (Ecology) via a National Pollutant Discharge Elimination System (NPDES) permit that must be renewed every five years.  The current permit expires in November 2010.

Over the past 40 years, the City has reduced its CSO volume by approximately 20 billion gallons with projects like those outlined in the current plan, including building diversion structures and wastewater storage facilities to store excess wastewater until flows decrease enough to return it to the system. However there is still much more work to be done, and the final reductions are some of the hardest to achieve because they require significant capital investments. 

So why should we care?

1)      Reducing overflows is a good thing for our environment.   Seattle’s CSO Reduction Plan will reduce the overflows into Lake Washington by 14 million gallons per year (60% of current discharge) over the next five years.  The plan also includes opportunities to expand and test green infrastructure projects (rain gardens, cisterns, green roofs etc.).  The first green infrastructure programs will be piloted in the Ballard basin, followed by full-scale implementation in North Union Bay, Interbay, Montlake and Fremont/Wallingford CSO basins.  

2)      It is a large capital program with potential impacts to rate payers.  The total cost of the proposed five year plan is estimated to be $162 million dollars.  SPU estimates increases to monthly residential wastewater and drainage bills of $1.75 in 2011 and $4.62 by 2015 on top of a typical monthly bill of $63.87.   

The Council has not actually adopted these rate increases, but we have endorsed a plan that suggests real rate impacts.  The Executive is currently developing his drainage and wastewater rates proposal that will come to the Council this summer.  At that time, we will have the chance to carefully look at all of our public utility costs and decide the right level of rates going forward.  In addition, King County is developing its own CSO reduction plan which will also have rate impacts – stay tuned for more details on this, this summer.

One final note – you may also be hearing about King County’s CSO program in your neighborhood.  The County, like the City, must also reduce outflows.  They have prioritized four basins in the City of Seattle, and are currently conducting a public outreach process to select alternatives.  You can learn more about these projects in your neighborhood on the King County website:

If you are still reading, and want to learn more, take a look at the full CSO Reduction Plan Amendment and let us know what you think.


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Comment from Larry Garrett
Time May 3, 2011 at 1:01 am

Mike, The Ballard raingarden “green” CSO approach has been a failure. The City should not use this approach anywhere else in the City. We have borrowed money to dig holes and now they have been filled in. Not what I want to hear about. SPU’s response to ask that residents shut down any further criticism of the Green project. Waste and then cover up — sad.

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