The steps on the scaffolding are steep and slippery in the rain. Slowly we navigate downward to an underground tank that will eventually hold 2 million gallons of raw sewage and polluted stormwater overflow to keep it from entering Lake Washington. Last year, more than 154 million gallons overflowed from city-owned pipes into all of the city’s dozen water bodies and, with it, 8,000 tons of toxic metals and volatile chemicals.
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Seattle is one of nearly 800 U.S. cities that uses a combined sewer system, which collects rainwater, industrial wastewater and raw sewage in one set of pipes. Built in times of smaller populations and less use, combined sewer systems often overflow during heavy rain or storms, shunting extra wastewater and sewage into nearby bodies of water.
Here on Lake Washington, Seattle Public Utilities (SPU) is building the tank to stop the city’s highest frequency overflow. In 2012 pipes in the basin on the western flank of the lake — which includes View Ridge, U Village and Sand Point — overflowed 149 times.
A map, updated hourly, of sewer overflows in Northeast Seattle on January 14th, 2014. Photo: King County
"Once the tank is used, we'll be able to operate and control this whole tank from back in our operations and control center down in Sodo," says Keith Ward, an engineer with Seattle Public Utilities. "And when the storm is over, we'll pump the combined sewage back up 65th, down Sandpoint Way and back into the system."
Real time controls and underground pipes will carry the toxic overflow to King County’s treatment plant at Discovery Park.
Roughly 20 feet deep, the tank has three vaults for odor control and massive amounts of electrical, instrumentation, controls and steel. Construction manager Dennis Davis of CDM Smith points to layers of rebar reinforcing the construction.
Lake Washington's Windermere Basin tank will hold 2 million gallons of wastewater when complete. Photo: SPU/ Susan Stoltzfus
"This is going to have a massive tipping bucket. This is what we're all waiting for — when they test this tipping bucket and it comes roaring down here and then washes this out," said Davis. The bucket, which was designed to work in both stormy and dry weather, will be filled with fresh water to clean out the holding tank.
The tank can handle 2 million gallons of stormwater, but since an estimated 47 million gallons overflowed into Lake Washington last year, it may need to operate continuously during big storms. Those generally happen only two or three times a year.
"The idea is when there's a big storm, we fill up this tank. We let the storm go out, we pump it back into the system. If there's another big storm we fill up the tank," said Ward.
The Windermere Basin tank — which hasn’t been given a name just yet — will cost the city $50 million and will be just one of a larger system of tanks dealing with Seattle's stormwater runoff and pollution problem, which violates the Clean Water Act. Their construction comes on the heels of years of water quality and public health violations, penalties and compliance orders issued by state and federal regulators.
In 2012 alone, there were 355 overflow events in the city’s 12 receiving waters. One hundred forty nine of those were in Lake Washington. Salmon Bay had 96, which spilled some 59 million gallons of polluted stormwater and sewage. Several years ago, one Seward Park storm caused a million gallons of sewage and stormwater to overflow.
SPU began excavation on the Windermere Basin tank a few years ago to show good faith to regulators, says Ward. Since 2001, the agency has also been working to contain the stormwater problem with rain gardens, retrofits and green infrastructure.
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