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Where Does all the Gunk Go?

Now that the rainy season is upon us, have you noticed those automobile oil slicks snaking their way toward the drains on our streets? Some people have and they’ve expressed concern that all that pollution is dumping into Puget Sound. The good news is SDOT implements several different strategies to protect the Sound from pollution.

runoff (rs)

Auto pollution from a downtown street enters a combined sewer system and will go through treatment at the West Point treatment plant

Seattle’s drainage system comes in two forms – in Downtown Seattle the existing sewer system is a “combined sewer” system meaning it conveys both waste water and stormwater to the West Point treatment plant prior to final discharge into Puget Sound. The rare exception is during an extraordinary weather event, such as a hundred year storm, when the system cannot keep up with the overflow. To prevent such combined sewer overflows from entering the Sound, SDOT and SPU are funding a larger regional flow control facility that will be built along the waterfront. 

swell (rs)

An “extended curb bulb” for water treatment of road surface water prior to dropping into the non-combined sewer system











In other parts of Seattle, which are not on combined sewer systems, we install water quality treatments as we make roadway improvements. For example, on two recent projects (First Avenue S from S Stacy to S Horton streets and Fauntleroy Avenue SW from SW Alaska to SW Holly streets) we installed Storm Filter Catch Basins and Filterra Vaults to treat storm water before it discharges to the storm system and eventually to Puget Sound. 

Seattle’s recently revised storm water, grading and drainage control code requires additional measures to protect water quality and flow control on virtually every project in the city. The new code requires that all projects implement “green infrastructure to the maximum extent feasible” such as bio-retention like swales, landscaping, permeable paving, and green roofs. Even before the new code was adopted, SDOT’s sidewalk installations have included bio-retention and permeable pavement where feasible. The upcoming Transit Rapid Ride projects will also reduce pollution by replacing some of the road surface with raised sidewalks where storm water runoff doesn’t carry vehicle pollution, and in other locations will incorporate permeable pavement.