Swale on Yale: A Breakthrough Discovery in Biofiltration Systems

In 2012, the City of Seattle proposed Swale on Yale; a project in the Cascade neighborhood aimed at reducing stormwater runoff from Capitol Hill to improve the water quality of Lake Union and Puget Sound as a whole. To accomplish this, Seattle Public Utilities (SPU) and Vulcan Inc. joined to construct a regional scale biofiltration swale to treat millions of gallons of stormwater from 435 acres of Capitol Hill streets and sidewalks. A series of four wide flat-bottom ditches planted with thick vegetation were built along two blocks of Yale Avenue North and Pontius Avenue North to slow the speed of runoff, reducing sediments and pollutants carried into Lake Union.

Two years later, Herrera and the City of Redmond conducted a study monitoring hydraulics and water quality at a bioretention system installed along 185th Ave NE in Redmond. After collecting several water samples, results confirmed that the compost in bioretention contributed large amounts of phosphorus, nitrate, and copper to the runoff. Ideally, stormwater filters should remove these pollutants, not contribute to the problem.   

This discovery led the region to search for alternatives to compost in bioretention and improve existing designs to eliminate pollutants. In 2019, Herrera, along with the City of Seattle, developed a bioretention “polishing layer” composed of sand, iron filings, and activated alumina. This same year, SPU conducted the first full-scale field deployment of a polishing layer at the Swale on Yale project.

In 2020, Herrera implemented an intensive monitoring program at three of the four swales. Our team found that the polishing layer not only eliminated the pollutant export issue from the compost layer above, but also reduced influent stormwater phosphorus concentrations by 76% and copper concentrations by 44%. With this, a polishing layer design approach has proven successful in optimizing the effectiveness of biofiltration systems in removing pollutants from runoff. Using this polishing layer approach, designers can now leverage the benefits of compost in their biofilters without worrying about pollutant leaching impacting downstream receiving waters.

Interested in learning more about the Swale on Yale project results? Join Dylan Ahearn (Herrera) and Shanti Colwell (SPU) at the APWA Stormwater Managers Meeting on March 19th as they present on their findings.

Make sure to tune into the 2021 Green Infrastructure Summit sponsored by Herrera on March 25th-26 to see Dylan Ahearn and Shanti Cowell discuss Herrera’s role in the first full-scale field result of a polishing layer. Purchase your tickets using this link. We look forward to seeing you there!