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WSDOT Modified Media Filter Drain


The Washington State Department of Transportation (WSDOT) Modified Media Filter Drain (MMFD) is a modification of the standard Media Filter Drain BMP. A standard media filter drain consists of three components: a vegetation-free gravel zone to assure sheet flow from the pavement and to provide initial infiltration losses; a three-foot-wide grass strip to provide pretreatment; and media filtration. The MMFD provides an end-of-pipe alternative to the standard media filter drain, using a detention pond or vault in place of the grass strip as pretreatment and vegetation-free gravel zone as initial infiltration losses. A flow spreader is used downstream of the detention pond or vault to return the flow to sheet flow before dispersing it over the media filtration.


WSDOT must use proven stormwater treatment BMPs when required by their municipal stormwater permit. These BMPs must either be included in the Highway Runoff Manual or approved through TAPE. One of the few BMPs in the Highway Runoff Manual that is approved for enhanced (dissolved metals) treatment is the Media Filter Drain. This BMP, however, is often difficult to install due to space constraints along the immediately adjacent right-of-way or retaining walls that don’t allow for sheet flow.

WSDOT designed the MMFD as an end-of-pipe alternative to the standard media filter drain, allowing for its installation in areas that a standard Media Filter Drain may not work. Because the MMFD was not listed in the Highway Runoff Manual, WSDOT had to pursue approval through the TAPE program. The MMFD was evaluated between October 2011 and October 2012 and ultimately received a General Use Level Designation for Basic (TSS) and Enhanced (Dissolved Metals) Treatment.

Point Defiance


The Point Defiance Regional Stormwater Treatment Facility is located just outside the entrance to the Point Defiance Park. The 5,500 square foot facility treats stormwater from 754 acres using a treatment train of two TAPE approved technologies – a hydrodynamic separator followed by media filtration cells. The hydrodynamic separator provides pretreatment, removing trash and larger solids and extending the maintenance cycle of the downstream media filtration cells.

After leaving the hydrodynamic separator, stormwater flows through a series of six cascade pools, troughs, and filtration cells filled with engineered media. The filtration cells also contain plants that improve the effectiveness of the treatment media. Stormwater filters through the engineered media, removing pollutants through a combination of filtration, adsorption, and absorption. Treated stormwater enters an underdrain system then flows into a bioswale before rejoining the city’s stormwater drainage system and being conveyed to Commencement Bay.

Overall, the treatment facility has the ability to remove sediment, heavy metals, nutrients, and oil and grease from up to 8 million gallons of water each day. These are contaminants that would otherwise flow into untreated and unimpeded to the Point Defiance Marina, Commencement Bay, and Puget Sound.

Prairie Line Trail


The Prairie Line Regional Stormwater Facility is located near the corner of Hood St. and S. 21st St in Tacoma, WA. The facility, which is incorporated into a portion of the old Prairie Rail Line railroad spur, uses a treatment train approach to treat stormwater runoff from 42 acres. The treatment train is made up of two TAPE-approved treatment technologies – a hydrodynamic separator located at the upstream end of the facility which provides pretreatment by removing trash and larger debris followed by a vegetated media filter to remove sediment and heavy metals.

The stormwater facility was completed in 2014 and is a part of a historic art installation, part stormwater treatment. Between 1873 and 2003 the rail line was part of the Northern Pacific Railroad, which extended from Tacoma to the Great Lakes. The vegetated media filter component of the treatment train was designed to sit within the existing railroad tracks. As water infiltrates through the media pollutants such as sediments, nutrients, heavy metals, and oil and grease are removed through a combination of filtration, adsorption, and absorption. The vegetation aids in the treatment of the stormwater by helping to restore soil permeability through the uptake and transpiration of moisture within the media.

After leaving the facility the treated stormwater rejoins the City of Tacoma’s stormwater drainage system before being discharged into the Thea Foss Waterway.

Lake Union Ship Canal Test Facility


The Lake Union Ship Canal (LUSC) site is located under the Interstate-5 bridge at the North end of Lake Union. This site, constructed as an ultra-urban stormwater research facility in 1996, is owned by the Washington State Department of Transportation but has been managed by Ecology since 2015. The facility receives runoff from approximately 32 acres – 23 acres of pavement and 9 acres of roadside landscaping.


The Washington State Department of Ecology started managing the LUSC site as a TAPE testing facility in 2015. Ecology recognized that finding a test site was one of the major obstacles facing manufacturers navigating the TAPE program. With four testing bays allowing for the simultaneous evaluation of four different technologies, LUSC was opened to lessen this burden on manufacturers and provide an accessible and secure testing location.