Polluted stormwater is one of the largest threats to the health of Puget Sound, so the city of Tacoma joined forces with Metro Parks Tacoma to host an innovative water treatment system designed to improve Puget Sound water quality – The Point Defiance Regional Stormwater Treatment Facility. The aim of this innovative facility is to capture the worst pollutants before they wash into an area of the Sound already overloaded with heavy metals from the Tacoma Smelter Plume.
Before construction of the facility, polluted stormwater flowed untreated into the Sound near Point Defiance Marina. The city evaluated multiple options to fix the problem. One option was to retrofit every street corner with a localized bioretention system. This option was estimated at $15 million vs. the $2.4 million it cost to build the new facility. It would also have made maintenance much more difficult and cost the city $3.5 million a year to maintain all of those facilities.
The City of Tacoma and Metro Parks Tacoma shared a vision to provide stormwater treatment through a park amenity at the entrance to Point Defiance Park. The city wanted a unique system that would be both functional and aesthetically pleasing. With over 2 million visitors each year, the city wanted this to be an educational piece for the public and make the community self-aware of the need for stormwater BMP’s.
The facility works as follows: stormwater is first directed to a CDS hydrodynamic separator for pretreatment before entering the bioretention facility. The CDS uses swirl concentration and continuous deflective separation to screen, separate and trap trash, debris, sediment, and hydrocarbons from stormwater runoff. Pretreating stormwater with a CDS unit increases water quality, provides a single point of maintenance, and increases the aesthetic appeal of the downstream bioretention facility by removing unsightly trash and debris.
The bioretention treatment facility consists of a series of cascade pools, distribution channels (troughs), and Filterra treatment cells. The nine precast Filterra treatment cells range from 30’-100’ in length and 10’ wide. As the water flows through, the Filterra system captures and immobilizes pollutants. Those pollutants are then decomposed, volatilized and incorporated into the biomass of the Filterra system’s micro/macro fauna and flora. Treated flows from the underdrian system at the bottom of the container enters the bioswale that conveys the stormwater towards the Point Defiance Marina and Commencement Bay.
The Filterra system incorporates Filterra engineered media, a specified gradation of washed aggregate and organic material homogeneously blended under strict quality controlled conditions. Filterra media has been optimized to operate at high flow rates (100 inches/hour) while removing harmful pollutants such as TSS, phosphorus, nitrogen, copper, and zinc. Filterra has been tested and approved by the Department of Ecology and has received a General Use Level Designation.
The facility provides treatment for 754 acres in a footprint of only 5,550 square feet, and will treat up to eight million gallons per day of the dirtiest runoff from as far as two miles away. It is also providing an educational benefit. Students at Tacoma’s Science and Math Institute may have the opportunity to do water quality testing or other, related experiments, and professors from local universities also have expressed interest in the project as a potential learning laboratory. This interest helped to drive the facility design, building it so people can see the water as it moves through the system.