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Cleaning Tacoma water without using energy or chemicals

Jessica Knickerbocker was watching water come out of a culvert and into a new bio-retention facility at Point Defiance Park.

“When it’s raining, it’s black,” said the city of Tacoma Environmental Services engineer. “You’d be amazed. It looks like tar.”

The facility doesn’t use energy or chemicals to filter pollutants from water that flows from 480 acres of Tacoma and Ruston streets, yards and parking lots.

The water, coming as far as North 30th Street, is intercepted, cleaned and sent on its way to Puget Sound.

Knickerbocker was project manager during construction of the facility, which is the largest of its kind in the area and innovative in many ways.

Surface water running off streets and gutters makes its way to just north of the park entrance and enters a vault. There, floating objects are trapped and large sediment sinks.

The remaining water is split, with some continuing into the bio-retention facility just inside the park and the rest — about 36 percent — spilling untreated into Commencement Bay near the Tacoma Yacht Club.

Knickerbocker said she would have liked to have built a facility that could treat all 756 acres in the collection area, but the cost and the real estate needed was prohibitive.

One aspect that made the facility attractive to planners is its economical use of space compared with other natural cleaning facilities.

“For a treatment wetland, I would have had to take the whole marina and the yacht club peninsula to provide the same treatment that we’re doing here,” Knickerbocker said.

“It’s the biggest bang for the buck.”

A $1 million grant from the state Department of Ecology and money from the $198 million Point Defiance bond issue helped pay for the facility, which is part of Metro Parks’s Waterfront Phase 1 improvement plan for Point Defiance.

After water enters the facility, it drops via gravity to a series of concrete vaults, troughs and pools that hold 5,500 square feet of media – a mix of gravel and organic matter – where pollutants are filtered and trapped.

Topping the media are drought-tolerant plants, such as evergreen huckleberry, that can tolerate wet winters.

The water then moves into a natural-looking creek where it flows to Puget Sound.

The facility, built roughly on the site of the former go-kart track, will not be open to the public but can be seen from all angles.

Students from Tacoma School District’s Science and Math Institute have monitored construction and might make the facility part of their curriculum, Knickerbocker said.

“You could easily take samples from the troughs,” she said.

The facility went online in early December. Dark, tarry-appearing sediment is already building up in the troughs and pools.

Knickerbocker scoops up a handful of gooey muck.

“This is treating the dirtiest of the dirty water,” she said.

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