County Wastewater Treatment Department Tackles Seattle’s High Runoff Flows in Environmentally Sound Fashion

A wet-weather treatment station in Washington’s King County will help control CSOs and furthers a wide range of sustainability goals.

County Wastewater Treatment Department Tackles Seattle’s High Runoff Flows in Environmentally Sound Fashion

When completed, the wet weather station will handle up to 70 mgd of combined stormwater and wastewater that would otherwise run into the Duwamish River.

A new wet-weather treatment station in Seattle’s Georgetown neighborhood won’t be completed until late 2022, but it has already won a Platinum Achievement Award for Sustainability from the Institute for Sustainable Infrastructure’s Envision rating system.

The station will serve an older area of the city that has combined storm and sanitary sewers.

The King County (Washington) Wastewater Treatment Division, which serves 17 sewer districts and more than 1.7 million people in the Seattle area, operates four other wet-weather stations. The older sites use settling and chlorination/dechlorination; the Georgetown station will use advanced settling and UV disinfection.

“When it is completed, the station will be able to treat up to 70 mgd of combined rain and wastewater that would otherwise run into the Duwamish River without treatment,” says Norm Mah, a spokesman for the division. “Heavy rains are commonplace for parts of the year here. This plant will enable us to treat the stormwater runoff before it goes through our system and to the outside pipes that go into the Puget Sound.”

The amount of rain needed to trigger operation of the wet-weather plants varies with rain intensity and other factors, but a half-inch of rain could do the trick. The Georgetown plant’s sustainability award reflects more than the environmental benefit of treating combined sewer overflows. It also reflects the county’s commitment to diverting demolition materials from landfills, reducing chemical use, restoring the Duwamish River shoreline and building green infrastructure, such as permeable pavement, green roofs, roadside rain gardens and cisterns.

Three-part project

The $250 million project began with community planning meetings in 2015 and 2016. Site preparation began in 2017. The project includes the treatment station itself, the outfall to the river and the pipes that connect the two. About half of the money will come from a $129 million low-interest loan from the U.S. Environmental Protection Agency’s competitive Water Infrastructure Finance and Innovation Act program. Qualifying for that loan could save ratepayers up to $34 million.

The construction of the outfall by Pacific Pile & Marine, including native planting to improve wildlife habitat at the entry point and logs to stabilize the river bank, will be completed in 2019. Construction of the treatment plant and the pipes connecting the plant to the outfall began in 2018 and will take more than three years to complete.

From the early planning stages, King County saw the Georgetown project as a way to spark economic equity. The county structured construction contracts to encourage small-business participation, require contractors to set aside a percentage of labor hours for apprentices, and set voluntary hiring goals for women and people of color.

The county also partnered with community colleges and unions on job-training programs and apprenticeships. The project is part of a pilot program, Priority Hire, that requires contractors to train and hire workers living in ZIP codes with high poverty and unemployment.

Learning opportunities

Besides performing its function, the green infrastructure component will serve as a demonstration. Instead of using underground stormwater storage tanks that can be pumped back to the sewer system during nonpeak times, the plant will have more visible methods for on-site stormwater management, such as landscaping, stormwater reuse, bioretention, permeable surfaces and a green roof.

The county plans to offer tours and on-site interactive lessons to school groups and the general public. The treatment station design includes public art for the building.

When the plant is operating, special lighting along the building will come on, and passersby will be able to see what is happening inside. The pipes will be colored to show how the water moves through the station, and informative signs will be posted. The plant is designed to start up automatically when the combined sewers overflow. Operators will be dispatched to collect samples and pump down and clean out tanks when the overflow event is over.

Listening to residents

Community input guided plans and grant funding to improve pedestrian access to the Duwamish River and the construction of a 135-foot-long, 12-foot-high green wall along one of the major nearby thoroughfares to reduce dust and improve air quality.

The Georgetown neighborhood was developed as residential, but since the 1960s, it has been mostly commercial and industrial. Now the Georgetown Merchants Association calls the neighborhood “Seattle’s industrial arts corridor.” Its red brick buildings are filled with art galleries, coffee shops, diners, bookstores and night clubs.

The wet-weather station site occupies what was technically a brownfield, since the state Department of Ecology identified it as having leaking underground storage tanks. A large amount of soil had to be removed, along with materials containing asbestos and lead-based paint, but to a large extent, the materials on the site were reused.

Five buildings were torn down and numerous concrete surfaces were removed, but old-growth timbers from a warehouse were salvaged, and the concrete was recycled. The project reused 85 percent of the demolition materials. The Institute for Sustainable Infrastructure cited the project for creatively addressing community needs, improving air and water quality and demonstrating leadership in planning and design.

Meeting demands

The Georgetown wet-weather station is designed to reduce untreated CSOs to the Duwamish River by 95 percent. It is one of 14 remaining projects in the county’s CSO control plan. “This is infrastructure that’s needed to meet the growth of the region,” Mah says. “We are one of the fastest-growing cities in the country.”

While the Georgetown project is the first in the state to win an Envision platinum award, the county has been following sustainable practices for many years. Those include programs to encourage use of Seattle’s abundant rainwater. For example, the county works with other entities, including Seattle Public Utilities, to install cisterns and rain gardens that encourage the capture and recycling of stormwater.

“There is a lot of awareness of Seattle as the Emerald City, but people also call it the Rain City,” Mah says. When the Georgetown station is complete, King County will have one more tool to keep rainfall from being a problem.


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