Seattle’s heavy rains test Georgetown’s new treatment facility



On a typical light rainy day in Seattle, rainwater can fall onto hard surfaces like asphalt, concrete, and roofs. It absorbs oil, fertilizer, bacteria, pet waste, and more from your car and ends up rolling down the drain.

In some of Seattle’s oldest neighborhoods, rainwater flows into pipes where it mixes with sewage. In the event of heavy rains that overwhelm treatment facilities, rainwater mixed with sewage flows untreated into Puget Sound and Lake Washington, threatening drinking water and harming wildlife such as salmon, killer whales and shellfish.

But Georgetown’s new treatment facility, which opened in November 2022, is treating some of the stormwater that becomes contaminated during the heaviest rains. The project is aimed at treating wastewater as winter storms are expected to last longer and become more intense due to climate change, said King County Executive Dow Constantine, who toured the plant Wednesday. He said it is one of several projects.

“This facility…was built to protect water quality for the next century,” Constantine said.

A 2017 study by Seattle Public Works found that extreme storms that dump more than 4 inches of rain in a 24-hour period have become more frequent since the company began measuring precipitation 40 years ago. I did.

The facility can process up to 70 million gallons per day and was activated approximately five times during heavy rain events. The Georgetown facility cost about $275 million and was designed to withstand flooding and 2 feet of sea level rise. Since opening, the facility has stopped approximately 32 million gallons of contaminated stormwater from reaching Puget Sound.

The facility was put to the test in December when an atmospheric river swept through the state, dumping nearly 10 inches of rain on parts of western Washington. This plant processed approximately 24 million gallons per day. The facility will only treat stormwater flowing from the Georgetown and Beacon Hill areas.

The water is first filtered through a metal screen that captures debris and flushable wipes to remove other solids from the water before being transferred to a settling tank. The water is then disinfected with ultraviolet light before being released into the Duwamish River.

According to King County, wastewater facilities treated a total of 35.4 billion gallons of stormwater and wastewater in 2022, and 1.67 billion gallons were discharged untreated.

While certainly an improvement over the 1960s, when between 20 billion and 30 billion gallons of untreated wastewater entered Puget Sound and Lake Washington, King County and Seattle utilities are attempting to further reduce that number. We are currently working on a project to do this.

King County and Seattle Public Works are building a 30 million-gallon tank to store untreated sewage and rainwater if the West Point treatment plant near Discovery Park becomes full. The project is expected to prevent 75 million gallons of wastewater from entering Puget Sound and Lake Washington each year.

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