Seattle to Overhaul Stormwater System

After more than four years of construction, King County celebrated completion Thursday of its newest and largest system for controlling overflows of untreated stormwater and sewage into Elliott Bay and Lake Union.

"These new clean-water facilities are typical of so much that we do," said King County Executive Ron Sims. "They won't be visible to most people and will be quietly doing the hard work of reducing pollution in our waters. But the value of this new system to water quality and its impact on our quality of life and aquatic life will live on and on.

"Because of these new facilities, Seattle's urban waterfronts will be cleaner and healthier for us to enjoy for generations to come," Sims said. "This joint project with the City of Seattle is one of the most significant public investments so far by King County. It's a sound investment for clean water."

King County's new Denny Way/Lake Union combined sewer overflow (CSO) control facilities will reduce controlled discharges during storms. They'll take place an average of only once a year at the Denny Way CSO outfall in Elliott Bay and several city and county outfalls on the south and east sides of Lake Union.

"I've been awed by this massive clean-water project -- the largest system for controlling combined sewer overflows in King County," Sims said. "It's an extraordinary new quintet of facilities that will work in harmony."

He described the new facilities:

* The Elliott West CSO Control Facility for managing the flow, storage and treatment combined wastewater and stormwater;

* A pair of pipelines ranging from 6 to 8 feet in diameter in Myrtle Edwards Park;

* A package of four pipelines tunneled and trenched south of Lake Union;

* Two outfalls up to 340 feet offshore and 60 feet deep in Elliott Bay; and

* A huge 14-ft-diameter tunnel running more than a mile under Mercer Street through the base of Queen Anne Hill.

"Our new system will work seamlessly with new and improved City of Seattle combined sewer pipelines," Sims said. "Together, our combined sewer systems will handle hundreds of millions of gallons of dirty water that flow from homes and business and off streets, parking lots and rooftops."

The new facilities will reduce both the volume and the frequency of untreated overflows. Until now, untreated CSOs discharged into Lake Union between 10 and 115 times a year, depending on rainfall and other weather conditions. And they discharged up to 50 times a year into Elliott Bay at Myrtle Edwards Park.

But with this new system, King County will store flows in the new Mercer Street Tunnel during small and moderate storms. After a storm subsides, the system will send the flows to the county's West Point Treatment Plant for treatment when capacity is available.

When the new Mercer Street tunnel completely fills with combined flows during major storms, the new CSO Control Facility will screen, disinfect and dechlorinate the flows. Those treated flows will discharge four to 20 times a year. Untreated overflows will take place an average of only once a year at each remaining outfall.

"Most of the new Denny Way/Lake Union facilities are underground or under water -- and out of sight," Sims said. "But they won't be out of mind -- and forgotten -- by the people who planned, designed and built them -- and by the people who operate and maintain them."

He noted that some people have dedicated nearly 15 years of their careers to planning, designing and building the new system for controlling overflows.

The U.S. Environmental Protection Agency awarded King County a $35 million grant to reduce the direct cost of the $140 million county project to sewer system ratepayers.

"This project is another example of our growing experience and expertise in building tunnels to carry wastewater with minimal impact on the communities above," Sims said.

Metropolitan King County Council Chair Larry Phillips, who represents the communities affected by the CSO project, called the facilities a triumph of cooperation that crossed jurisdictional boundaries.

Source: King County