It cost $140 million and took 12 years to plan and build, but a Combined Sewer Overflow (CSO) project has been completed, King County officials announced last week.
CSOs result when water from rainstorms combines with sewage and overwhelms the handling capacity of the West Point sewage plant, leading to an untreated discharge into local waters such as Lake Union and Elliott Bay.
The problem, explains project manager Judy Cochran, is that city pipes were designed to handle both stormwater and sewage "back in the horse-and-buggy days of 150 years ago."
The result was that untreated wastewater was dumped between 10 and 115 times a year into Lake Union and up to 50 times a year into Elliott Bay, according to King County's Wastewater Treatment Division figures.
Federal regulations allow an average of four discharges of untreated wastewater a year, said King County Council member Larry Phillips. By contrast, state regulations allow for an average of only one a year, he said.
"That's the goal," added Phillips. "The point is, the system has to be moving toward that goal."
Nuts and bolts
The most visible element of the system is a brick building on Elliott Avenue West near the grain terminal, but it connects to a 6,000-foot-long, 14-foot, 8-inch tunnel leading to Lake Union, Cochran said.
The Frank Collucio Construction Company used a huge boring machine to dig the tunnel, which runs under Mercer and West Mercer streets. It can hold approximately 7 million gallons of wastewater, making it an effective holding tank during severe storms.
If the tunnel fills up, some or all of the six electric pumps located around 50 feet below ground level in the building will pump the wastewater through a treatment system.
The treatment system in the building screens out solids, adds a strong bleach to disinfect the water and neutralizes the bleach with sodium bisulfate before it is pumped out through two outfall pipes into Elliott Bay. One of the outfall pipes extends 400 feet into the bay, and one from an existing Denny regulator goes out 100 feet.
Metro estimates treated wastewater will be pumped into Elliott Bay from four to 20 times a year, and neutralizing the bleach is important because it can harm sea life, Cochran noted.
The wastewater will be treated onsite if the storm is still going on when the tunnel is filled to capacity. If the storm ends in time, the wastewater will be pumped to the West Point plant for treatment, she said.
The CSO facility also includes odor-control units on the exterior of the building, where large tanks containing carbon filters will remove the smell, Cochran said. "Judging from a recent tour of the facility, we can report that the system works well."
That's significant, according to Phillips, who has worked on wastewater issues for years. "It's right across the street from one of the most densely populated places in Seattle or King County," he said of Lower Queen Anne.
Bottom line
King County is responsible for the lion's share of the $140 million price tag, Cochran noted, but the city of Seattle will pay 17 percent of the cost, and the U.S. Environmental Protection Agency ponied up $35 million for the project. "It's very rare," she said of the federal grant.
The money will be paid on a long-term basis, so homeowners aren't in for any sticker shock, according to Phillips. "The difference in your rates is actually very small," he said.
The cost is worth it, according to King County Executive Ron Sims. "This joint project with the city of Seattle is one of the most significant public investments so far by King County," he said in a press release. "It's a sound investment for clean water."
Staff senior reporter Russ Zabel can be reached by phone at 461-1309 or via email at the address rzabel@nwlink.com.
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