An estimated 10 million gallons of untreated wastewater was discharged into Elliott Bay on Monday night from the West Point Treatment Plant in Seattle's Magnolia neighborhood.

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About 10 million gallons of raw sewage spilled into Elliott Bay on Monday night and Tuesday morning because of an equipment failure at the West Point treatment plant in Magnolia.

The overflow, during a three-hour period ending at 1 a.m. Tuesday, happened when a switch in a tank malfunctioned, opening a gate and diverting all raw sewage arriving at the plant directly into Puget Sound.

It took three hours to repair the switch so the gate would shut, said plant manager Pam Elardo.

County officials have closed the nearby beach and are taking water samples to determine the overflow’s effects.

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Elardo dispatched a crew of six to walk the beach Tuesday morning and said no waste was found on the beaches. “It’s not a discernible effect on the receiving water,” Elardo said. “But this is a big deal, it is not something that should be happening and we want to be sure it doesn’t happen again.”

The spill represents a fraction of the plant’s volume — it treats an average of 130 million gallons of wastewater a day from Seattle and surrounding communities — but nonetheless shocked clean-water advocates around the Sound.

“Obviously that’s bad,” said David Dicks, executive director of the Puget Sound Partnership, a state agency charged with protecting and preserving the Sound. “The input of raw sewage into Puget Sound is bad and we need to be very vigilant about how treatment plants operate and make sure they are fully operational at all times.”

The last major spill was during five days in June, when as many as 493,000 gallons of untreated sewage spilled into Puget Sound from a corroded Bainbridge Island pipe. In 1993, 8 million gallons of untreated sewer water spilled from an Edmonds plant onto streets, wetlands and finally into Puget Sound.

King County notified the state Department of Ecology at 10:40 p.m. Monday of the recent spill, said Mark Henley, permit manager at Ecology for King County wastewater treatment. “The department is very disappointed and concerned that this happened last night,” Henley said.

The malfunction happened when workers tried to engage a standby position for the emergency bypass gate at the front of the plant, which is routinely set during heavy rains. Instead, the switch malfunctioned and swung the gate open.

The plant management team will be dissecting the event, the crew’s response, and maintenance practices, Elardo said.

“In the context of how many pieces of equipment there are at this plant, it’s not surprising, things happen,” Elardo said. “But it is such a critical piece of equipment. I am not happy this happened.”

The county has five days to file a written report about what happened. Enforcement action, including fines of up to $10,000 per day and a regulatory order, could result from the spill. “Ten million gallons is a very large spill, it is unacceptable,” Henley said.

Neighbors of the plant were surprised by the news — and aghast.

“That is my front yard,” said Mary Beth Dols, a Magnolia resident. “I think it’s appalling.”

Lynda V. Mapes: 206-464-2736 or lmapes@seattletimes.com