Operator error was the cause of an 8.7 million-gallon spill of raw sewage from the West Point Treatment Plant the night of Dec. 14, county and state reviews have found.

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Operator error was the cause of an 8.7 million-gallon spill of raw sewage from the West Point Treatment Plant in Seattle the night of Dec. 14, county and state reviews have found.

The state Department of Ecology announced Wednesday it has fined King County $24,000 for the error. The county operates the plant under a permit from the state.

The trouble started when plant employees readied a bypass gate to open if needed in an emergency during high flows in rainy weather. But an electrical problem caused the gate to open rather than remain on standby.

Raw sewage was diverted around the plant, located adjacent to Discovery Park, and piped directly into Puget Sound for two hours and 52 minutes.

Workers should have used a backup system to close the gate within minutes, but instead took nearly three hours to troubleshoot the problem and close the gate, according to the Department of Ecology.

In a written statement released Wednesday, King County Executive Dow Constantine concurred with the findings. Constantine also called for reassignment and retraining of some plant employees and a “no-excuses” policy for spills.

The North Beach recreation area near Discovery Park was closed after the spill. Lab samples showed water quality improved within days of the overflow and the beach was reopened four days later.

The West Point facility opened in 1966 and cleans an average 130 million gallons of wastewater a day for Seattle and several other cities and sewer districts in North and Central King County.

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