Millions of gallons of untreated wastewater and stormwater began dumping into Puget Sound Thursday after high tides and heavy rains overwhelmed a King County wastewater-treatment center in Seattle.

Flooding at West Point Treatment Plant in Magnolia’s Discovery Park caused damage that apparently fried an electrical circuit and triggered a system shutdown, a spokesman said.

That has caused the county to operate the facility much of Thursday in “emergency bypass mode” — dumping untreated effluent directly into Puget Sound.

Officials were still calculating how much untreated wastewater had flowed into Puget Sound. Doug Williams, a spokesman for the county’s Department of Natural Resources and Parks, estimated more than 150 million to 200 million gallons, with that number likely to grow.

By Thursday night, Williams said the plant was partially back on line and was providing initial treatment to some of the water which had been flowing untreated into the Sound or diverted to other treatment plants.

The dumped sewage is a mix of about 90 percent stormwater and 10 percent wastewater, he said.

The county has managed to divert nearly 200 million gallons of sewage water headed for West Point to four other treatment facilities, Williams said.

Chris Wilke, executive director of Puget Soundkeeper, an environmental watchdog group, said the amount of untreated sewage dumped so far comprises about one-fifth of the typical overflow amount for the area’s sewers annually.

“So this is a very significant event,” Wilke said. “This is alarming.”

The shutdown at the plant happened about 2 a.m. Thursday, at a time when the facility was receiving peak levels of stormwater and wastewater from sewer discharges amid heavy rains, Williams said.

That surge of water was being treated and discharged normally from the plant. But at the same time, high tides off Discovery Park were “pushing the treated effluent back into the outflow pipes,” Williams said.

“As a result, we ended up with a flood of wastewater and stormwater into the facility itself, and at the time we had a circuit break,” he said. “A portion of the system went offline, so we have been bypassing flows since then.”

The untreated water is being piped about three-quarters of a mile offshore and 240 feet deep into the Sound, Williams said.

The department posted signs along the beach to alert people to stay out of the water, as well as contacted the state’s Department of Ecology.

Williams said crews also would conduct water-quality sampling to assess any environmental impacts.

Such large sewage discharges can pose health risks and other environmental concerns, Wilke said.

“It’s good that the discharge is happening at depth,” he added. “That reduces some of the risks.”

Repair crews worked to pump out the treatment plant throughout the day Thursday.

The West Point Treatment Plant serves homes and businesses in Seattle, Shoreline, north Lake Washington, north King County and parts of south Snohomish County. During the area’s rainy season, West Point provides secondary treatment for flows up to 300 million gallons a day, according to the plant’s webpage.

“I think this is a reminder that we need to have contingencies designed into our wastewater systems to prevent these kinds of things from happening,” Wilke said.