An employee released 952,000 gallons of partially treated sewage — enough to fill 50 backyard swimming pools — by accident into the San Francisco Bay but the spill didn't cause any harm to public health or the environment, officials said.

The employee at San Jose's Alviso wastewater treatment plant turned the wrong valve Wednesday, releasing the partially treated water, the San Jose Mercury News reported Saturday.

Normally water from the toilets, showers, dishwashers, washing machines and other appliances of 1.4 million residents in eight South Bay cities flows into the plant, where it goes through three levels of treatment, San Jose's director of environmental services Kerrie Romanow said.

She also said the spilled wastewater had gone through two of the three levels and that most of the impurities had been removed.

"It's not raw sewage into the bay," Romanow said. "We are thankful for that."

The wastewater was heading to a building for the last treatment, but the filters there were being replaced and a staff member turned a manual valve. That valve was not labeled resulting in the partially treated wastewater flowing into Artesian Slough, which flows into the bay, she said.

The error was quickly discovered, she said, and the valve was closed 10 minutes later.

Plant officials took water quality samples near the outflow pipe, and the results showed that the wastewater did not exceed state or federal health limits, Romanow said.

Environmentalists said they are monitoring the situation.

"It's disappointing," said Sejal Choksi-Chugh, executive director of San Francisco Baykeeper, an environmental group based in Oakland. "When you have human error, it's really unfortunate. Ten minutes is a long time, and 950,000 gallons is a lot. But luckily it was secondarily treated, so it could have been worse."