SAN JOSE — An employee who turned the wrong valve at San Jose’s wastewater treatment plant this week sent 952,000 gallons of partially treated sewage — enough to fill 50 backyard swimming pools — into San Francisco Bay.

But the accident apparently did not cause any harm to public health or the environment because most of the pollutants already had been removed before the wastewater reached the bay, state water regulators and city officials said Friday.

The spill occurred from 8:19 a.m. to 8:29 a.m. Wednesday, said Kerrie Romanow, San Jose’s director of environmental services.

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 Alviso plant. The wastewater undergoes three levels of treatment: “primary,” which removes 50 percent of what wastewater engineers call “suspended solids”; “secondary,” which removes 95 percent of the solids; and “tertiary,” which removes 99 percent.

The spilled wastewater Wednesday already had undergone primary and secondary treatment, Romanow said, so most of the impurities were gone. It was heading to a building where it was to receive tertiary treatment, but the filters there were being replaced, she said, so rather than the usual computer-operated system of wastewater routing, a staff member turned a manual valve. That valve was not labeled, she said, resulting in the partially treated wastewater flowing into Artesian Slough, which flows into the bay.

“It’s not raw sewage into the bay,” Romanow said. “We are thankful for that.”

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 when the results came back from the lab Friday afternoon, they showed that the wastewater did not exceed state or federal health limits.

No dead fish were observed, Romanow said, adding that the plant discharges roughly 90 million gallons a day of treated wastewater into the bay. So the wastewater in Wednesday’s mishap represented just 1 percent of that.

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.”

Choksi-Chugh said the San Jose-Santa Clara Regional Wastewater Facility has not had a history of pollution problems in recent years.

Few wastewater plants treat to tertiary levels. San Jose’s is required to by state and federal authorities because it discharges into very shallow parts of San Francisco Bay without much tidal action.

Ironically, state water regulators said Friday, it was good that the spilled wastewater had not yet been treated with chlorine. That’s because chlorinated water kills fish.

Typically, the San Jose plant and other sewage treatment plants mix chlorine into treated wastewater to kill pathogens that can cause diseases like cholera or dysentery. Before releasing it into the bay, a river or the ocean, they routinely mix it with other chemicals that neutralize the chlorine so it does not harm fish.

Highlighting that issue, officials at the San Francisco Bay Regional Water Quality Control Board on Friday announced a $1 million settlement with the California Water Service Company to close a case in which one of the company’s 12-inch drinking water pipes broke, spilling 7.2 million gallons of chlorinated water into San Mateo Creek in October 2013. That spill killed more than 200 fish, including roughly 70 steelhead and rainbow trout, said Dyan Whyte, the board’s assistant executive officer. The company will be required to spend $500,000 replacing old pipes in the area, she said.

San Jose will likely face some kind of fine for this week’s spill, but it could take a year for the case to be closed, said Bill Johnson, a senior environmental scientist at the board.

“Our take on it is that it is a really large volume of water, but so far it doesn’t seem like it has done a whole lot of harm,” he said. “We’re pleased that they got it under control quickly.”

Paul Rogers covers resources and environmental issues. Contact him at 408-920-5045. Follow him at Twitter.com/PaulRogersSJMN.