Peninsula and South Bay customers of the region’s largest water system could be drinking fluoridated water if a new plant is built in the East Bay.

San Francisco Public Utilities Commission spokeswoman Beverly Hennessey said Tuesday that the agency’s fluoride plant on Polhemus Road near San Mateo is aging and needs to be upgraded, or moved to a new site.

Upgrading would cost $11.2 million, and relocating it to the Sunol Valley would cost about $10.1 million, she said.

Put simply, the plant built in 1972 becomes too concentrated with fluoride when the water system’s reserves get low, and the plant has to be shut down during these periods. If a new plant is built in the East Bay, it could fluoridate all of the PUC’s customers because of how the water flows, officials said. If the plant remains at its current site, cities south of San Mateo that are not fluoridated won’t receive the service, because it’s technically impossible.

So far, one closed meeting has been held on the topic, but a schedule of public meetings is being planned, Hennessey said. There is no timeline as to when this issue will be decided, she said.

“We’re not rushing this,” she said. “We’re also not advocating one way or the other if people’s water should be fluoridated.”

The San Francisco PUC, which draws its water from the Hetch-Hetchy system in the Sierra, serves 2.4 million people in San Francisco and 33 communities stretching from Milpitas and San Jose to Daly City and the water district in southern Alameda County.

About 60 percent of these cities already are fluoridated. But about 30 percent don’t receive any fluoride, including Sunnyvale, Milpitas, Mountain View, San Mateo, northern Santa Clara, Redwood City, Menlo Park, Atherton, Los Altos Hills and East Palo Alto.

Cities in the remaining 10 percent fluoridate their own water, which could become less expensive if they hooked up to a larger system, officials said.

Because the San Francisco agency has only begun to float the idea to rebuild or relocate its fluoride plant, it’s unclear if the cities that don’t get fluoridated water will want the new service, according to Art Jensen, General Manager for the Bay Area Water Users Association — a coalition made up of the majority of PUC customers.