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Don’t be alarmed if water you get from the San Francisco Public Utilities Commission looks a little cloudy or milky white beginning today.

Officials with the utility say the phenomenon — which may be noticeable over the next two months to many of their 1.7 million customers in San Mateo, northern Santa Clara and southern Alameda counties —is perfectly harmless.

“We just want people to know, it’s nothing to worry about,” said the utility’s spokeswoman, Maureen Barry.

Because of planned maintenance on the Hetch Hetchy Water System, the utility will increase the flow of water through its Sunol Valley Water Treatment Plant, resulting in an increase in bubbles that cause the cloudy appearance. Two main factors contribute to the bubbles, according to David Briggs, the utility’s water supply and treatment manager.

For one thing, he said, the Sunol plant’s water pipe isn’t as full as those used by the Hetch Hetchy system, which results in more air mixing with the Sunol plant’s water. Also, he said, because the Sunol plant’s output will be doubled during the maintenance, the water running through its pipe will be more turbulent, which also mixes air with the water.

Because the bubbles dissipate the farther they travel, the cloudiness should be especially noticeable to residents in southern Alameda and Santa Clara counties, who are relatively close to the Sunol facility. Santa Clara County cities served by the utility include

San Jose, Milpitas, Mountain View, Santa Clara, Sunnyvale and Palo Alto.

The maintenance is expected to be completed by Feb. 20. After that, the discoloration no longer should be noticeable, the officials said. They added that this is the fourth year such maintenance has been done and that despite some past complaints about the cloudiness, they know of no reports of anyone becoming ill from it.

Contact Steve Johnson at sjohnson@mercurynews.com or 408-920-5043.