The San Francisco Public Utilities Commission (SFPUC) is warning that tap water on the Peninsula and in the East Bay may turn cloudy between Dec. 5 and early January because of routine maintenance and seismic upgrades in the Central Valley.

According to the SFPUC, the change in the water is "only aesthetic, with water continuing to meet all state and federal regulatory requirements."

During the time of the maintenance work, customers will not be receiving water from the Hetch Hetchy Reservoir -- the SFPUC's usual source -- but from local reservoirs.

According to the water agency, the increased treatment of the local-reservoir water at the SFPUC's Sunol Valley Water Treatment Plant will cause air to be mixed into the water, which makes water appear "cloudy or milky."

The cloudiness dissipates if the water is allowed to stand for a few minutes, which causes the air bubbles to rise to the surface and dissipate.

Water officials expect water to return to normal when the work is completed in the week of Jan. 7.