Southwest Water Co. said Tuesday that the California Public Utilities Commission unanimously approved a multi-year rate increases for the company’s southern California service region.

The decision authorized a rate increase of 6.4 percent, which became effective in late August, and should add about $3 million to the Los Angeles-based utility company’s annual revenues.

Also approved were two more rate increases that will take effect over the next two summers, estimated at 2.9 percent and 2.6 percent respectively. The increases will mainly affect residential users; only 15 percent of Southwest’s revenue is derived from commercial users.

DeLise Keim, a spokeswoman for Southwest, said the revenue will be used to fund improvements to the company’s water delivery infrastructure, in addition to $21 million that already has been spent the past three years.

Shares in the utility company were flat at $12.61 in early trading Tuesday.