Water rates across much of Atlanta's northern and western suburbs will increase by at least $3.25 monthly for the average family, beginning in October.

Blaming rapid increases in fuel and construction costs, the Cobb County-Marietta Water Authority on Monday voted for the increase, about 50 cents per thousand gallons. The increase includes a previously approved increase of 15 cents per thousand gallons that was to have taken effect next January.

Authority General Manager Glenn Page said construction costs have tripled in five years, at a time when Cobb's aging system needs to be upgraded and new federal water treatment regulations are taking effect.

"With the trends in energy costs, we expect further price increases. These are things that hit us hard," said Page, whose agency supplies systems from Douglas County around the northern arc to Mountain Park in north Fulton County.

Cobb is its largest customer.

A decrease in demand brought about by water conservation was responsible for " 25 to 33 percent" of the rate hike, Page said. Chemical supplies for water treatment were more of a problem. The price of alum, for example, has risen 75 percent in five years, he said.

Officials at the Cobb Water System, which supplies homeowners and businesses, could not immediately say how much retail prices would increase. When the wholesaler last increased its thousand-gallon price by 13 cents in January, the water system base charge went up 18 cents: an 8-cent previously approved boost and another dime in response to the wholesaler's increase.