Waterford — An increase in costs associated with treating the approximately 1 billion gallons of wastewater that pass through the town’s sewer system each year was the subject of a public hearing Tuesday.

A 63.5 percent increase, or $684,598 increase, in wastewater treatment costs has prompted the town to pursue its first raise of sewer rates in three years. In 2011-12, the town spent $1.7 million in treatment costs but budgeted $1.3 million.

“We underestimated what we thought the volume and the cost of treatment would be,” Utility Commission Chairman Peter Green said.

The town’s 6,812 active residential water customers may see their bills increase beginning in January if the measure is approved by members of the town’s Representative Town Meeting next month.

Waterford has a rate fee structure based on a two-tier charge system. The first tier is a $45 per quarter, or $180 per year, facility service charge that would not change under the proposed rate increase, Green said. The second tier, based on water consumption, would increase from $3.50 per 100 cubic feet to $3.85 per 100 cubic feet.

Green said “typical consumption” is about 65,000 gallons of water, or about $304 a year. Currently, the average cost to a family is about $414.14 a year. Under the proposed rate, that amount would increase to about $514.56 per year, a 6.28 percent increase.

Copies of the proposed changes are available in the Town Clerk’s office and in the office of the Waterford Utility Commission.