SOUTHWEST HARBOR — Satisfied that a series of modifications to the treatment process are reducing the levels of contaminants in the town’s public water supply, selectmen on Tuesday gave the approval for engineers to proceed with the next phase of their plan.

The 4-0 decision followed a presentation by William Olver and Annaleis Hafford of Olver Associates, the Winterport engineering firm working on the project.

Southwest Harbor, which gets its water from Long Pond, has been seeking to reduce levels of haloacetic acids (HAAs) and trihalomethanes (THMs) in the water since 2004. That’s when the Maine Drinking Water Program notified town officials that levels of the two contaminants exceeded allowable limits. The contaminants form when chlorine used in the disinfection process reacts with organic matter in the water.

Olver Associates was hired in 2006 to study the problem and recommend a solution. The town currently has a consent order with the state allowing the water treatment plant to continue to operate while the problem is being addressed. The engineering firm recommended a phased approach to tackling the issue, beginning with operational changes at the plant.

Those changes, which include eliminating the use of acid and reducing the amount of chlorine during the treatment process, have substantially reduced the levels of HAAs, bringing them into compliance with state and federal standards, Mr. Olver said. There has been a moderate reduction in the levels of THMs but they are still above acceptable limits.

As the next step, Mr. Olver is recommending that an aeration system be added to the treatment plant. The cost of installing the system is estimated at $230,000. However, Mr. Olver cautioned that the estimate is based on 2007 figures; he said the costs are now likely to be about 30 percent greater.

THMs can easily be transformed into chloroform, which is readily removed through the aeration process, Mr. Olver explained. This step might bring the water into compliance without other measures being taken, he said. He suggested the town officials meet with the state officials in August to discuss implementing the aeration system.

Town manager Robin Bennett asked if any grants were available to pay for the system.

Probably not, Ms. Hafford responded, but there may be low-interest loans available through the Maine Drinking Water Program.

Opinions about the health effects of drinking water containing HAAs and THMs vary. Several studies suggest that consuming large quantities of water containing THMs can cause cancer. The Environmental Protection Agency (EPA) states that both contaminants increase the risk of cancer and, in the case of THMs, can lead to problems with the liver, kidneys and central nervous system.

Town officials have noted that levels of the contaminants have remained basically the same over the past several years. The town has only been out of compliance after the EPA adopted new standards in 2004.