FALMOUTH — The unexpectedly high cost of replacing an important sewer main could leave the town on the hook for at least $96,999 in extra expenses.

The increase means the Town Council may have to make a supplemental amendment to an order it approved in January to borrow $5.9 million to pay for the project, according to Town Manager Nathan Poore.

The council was scheduled to discuss the sewer upgrade at its meeting Monday. As of Monday afternoon, town staff members were still working on updated financial figures, Poore said, but the net impact on ratepayers could be less than predicted.

“It is a work in progress right now. At the end of the day, over the course of the loan it will be somewhat minimal,” he said.

Falmouth is in the middle of replacing the Mill Creek pump station and force main. The Mill Creek pump station is the largest and most important station in the wastewater system and processes flow from residential and commercial areas on routes 1 and 100 and the town of Cumberland. That town’s ratepayers will pay 44 percent of the project’s costs.

The system, constructed in 1971, is outdated and has insufficient pumping capacity, according to an environmental assessment from the Maine Department of Environmental Protection. Heavy rains cause the system to back up and overflow directly into Mill Creek, according to the report.

In January, the council approved the replacement project with a cost limit of $5.9 million. Most of the project will be financed with a low-interest loan, but the town may use $1 million from uncommitted wastewater enterprise funds to pay for the project.

In April, the town implemented an 8.5 percent sewer rate increase to pay for the project, increasing residents’ monthly minimum charges to $40.44.

While the pump station portion of the project came in within budget, the force main part was well in excess of engineering estimates, according to a July memo from the Wright-Pierce engineering firm.

Shaw Brothers Construction, a company based in Gorham, was the only bidder for the force main project. The high cost of the force main replacement will increase the total project cost from $5.8 million to $6.2 million, according to a memo to the Town Council.

The original bid was considerably higher, but Shaw Brothers brought the price down by proposing to use a different type of pipe and lower its traffic control costs.

In its July memo, Wright-Pierce said the town could rebid the project, but it was “impossible to guarantee a lower bid cost” because of unknown factors and the tight bidding climate. Other companies hadn’t bid on the project initially because they were unavailable to complete the work this year, the engineering company said. It would cost about $45,000 to delay the force main construction and rebid the project, it said.