TICONDEROGA — Construction work was partly to blame for a nearly 18-day discharge of millions of gallons of partially treated wastewater from the town’s treatment plant into the LaChute River.

The bypass from the wastewater treatment system ended Sunday, and is permitted as a combined sewer overflow discharge. The discharge occurred at 219 Montcalm St., down river of Bicentennial Park and the falls, and eventually flows into Lake Champlain.

Tracy Smith, chief wastewater operator, said about 15.596 million gallons released over that period, and was incorrectly reported Sunday on a NY-Alert notification as 35.58 million gallons.

“Quite frankly, the discharge is very, very, very diluted,” Smith said in a phone interview Monday. “If you looked at it, you’d think it was treated wastewater.”

The treatment plant is under construction until potentially next fall, leaving one of its clarifiers, a kind of settling tank used during the treatment process, offline. That was the main reason for the length of the discharge, Smith said, combined with heavy rain and snow melt over the past couple of weeks.

The plant treats nearly all of the stormwater coming from Ticonderoga, and flow rates can vary from about 500,000 gallons per day to 60 million gallons per day, Smith added. Some of the days during this latest incident, the discharge was a few gallons, while other days it was much more. Smith said they’re required to report all bypasses, no matter how small.

Smith said the plant does have stormwater equalization basins, which gave the overflow some treatment, including screening and grit removal.

The construction happening is in response to a state Department of Environmental Conservation order to incorporate disinfection to the treatment process to satisfy the plant’s long-term control plan.

Engineers are constructing an ultraviolet disinfection system, according to a NY-Alert, and Smith said they are also building new clarifiers to handle more flow. The completion date is set for October next year, though Smith said construction has been ahead of schedule and could be done sooner.

“This is a big upgrade right here, and again, they’re working,” Smith said. “They’re moving along as quickly as they can.”

DEC was notified of the overflow, and the agency said it continues to investigate.