Remaining work for an expansion to the Forsyth County Water Treatment Plant added cost to the contract, but the project remains under budget.

The Forsyth County commission approved an extra payment Tuesday of about $372,000 for Garney Construction to complete the facility’s expansion on Antioch Road and to repair parts of the old portion.

The last anticipated change order brings the total to about $20.3 million, which keeps the project under the $21 million budget, said Donna Kukarola, county procurement director.

Of the change order approved Tuesday, the funding broke down to about $117,000 extra for the expansion and about $255,000 for work at the older portion of the facility, Kukarola said.

“Each of these items has been vetted and looked at very closely,” she said.

The project funding came from the county’s water and sewer fund, which operates from revenues generated by users of the system.

Work on the plant began after commissioners accepted a proposal from Garney in January 2010 for about $19.8 million.

The expansion brought the plant’s maximum treatment capacity from 14 million gallons per day to 26 million gallons per day.

The plant expansion has been operating for several months, and the change order approved Tuesday is expected to cover the remaining work, said Tim Perkins, water and sewer director.

Those tasks are estimated to take about three months to complete.

Perkins said the recent change order tasks were related to “things we probably should’ve put in the original design.”

Perkins said the biggest additional expense, at about $33,000, is for installation of “magmeters,” instruments that measure the amount of backwash recovery.

“We have the ability to recover our backwash water and recirculate it to our 20-million-gallon tank and not discharge it to the stream, so we take less water out of Lake Lanier.

“Our withdrawal will go further because we’re recycling that water,” Perkins said, “but to measure that flow, we don’t have any meters … so our numbers are off.”

Keeping records accurate will help the staff detect any water leakage, he said.

Perkins said the “big dollar item” for the older plant is about $117,000 to replace plate settlers that had eroded away, causing sediment buildup.

“We’re very lucky the new plant’s online,” he said. “I don’t think the old plant would’ve made it through this summer and we would have had trouble meeting our water demands.”