The City Council on Tuesday approved borrowing up to $2.5 million for upgrades to Belfast's wastewater treatment plant. The overhaul is expected to result in dramatic energy savings over the next 10 years with little or no change to sewer rates.

The work, recommended after an energy audit by Winterport-based Olver Associates, would include converting the plant's aeration system, replacing a sludge dewatering machine, upgrading a non-functioning ventilation system and replacing the boiler that heats the building.

The changes are designed to increase energy efficiency, which is significant because the plant is the city's largest energy user.

The 20-year federally subsidized Maine Department of Environmental Protection loan that would pay for the project would carry an annual payment of $145,000, of which about 42 percent will be offset by yearly energy savings, according to Bill Olver of Olver Associates, who presented the plan to the City Council Oct. 2.

The remaining $83,000 per year would come due at about the same time a $90,000 annual debt payment is set to come off the books. As a result, the wastewater treatment plant, which draws its operating budget from sewer fees, will see a savings of about $7,000 per year.

"It's basically a net zero to the bottom line," Olver said.

Within most plants, aeration blowers account for the lion's share of energy use, according to the energy audit report. The machines force air through a membrane to create bubbles that transfer oxygen to the bacteria responsible for breaking down solid waste.

Belfast's antiquated coarse-bubble blowers — a concession to oily water coming from the Stinson sardine cannery at the time — would be replaced by fine-bubble blowers that distribute oxygen better and use less electricity.

Replacing the aerators would add $150,000 (or 21 percent) to the cost of that part of process over the next 10 years. But any short-term gains from keeping the old system would be fleeting, Olver said, because waiting 10 years would add $150,000 to the conversion alone and another $500,000 accounting for the energy used by the old boiler during that time.

Workers will have cleaner air to breathe after the renovations. The plant's 50-year-old system is no longer operating and wouldn't meet current safety codes if it were, according to the report. The planned replacement is expected to save $428,000 over its 20-year life, owing to a feature that recovers 90 percent of heat from air before it leaves the building.

A 25-year-old belt press that removes water from sludge would be replaced with a screw press that removes almost twice as much water and uses a fraction of the electricity. The denser sludge produced by the new million-dollar machine would lower the cost of trucking it to Hawk Ridge Compost in Unity, saving $40,000 (or 44 percent) per year.

Finally, the oil boiler that heats the plant would be replaced with a wood pellet boiler. While this upgrade would make it slightly more expensive to heat the building — up $20,000 (or 5 percent) over the next 20 years, including the cost of the new boiler — the city's Energy and Climate Committee backed the switch to wood on grounds that oil prices could go up and the city is trying to reduce its fossil fuel use.

Olver said the upgrades are likely to start in late 2019 or early 2020.

Editor's note: This article has been updated to correct the net cost of converting to a wood pellet boiler.