Co-Digestion Is a Key Component in Montpelier's Quest for Net Zero Energy Status

Septage and high-strength food processing waste provide a substantial source of renewable energy for the clean-water plant in Vermont’s capital.

Co-Digestion Is a Key Component in Montpelier's Quest for Net Zero Energy Status

The biogas goes directly to dual-fuel boilers. The Montpelier plant produces more biogas than it can use in summer. The excess is flared (lower right of the photo), but the staff is developing a project to use more of the gas.

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Taking in food waste has enabled a Vermont clean-water plant to upgrade its biosolids dewatering equipment and produce enough biogas to nearly eliminate fuel oil for heating buildings and digesters. It also helps the host city toward its goal of net-zero energy.

The project at the Montpelier Water Resource Recovery Facility also included the addition of a receiving station for high-strength organics.

The three digesters were also upgraded. One was converted to a blend tank to feed internally generated sludge and high-strength organic waste to the two other digesters. The blend tank and one of the digesters were fitted with new steel covers and linear motion mixers (Ovivo) and the third digester received a dual-membrane cover (also Ovivo) and a Vaughan Rotamix mixing system.

In 2021 the American Public Works Association honored the project as the Rural Communities Project of the Year/Environment. 


Even before it started accepting food waste, the Montpelier facility was handling a lot of solids. The plant takes in about one third of all septage in Vermont, which has a high percentage of its population on septic tanks. It also takes in sludge from other municipal treatment plants as well as landfill leachate.

“Our average flow is about 2 mgd, but because of the solids we take in, we actually operate like a 10 mgd facility if you look at how much solids we produce each day,” says Christopher Cox, chief operator.

The septage and imported sludges go directly to two new incline screw presses (BDP Industries) for dewatering. The old belt presses produced material 16-18% solids; the new ones achieved 26% solids.


“We really pushed to have the presses be fully automatic,” says Cox, whose plant operates with a staff of four. “It’s like a push of a button. We can run them into the night. Whoever is on call checks them once a night. We were running both belt presses eight hours. Now we’re running two screw presses 12 to 16 hours a day.”

The dewatered material goes to landfill, and the filtrate returns to the headworks. The digesters are reserved for the high-strength organic waste and the sludge produced on site.

“There’s not a lot of organics in septage compared to FOG, dairy waste or brewery waste,” says Cox. “Those have much higher organic concentrations. We’re trying to make gas to offset fuel oil, so we don’t want to waste any of that space digesting septage. It dewaters very well. It’s a very fibrous material.”

Tipping fees for receiving septage, sludges and leachate bring significant revenue, which increased from about $1 million a year to $1.25 million in the first full year of taking in waste from food processors, dairies, breweries and ice cream plants. The material boosted biogas production significantly. Previously the plant produced only enough to heat the digesters. Now it heats all the buildings as well.

The biogas goes directly to two dual-fuel boilers without refinement. “There’s no treatment in between,” says Cox. “All it needs is pressure and a good concentration of methane, which comes from a well-run anaerobic digester. We’re not trying to strip out siloxanes or other things you normally remove when trying to make power.

“The simplicity of burning gas with no conditioning is pretty nice. Once you start cleaning the gas, you have a whole other process. The break-even point for a generator didn’t make sense for the city.” In winter all the gas is used to heat the digesters and buildings. In warmer weather the digesters produce excess gas, which at present is flared.


Montpelier has committed to becoming a net-zero energy city by 2030, so flaring excess methane is not in the long-range plan. The aim is to add a new process to use the methane but also to be flexible, in case methane production should drop.

“Not flaring is a huge priority,” Cox says. “But if we can’t figure out how to not flare, do it economically and have the correct payback, it’s not worth doing. We’re not going to do it and lose tons of money.”

The city considered using the excess methane to fuel additional biosolids drying and make a fertilizer product, but land application is difficult now because of regulation of emerging contaminants, such as PFAS.

“At first we were all excited,” Cox says. “We thought farmers could use it and maybe we could sell it. But that was a few years ago. The climate of managing biosolids in New England is just crazy now because of PFAS.  Land application sites are going away.”

Now the city is considering a process to gasify the sludge. The excess methane could preheat the dryer for that process, which would significantly reduce the volume of solids landfilled. “Everyone’s competing to get material into the landfills,” Cox says. “Anything we can do to reduce the amount of solids we produce will be a huge benefit.”

Cox, who has a degree in environmental science from Colby-Sawyer College in New London, New Hampshire, has worked in the wastewater treatment industry for 10 years: “I got into the field thinking my job was to protect the environment by keeping the water clean, which is the main goal. But a lot of my job is figuring out how to get all the energy out of the waste we take in. It’s exciting.”  


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