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Lots of farms have biodigesters. But how do they work, and do they always cut emissions?

Cows release a super potent greenhouse gas when they burp and also when their poop breaks down. It’s called methane and a lot of farms in Vermont and around the country are trapping it to make electricity or natural gas.

Vermont Public partnered with NOVA to find out how it all works.

The James family has been milking cows — and bottling that milk — at their farm in Weybridge, Vermont since 1930. Today, they milk about 540 Holsteins, and the fourth generation is getting ready to take over the business.

On a chilly spring day, Bob James gave us a tour.

“It’s a free stall farm, so cows can get up, walk around, eat, drink, sleep whenever they want,” he said, as we walked through one of the barns.

Even though this is a small farm by national standards, all of those cows produce a lot of waste. But at Monument Farms, that waste has a second and even third life. They use it to make electricity and bedding for the herd.

The milking parlor at Monument Farms.
Abagael Giles
Vermont Public
The milking parlor at Monument Farms.

That’s thanks to something called an anaerobic digester.

“A digester is … it’s like a pet, in a way. I tell people and they don’t understand,” James said. “But it’s a living thing in there.”

How a biodigester works

As much of the cow manure as possible gets scraped from the barns and fed into a big airtight tank.

“Ours is a concrete tank, like an underground swimming pool that's capped off, there's no oxygen in it,” James said.

It takes about a month in there for naturally occurring bacteria to eat up the bits of haylage and grain in the manure that the cows’ stomachs missed. In fact, the process is very similar to what happens in a cow’s stomach.

All that eating produces a methane-rich gas called biogas.

The gas gets siphoned from the tank into a very cold pipe. It cools really quickly, and all the water in the gas condenses, so it drops out like dew.

“The gas then goes through a pressurized pump that feeds, in our case, the engine,” James said.

That engine burns the gas to make electricity, which gets sent into the grid. In return, Monument Farms gets a credit on their electric bill.

The electricity is considered renewable — in Vermont and by federal standards.

To be clear, that doesn’t mean biodigesters that are used to create electricity don’t also create planet-warming emissions.

Biodigesters, like the one at Monument Farms, take one super potent greenhouse gas — methane from cow waste — and burn it to make a less potent greenhouse gas — carbon dioxide — when they make electricity.

Abagael Giles
Vermont Public
Burning biogas captured from manure creates carbon dioxide and water.

In the short term, methane has 80 times the warming power of carbon dioxide.

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The climate impacts

Before Monument Farms had a biodigester, they let their manure sit in a lagoon, where bacteria broke it down into nitrogen-rich fertilizer in the open air. Methane that now gets trapped in the digester was going into the atmosphere.

Joe Rudek is a scientist with the Environmental Defense Fund.

“Studies have shown that given the potential that we currently have for reducing methane from oil, gas and from livestock, we could avoid a quarter of a degree Celsius of warming by mid-century, which is very significant,” Rudek said.

According to the Environmental Protection Agency, agriculture as an industry is responsible for almost as to much of America’s methane emissions as fossil fuel extraction and processing, and coal mining.

States like California and Oregon, as well as Vermont, are trying to make it profitable to capture methane.

Treating methane as an energy source is one way to do that.

But some experts say there’s a risk in that, because you might incentivize farmers to farm in a way that produces more methane rather than less.

Tom Richard is a professor of agricultural and biological engineering at Pennsylvania State University. He says ideally, these policies would result in a net reduction in methane emissions.

“If we're trying to mitigate climate change, and people are investing money, whether that's through incentives for renewable energy or, or carbon sequestration in a forest or some other form, they want to make sure that that new funding and that investment, and that incentive, is actually getting people to do something different than they would have done otherwise,” Richard said.

Scraping the manure out of the barn for the digester at Monument Farms.
Abagael Giles
Vermont Public
Scraping the manure out of the barn for the digester at Monument Farms.

“Right now the economics are really driving it to injection into pipelines,” said Rudek with the Environmental Defense Fund.

Here’s what that means. At some farms, gas produced by the digester is refined on site. At that point, it’s almost pure methane and is injected into a natural gas pipeline.

Rudek says there’s a risk to the climate if farms decide to get bigger in order to send more methane to gas pipelines, as opposed to harvesting methane from waste that would previously have gone into the atmosphere.

“Natural gas, remember, is a bridge fuel, and we need to get over that bridge,” Rudek said. “And whether it’s renewable natural gas or fossil natural gas, we need to get to the other side of that bridge.”

But aside from reducing our methane emissions by doing things like helping farmers adapt their work and how they feed their cows, right now, there just isn’t a better way to deal with that methane than burning it.

Scientists are working on that, and Richard in particular is optimistic that there are better options than burning methane on the horizon.

The digester at Monument Farms is a bit like a big, underground concrete swimming pool -- but with no oxygen.
Abagael Giles
Vermont Public
The digester at Monument Farms is a bit like a big, underground concrete swimming pool -- but with no oxygen.

But as some states look toward helping more farms capture methane from waste, he says they need to make sure these new policies don’t exclude smaller farms — or ones that don’t have access to a pipeline.

Costs and benefits for small farms

For small farms, it can be too expensive to hook up to a natural gas pipeline — and right now, that’s where the money is in this burgeoning industry.

“In Vermont, and here in Pennsylvania as well, we've got a lot of smaller farm operations, smaller dairies,” Richard said. “And the incentives for them actually have to be even higher to get them to be able to afford to upgrade their manure management systems. And so thinking about: what are the right timescales for these incentives? And how do we actually structure them so that the distribution of profits is appropriate for different sizes of farms? Those are things which we haven't really taken on in the policy arena yet.”

At Monument Farms, they burn their biogas onsite to make electricity, which goes back into the grid.

Some environmentalists, like Rudek at the Environmental Defense Fund, say this is the gold standard.

Most months, the digester at Monument Farms makes enough power to offset all of the farm’s electric bill, including the electricity for the bottling plant.

And there’s another benefit: they squeeze the mostly sterile manure through a screen to make bedding for their cows. The extra they sell to a potting soil company in Middlebury.

Bob James of Monument Farms. They've been capturing the methane from their manure to make electricity for years now.
Abagael Giles
Vermont Public
Bob James of Monument Farms. They've been capturing the methane from their manure to make electricity for years now.

New federal incentives on the horizon could help farms like Monument get paid more for the electricity they make.

And Vermont’s Public Utility Commission is designing new regulations through the Affordable Heat Act, that — if approved by lawmakers — could incentivize utilities to buy more natural gas from digesters in Vermont.

Bob James says the digester at Monument Farms has been helpful for business, but it’s not incentivizing them to grow and make more methane.

“It's not like we're making money, but we're not spending the money,” he said. “So that's a nice savings.”

In the end, it’s Vermont’s milk market that shapes their business size.

NOVA's national Climate Across America initiative has been made possible by the Corporation for Public Broadcasting, deepening its efforts to build the capacity of local PBS stations to tell climate solution science stories that have an impact in their communities.

Abagael is Vermont Public's climate and environment reporter, focusing on the energy transition and how the climate crisis is impacting Vermonters — and Vermont’s landscape.

Abagael joined Vermont Public in 2020. Previously, she was the assistant editor at Vermont Sports and Vermont Ski + Ride magazines. She covered dairy and agriculture for The Addison Independent and got her start covering land use, water and the Los Angeles Aqueduct for The Sheet: News, Views & Culture of the Eastern Sierra in Mammoth Lakes, Ca.
As a Producer, Mike helps cultivate and develop stories from and about our community for visual presentation. His 20 years of technical experience as a Director and Editor enables him to help deliver our content across multiple platforms to connect our stories to as many folks as possible.

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