These Manure Digesters Incorporate Food Scraps. Does That Make Them Better? | New England Renewable Energy May Get a Boost from Dairy Manure

These Manure Digesters Incorporate Food Scraps. Does That Make Them Better?

The technology allows small and mid-sized dairy operations to lower their carbon footprints, earn extra money, and create organic fertilizers for farm use, though many worry about the PFAS contamination in food waste.

A view of Longview farm. The tank in the foreground stores the fertilizer produced by the co-digester. (Photo credit: Meg Wilcox)

A view of Longview farm. The tank in the foreground stores the fertilizer produced by the co-digester. (Photo credit: Meg Wilcox)

A bright blue tanker truck rumbles up the road to Longview Farm, a dairy operation in western Massachusetts. The driver, Luke Page, hops down from the cab and wrestles to connect a fat hose from the truck to pipe protruding from the ground. The pipe leads to a food waste collection tank and, after securing the hose, Page twists open a valve and begins pumping 2,000 gallons of spent cooking oil into the tank.

The waste grease, collected from a local pizzeria, a Mexican restaurant, and a pub, will be mixed with manure in the dairy farm’s anerobic co-digester and converted into renewable energy. The system generates nearly 6,000 megawatt hours of renewable electricity annually—enough to power 550 homes per year.

Methane gas digesters are used by dairy farms to convert manure into energy and reduce their greenhouse gas (GHG) emissions. The addition of food waste in co-digesters enables small and mid-sized dairies like Longview, which has 600 cows and doesn’t produce enough manure for a traditional methane digester, to access the technology. The approach allows them to lower their carbon footprints, earn extra revenue, and tap into methane digesters’ other benefits, which include the creation of organic fertilizers, also called “digestates,” for use on the farm.

Co-digesters are designed for dairies with anywhere from 75 to 2,000 cows and may be an improvement over methane digesters built on top of large waste lagoons on centralized animal feeding operations (CAFOs). Those systems are expensive, often dangerous for workers, and are associated with a bevy of problems, including propping up industrial-scale agriculture.

Longview Farm’s co-digester is one of six in New England owned and operated by Vanguard Renewables, a U.S. pioneer of the technology. Vanguard was acquired last year by BlackRock, one of the world’s largest investment managers, and has big plans to build dozens more co-digesters across the U.S.

Though farmers have long added organic wastes to their methane digesters, Vanguard’s concerted efforts to develop anerobic digesters that handle both food waste and manure for smaller dairies is a relatively new development in the U.S. The technology offers an alternative approach to turning dairy waste—and food waste—into renewable energy, but questions remain about whether it can scale up and still benefit small dairies and the environment simultaneously. PFAS contamination in food waste getting spread on farm fields is a potential concern.

Co-digestion has “been going on in Europe for 30 years,” while the U.S. is just ramping up the technology, said John Hanselman, chief strategy officer and cofounder of Vanguard Renewables. “We’ve kind of cracked the code [of the logistics, technology, and economics for the U.S.,] and are hoping to make it a huge part of America’s lifestyle.”

Matthew Lillie of Vanguard Renewables stands at the top of the co-digester, which is entirely underground. The fertilizer produced by the co-digester is stored in the tank behind it.

Matthew Lillie of Vanguard Renewables stands at the top of the co-digester at Longview Farm. The fertilizer produced by the co-digester is stored in the tank behind it. Photo by Meg Wilcox.

Longview Farm’s Closed-Loop System

The co-digester at Longview Farm is an enclosed tank buried 16 feet underground. Only the top of the 600,000-gallon tank is visible. Standing at its edge, Denise Barstow Manz says the co-digester works like a stomach: Microorganisms convert the carbs, fats, and proteins in the wastes into methane gas, which is stripped of air pollutants like sulfur before it’s burned to create electricity.

Barstow-Manz, the education director at Longview Farm, is part of the seventh generation to farm the land that has been in her family since 1806. She recently had her first child and now devotes her time to engaging with the public, while her father, uncle, and cousins run the day-to-day farm operations.

The Barstows installed the co-digester in 2013, following years of unstable milk prices. Barstow Manz was in college then. “It was one of the things that made me want to come back” to the farm, she says.

Roughly three-quarters of the 38,000 tons of waste digested by the system annually comes from area food manufacturers, restaurants, and grocery stores.

Cabot Creamery, a regional dairy cooperative, delivers its manufacturing waste to the co-digester, and has created something of a closed-loop relationship with the farm. Longview sells all its milk to the dairy processor, which produces butter and powdered milk at its nearby facility in Springfield. Cabot not only recycles its processing waste at the farm, but it also purchases the bulk of the energy the co-digester produces through renewable energy credits, buying enough electricity to power its entire butter-processing operation.

“That’s sort of the holy grail; the commercial side of our business and the cooperative side of our business are helping each other in a way that’s dynamically beneficial for both,” Jed Davis, director of sustainability at Cabot Creamery, told Civil Eats.

Cabot took an early interest in efforts to scale down anaerobic digesters for New England-sized dairies because it believed its dairies could benefit economically, environmentally, and socially from co-digestion, Davis said.

By benefiting socially, Davis means odor control: co-digesters produce an odorless organic fertilizer. That’s important for dairies in regions where a growing number of people live. Longview, for example, is located within an 18-mile radius of five colleges and the city of Springfield. Before installing the co-digester, the farm received odor complaints when it spread manure on the fields it rents from landowners, and that was a problem.

“We are very reliant on our neighbors and the people in our communities who value agriculture and have decided to let us farm their grandpa’s old farm . . . rather than sell it for development,” said Barstow Manz.

Denise Barstow Manz inside the dairy barn at Longview Farm. Photo by Meg Wilcox.

Co-digester Benefits

The co-digester produces enough liquid, organic fertilizer to meet 90 percent of the farm’s annual fertilizer needs for its hay, alfalfa, and corn crops. It’s a “big savings for us . . . and we see increased crop yields. We see enhanced soil health,” Barstow Manz said.

Co-digesters also produce bedding for animals by separating out large solids and the woody parts of the cow’s diet that aren’t broken down by the digester. The heat from the digestion process produces a pathogen-free fiber.

It’s a “big savings for us … and we see increased crop yields. We see enhanced soil health.”

Longview’s system also generates enough waste heat to meet the farm’s needs plus eight homes in the community. Vanguard developed a prototype community waste heat system at the farm, with a state grant.

Co-digesters help keep food waste out of landfills and incinerators and reduce the need for synthetic fertilizers, yielding potentially significant GHG emissions reductions. U.S. food loss and waste from farm to kitchen generates the equivalent GHG of 42 coal-fired power plants, according to the U.S. Environmental Protection Agency (EPA). That figure includes food losses during farming, processing, distribution, retail operations, and consumption (e.g., in homes, institutions, and restaurants).

Synthetic fertilizer production and use meanwhile contributes 2 percent of global GHG emissions annually, while livestock accounts for nearly one-third of all human-caused methane gas emissions in the U.S.—though the majority of that comes from cow burps, not the manure itself.

PFAS and Other Risks

Adding food waste to methane digesters, however, raises the thorny issue of potential microplastic and PFAS contamination in fertilizer. Farms that take in food waste and then spread the digestate on their fields, could unwittingly be contributing to PFAS contamination in the environment, warned Tyler Lobdell, staff attorney at Food and Water Watch.

The EPA has found rising levels of PFAS in food waste, originating possibly from the use of contaminated irrigation water, food processing equipment, or packaging materials that come in contact with food.  The limited data suggest that food packaging and compostable serviceware may be the largest contributors of PFAS in food waste, though fish and meat are also significant contributors, according to the EPA.

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Farms that take in food waste and then spread the digestate on their fields could unwittingly be contributing to PFAS contamination in the environment, warned a Food and Water Watch attorney.

Researchers at the University of Vermont additionally found “early evidence” that microplastics and larger plastic pieces may be present in many food waste-derived composts and digestates, and that those plastics could be transferred to farm fields when applied as soil amendments. Over time these plastics may accumulate in soils, break down, and release chemicals that are harmful to human health and the ecosystem.

In response, Vanguard public relations manager Billy Kepner told Civil Eats, “We are concerned about [PFAS], but it’s something that we try to mitigate as much as possible.”

He later elaborated, “Maintaining a diversified input stream—of dairy farm manure, bulk fluid processing wastes, and organic waste from packaged materials—is a key component to our environmental risk management for matters such as microplastics and PFAS. We do not receive or process higher-risk materials such as biosolids.”

Biosolids, or sewage sludge, are sometimes mixed with food waste at compost facilities. They have far higher levels of PFAS than food waste alone, according to the EPA, and have caused widespread contamination across many farms.

Vanguard doesn’t allow food packaging or compostable serviceware in its digesters, either. It sends packaged food waste to a de-packaging facility before adding it to its co-digesters, said Kepner. Nonetheless, the Vermont researchers cite studies showing that some portion of packaging remains in food waste even when mechanical de-packaging machines, or humans, remove it.

Food and Water Watch’s Lobdell would prefer to see food waste reduced at the source, rather than sent to a digester. Reducing food waste at the source, at the scale that’s needed on a rapidly over-heating planet, remains a challenge.

Other advocates would like to see tighter laws to ban PFAS from food packaging and manufacturing, and to require testing at compost facilities and co-digesters.

“At this point, people are considering food waste to be a relatively clean source, regarding PFAS and other toxics, compared to sewage sludge, but I think testing is needed to verify that,” said Tracy Frisch, author of a Sierra Club report on PFAS contamination on farms and chair of the Clean Air Action Network of Glens Falls, New York.

Caleb Goossen, organic crop and conservation specialist at Maine Organic Farmers and Gardeners Association, agrees with Frisch, though “packaging is a different matter,” he said. Goossen is more concerned about food packaging, compostable serviceware, and biosolids getting mixed into compost and digestates.

Barstow Manz deferred to Vanguard’s expertise on the question of potential PFAS pollution at Longview Farm.

Additional environmental problems have been associated with methane digesters at CAFOs, from air pollution in disadvantaged communities to ammonia releases from the digestate to methane gas leaking from anaerobic digesters built on top of large waste lagoons. Worker protection on CAFOs have also been found wanting in Civil Eats’ investigative series, Injured and Invisible.

Methane digesters at CAFOs are also viewed as propping up a highly unsustainable industry.

The “significant concern of methane emissions in agriculture . . . directly correlates with the rise in mega-dairies and the necessity to handle waste in liquid lagoons,” said Lobdell.

Enclosed, well managed co-digesters on small farms may have fewer downsides, and there are a lot of dairies in that category that could potentially benefit from them. Roughly one-third of milk produced in the U.S. comes from farms with fewer than 500 cows, and about a half from farms with fewer than 1,000 cows.

“Diverse income streams will always make a farm more resilient, which is going to be really important in a year like this,” when epic floods have devastated many Vermont farms.

Still, Lobdell argues, “As a general matter, we don’t need to be capturing methane from manure—we should just manage it differently and not have the pollution to begin with.” The National Sustainable Agriculture Coalition, for instance, recently featured information on the rise of dry manure management in California dairies. Ultimately, he also acknowledges, “Our concerns are pretty focused around the largest of these [digester] facilities.”

Goossen in contrast thinks that co-digesters could be beneficial, particularly for farms that are managing their manure anaerobically, or in a way that generates methane. “It’s better to capture it and burn it, climate wise,” he said. For farms that can manage their manure in a way that doesn’t generate methane, he’s less convinced and wants to see how the environmental benefits and impacts pencil out.

Nevertheless, “diverse income streams will always make a farm more resilient, which is going to be really important in a year like this,” he said, referring to epic flooding that devastated many Vermont farms.

Can Co-Digesters Scale Up?

As some of the first farmers to install a co-digester in New England, the Barstows raised much of the capital themselves. Today, Vanguard assumes the upfront building costs, and has since bought their system.

Vanguard therefore owns the co-digesters and pays farmers rent to operate them on their farms. The company profits from the sale of the biogas to the electric grid or to natural gas companies.

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Farmers may not have to front the capital, but for a project to work, a grid operator must be able to profit from buying the biogas. In states with favorable renewable energy policies, such as net metering, or a renewable portfolio standard, the economics work better for co-digesters.

California’s Low Carbon Fuel Standard, however, acts as a bit of a disincentive for co-digesters because of the way it calculates the carbon intensity of manure versus food waste. Manure-only digesters receive better tax incentives.

States with food waste landfill bans also make co-digesters more affordable because the farms aren’t competing with landfills for the food waste.

But in states like Indiana, with low electricity prices and no food-to-landfill bans, co-digesters are economically out of reach for small dairies, said Mark Stoermann, chief operating officer for Newtrient, an independent service provider formed by dairy cooperatives.

A recent Cornell University study on the economic feasibility of co-digesters in New York, for example, pegged the price for an 1,800-cow farm at about $9 million and the annual benefits at $3.8 million. That estimate factors in a 30 percent tax credit (yet to go into effect) that the Inflation Reduction Act grants dairy farms installing anaerobic digesters.

Will Vanguard Stay Focused on Co-digesters?

Vanguard is the primary company focused on co-digestion, though other companies are emerging, Stoermann said. “They’ve really built a more holistic co-digestion model, working with smaller farms and all of the other feedstocks that might be in the area, in a way that other companies haven’t.”

Last year’s acquisition by BlackRock, the IRA tax credit, and growing interest in anaerobic digestion may be pulling Vanguard in other directions, however. Of the 130 systems the company is developing across 22 states, roughly half will be co-digesters, according to Hanselman. The remainder will be manure-only digesters on large dairies. Co-digesters don’t work on mega-dairies because they produce too much manure and don’t have the land base to spread the digestate.

Notably, BlackRock is the focus of accusations of greenwashing through investments in climate-friendly companies and projects, as well as revolving-door hiring practices with the oil and gas industries.

“[Dairy production] is a huge methane generator. If you continue to let it go, it’s going to impact our environment so negatively.”

Vanguard says that tackling food waste remains an integral part of its mission, but there are some situations where a methane-only digester is the best solution. “I fully understand people who are [concerned] this is helping very large ag,” said Hanselman, “but [dairy production] is a huge methane generator. If you continue to let it go, it’s going to impact our environment so negatively.”

Vanguard also operates some co-digesters that pump methane or “biogas” directly into gas pipelines and is starting to supply large companies like AstraZeneca with biogas to help them offset greenhouse gas emissions from other sources.

Lobdell calls those deals myopic. “We find that approach to be problematic and distracting from the need to, in an absolute sense, reduce our greenhouse gas emissions in all sectors, he said. “The business model depends on long-term waste generation and continued reliance on climate-destroying methane gas.”

Meanwhile, back on Longview Farm, a Vanguard technician hunches over two computer monitors in a building near the digester, tracking everything inside the tank from atmospheric pressure to the chemistry composition of the digester’s contents. Technicians are on the farm every day, ensuring that the system runs optimally, balancing the input of food waste, and coordinating the deliveries that come in.

“There’s really not a lot of downsides” for farmers, said Barstow Manz. “The beauty of working with Vanguard . . . is that we don’t have to do all the chemistry and the maintenance and dealing with the food waste contracts. They handle that so we can focus on the day-to-day on the farm.”

“I think it’s a smart investment my family made,” she said. “It’s a thoughtful way to fit into our community. It’s not a haunted hayride or something fun and nice, but we’re really adding value to our community.”

Meg Wilcox is a freelance writer based in Boston focused on solutions-oriented stories about the ways people are fighting climate change, protecting the environment and making our agriculture systems more sustainable, including by addressing poverty. Read more >

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  1. Meg, this is a great overview, thanks for updating us on what is going on with co-digestion in New England. Could you comment -- what are the prospects for states developing more requirements for PFA testing along the food waste processing stream? And is there any similar co-digestion model operating for seafood processors, who also produce a high volume of processing waste that could be feedstock for co-digestion?

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