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Digesting a New Energy Future


Increased focus on sustainable energy and the push for low-carbon fuel sources are powering renewed interest in the potential for a proven technology which adds value to underutilized products.

Anaerobic digestion uses bacteria to break down organic materials in the absence of oxygen. The process results in several products, including solids called digestate, and liquids, which are sometimes rich in nutrients like nitrogen, potassium, and phosphorus. The key output from digestion, though is biogas, consisting primarily of methane, carbon dioxide, ammonia, and some sulfur compounds. Biogas is a promising source of renewable natural gas that is usable as an alternative to fossil fuel-derived natural gas.

In the United States, anaerobic digesters are commonly associated with large dairy operations. Larger animal agriculture operations sometimes require digesters as a way to manage manure. However, there is also tremendous potential in using municipal waste and food processing waste as digester feedstocks for significant biogas production.

“Most digesters have historically been on dairies, but there are digesters associated with poultry, swine, and beef cattle waste as well,” says Rod Larkins, recently retired AURI senior director of science technology. “The reality is that animal waste has already been digested by the animals, so it’s not as rich of a stream for anaerobic digestion as, for example, food waste or vegetable processing waste. Minnesota is fortunate to have several ag processing industries that have organic waste streams.”

Larkins says many processors struggle to generate value from those processing leftovers and often have to pay for disposal. Turning those coproducts into a low-carbon energy source is appealing on many fronts.

“We naturally think about sugar beet and vegetable processing in the state, they all end up with high levels of organic waste streams that are candidates for anaerobic digestion,” Dr. Luca Zullo, AURI’s current senior director of science technology explains. “So is food waste from restaurants, waste from grocery stores, and any organic waste that ends up in the garbage every day. We are exploring those different waste streams as potential for anaerobic digestion.”

AURI has researched anaerobic digestion for years using a variety of ag-based feedstocks. Currently, a semi-trailer sized portable anaerobic digester is set up at the AURI Waseca facility to help evaluate various feedstocks and evaluate various feedstocks. But the organization is also working with thought leaders across the state to identify opportunities and roadblocks for larger-scale anaerobic digester projects.

“The low carbon fuels interest creates new opportunities for renewable natural gas and anaerobic digestion is a method to create value from agricultural products produced in the state,” says Shannon Schlecht, AURI executive director. “The formation of a thought leader group and identifying collaborative opportunities with this group brings multiple benefits to AURI as we do not have the expertise in-house to advance this on its own and utilizing other industry experts helps us better define constraints and opportunities to accelerate investment and implementation of this technology.”

“As we looked at this whole area, we saw it was ripe with opportunity,” Larkins says. “As the country and the world is turning more towards green feedstocks, this is certainly a good opportunity to generate green, renewable natural gas and fertilizer systems to support agriculture.” The thought leaders group includes representatives from a range of small and large businesses, government and academia.

AURI has long focused on helping Minnesota’s agribusinesses utilize various processing streams to generate new revenue streams. The effort to use ag processing products and other organic waste streams for energy production is being augmented by a push for more sustainable fuel sources. “What’s driving this interest is commitments by industry to become carbon neutral or net-zero greenhouse gas emitters,” Schlecht says. “This movement requires new investment and changing processes to meet those bold commitments. Anaerobic digestors utilizing agricultural materials can play an important role in meeting those commitments.”

Larkins says among the issues holding back large-scale digester projects is permitting. He says some people don’t want a digester in their area because they have the impression a facility will have an odor. Larkins says digester technology has advanced, making them less visible and more efficient.

Economics also play a limiting role, but economic incentives could help grow the potential for large-scale anaerobic digesters.

Funding of digester systems is also a limiting factor. Depending on the size, farmbased systems may cost from several hundred thousand dollars to up to $15M, while larger-scale facilities, often aggregating feedstock from multiple sources or integrated with industrial or public wastewater facilities, are considerably more expensive and may cost over $50M and up to $100M.

“Finding financing to do that is another blocking issue that wev’e identified,” Larkins adds.

There are challenges to establishing a large digester, but the tide seems to be turning in favor of the technology.

“California has significant incentives for renewable natural gas. California’s transit system is using natural gas as a transportation fuel. Renewable natural gas is a substantial component of what’s currently being used in California to fuel vehicles,” Larkins explains.

AURI has developed a project proposal in partnership with the University of Minnesota to identify and quantify organic waste streams in the state of Minnesota. Having a handle on the available feedstocks is a key step to understanding the economic feasibility of using digesters to produce renewable natural gas.

“Anaerobic digestion has typically only worked for larger operations and part of this effort will be to look at community models to determine if a collaborative model utilizing waste streams from producers, processors, municipalities, etc. is economically feasible in the current environment,” Zullo says.

In many previous instances, farmers have been asked to be ones to operate digesters on their farms. Larkins envisions a system more closely modeled after other ag processing systems.

“The highly successful anaerobic digestion systems nationally and internationally are systems that are set up independent of a farmers’ daily operations. Think about an ethanol plant. Farmers bring corn into a facility run by professionals,” Larkins says. “The way anaerobic digestion systems will likely operate best is when they’re run by professionals with farmers or the ag processing companies as customers that provide feedstocks and utilize the digestate.”

Schlecht says AURI’s formation of an industry thought leader group for Minnesota is a key step forward to ensuring that both the business and technical aspects of larger scale anaerobic digestion align for de-risking opportunities and adding new value streams for farmers and ag processors across the state. With more states considering low-carbon fuel standards and more companies pledging to reduce their carbon footprint, the time to invest in utilizing livestock and processing waste streams to produce renewable natural gas could be now.

“I’m excited about the future, and I believe this thought leaders group can drive it,” Larkins says.

To learn more about AURI’s work with anaerobic digesters, visit