UNIVERSITY PARK, Pa. — An advanced technology that could benefit farmers’ bottom line and help answer the rising demand for domestic energy production borrows a key ingredient from a centuries-old practice of burning dried manure to warm homes where trees were sparse or expensive: animal waste. According to Christine Costello, associate professor of agricultural and biological engineering in Penn State’s College of Agricultural Sciences, anaerobic digesters may help not only manage animal waste but also generate renewable energy and additional income streams.
Anaerobic digesters convert raw manure into nutrient-rich fertilizer and biogas, which can be sold or used to power the farm. These manure-based anaerobic digester systems help reduce greenhouse gas emissions by creating and capturing methane — which can offset fossil fuel use when used to produce energy — from organic waste.
There are approximately 400 operational anaerobic digesters located on large livestock farms across the United States, according to the U.S. Department of Agriculture (USDA). Roughly 86% of these digesters are installed on dairy farms, and California, Wisconsin and Pennsylvania have more than any other state.
Costello, who studies the environmental outcomes of anaerobic digestion in the U.S., pointed out that the technology has far more potential, though. The U.S. Environmental Protection Agency estimates biogas recovery systems are technically feasible for more than 8,000 large dairy and hog operations. Also, anaerobic digestion is a growing technology can be applied to other waste, like food waste from food and beverage processing. While most of anaerobic digestion of food waste occurs at stand-alone digesters and wastewater treatment facilities, some farms are finding co-digestion of food and manure together a viable path.
In the following Q&A, Costello discussed the potential for expanded anaerobic digestion and her work to help make it happen.
Q: How does anaerobic digestion work? What are the benefits?
Costello: Anaerobic digestion is a biological process that breaks down organic matter in the absence of oxygen. While frequently viewed simply as a waste management tactic, it can play a positive role in energy, agriculture and environmental outcomes.
The benefits of anaerobic digestion include advanced waste management — it decreases the physical bulk of organic waste by up to 60%; pathogen destruction — high operating temperatures kill harmful bacteria and weed seeds; and renewable energy generation — the process produces biogas and captures methane formed by microbes that can be harnessed to generate energy. The remaining liquid and solid material, the digestate, retains the nutrients in manure or other feedstocks, which can be used as organic fertilizer. On farms, solid digestate can be used as animal bedding and liquid digestate replaces manure application to land.
The environmental and climate-protection benefits of anaerobic digestion to manage dairy manure and improve nutrient cycling on farms can be significant when well managed. Co-digestion with grasses harvested from winter cover crops or grass buffer strips — both of which are used to help maintain soil nutrients and reduce erosion — could provide habitat for wildlife and nutrient benefits and modestly increase biomethane production.
Co-digestion with food waste has been shown to be a profitable pathway for some dairy farms. It also reduces the greenhouse gas emissions over the farm and provides additional nutrients to support crop growth. Anaerobic digestion of food waste, when in balance with the farm’s crop needs, is a form of circular waste management in cases where food waste cannot be avoided or edible fractions donated.
In addition, as municipalities and governments seek to remove food waste from entering landfills, following avoiding and donating food waste, anaerobic digestion is a primary candidate for alternative treatment as it can recover value in the form of energy and nutrients that are otherwise lost in landfills. Given increased demands for this service, our Pennsylvania farmer partners have found co-digestion of manure with food waste to be an economically viable pathway, primarily through tipping fees received. The digestate is used as a nutrient source to meet the cropland’s needs and the additional biomethane results in electricity or renewable natural gas beyond the farm’s needs and can be sold to a local utility.
Q: You’re part of a project called Grass2Gas, focused on developing a biobased value chain using perennials and winter crops to feed anaerobic digestion. Recently, your team explored growing grassy biomass on dairy farms to create fuel for the digester, along with the manure. What did you find?
Costello: Our research has been focused on the co-digestion of manure with herbaceous feedstocks, which is not common. The motivation was to learn whether the additional products from the digester — such as energy, nutrients, bedding, etc. — could create sufficient additional economic value to support the cultivation of perennial grasses and winter cover crops, which are difficult for farmers to economically support.
While the team — we at Penn State are working with researchers at Iowa State University — found that most systems would still require an economic incentive such as a government-sponsored premium pricing for renewable energy to benefit farmers, we were also were able to document the ecosystem services potential by integrating grasses and cover crops into agricultural landscapes. More details can be found on the project’s page.
Q: How can farmers decide if their operation would be a good candidate for an anaerobic digester?
Costello: Our team developed a tool to help farmers make this assessment. We published it in the Journal of Environmental Management. In short, it’s a spreadsheet that allows a user to design their own farm digester system and calculates the change in greenhouse gas emissions from the farm, the nutrient content of the digestate, the renewable energy produced and the cost of the system, as well as the payback period as a function of user-specified economic variables.
Each farm and digester can be unique. There are many possible configurations, depending on feedstocks — manure, grasses and food waste; digester sizes — from hundreds to hundreds of thousands of gallons; and system type — combined heat and power or renewable natural gas. We wanted to create a tool that allowed users to explore options relevant to their situation. They can enter their information and examine the projected outcomes to learn if anaerobic digestion is likely to be economically feasible. They can see if it will reduce greenhouse gas emissions, how much energy will be produced and whether there will be more nutrients in the digestate than the local cropland can absorb.
Q: What do you think the future holds for anaerobic digestion in the U.S.?
Costello: The process could play a significant role in providing clean energy here, both on and off the farm. Denmark, which leads the world in anaerobic digestion, suggests what is possible. That country primarily converts agricultural and industrial waste into renewable natural gas. Denmark’s 150 or so centralized biogas plants supply a massive portion of the nation’s gas demand — with green biomethane often making up 35% to 45% of overall gas consumption. Denmark has regulations that prohibit food waste from entering landfills and enforce rules on land application of nitrogen and phosphorous.
A little closer to home, New York State has passed the Food Donation and Food Scraps Recycling Law, which requires single businesses and institutions producing two tons of food waste per week to donate excess edible food and recycle the remaining if there is an organics recycler within 25 miles; by 2029 the rule will be 0.5 tons per week and within 50 miles. This could lead to more projects of manure and food waste co-digestion in the region. I hope we will see discussions across the agricultural, energy and waste management communities that lead to great projects and positive outcomes to manage these organic materials.