Q&A: How anaerobic digestion can help reduce emissions and increase income streams Sadie Harley Scientific Editor Andrew Zinin Chief Editor An advanced technology that could benefit farmers' bottom line and help meet 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 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. 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, a growing technology, can be applied to other waste, such as food waste from food and beverage processing. While most anaerobic digestion of food waste occurs at stand-alone digesters and wastewater treatment facilities, some farms are finding co-digestion of food waste and manure a viable path.

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.