By Dr. Michelle Young
Arizona State University, Swette Center for Environmental Biotechnology
Food waste is a problem in the United States. About 30 to 40% of the U.S. food supply, or roughly 40 million tons, ends up as pre- or post-consumer food waste (1). Of that, 76% of food waste and its nutrients is landfilled, which accounts for 15% of the material landfilled in the US (2). Food waste accounts for one-third of landfilling-related emissions, or 36.2 million metric tons of CO2 equivalents. Isn’t composting an option? While composting provides the added benefit of producing fertilizer and soil amendments, about 2.2 million metric tons of CO2 equivalents are emitted from the biodegradation of food waste (3).
What about making energy from food waste instead? This is one of the hottest topics in the food waste and municipal wastewater treatments sectors: co-digesting food waste in anaerobic digesters at your local wastewater treatment plant to produce natural gas as methane. Medium to large municipalities often have underutilized anaerobic digesters at their wastewater treatment plants as part of sewage treatment. These digesters provide a financial benefit to the treatment plant: Solids produced during wastewater treatment (i.e., bacteria) are broken down to soluble organic molecules, such as acetate, reducing the amount of solids sent to landfills. Acetate can be consumed by methanogenic bacteria to produce methane–essentially natural gas—for combustion in engines and turbines to produce electricity for plant operations.
During co-digestion, wastewater treatment plants supply food waste to the anaerobic digesters from streams as varied as industrial waste streams from food and beverage manufacturing, household food wastes, restaurant wastes, and grease from grease interceptors and traps. Because these organics are rich in energy and biodegradable, adding food waste to the digesters can double methane production. For example, East Bay Municipal Utility District in Oakland, CA, produces 30% more energy than needed to operate the plant and returns excess to the electrical grid. In total, it is estimated that municipal co-digestion produces 551 million kWh per year of electricity – enough energy to power about 50,000 homes.
One largely overlooked area in the food waste co-digestion literature is nutrient recovery from the solids and liquids produced during digestion. To some extent, phosphorus is discussed as a necessary nutrient for healthy microbial communities during anaerobic co-digestion. The few review papers that discuss phosphorus liken its recovery to that of traditional anaerobic digestion processes, where most of the phosphorus remains in the biosolids, making recovery difficult. Most traditional anaerobic digesters recover phosphate for agricultural purposes using struvite precipitation, biosolids application, or incineration of sludge ash. Each of these processes has its own obstacles to larger commercialization due to the energy required. Ultimately, how food waste digestion effects these processes is unknown and provides an excellent opportunity for future research.
(1) https://www.usda.gov/foodwaste/faqs
(2) US EPA https://www.epa.gov/sites/production/files/2018-07/documents/2015_smm_msw_factsheet_07242018_fnl_508_002.pdf
(3) https://www.epa.gov/sites/production/files/2019-04/documents/us-ghg-inventory-2019-chapter-7-waste.pdf
Michelle Young is a post-doctoral researcher in the Biodesign Swette Center for Environmental Biotechnology. She received her Ph.D. in Environmental Engineering in 2018. Her research focuses on utilizing existing wastewater treatment resources for improved nutrient and energy recovery from wastewater and non-traditional waste streams.
Opinions expressed here do not necessarily reflect those of the Sustainable Phosphorus Alliance.