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Characterizing the heterogeneity of microplastics in products from the anaerobic digestion of food waste
Summary
Food waste is increasingly being turned into fertilizer through a composting-like process called anaerobic digestion, but this study found that the leftover liquid (digestate) can carry thousands of microplastic particles per liter, likely from plastic packaging tossed out with food scraps. When this digestate is spread on farm fields as fertilizer, it could deposit thousands of microplastic particles per square meter into the soil, raising concerns about plastics building up in the environment (and potentially our food and water supply) unless waste facilities do a better job filtering out plastic before processing.
Abstract The food industry consumes more than a third of all plastic packaging, much of which is discarded along with excess and wasted food. As the diversion of food waste (FW) from landfills to more sustainable disposal pathways like anaerobic digestion (AD) increases, food-associated plastics may follow, degrading to form microplastics (MPs) along the way. MPs pose a threat to ecosystems when digestate, the liquid residue of AD, is land-applied as fertilizer or sent to a wastewater treatment plant. To improve understanding of the prevalence, sources, and attributes of MPs associated with AD, we quantified and characterized MPs in liquid digestate from three full-scale digesters that vary in feedstock composition and digester operating conditions. MP abundance ranged from 400 to 25,033 particles L-1across the three digesters with substantial variation in polymer type, color, and size. Polyethylene terephthalate (PET) and polyethylene (PE) fragments and films were most common. MP abundance and heterogeneity were higher where facilities accept FW from a greater number of sources. However, there was also variability in MP abundance and composition within each site, related to operational changes or disruptions. We estimated that at typical field spreading rates, approximately 2,000 - 13,200 MP particles m-2 may enter the environment per application. These results point to a need for improved separation of waste streams prior to AD, especially for facilities that accept organic waste from grocery stores and other post-consumer sources.