“The recycling of organic waste through composting or fermentation and subsequent application on agricultural land is, in principle, an environmentally sound practice to return nutrients, trace elements, and humus to the soil,” the study authors write. “However, most household and municipal biowaste is contaminated by plastic material.”
Microplastics, which the new study defines as plastic particles smaller than 5 mm, result from the breakdown of plastics, and are pervasive both on land and in the oceans. While the extent of their environmental and health effects is not clear, studies have found they’re detrimental to the health of organisms such as earthworms and rodents, and that they make their way into human food supplies.
In the new study, researchers at the University of Bayreuth in Germany investigated fertilizer produced by a biowaste composting plant (which treats waste aerobically) and a biowaste digester (which uses an anaerobic process). There were fewer than 25 microplastic particles per kilogram in the compost from the first plant, while the freshly-digested fertilizer from the second plant had up to 146 particles per kilogram of the pollutants. By contrast, no microplastics were found in digestate from an agricultural energy crop digester, suggesting that the contamination in the products from the first two plants originated in the homes and businesses that were the source of the waste used.