Microplastic Contamination of Composts and Liquid Fertilizers from Municipal Biowaste Treatment Plants: Effects of the Operating Conditions

High-caloric-value household biowaste is an attractive substrate for the production of biogas and fertilizer. Most household biowaste is contaminated by plastics, typically in the form of bags and foils from packaging. Operators of municipal biowaste treatment plans take great care to remove these contaminants, often at the cost of reducing the organic material entering the process. This study compares the residual plastic contamination of fertilizer (composts, digestates) from biowaste treatment plants with compost produced from greenery and digestates produced by agricultural biogas plants processing manure and energy crops. While the fertilizers from the agricultural biogas plants and greenery composts were minimally contaminated by plastic, we found considerable numbers of plastic fragments in the composts/fertilizers from the biowaste treatment plants. Moreover, while certainly being influenced by the quality of the incoming biowaste, this residual contamination appeared to depend largely on the operating conditions. In particular, shredding of the incoming material increased the degree of contamination. Sieving was an efficient method for the removal of fragments > 5 mm but was less efficient for the removal of smaller fragments. In view of the number of the recovered fragments in that size range, it is likely that still none of the finished composts surpassed the current dry weight limits imposed for the plastic contamination of high-quality composts with fragments > 1 mm in Europe (0.3% of dry weight) nor even in Germany (0.1% of dry weight). The contamination of the liquid fertilizer produced via anaerobic digestion by three of the investigated biowaste treatment plants (up to 10,000 particles with a size between 10 and 1000 μm −1 ) may pose a more serious concern. Graphical Abstract