Extraction and analysis of microplastics in wastewater sludges of a multi-product pulp and paper mill

Pulp and paper wastewater sludges are waste streams produced in major quantities across the world. The recycling of these organic sludges, for example to soil amendments, is desired in the circular economy but carries the risk of potential pollutants to be also introduced into the environment. Pulp and paper wastewater sludges have been scarcely studied matrices in the microplastic research due to their complex composition. In this study, we optimized an extraction process for microplastics from pulp and paper wastewater sludges, and quantified and characterized microplastics down to 20 μm in primary sludge and biosludge generated at the wastewater treatment plant of a multi-product pulp and paper mill in Finland. The occurrence of microplastics was high in primary sludge, 900-1600 microplastics g^-1 dry weight, while the maximum number of detected microplastics in biosludge samples remained at 210 g^-1 dry weight. Biosludge samples suffered from larger amounts of remaining solids after the extraction process, thus compromising the detection of smaller microplastics (<100 μm) and increasing the uncertainty related to the interpretation of the results. The most prevalent microplastic shape in all samples was fragment, and the most recurring polymer types were polyethylene and polypropylene, while a polystyrene-based copolymer represented approximately 10% of identified microplastics in primary sludge. The present study advances the development of microplastic analysis of the challenging pulp and paper wastewater sludges and brings novel information to the progressing discussion of their circulation potential.