Microplastic budgets and discharges to Lake Saimaa from three Finnish wastewater treatment plants

Microplastic (MP) pollution has been widely related to the discharges from wastewater treatment plants (WWTPs), but studies evaluating MP abundancies and qualities in both discharges and recipient water bodies have rarely been reported. To assess this, microplastics (≥100 μm) were analyzed from influent, effluent, and sludge of three Finnish WWTPs as well as from the sediments of their recipient waterbodies. The studied WWTPs utilized different treatment methods varying from secondary treatment to tertiary treatments with membrane bioreactor (MBR) and sand filtration. Results demonstrated high removal of MPs in all studied processes (>99 %), but the highest removal of MPs was reached at the MBR-based WWTP. Based on our calculations, over one third of the MPs entering the studied WWTPs were likely removed from the wastewater treatment process to other fractions than final sludge or effluent, such as materials removed with screening or grit separation. The role of the fractions in the environmental MP pollution would need further studies. It was estimated that at least 258 million MPs (≥100 μm) were discharged to the Lake Saimaa via studied WWTPs daily. MP concentrations in sediment samples were relatively low, and the types of detected MPs did not show direct linkages with the discharged MPs from the WWTPs. Blank correction was applied to all provided data together with the original data. As a result, the need for more precise blank correction was highlighted.