Niina Kärkkäinen, Salla Selonen, Kari Hartonen, Markus Sillanpää
Abstract Despite the high microplastic (MP) removal efficiencies modern wastewater treatment plants can reach, wastewater effluent remains a substantial source of microplastic pollution due to the sheer volume of daily discharges. Additionally, sewage sludge generated during treatment has the potential to majorly contribute to microplastic pollution, depending on its end-use. This study measured the microplastic number concentrations in effluent and composted sewage sludge, with a special focus put on determining microplastic fibres in the compost. The effluent samples were sampled from two opposite seasons, summer and winter, since it was expected that the seasons would have a noticeable effect on the microplastic composition and number concentrations of each season. The effluent samples contained non-fibrous microplastics with average concentrations ranging from 0.18 to 1.81 MP/L in summer, and from 0.41 to 1.99 MP/L in winter. Microplastic fibre concentrations in a single replicate sample ranged from 0.00 to 0.53 MP/L in summer and from 0.00 to 1.58 MP/L in winter. For composted sewage sludge, the microplastic fibre concentration was on average 278 MP/g of dry compost, with polyester being the dominant polymer type . These findings highlight that both the effluent and the sewage sludge-based compost serve as notable sources of microplastic pollution to the environment, with the emphasis on needing to develop less time-intensive and laborious analysis methods for microplastic fibres in the future.