Three to five replicate samples of surface water and treated wastewater were processed at seven locations in Örebro between September and November 2019. The levels of microplastics >300 µm, excluding fibers, in surface water varied between 0 - 15 MP/m' (0-10 µg/L), and 32-159 Mp/m' (2-20 µg/L) in treated wastewater. Örebro city contributed with more than 90% to the microplastics discharged into lake Hjälmaren via Svartån. Precipitation and flow varied <luring the sampling period, and high recordings were associated with both high and low concentrations of microplastics depending on the sampling location. Calculations of the yearly load of microplastics from treated wastewater and stormwater indicated that the STP contributed with up to four times more to the total load. However, it was recognized that the uncertainty in the data was high and that more samples were needed for a more solid conclusion.

 Visual characterization was performed to obtain a detailed level of classification to aid in identifying source contributions of microplastics. However, the relatively low number of microplastics, in combination with variations in levels and types of microplastics between the replicate samples, made it difficult to appoint and quantify sources. Primary microplastics in different colours and in the shape of spheres made up 4%, and non­spherical semitransparent primary microplastics made up 44%, of the total number of MP particles from the STP. Films made up a larger part of microplastics in stormwater recipients compared to in effluent from the STP, which were attributed to littering being an important source of this microplastic type. Artificial turfs were estimated to contribute with 3-7 % of the microplastics in stormwater in the form of green polyethylene straws.