Mapping the invisible – flows of microplastics in a model city

Microplastics have received increased attention in the last decade as a pollution of concern. As part of the zero pollution vision for 2050, the European Commission has formulated a target to reduce emissions of microplastics to the environment with 30% by 2030. Most of the microplastics originate from sources on land and urban wastewater and stormwater have been identified as pathways to the environment. In this study, flows of microplastics in wastewater and stormwater were quantified for a hypothetic city of 100 000 inhabitants. Two types of data were used for the quantification: 1) source estimates based on literature values and, 2) estimates based on extrapolation of measurements in urban waters. The quantified flows were also connected to potential mitigation efforts that have been brought up in policies, both preventive and in terms of treatment. The results show that the emissions of microplastics to stormwater were estimated to be much higher than those to wastewater. The largest source to wastewater was estimated to be synthetic fibres (mainly polyester) released during laundry washing. Polyester also had the largest share in the wastewater measurements. Tyre wear particles and cigarette butts were estimated to be the largest sources to the stormwater. While measurements confirmed tyre wear particles, other source could not be confirmed. Instead, the samples contained other types of microplastics. This mismatch indicates that large flows of microplastics may come from sources that have yet to be identified and quantified. The measures introduced to wastewater had the potential to reduce the load of microplastics in the wastewater by half. However, as wastewater treatment retain most of the microplastics, and because the largest emissions came from stormwater, this reduction made a small difference in terms of total loads to receiving waters. Reduced urban traffic, changed driving behaviour, and more stormwater treatment, were also required to reduce emissions of microplastics from the urban water system to receiving waters by 30% in accordance with the EU target.