Water pollution has long been considered a challenge in urban areas, and new types of pollution are continuously introduced to society. Urban wastewater and stormwater act as transport pathways of pollution from urban areas to receiving waterways. Microplastics are a new type of pollution that is being highlighted as problematic, and the presence of microplastics is widespread in the environment. The knowledge about behaviour, sources and transport pathways is still limited. Still, there have been suggestions for ways to control microplastic pollution. Moreover, many other pollutants have been introduced, and to a various extent controlled, in the urban water system before, which means that there are strategies for pollution control in place.

The problem of microplastic pollution is receiving attention from both research and policy. Therefore, there is a need to simultaneously explore ways to handle the pollution and learn more about the flows of microplastics. The aim of this thesis is to contribute to the understanding of flows of microplastics in urban waters and investigate strategies and measures that can control the flows. This aim has been addressed through three different studies presented in the three appended papers. The empirical material was gathered between 2017 and 2020 using both quantitative and qualitative methods. The flows of microplastics were investigated using a quantitative approach with inspiration from substance flow analysis. Strategies and measures to control pollution were investigated by means of analyses of documents, literature review and interviews.

In terms of flows, the results show that laundry seems to be a large contributor to the wastewater system, while the contribution from cigarette filters was smaller than anticipated. Littering, in general, as well as paint, are potentially important sources of microplastics where the understanding is very limited. In terms of strategies and measures to control the flows, there are both preventive and treatment options for microplastics. As microplastics are a diverse pollutant group, several different solutions are needed. Some aspects that influence control efforts are related to the properties and behaviour of the specific pollutant and can therefore be more or less suitable for different pollutants. Other aspects are more general, such as what compartments are seen as valuable and prioritised in terms of protection. Further, treatment moves the pollution from the water to another medium, and this movement also needs to be considered.

Challenges in terms of changing, limited or even lack of legislation, as well as unclear responsibility, and problems with collaboration among urban actors also impact the possibilities to control pollution. The case of microplastics also raises questions about when and how pollution should be controlled and by whom. Further, emerging practices in the urban water system, such as source separation, pipe-separation, and water reuse, will have implications for how pollution will be handled in the future.