Mitigating Microplastics Pollution in Urban Water Systems : Flow, actors, and control measures

Abstract: Water pollution has long been considered a challenge in urban areas. Urban wastewater and stormwater act as transport pathways for pollutants from urban areas to receiving waters. Microplastics are a pollution problem of growing concern. As part of the zero-pollution action plan for 2050, the European Commission has set the target to reduce emissions of microplastics to the environment by 30% from 2016 to 2030. The majority of the microplastics originate from sources on land, and urban areas are viewed as particular hotspots. More knowledge is needed on the sources and pathways of microplastics within the urban area, such as to and from urban water systems. To be able to reduce microplastics pollution, there is also a need to understand where control measures can be introduced and make an impact in terms of reduced loads to the environment. For control measures to be implemented, there is also a need to understand the incentives and challenges that the actors, who have a responsibility related to the flows, encounter when working with pollution control.   The aim of this thesis was to contribute to enhanced understanding of flows of microplastics in urban water systems and how the flows can be controlled, in order to facilitate mitigation actions for these flows. This aim was addressed by identifying and quantifying sources with emissions to urban wastewater and stormwater and pathways from the urban water system, with inspiration from the method substance flow analysis. The estimated sources were also compared to measurements in raw wastewater and stormwater from four different catchments to identify matches and differences. Further, control measures were introduced to the quantified sources and pathways. Municipal officials, who can implement the control measures, were also interviewed about their incentives and challenges when controlling microplastics and other pollutants in the urban water system.   The emissions of microplastics to stormwater were estimated to be much higher than those to wastewater. Tyre wear particles and cigarette butts were estimated to be the largest sources. While stormwater measurements confirmed tyre wear particles, cigarette butts, and paint particles, which was another large source according to the source estimates, could not be confirmed. Instead, the samples contained other types of microplastics. These mismatches indicate that large flows of microplastics may come from sources that have yet to be identified and quantified. 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. Untreated stormwater was the largest pathway to receiving waters, followed by treated stormwater. Treated wastewater and overflows had comparatively small emissions. 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.   Three types of incentives for pollution control among the municipal officials were identified. One incentive was that pollution control positively affected other aspects that were seen as important, such as that it had economic benefits or increased water- and sludge quality. Another incentive was a perceived responsibility for water quality, for the environment, or to be role models. A third incentive was a “pressure” to act from the local environmental control authorities or through local politics. The challenges for pollution control were primarily related to insufficient resources in terms of time, finances, and knowledge. Changing, limited, or lack of legislation was also identified as a challenge. Other identified challenges, such as unclear responsibilities and unclear expectations of the work, can be alleviated with a strategy or plan, but a plan did not ease all challenges. Support, in terms of financial means and guidance, is also needed so that actions set out in the plans can be implemented.  So far, the control measures for microplastics related to urban water systems have primarily been preventive and focused on restricting use of microplastics in certain applications. There are no restrictions on emissions yet. If such restrictions come, where they are placed will impact the type of measures that will be introduced. This, in turn, will have consequences at the system level: for example, if it will lead to emissions in another part of the system, and which actors will be responsible for managing the pollution.