Modelling the Occurrence and Fate of Microplastics Caused by Tyre Wear on Highway E18

Abstract: The accumulation of microplastics (MPs) are a growing concern for the environment, the maximum contributor to the MP pollution is tyre wear particles or TWPs. TWPs can accumulate in the environment and remain there for long periods of time and hence have the ability to disrupt ecosystems. With nearly 6,000,000 tons of TWPs being emitted each year globally there is a need to study theiroccurrence and fate in terms of why and how these particles get created and where they end up in the environment. The occurrence and fate of tyre wear particles (TWPs) were investigated, and a model was created in Python to simulate the flow of TWPs in a stormwater system at a test site on Highway E18 in Sweden. In the literature review it was found that the generation of TWPs is highly dependent on annual average daily traffic and the type of the road; it is also dependent on the driving behaviour such as acceleration, making unnecessary turns and the use of studded tyres in the winter. The TWPs are generally transported via stormwater but wind also plays a crucial role in transporting TWPs which are of a smaller size than 10 µm. Although the common notion is that the TWPs get washed into rivers,the literature review also suggests that most TWPs get trapped in soil that is covered with vegetation. Three processes were simulated in the developed model, namely transport, hetero-aggregation and settling. The results clearly show that a rainfall with an intensity of 40 mm during 15 min providesminimum settling in the stormwater system, and a 10 mm rainfall with a duration of more than 30 hasmaximum settling. Hetero-aggregation and settling is directly dependent on particle size and hence larger particle sizes have a higher probability of attaching to another particle or to settle in the pipes or wells of the stormwater system. It was also observed that the flow patterns such as depth, discharge and velocity of flow largely affect the settling and hetero-aggregation in the stormwater system. Most particles remain unaffected in short duration and high intensity rainfalls; the opposite is true for the longer duration and lower intensity rainfalls. Using the model, a stormwater system can be designed to trap or to flush out TWPs from the system. Also, the concentration of TWPs in the sinks can be known using the model that is created in this study