Use of computational fluid dynamics to model microplastic transport in the stormwater runoff system

Abstract

Microplastics (MPs) are increasingly accumulating in stormwater runoff systems, particularly wetlands. In this study, the impact of various factors on the motion and distribution of MPs within a wetland water environment was explored through computational fluid dynamics (CFD) simulations, utilizing a Volume of fluid (VOF) model coupled with the Discrete particle model (DPM). To understand those aspects, different MPs (e.g., Polyethylene Terephthalate (PET), Polyvinyl Chloride (PVC), Polystyrene (PS), and Polypropylene (PP)) were released from the inlet to examine how factors, such as their type, size, and shape, in two different water flow velocities under a constant air current, influenced their behavior. The spatial distribution of MPs due to the impacts of different variables was examined through the tracking of particle positions. It was found that buoyancy and particle size significantly affected the distribution of MPs, which emerged as a key discovery. The vertical and horizontal distributions of MP particles indicate that under 0.3 m/s water velocity conditions, the majority of large spherical PET and PVC MPs tend to sink toward the bottom. In contrast, numerous smaller non-spherical particles tend to float near the surface. Among the four types of MPs examined, PP and PS large spherical particles showed the highest mobility, particularly with increasing water velocity. Smaller particles travel longer distances because they have less mass and are more sensitive to air currents. In contrast, larger particles settle more quickly due to gravity, resulting in shorter travel distances. In summary, using the CFD approach improves the ability to predict the dispersion of MPs in aquatic environments.