Numerical Modelling of Microplastic Transport in the Confluence Area of the East Jiu River and the Maleia Stream Using a CFD – DPM Approach

Microplastic pollution in aquatic ecosystems represents a major environmental concern, and understanding the mechanisms governing transport, dispersion, and accumulation requires methodologies capable of describing particle – turbulence interactions. This study proposes an integrated methodology for investigating microplastic transport in the confluence area of the East Jiu River and the Maleia Stream, combining experimental data on polymer typology (reported in a previous study) with Computational Fluid Dynamics (CFD) numerical simulations. The numerical modelling was performed in ANSYS Fluent over a representative river reach, employing the Reynolds-Averaged Navier – Stokes (RANS) equations to describe the continuous phase and the Discrete Phase Model (DPM) within a Euler – Lagrange framework to track microplastic particles. Turbulence effects on particle trajectories were incorporated using stochastic tracking methods, enabling a realistic representation of particle dispersion within the flow field. The numerical model consistency was verified through hydrostatic pressure distribution analysis, velocity field evaluation, mass balance closure, and residual convergence assessment. Simulations were conducted for commonly encountered polymers in aquatic environments (PET, HDPE, LDPE, and PVC) with particle diameters ranging from 25 to 250 μm. The results reveal distinct transport behaviors governed by material density and particle size. Polymers with densities lower than water (HDPE, LDPE) are predominantly transported within the upper flow layers, polyethylene terephthalate (PET) exhibits turbulence-controlled dispersion, while polyvinyl chloride (PVC) demonstrates rapid and stable sedimentation. The proposed CFD – DPM approach proves effective in identifying potential accumulation zones and supporting monitoring and mitigation strategies for microplastic pollution in riverine systems, providing a transferable methodological framework for similar studies in other hydrological contexts.