Author(s): Diego Lopez; Okba Mostefaoui; Emmanuel Mignot; Valerie Massardier-Nageotte

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Keywords: Bifurcation flow; Recirculation flow; Particle transport

Abstract: Many microplastic particles, originating from cosmetics, textile and tires residuals or from macro-plastic fragmentation, are found in large quantities in sewer systems, and due to limitations in water treatment abilities end up in the ocean through the river network. In the sewer system, many singularities (such as junctions, bifurcations, cavities, etc.) result in a flow separation with one or several large-scale recirculation zones. The velocity magnitude in these recirculation cells is typically one order of magnitude smaller than in the mainstream, so that particles could be trapped for long times, and float or settle depending on their density. Whereas there exists several studies and models for microplastic advection at large scale (Cai et al. 2023), little is known about local dynamics and the coupled effect of large structures and turbulence in a confined environment. The present work aims at studying experimentally the dynamics of microplastic particles in a bifurcation flow (which can be seen as a simplified combined sewer overflow). To that end, the bifurcation flow is first analyzed using 3-D velocimetry, and then microplastic particles of various sizes and densities are seeded and tracked in the lateral branch. An analytical model is then proposed in order to account for local particle dynamics and flow turbulence effects.

DOI:

Year: 2025