Study of the transport of model plastic microparticles in an experimental open channel bifurcation apparentus
Many end-of-life plastic products escape treatment and recovery channels and end up, intentionally or not, in different biosphere compartments, particularly in aquatic environments (rivers, lakes, oceans). Urban areas are the main source of microplastics, which come from the fragmentation of plastic packaging, the abrasion of tyres on roads and the release of synthetic fibers in washing machines, among other sources. This thesis focuses on the transport of microplastics within combined sewer overflows, the interface between the urban wastewater system and the environment. The main objective is to identify the accumulation zones and dispersion modes of microplastics in a free-surface bifurcation flow modelling a storm overflow. The challenge is therefore to understand "how microplastics, depending on their physico-chemical characteristics, are distributed in the lateral branch of a bifurcation ?".
To address this issue, protocols for developing model microparticles that reproduce the characteristics of environmental microplastics were developed for use in experimental apparatus. The first protocol enabled the elaboration of particles with controlled physical properties, incorporating in the bulk a fluorescent dye to improve their tracking by optical methods. The second protocol enabled accelerated ageing of the microparticles by UV photo-oxidation, simulating the chemical degradation of microplastics taken from an urban retention basin. As for the bifurcation flow, a 3D measurement method was used to characterize the three-dimensional structures present in the lateral branch. The measurements revealed that there was no systematic closure of the separation zone. Two forms of helical recirculation flow were identified : one carried by a vertical axis, associated with a longer residence time in the recirculation zone, and the other carried by a horizontal axis, favoring better transverse mixing. Downstream, these two structures generate secondary flows that accentuate the mixing between the slow flow in the separation zone and the fast flow in the vena contracta. The dynamics of the model microparticles were then studied experimentally using PTV-4D in a bifurcation flow, by varying their physical characteristics and injection position. A low Stokes number associated with the particles significantly increases the dispersion of the microparticles and their intrusion into the recirculation zone. In addition, a high settling or rising velocity increases accumulation near the bed and on the surface. In addition, the injection position plays a key role in the initial formation of accumulation zones, although this heterogeneity tends to diminish downstream.
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