Okba Mostefaoui

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 mainly from the fragmentation of plastic packaging, the abrasion of tyres on roads and the release of synthetic fibers in washing machines. This thesis focuses on the transport of microplastics within combined sewer overflows, the interface between the urban wastewater system and the environment and the main urban vector of microplastic pollution. The main objective is to identify the accumulation zones and dispersion modes of microplastics in a free-surface bifurcation flow modelling a combined sewer 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 microplastics found in the environment, were developed for use in laboratory experiments. 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 the absence of a closed 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 outside. The dynamics of the model microparticles were then studied experimentally using 4D-PTV in a bifurcation flow, by varying their physical characteristics and injection position. Apart from the density effect, which leads to an accumulation near the bed or on the surface, it has been observed that microplastics with a low Stokes number (characterizing the response time of the particles) disperse more and are more likely to enter the recirculation zone. In addition, the injection position plays a key role in the initial formation of accumulation zones, although this heterogeneity tends to diminish downstream. This thesis also led to the development of a numerical code for microparticle transport, without coefficient adjustment, suited to small-scale aqueous flows. The results of the thesis improve our understanding of the behavior of microplastics in a turbulent open channel bifurcation flow modelling a combined sewer overflow. They highlight the influence of the physico-chemical characteristics of the particles on their dispersion and accumulation rate, driven by the shapes of the flow structures. In addition, the development of protocols for the elaboration of model microparticles, representative of microplastics, and of an adapted numerical code, open the way to new studies and better prediction of their dynamics in urban aquatic environments.