Zoé Fau will defend her PhD at INRAE salle Rhône du Centre Lyon-Grenoble Auvergne Rhône-Alpes. Her PhD focuses on bioflocculation in activated sludge processes, and in particular the role of Extracellular Polymeric Substances (EPS). Optimising activated sludge processes (reducing energy demand and recovering pollutants) depends in part on a better understanding of the mechanisms, particularly bioflocculation, that drive the capture of organic matter. The role of EPS, secreted by bacteria, in these mechanisms is being studied more and more, but the conclusions are divergent. This PhD thesis proposes a dual approach, methodological and mechanistic, based on experiments at different scales (laboratory, pilot and full scale) on activated sludge fed with real wastewater, with the aim of improving our understanding of the role of EPS in bioflocculation in activated sludge processes.