AUTOEPURATION EN EAU COURANTE : EVALUATION DE L'OXYGENATION DANS LES CASCADES EN MARCHES D'ESCALIER/Self-purification in running water: experimental evaluation of the oxygenation capacity of stepped cascade

To estimate the aeration potential of these structures in low discharges corresponding to small natural streams, a study was realized on a laboratory cascade model equipped with a modular number of size-adjustable steps. Oxygen transfer in stepped cascades is strongly dependent on the interfacial exchange area per water volume, the multiphase flow structure and the turbulence in the fluid. From the hydrodynamic point of view, three flow regimes were identified for a flow rate lower than 3 L.s super( -1: two nappe regimes and one transition regime. The measured oxygen transfer efficiency varies between 15% and 40% depending on water flow rates, and the number and length of steps in the cascade. The results showed an improvement of the aeration with the flow rate and with the number of steps. On the various studied systems of cascades, a linear relation between steps number and aeration efficiency was demonstrated. The addition of gravel on the step surfaces increases the cascade aeration capacity slightly, depending on the immersion level of the gravel.)