Self-purification in running water: experimental evaluation of the oxygenation capacity of stepped cascade
In aquatic ecosystems, dissolved oxygen is consumed by the chemical and biological processes involved in natural self-purification (oxidation-reduction, degradation of organic matter,...) and by the respiration of the aquatic species. The self-purification can be enhanced by water aeration improvement and the dissolved oxygen level is thus an indicator of the water quality. Hydraulic structures such as cascades play the role of an aeration system by improving the dissolution of atmospheric oxygen in water. 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-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 (in the nappe flow) 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. For a given total height, an increase in the cascade length disadvantages the oxygen transfer.