Two-phase flow modelling for oxygen renewal estimation in vertical flow filter: luxury or necessity?

Scientists and practitioners exhibit an increasing interest on effluent transfer and degradation modelling in Vertical Flow Sand Filters (VFSF) and Vertical Flow Constructed Wetland (VFCW). Modelling software used to this purpose is mainly monophasic: in the unsaturated zone, only water flow is taken into account and air phase influence is assumed to be negligible. In hydrology, many studies have point out the limitations of this assumption in order to quantify air phase movement but little has been done in the modelling of vertical flow filter. Despite its complexity, two-phase flow modelling allows to overcome these difficulties. In this work, we describe the complex air and water flows in the particular case of vertical flow filter fed intermittently using both numerical and experimental results. Complete different behaviour is observed depending on ponding occurs or not. If it does, flow is clearly influenced by air entrapment which is responsible of a reduction of the infiltration speed and of the drainage of a part of the water kept at the interface between the sand and the drainage layer. Finally, we study the dependency of oxygen income by convection on hydraulic load and compare numerical results with experimental results obtained on oxygen consumption.