Diphasic transfer of oxygen in vertical flow filters: a modelling approach

Oxygen renewal, as a prominent phenomenon for aerobic bacterial activity, deeply impacts vertical flow constructed wetland (VFCW) treatment efficiency. The authors introduce a multiphase model able to simulate oxygen transfer in VFCWs. It is based on a two-phase flow module, and a transport module. The transport module is able to deal with convection/diffusion phenomena, inter-phase (air-water) mass exchange, and first-order kinetics. The first results displayed for the air phase allow us to draw the following ideas on the design of vertical filters. The ponding phenomenon is more efficient for oxygen renewal than non-ponding batch loading: it provides a higher value, sooner, and deeper in the filter. In non-colonised filters and for standard batch loading, oxygen convection in the air phase is predominant for oxygen renewal. The seepage front limits oxygen renewal through the bottom of the filter and leads to an insufficient oxygen concentration on the lowest part of the filter.