Measurements of water content distribution in vertical subsurface flow constructed wetlands using a capacitance probe: benefits and limitations

Water content profoundly affects the efficiency of vertical flow constructed wetlands. Detailed vertical water content profiles could help to improve our understanding of the behaviour of these systems. in this study we tested a capacitance probe to measure water content in a vertical flow constructed wetland pilot plant. Field measurements were compared with data recorded in a purpose-built laboratory apparatus. Our results show that the on-site assessment of water content profiles should help to identify the real layout of the vertical systems, and this in turn should prevent or at least highlight possible running failures. Theoretical hydraulic retention time, oxygen gas availability and the calibration of van Genuchten's equation for unsaturated flow were easily assessed thanks to the water content measurements. Furthermore, water content measurements performed in dynamic conditions enabled us to define the actual specific flow for the various phases of the filling-emptying processes and to check the influent ponding for a uniform distribution of wastewater over the bed. Water content measurements are quick, cheap and safe, providing a useful tool in many application areas. However, calibrating the probe for the media typically used in vertical flow bed is not simple. In addition seriously misleading measurements can arise when steep variations in water content occur in the system.