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