Characterizing hydraulic properties of filter material of a vertical flow constructed wetland

Characterizing the hydraulic properties of filter material used in a vertical flow constructed wetland (VFCW) is a prerequisite to model wastewater treatment using process-based models. The filter material is a matrix of porous mineral material and organic matter that makes hydraulic characterization a difficult task. Here, we present a combined laboratory and in situ approach to assess the hydraulic properties of a VFCW installed at Evieu in Ain, France. The laboratory approach produces prior estimates of the local properties of the different VFCW system layers. These prior estimates are subsequently refined with inversely estimated parameters using the HYDRUS-1D code in combination with in situ hydrodynamic measurements. Laboratory experiments consisted of both direct (sand box, pressure chamber, and permeameter experiments) and inverse estimates (evaporation method) of hydraulic parameters. In situ measurements were based on 5.5-day monitoring of the full-scale filter using 24 time domain reflectometry (TDR) probes installed at different depths. Applying the methodology with successive optimizations led to a reliable assessment of the hydraulic parameters of the VFCW. We conclude that the consistent representation of the hydraulic behavior of the VFCW requires in situ hydrodynamic observations combined with inverse modeling. However, to avoid the ill-posedness of the inverse problem, the number of fitted parameters should be kept to a minimum and parameter initialization need to be based on local-scale laboratory measurements.