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The present work deals with modeling of the fate of nitrogen through a vertical flow constructed wetland (VFCW) using gravel, treating directly domestic raw wastewater. The experimental design of the work involved lab-scale and full-scale experiments to calibrate the multi-component reactive transport model for constructed wetlands (CW2D). Besides measured values for the hydraulic parameters and the maximum autotrophic growth rate, we calibrated two other parameters (oxygen re-aeration rate and adsorption coefficients of ammonium) to reduce the difference between predictions and measurements. The obtained model determined the time-variation of nitrogen concentrations in the effluent with reasonable performances. With the use of the modelwedemonstrate that, during feeding period, the ammonium was significantly adsorbed onto organic matter besides conversion into nitrates; the adsorbed mass of ammonium was nitrified during the rest period provoking high nitrates concentrations during the first two subsequent batches. We also demonstrated that heterotrophic biomass was mainly present in the sludge layer (first 20 cm), whereas autotrophic biomass was located in the first 50cm of the VFCW (sludge and 30cm biomat).

Constructed wetlands (CWs) are attractive system for wastewater treatment for small communities, but interactions between the removal processes are often badly understood. Modelling of CW systems is a way to unravel them and improve their design. A modelling study with the HYDRUS/CW2D model was carried out on a French first-stage vertical flow constructed wetland (VFCW) at Evieu (Ain). Characterizing the hydraulic properties of the filtering material is a prerequisite. However hydraulic characterization is difficult due to varying proportion of organic matter in the filter. I present a combined laboratory and in-situ approach to achieve it. The laboratory approach gives prior estimates that are later refined by in-situ measurements and inverse modelling. Thanks to a large experimental dataset, it led to a reliable hydraulic parameter set allowing accurate simulation of water content within the VFCW. Prior to biological modelling, i developed and validated an original method to measure nitrification rates at the different depths of the VFCW and consequently at different organic matter contents. It is specifically adapted for partially saturated matrix and is based on respirometric principles. The obtained maximum growth rate coefficients for the autotrophic biomass are critical inputs for the VFCW plant modelling. Once the hydraulic calibration carried out and some of the biological parameters assessed, we simulate reactive transport. Particular attention was devoted to the nitrogen fate. During a feeding period, influent fractionation was determined, influent and effluent pollutant concentrations continuously monitored as well as in-situ oxygen concentrations. These measurements have been used to fit the biological model by adjusting kinetic parameters highlighting the need to include adsorption of ammonium. Good agreement between model and experimental data is obtained based on 2-4 h average concentrations, while instantaneous measurements are badly fitted., (AGRO 3) -- UCL, 2012

Modelling water flow in a Vertical Flow Constructed Wetland (VFWC) is a prerequisite to model wastewater treatment using process based filtering models. The material used in VFCW is very susceptible to generate preferential flow. If this occurs, water will bypass most of the soil porous matrix in a largely unpredictable way. Even if it is possible to simulate water content variations within a VFCW, we can not correctly model outflow with the standard van Genuchten-Mualem function if preferential flow occurs. A number of various model approaches have been proposed to overcome this problem. These models mostly try to separately describe flow and transport in preferred flow paths and slow or stagnant pore regions. The objective of this study was to study and simulate the hydrodynamic behaviour and the solute transport through a simplified representation of a French VFCW by using both a classical equilibrium model and a non-equilibrium model (dual-porosity model: mobile-immobile water model, with water content mass transfer) included in the HYDRUS-1D software package. Modelling results were compared to a solute breakthrough curve obtained from a tracer experiment, carried out on an existing VFCW. The comparison between measured and simulated tracer breakthrough curves indicates that the non-equilibrium approach seem to be the most appropriate for simulating preferential flow paths.

We developed an original method to measure nitrification rates at different depths of a vertical flow constructed wetland (VFCW) with variable contents of organic matter (sludge, colonized gravel). The method was adapted for organic matter sampled in constructed wetland (sludge, colonized gravel) operated under partially saturated conditions and is based on respirometric principles. Measurements were performed on a reactor, containing a mixture of organic matter (sludge, colonized gravel) mixed with a bulking agent (wood), on which an ammonium-containing liquid was applied. The oxygen demand was determined from analysing oxygen concentration of the gas passing through the reactor with an on-line analyzer equipped with a paramagnetic detector. Within this paper we present the overall methodology, the factors influencing the measurement (sample volume, nature and concentration of the applied liquid, number of successive applications), and the robustness of the method. The combination of this new method with a mass balance approach also allowed determining the concentration and maximum growth rate of the autotrophic biomass in different layers of a VFCW. These latter parameters are essential inputs for the VFCW plant modelling.