Filling hydraulics and nitrogen dynamics in constructed wetlands treating combined sewer overflows
Résumé
According to French standards, constructed wetlands for combined sewer overflow treatment (CSO CWs) are vertical flow filters with detention basin and outflow rate limitation. Their purpose is to treat rapid loads of wastewater with stochastic volumes, concentrations and periodicity. The first full-scale CSO CW has been monitored for three years, involving online equipment. This provided in-depth understanding of hydraulics and nitrogen dynamics. The saturation of the filter was visualized along a longitudinal section to follow hydraulics at filling. Tracer tests showed that short-circuiting effects, which are adverse, weaken during the process. This was confirmed by the decreasing NH4-N concentrations in the outflow. As such, short-circuiting can be addressed by minimizing filling time. As for nitrogen dynamics, NH4-N adsorption capacities were identical for the applied sand-zeolite mixture and pozzolana materials. To calculate inter-event nitrification of adsorbed masses, an equation was fitted to temperature and mass measurements. The rate doubled with every 5.7 °C. Finally, nitrate was found to get washed out by low hydraulic loads (median: 0.95 m3/m2), highlighting the possibility of developing a second filter stage for denitrification. The results helped to calibrate the design-support software Orage.