Influence of partial saturation on total nitrogen removal in a single-stage French constructed wetland treating raw domestic wastewater

[Display omitted] •The performance of constructed wetlands of the French system was assessed.•Nitrogen removal by filters with a saturated layer was investigated.•Nitrification and denitrification were induced in a single VFCW stage.•Change of the inner water saturation level of the VFCWs was tested.•The effect of Eh and O2 concentrations on treatment performance was analyzed. The objective of this research was to evaluate the nitrogen removal performance of vertical flow constructed wetlands implemented with a bottom saturated layer in the treatment of raw wastewater (French system). This research was carried out in two pilot systems over 16 months with different saturation depths (25cm and 15cm) to determine the optimal level for denitrification promotion. Monitoring included regular analysis of the physico-chemical parameters (TSS, dissolved COD, NH4+-N, TKN and NO3−-N) as well as online measurements (redox potential, oxygen and temperature, hydraulic behavior) to explain the filter’s behavior. Statistical performance analysis showed that a higher removal rate was obtained when the filters operated with 25cm saturation in comparison with 15cm saturation depth. The hydraulic study revealed the positive evolution of the system from the winter, where the clogging was observed, to the summer, where this phenomenon was not observed, demonstrating that the hydraulic conditions affect the nitrification efficiency. The redox potential measured at three different depths inside the filters clearly showed the variation in the Eh values as a function of the saturation level and feeding–resting cycles, indicating that such measurements are capable of revealing the existing biogeochemical processes in highly dynamic systems, such as constructed wetlands. The results demonstrated variations in performance according to the saturation condition and the system’s hydraulics; however, improvement of the design or the introduction of an additional treatment stage is necessary to achieve complete TN removal.