Influence of saturated zone depth and vegetation on the performance of vertical flow-constructed wetland with continuous feeding

The object of this study was to investigate the effect of saturated zone depth (SZD) and plant on the removal of organics and nitrogen in four continuous-feed vertical flow-constructed wetlands (VFCWs). Three VFCWs were planted with Iris pseudacorus and operated at different SZDs (19, 51, and 84 cm), and the other one was non-planted and operated at 51 cm SZD. The VFCWs were operated with an organic loading rate (OLR) of 79 g chemical oxygen demand (COD) m−2 day−1, a total nitrogen loading rate (NLR) of 11 g N m−2 day−1, and a hydraulic loading rate (HLR) of 0.35 m3 m−2 day−1. Simultaneous transformation of ammonium and nitrate occurred in all of the four systems. In the planted bed with 51 cm SZD, suitable conditions for nitrification and denitrification could be created and the best performance for total nitrogen (TN) removal was realized via simultaneous nitrification and denitrification (SND), achieving TN removal efficiency of 67.4–80.3%. Higher ammonium nitrogen (NH4+-N) and COD removal efficiency was obtained in the system operated with 19 cm SZD, whereas higher NO3−-N removal could be achieved in the bed with 84 cm SZD. With the same SZD of 51 cm, the planted VFCW performed preferable removal of COD, NH4+-N, and TN in comparison with the non-planted one. All the VFCWs showed high removal efficiencies for total phosphorus (> 60.15%). Adsorption of phosphorus was primarily observed in the top and upper-middle layers filled with carbon burn slag. It has been proved that the partially saturated VFCW operated with continuous feed could achieve good performance in removal of organic matter and nitrogen by SZD adjustment to develop appropriate aerobic and anoxic regions.