Impact of different feeding strategies and plant presence on the performance of shallow horizontal subsurface-flow constructed wetlands

The aim of this investigation was to evaluate the effect of continuous and intermittent feeding strategies on contaminant removal efficiency of shallow horizontal subsurface-flow constructed wetlands (SSF CWs). Also it was tested the effect of the presence of plant aboveground biomass on removal efficiency. Two experimental wetlands planted with common reed were subjected to a three-phase, 10-month experiment involving a common source of settled urban wastewater with a hydraulic loading rate of 26 mm/d during the first and second phases and 39 mm/d during the third. In the first and second phases one of the wetlands was fed continuously while the other was fed intermittently. In the third phase both systems were operated intermittently, but in one the macrophyte aboveground biomass was cut in order to study the effect of plant aboveground biomass on the removal efficiency. The intermittently fed system presented systematically more oxidised environmental conditions and higher ammonium removal efficiencies (on average 80 and 99% for the first and the second phases respectively) compared with the continuously fed system (71 and 85%). The mass amount of ammonium removed ranged from 0.58 to 0.67 g N/m2 d for the intermittently fed system and from 0.52 to 0.58 g N/m2 d for the continuously fed system. Sulphate removal was higher in the continuously fed system (on average 76 and 79% for the first and second phases respectively) compared with the intermittently fed system (51 and 58%). In the third phase the wetland that operated with aboveground biomass exhibited more oxidised environmental conditions and better removal efficiencies (on average 81% for COD and 98% for ammonium) than the wetland operated without aboveground biomass (73% for COD and 72% for ammonium). The results of this study indicate that the intermittent feeding strategy improved the removal of ammonium and the presence of aboveground biomass enhanced the removal of COD and ammonium.