Treatment performance variation at different depths within vertical subsurface-flow experimental wetlands fed with simulated domestic sewage

The aim of this study was to examine the variation in treatment performance, regarding the main pollutant parameters, i.e. chemical oxygen demand (COD), total nitrogen (TN), nitrate-nitrogen (NO 3 -N) and ortho-phosphate (PO 4 -P), at two different depths and at the outlet of duplicated vertical subsurface-flow constructed wetland (VSSFCW), with dimensions 0.6 m deep and 0.24 m 2 surface, planted with Pragmites australis and fed with simulated domestic sewage in a batch mode for four successive experimental trials, each one lasting three months. Four combinations of two hydraulic loads (0.08 or 0.17 m 3 m −2 ) and two feeding frequencies (2 or 3 times per week) were randomly applied to the four trials. The analysis of variance demonstrated that the pollutants concentration varied significant with the wetland’s depth, with the majority of COD and PO 4 -P removal occurring at the lower layer of the wetlands (i.e. in depths between 40–60 cm). High pollutant removal rates were obtained at the end of the four successive experimental trials at the wetlands outlet: COD 96%, PO 4 -P 52% and more than 60% of TN was transformed to NO 3 -N.