Performance study of vertical flow constructed wetlands for phosphorus removal with water quenched slag as a substrate

Two pilot-scale vertical flow constructed wetlands (VFCW) were designed, with one system containing water quenched slag (WQS) from a steel plant as a substrate and the other one as a control without WQS. Each VFCW consisted of a down-flow unit and an up-flow unit. Both of the VFCWs were planted with Canna indica Linn. This study focused on evaluation of the ability to remove phosphorus (P) for both VFCWs. Batch P adsorption and desorption tests were conducted. The influence of hydraulic retention time (HRT) and phosphorus loadings on phosphorus removal were investigated. Results showed that the WQS had much higher P adsorption capacity than gravel. The amounts of phosphorus adsorbed on the WQS were 0.17 g/kg and 0.05 g/kg for the down-flow and up-flow units, respectively, and were 0.09 g/kg and 0.025 g/kg on the corresponding gravel, respectively. Dates from the adsorption experiment were fitted to both Freundlich and Langmuir adsorption isotherm equations. Desorption test demonstrated that the adsorption of phosphorus to the WQS was not a reversible process. An HRT of 1 d, which favored the VFCW system operation, was proposed. More than 80% of phosphorus removal was achieved for the VFCW with WQS when phosphorus loadings were in the range of 12.2–36.8 g/m2 d. The down-flow and up-flow units of the wetland system ensured sequential and efficient removal of phosphorus from wastewater, and also easy and flexible replacement of the WQS substrate in case of saturation. The WQS used in this study can be a promising substrate for the VFCW for improving phosphorus removal.