Two variable residence time-based models for removal of total phosphorus and ammonium in free-water surface wetlands

Abstract

This paper presents two powerful and productive models, called VART-TP model and VART-NH4 model, for simulating the removal processes of total phosphorus (TP) and ammonium (NH4+), respectively, in constructed wetlands with free water surface (FWS). A distinctive feature of the models is the incorporation of a dynamic diffusion-dominated root-zone. The VART-TP model was tested with data collected from four small test cells of constructed wetlands (TCW) planted with submerged aquatic vegetations in south Florida, USA. The test cells were divided into two groups with the north test cells (N1 & N15) receiving an average phosphorus (TP) concentration of 71μg/l and the south test cells receiving an average TP concentration of 27μg/l. Testing results showed that root mean square error (RMSE) of the VART-TP model varied in the range of 4.45-17.50μg TP/l., respectively, for north test cells while the RMSE ranged between 3.58-7.63μg TP/l for south test cells. The result suggested that (1) the N1 and N15 test cells receiving high TP concentrations are capable of removing TP with high efficiency and (2) the VART-TP model is capable of simulating the removal processes of TP. The VART-NH4 model was tested with data collected from Lake Manzala Engineered Wetland (LMEW), Egypt, which generally received the influent of high ammonia concentration (average=2.26mg NH4/l). The testing results showed that the NH4+ concentrations simulated with the VART-NH4 model were in reasonable agreement with those monitored at the wetland outlet, as indicated with the RMSE of 0.06mg/l. The results demonstrated that the VART-TP and VART-NH4 models are reliable and efficient modeling tools for designing constructed wetlands.