Modelling phosphorus dynamics in constructed wetlands upgraded with reactive filter media

University dissertation from KTH : KTH Royal Institute of Technology

Abstract: Phosphorus mobility and sorption mechanisms underlying the removal efficiency of different adsorbent media were evaluated using the SWAT model and 3Dreactive transport model. The aims of the thesis were to: i) improve understanding of phosphorus mobility in the Oxundaån catchment; ii) identify suitable sites for constructing wetlands; iii) develop the numerical reactive transport model in order to identify key parameters which affects sorption efficiencies of the reactive adsorbent media; iv) predict the media's sorption efficiencies; and v) evaluate the viable reactive adsorbent media for mitigating eutrophication in the Baltic Sea. To predict phosphorus removal efficiencies of the adsorbent media and visualize the sorption process within the constructed wetlands, a three-dimensional model was developed within the COMSOL Multiphysics®. The reactive transport model was developed by coupling four physics interfaces to simulate the processes of water flow dynamics, transport of diluted phosphorus species, reaction kinetics and heat transfer in the porous media. The SWAT modelling results showed that arable land with the less background phosphorus retention, lower soil permeability and lower land slope could provide suitable sites for constructing wetlands. The simulated phosphorus sorption efficiency of the reactive filter media was ranked: 88 % (113 g P kg-1) for Polonite®, 85 % (81gP kg-1) for Filtralite P®, 62 % (61 gPkg-1) for Blast furnace slag, 57 % (44gP kg-1) for Wollastonite. In comparison to other media, Polonite® was observed to be a suitable reactive adsorbent media for wetland applications under different hydraulic loading rates and pH change, whose P-removal efficiency last for six years. Moreover, the modelling results showed less significant effects of particle size on the phosphorus removal efficiency as compared to solution pH and Ca mineral content. Precipitation was identified to be the dominant mechanisms for phosphorus removal in these media with the positive correlation to pH increase (R2 = 0.94, p-value = 0.002) and Calcium content (R2 = 0.73, p-value = 0.004). The good agreements between the simulated outputs and experimental data accurately captured the processes of phosphorus mobility and removal. The results suggest that the reactive transport models are valuable tools for providing insight into sorption processes in subsurface systems and improving design criteria for constructed wetlands. More experimental data are needed to calibrate the sensitivity of local parameters in order to better assess the performance of subsurface flow constructed wetlands.