Constructed wetlands (CWs) represent a natural solution for sustainable wastewater treatment, combining hydraulic processes with biochemical pathways for the removal of organic matter and nutrients. Despite their widespread application, vertical flow CWs under unsaturated conditions remain underexplored, especially with regard to oxygen dynamics and substrate degradation. This thesis develops and applies a one-dimensional numerical model of an unsaturated vertical CW, designed with reference to the HYDROUSA project facility in Lesbos, Greece, to simulate water flow and pollutant removal processes under realistic operating conditions. The model combines the Richards equation for variably saturated flow with the Dilute Species Transport module of COMSOL Multiphysics, allowing for the simultaneous description of water movement and solute transport.