Numerical model of a constructed wetland in Lesbos, Greece

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. To capture the biochemical transformations of organic matter, nitrogen species, and oxygen, the Bio-Pore conceptual framework was implemented and adapted to unsaturated vertical conditions. Particular attention was paid to oxygen exchange between pore water and the atmosphere, reformulating it to reflect its dependence on water content and saturation, thus expanding the model's applicability beyond traditional saturated systems. Calibration and validation were performed using field data of inlet and outlet chemical oxygen demand (COD), measured weekly at the Lesvos pilot plant. COD was separated into soluble and particulate fractions, which were dynamically simulated through hydrolysis reactions, substrate degradation, and microbial growth. The oxygen balance was analyzed using an additional source-sink formulation that considers diffusion and replenishment from the atmosphere. Sensitivity analyses were performed to assess the role of hydraulic parameters (Van Genuchten-Mualem functions), biodegradation constants, and oxygen mass transfer coefficients in treatment efficiency. The results demonstrate that the adapted Bio-Pore scheme can successfully reproduce the temporal dynamics of COD removal in an unsaturated vertical wetland. The integration of an oxygen exchange mechanism significantly improved the accuracy of dissolved oxygen predictions, crucial for maintaining aerobic degradation. The model also provided insight into the interaction between flow variability, substrate removal pathways, and long-term treatment performance. This thesis helps bridge the gap between empirical observations and process-based modeling of unsaturated constructed wetlands. By integrating advanced hydraulic formulations with a network of biogeochemical reactions, it offers a flexible framework for simulating pollutant removal and optimizing wetland design. This approach supports the development of more resilient and efficient constructed wetlands, highlighting their potential to provide decentralized wastewater treatment while protecting environmental quality and promoting circular water use.