Multiphysics modelling of flow dynamics, biofilm development and wastewater treatment in a subsurface vertical flow constructed wetland mesocosm

A robust computational fluid dynamics (CFD) model accounting for both spatial and temporal dynamics of a subsurface vertical flow treatment wetland system was developed by combining fluid transport, solute transport, biokinetics, biofilm development, and biofilm detachment/sloughing using COMSOL Multiphysics™. The local porosity of the porous media was calculated based on the estimated biofilm development over time. The biofilm development sub-model considered both organic pollutant metabolism/degradation as a mechanism for microbial growth and the effect of fluid shear stress on the local biofilm detachment. The influence of biofilm accumulation on the permeability of the porous media was considered using the Kozeney–Carman relation. The permeability of the porous media was estimated by calibrating the model against experimental tracer data that had been collected in a full water-recirculation batch operation. As the biofilm developed from start-up, removal efficiency of readily biodegradable organic matter improved during the first 10 weeks of operation while no significant change was predicted after 10 weeks. A dead-zone region was predicted near the bottom of the system opposite to the outflow port after 100 days of operation attributed to exponential biofilm growth and low biofilm detachment rates. A decrease of 95% in the advective transport of organic matter in the dead-zone region occurred after 280 days of operation. No appreciable biofilm accumulation was predicted in the region near the outlet due to the high fluid shear stress acting on the biofilm surface. The averaged porosity of the mesocosm decreased by 6.6% after 365 days of operation while the local porosity in the dead-zone region decreased by 71%. Good agreement was found between the averaged porosity of the entire system and experimental data. This is the first model to integrate hydrodynamics, solute transport, biokinetics relating to pollutant removal, biofilm development, and biofilm detachment into a comprehensive model. Although computationally intensive, this is the first model to predict bio-clogging processes in a spatial manner similar to what would be realistically expected.