Quantification of hydraulic characteristics and validation of CFD simulation of subsurface flow constructed wetlands using tracer

The analysis of residence time distributions (RTDs) is of great significance in the hydraulic characterization of subsurface flow constructed wetlands (SSF-CWs), and tracer experiments and simulations are generally used methods to obtain RTDs. This study developed a computational fluid dynamics (CFD) model involving two tracer tracking approaches in order to understand and quantify the flow behavior in SSF-CW. To validate the CFD model, RTDs were obtained from bench-scale impulse response tests with finite-constant injection of tracer, and the hydraulic indexes were corrected by introducing tracer injection time and nominal residence time in the formulae. The shape similarities of RTDs between the CFD model, the gamma model and observation were evaluated by a dynamic time warping algorithm, where the CFD model incorporating advection, diffusion and dispersion processes had the highest shape similarity (0.94-0.98). The proposed CFD model gave the tracer the same velocity field as the fluid, characterizing more accurate hydraulic indexes compared to the gamma model. The CFD model is appropriate for design optimization prior to full-scale construction since it saves time and is environmentally benign. It also gives visualization of the tracer trajectory, which aids in understanding contaminant fate and transport within the SSF-CW.