Hydraulic Model Comparison of Vertical-flow Regimes in Constructed Wetlands

Vertical-flow treatment wetlands have come into use because of their ability to provide highlevel secondary and tertiary wastewater treatment at a fraction of the land use of horizontal subsurface flow or free water surface wetlands. The flow regime typically selected is vertically downward, allowing wastewater to infiltrate through the system by gravity. However, recent research suggests that upflow wetlands may provide high-level treatment in terms of nitrogen removal. This paper compares upflow and downflow wetlands, focusing on the hydraulic function and natural aeration potential of each flow regime. Using HYDRUS-2D, a flow simulation and sensitivity analysis of plant uptake (Feddes parameters) and oxygen transport (kinetic parameters) are performed on hypothetical upflow and downflow wetland configurations. Comparison simulations of wetland mesic and prairie vegetation show that true wetland vegetation will thrive in saturated conditions and allow more oxygen transfer into the soil. The kinetics indicate that dissolved oxygen within the wastewater will be consumed very rapidly, reaching less than 30% of the soil depths in both the upflow and downflow cases. Adding gaseous diffusion of oxygen from the soil surface results in oxygen transfer down through 40% of the depth and shows that the upflow hydraulic regime may have better aeration capacity than downflow. This data may useful for choosing the hydraulic regimen, plant type and wetland bed depth to make nitrification and denitrification in constructed wetlands more effective and efficient. Better design of constructed wetlands will enable their use and benefits in developing communities in need of economic, environmentally-based wastewater treatment.