Numerical simulation of contaminant removal in a vertical subsurface-flow constructed wetland

Hydraulic characteristics and contaminant removal effect in a vertical subsurface-flow constructed wetland were investigated by establishing a numerical model. The results demonstrated that retardation factor decreased with time while wetland substrate was gradually filled with the importation of contaminated water until reaching the saturation state. The flow velocity increased with time when the substrate was not on saturated condition, and decreased rapidly after saturation. The process of the substrate reaching saturation state was layered and gradual. The increased rate of effective saturation in substrate was less than that of the post-period: the higher the saturation level, the more easily water flowed. When the substrate was not completely saturated, pollutant concentration increased sharply to the max value. After the substrate was at the saturation state, concentrations of contaminants decreased slowly with time as adsorption within the solid particles and biodegradation. At the early stage the retardation factor gradually decreased as water depth increased, resulting in a close relationship between effective saturation and the retardation factor. With the increase of porosity and partition coefficient and decrease of degradation rate, hydraulic efficiency of the vertical subsurface-flow constructed wetland lessened.