Variation in contaminant removal efficiency in free-water surface wetlands with heterogeneous vegetation density

Abstract

The interaction between flow and vegetation in constructed wetlands plays a major role in determining wetland performance. In this study, a two-dimensional depth-averaged model was used to simulate flow, mass transport and contaminant removal in a conceptual free-water surface (FWS) wetland with heterogeneous vegetation patterns. The main objectives of this study were (1) to quantify the effectiveness of FWS wetlands with different vegetation patterns in reducing pollutant load, and (2) to identify optimal vegetation distributions that maximize contaminant removal. Simulations were performed for different random vegetation fields characterized by imposed mean, variance and correlation lengths of stem density. The wetland was assumed to receive water from a stream and to deliver it back to the same stream according to an imposed head drop. The simulations show that the concentration reduction efficiency increases monotonically with average stem density, whereas mass removal has a peak for an intermediate value of average stem density. The ensemble average of the total mass removal decreases for increasing stem density variance and correlation length, because the presence of vegetation patches with significantly different stem density promotes preferential flow paths. Preferential flow paths parallel to the mean flow direction were found to reduce the hydraulic efficiency of wetlands by producing short-circuiting, whereas, for the same mean stem density, alternating stripes of stem density perpendicular to the flow direction provided higher concentration and mass reduction. By providing a quantitative understanding of the impact of spatial vegetation heterogeneity, the results provide useful guidelines for design and maintenance of constructed wetlands for wastewater treatment.