Variable residence time-based model for BOD removal in free-water surface wetlands

Constructed wetlands have been increasingly used for treatment of secondary wastewater containing various contaminants. This paper presents a Variable Residence Time (VART)-based model, called VART-BOD model, for simulation of BOD removal processes in constructed wetlands with free water surface (FWS). Mathematically, the VART-BOD model describes a free water surface wetland with three vertical layers, including vegetated water column layer, advection-dominated upper root layer, and diffusion-dominated lower root layer. Biogeochemically, the VART-BOD model simulates various BOD removal processses and mechanisms, including monod kinetics of bacterial growth, mass exchange between water column and root layers, advection, dispersion, and diffusion. A unique feature of the VART-BOD model is the incorporation of a dynamic diffusion-dominated root-zone. The VART-BOD model was tested with data collected from two distinct FWS wetlands: Gustine Wetland, USA and Manzala Wetland, Egypt. Testing results showed that the coefficient of determination (r2) and root mean square error (RMSE) values vary in the ranges of 0.73–0.99 and 0.41–8.70 mg/L, respectively, for Gustine Wetland cells while r2 = 0.98 and RMSE = 0.72 mg/L for Manzala Wetland. The VART-BOD model is a reliable and efficient tool for designing constructed wetlands and for understanding effects of various processes and mechanisms on the treatment efficiency of wastewater in constructed wetlands