Graphical representation of nutrient removal in constructed wetlands

A simple steady state model that is conceptually similar to those of Vollenweider and Dillon was developed to predict phosphorus retention in a wetland treatment system. The proposed model calculates wetland TP outflow concentration (TPout) as a function of TP mass loading rate (L), hydraulic loading rate (qs), and phosphorus settling velocity (νs). A phosphorus-removal diagram was developed, based on this simple model, and plots L, qs, νs against TPout on a three-axis graph. Comparisons between predicted and observed TPout in operating wetlands were in good agreement and indicated that a simple relationship governing phosphorus retention in wetlands exists and this relationship can be used to predict and evaluate wetland treatment performance. The proposed diagram can also be used to answer management questions by manipulating design parameters, such as mass loading and hydraulic loading rate, to evaluate economic impacts due to alternatives such as increasing the size of a wetland surface area, changing water depth, or reducing mass loading rates with pre-treatment options.