Relationship between hydraulic efficiency and phosphorus removal in a submerged aquatic vegetation-dominated treatment wetland

Abstract

A tracer study (Rhodamine-WT dye) was performed on a 147-ha submerged aquatic vegetation (SAV)-dominated free-water surface treatment wetland in south Florida that received agricultural drainage waters (ADW). Two dimensional, time series plots of the dye concentrations revealed that a disproportionate amount of tracer flowed along the eastern and western levees of the cell. The tracer response curve developed from the outflow data indicated a prominent short circuit, which conveyed 44% of the volumetric flow based on model analysis. This large fraction of flow bypassed the SAV community and exited the wetland with only partial treatment for total phosphorus (TP) removal. The volumetric efficiency of the wetland was high (∼100%), but the hydraulic efficiency was low due to the low tanks-in-series number ( N = 1.3). Even though the short-circuit channels promoted near-plug flow, the dispersion number for the wetland was nevertheless still high ( D = 1.25), due to the wide distribution of detention times among the multiple flow paths. A dynamic hydraulic model based on a tanks-in-series network with mixing between the short-circuited and shallower, vegetated zones simulated the tracer response curve to an accuracy of 0.99 (as the coefficient of determination). The amount of time that a parcel of water resided within the wetland affected TP removal. Hydraulic retention times (HRTs) > 2.5 days consistently produced TP concentrations less than 35 μg/L, or 64% TP removal based on the inflow concentration of 96 μg/L. “Steady-state” TP concentrations among the internal and outfall stations during the tracer study enabled direct analysis of the impact of short-circuits on P concentrations and removal effectiveness. Based on longitudinal P concentration gradients, faster flowing short-circuit channels that contain little SAV were less effective in reducing TP concentrations ( k = 0.24 day −1) than the more densely SAV-occupied areas ( k = 0.50 day −1) that characterize 56% of the wetland. This was likely due to both the shorter detention time and the sparse SAV community within the channels. The lack of SAV contributed to an environment that reduced the sedimentation rate, direct plant uptake, and the pH, all of which contribute to removal and retention of P in the more quiescent, shallower SAV zones. Our results suggest that tracer studies, coupled with internal P concentration gradient analyses, are useful for determining hydraulic efficiency of a wetland and its effect on P removal. These types of investigations can also serve as the basis for modeling, optimization, and design studies.