Evaluating Floating Treatment Wetlands to Improve Nitrogen Removal in a Wet Detention Pond

Wet detention ponds are used for stormwater treatment across the United States and reduce most pollutants by at least 60%, but only remove 30% of total nitrogen. Floating Treatment Wetlands (FTWs) are an emerging technology that uses aquatic plants suspended in the pelagic zone to remove nitrogen through vegetative assimilation and microbial denitrification. A before-after field experiment evaluated nitrogen removal in a an existing pond in Orlando, FL, retrofitted with BioHaven® FTWs planted with the aquatic macrophytes Juncus effusus (Soft Rush) and Pontederia cordata (Pickerelweed). Surface water samples were used to compare the nitrogen removal performance of the pond under both storm and non-storm conditions during a pre-analysis phase (control) to post-analysis after FTW deployment. The evaluation revealed similar TN removals in non-storm conditions during pre-analysis and post-analysis periods (-1% and -3%, respectively). During storm conditions, there was a negative TN removal of -26% in the preanalysis compared to the positive 29% removal post-analysis. In addition, nitrogen concentrations for organic-nitrogen, ammonia/ammonium, and nitrites/nitrates were used as input for calibrating and validating a system dynamics model to predict multiple, interacting nitrogen species\u27 transformation and translocation across the abiotic and biotic components of water, sediment, plants, and atmosphere. The validated model created in STELLA v.9.4.1 was used to simulate alternative designs to achieve maximum nitrogen removal based on the treatment efficiency in the evaluation. Simulations predicted 60% FTW coverage at the experimental planting density (22 per m2) could achieve maximum nitrogen removal. Alternatively, similar nitrogen removal could be achieved at only 15% FTW coverage by increasing plant density. The model can be used as a lowcost tool for designing FTW technology applications and monitoring nitrogen transport