Floating Treatment Island in a Stormwater Pond: Numerical Insights into the Effects of Root Zone Permeability on Pond Hydraulics and Nutrient Removal

This study used computational fluid dynamics to investigate how the root zone density affected the hydraulics and nutrient removal of a full-scale stormwater pond retrofitted with a floating treatment island (FTI). The root zone density was represented by permeability, which describes the capacity of fluid to flow through the porous root zone of the FTI. Permeability ranged between 10−20 and 10−2 m2. Nutrient removal was represented by the first-order reaction model, and the FTI occupied 11% of the pond volume. The simulations showed that low permeability caused strong flow deflection around the FTI, resulting in a modified flow structure within the pond that reduced short-circuiting and mixing, compared to ponds without FTIs or with high permeability FTIs. Importantly, higher limits of mass removal were achieved for FTIs with higher permeability, suggesting that increasing removal does not require increasing the number of plants placed in the FTIs, thus reducing the cost of building FTIs. The inverse relationship between root zone density and treatment is explained by the fact that, for higher root density, most pond water bypasses the root zone, receiving no treatment. The relationship between mass removal in the pond system and root zone permeability is described for a comprehensive range of permeability values.