Evidences in Hydrodynamic Behavior Along a Float Treatment Wetland (FTW) on a Tropical Urban Stream

Floating Treatment Islands (FTIs) or Floating Treatment Wetland (FTW) are nature-based solutions to improve water quality and have been implemented in lentic and lotic ecosystems. The water treatment process occurs by a biofilm developed along the plant’s root zone, thus enabling the adsorption of components such as nitrogen, phosphorus, heavy metals, and suspended solids. The efficiency of an FTW system can be understood as a function of hydraulic retention time (HRT), water depth, the cultivated biomass, and the species of the plant. Studies carried out in laboratory and computer modeling describe the behavior of the fluid in the root zone and suggest optimal conditions to the FTW implementation, such as the number of FTW required, their distance, and dimensions to increase water treatment efficiency. However, physical and computational models generally consider a rigid behavior of the root zone, unlike the real conditions of plant roots that show movements along their extension caused by the stresses exerted by the fluid. This consideration can lead to differences between the behavior of physical and computational models developed under these conditions to a real physical model emplaced in a natural environment. Understanding the hydrodynamics of FTW systems is critical to measuring the positive impacts on aquatic ecosystems and the economics of urban water management. We quantified suspended solids adsorbed by the roots and their relationship with the characteristics of the root zone used in the experiment. The results of this pilot project were positive in relation to the theoretical descriptions. The water velocity through the root zone showed an average reduction above 60%. Along the length of the FTW, the capture of suspended sediments was related to the root zone mass of Pistia stratiotes (r2 = 0.5). However, there was no progressive reduction in the amount of solid mass captured, which may be associated with the non-uniformity of root biomass along the FTW and fluid behavior. As a way of complementing the experiments carried out in the laboratory and computer modeling, the real-scale experiments in an urban stream bring us hydrodynamics data and suspended sediment mass reduction in a pilot project of FTW, which may contribute to the improvement of hydrodynamic models.