Evaluating the influence of hydrodynamics on the biological efficiency of Floating Treatment Wetland-retrofitted stormwater ponds.

Constructed wetlands incorporating floating treatment islands are a type of Nature-Based Solution that is gaining recognition as a promising approach for managing, treating, and reusing urban runoff. However, the hydrodynamics within these systems are highly complex and interfere in their treatment efficiency, requiring advanced simulation tools for effective analysis and design. At this aim, this work focuses on the bio-hydrodynamic analysis and optimisation of Floating Treatment Wetlands (FTW) using a validated, open-source, three-dimensional numerical model based on Computational Fluid Dynamics. This work addresses some key limitations observed in prior studies: (i) on one hand, it assesses the influence of a new set of geometric parameters, i.e. the FTW's submergence depth ratio and coverage area and, (ii) on the other hand, this study explores and quantifies the influence of hydrodynamics on the biological performance of FTWs. The research has shown that the depth of the floating treatment islands is the geometric parameter with the greatest influence on the system's hydrodynamics, while the length of the islands plays a more significant role in determining treatment performance. This apparent paradox highlights that, to enhance the purification efficiency of these systems, it is not the overall hydrodynamics of the wetland that must be optimised, but rather the fluid dynamics occurring within the floating islands themselves.