Hydrodynamics Induced by Floating Treatment Wetlands: Impact of the Flow Velocity and Root Network on Flow Structure and Suspended Solids Removal

Floating treatment wetlands (FTWs) have been increasingly studied and applied over the last 15 years, yet the conditions for their effective implementation are still not fully understood. To refine current understanding and identify conditions that maximize particulate pollutant removal, a flume experiment with a real FTW was conducted to investigate how the mean flow velocity ( U 1 = 0.018 m · s − 1 ; U 2 = 0.083 m · s − 1 ; and U 3 = 0.171 m · s − 1 ) and root-network characteristics (i.e., R = ratio of the water column occupied by the FTW over the total pond water depth) influence the flow structure and associated suspended solids (SS) retention within FTWs. Flow acceleration (up to twice the mean flow velocity) and deceleration occurred underneath and downstream, respectively, of the FTW having a rooting depth ratio R of 0.56 ( FTW 0.56 ). A substantial fraction of the flow was diverted toward the flume bottom—particularly under U 1 —reducing root–flow contact and particle entrapment. Under higher velocities, the flow could penetrate deeper into the root network, increasing opportunities for particles’ entrapment and increased treatment benefits. The results indicated that FTW 0.56 could enhance pond treatment efficiency by up to ∼ 35 % , with greater benefits at higher velocities. This is partly due to reduced performances of conventional ponds to settle particles under these higher flow velocities ( U 2 and U 3 ). Increasing the rooting depth ratio (e.g., 0.87) would further improve efficiency, especially under low flow (up to three times higher removal efficiency), where FTW 0.56 offered limited additional benefit because of greater short-circuiting around the root network. Overall, the results of the present study suggest that careful consideration should be given to the flow the FTWs are subject to, the original pond design and its performance toward natural settling, to evaluate whether inclusion of FTWs would provide sufficient additional treatment benefits.