Turbidity reduction induced by Floating Treatment Wetlands (FTW): A flume experiment to assess the impact of flow velocity

Abstract The aim of the present study was to investigate the impact of the mean flow velocity on the Floating Treatment Wetland (FTW) roots' network behaviour and associated opportunity for suspended solids entrapment. The objective was to identify the appropriateness of a FTW to reduce turbidity in a retention pond subject to specific flow regimes. A flume experiment with and without FTW was run under three mean flow velocities impacting roots' inclinations and roots' water column occupancy. For initial turbidity of 60–70 FNU, the results indicate that inclusion of a FTW would increase turbidity reduction by up to 37%, 34% and 45% under flow velocity of 0.023 m/s (V1), 0.097 m/s (V2) and 0.181 m/s (V3), respectively, compared to the Control (experiments without FTW). This suggests that even under high flow velocity, inducing the roots to flatten under the mat, the FTW provided an efficient additional particles removal process to settlement, i.e. entrapment of suspended solids into FTW roots' network. For V1, turbidity thresholds for good and very good water quality status (EU guidelines) were reached about 1:00 and 5:30 (h:mm) faster with the inclusion of a FTW. Providing that flow ratio through the FTW roots' network could be promoted in a real retention pond, such as in the present flume experiment, potential turbidity removal efficiency solely induced by a FTW covering 16% of a pond length and 70% of pond width could be extrapolated for various pond sizes. As an example, the inclusion of a 8 m long FTW in a 50 m long pond with inlet turbidity of 60–70 FNU, could improve turbidity removal efficiency by ~32% at low flow velocity (V1) and by ~10% at medium and high flow velocities (V2 and V3).