Solute mixing in full-scale constructed wetlands: seasonal variation of vegetation & hydraulic performance

Within the last decades the importance of sustainable treatment technologies, such as constructed wetlands (CWs) and vegetated ponds, has raised due legislation (e.g. WFD), directing toward green infrastructure to mitigate water pollution. The efficiency of pond and CW treatment systems depends on the internal hydrodynamics and mixing interactions between water and aquatic vegetation. In order to contribute to the current knowledge of how emergent real vegetation affects solute mixing, and physical flow characteristics in full-scale aqueous systems, an understanding and quantification of those processes and interactions was sought under the: i) natural seasonal vegetation and flow rate variation in two CWs, and ii) physical flow characteristics in overall six different full-size treatment units. To address these issues, outdoor tracer field studies were undertaken in each treatment unit. Regarding the seasonal plant variation, an intelligent automated tracer injection system was developed to achieve autonomous remote measurements in two CWs, vegetated by Phragmites australis, in different seasons and flow rates. Experiments involved measurements of longitudinal mixing, physical flow characteristics and vegetation characteristics in different plant ages and various discharges. It was shown that seasonal vegetation variation influences the longitudinal mixing coefficient by up to four times, and the physical flow characteristics by increasing the flow resistance and creating stagnant backwaters at the end of plant cycle, achieving reduction of the peak concentration by three times. Longitudinal mixing decreased with discharge in all plant ages. Furthermore, it was shown that internal design (i.e. bed topography or vegetation distribution) overwhelm the seasonal plant variation effects on mixing and flow characteristics. Moreover, relative comparison of outlet configuration, inflow conditions, and internal features, between the six different treatment units demonstrated an increase in residence time by up to three times. Results underlined the importance of investigating hydrodynamics and physics of flow in full-size units to enhance treatment efficiency and predictions of water quality models