Sedimentation and resuspension modelling in free water surface constructed wetlands

Abstract

Eutrophication is a widespread problem that is being tackled from many perspectives and the recently applied technology of constructed wetlands is being used in the treatment of eutrophic water. However, process-based models to simulate their performance are scarce, so in this work a mechanistic model was developed to simulate the removal of total suspended solids, phytoplankton and total phosphorus in free water surface constructed wetlands treating eutrophic water. The model represents the influence of the main factors of the biotope and biota on these water quality variables, and particular attention is paid to resuspension produced by wind and by avifauna. Likewise, the effect of emergent vegetation cover in sedimentation, resuspension and phytoplankton growth is included. Phytoplankton is considered to store phosphorus internally in order to use it when growing, and the contribution of phytoplankton concentration to the suspended solids budget is included. The software AQUASIM was used to calibrate and validate the model in two full-scale constructed wetlands treating eutrophic water from Lake l′Albufera de València (Spain) for three years. The simulated data and field measurements showed satisfactory adjustments for the three studied variables. The budgets obtained for each variable reveal that sedimentation and resuspension are the main processes in total suspended solids performance. Sedimentation of organic particulate phosphorus is the most important process in total phosphorus removal. The sum of the effect of resuspension by avifauna and by wind increases by more than 50% the quantity of solids that enters the water column. The model reveals that simulating the effects of the emergent vegetation cover and resuspension is crucial for representing the performance of the studied variables.