Abstract
Free water surface constructed wetlands (FSCWs) can be used to complement conventional waste water treatment but removal efficiencies are often limited by a high ratio of water volume to biofilm surface area (i.e. high water depth). Floating treatment wetlands (FTWs) consist of floating matrices which can enhance the surface area available for the development of fixed microbial biofilms and provide a platform for plant growth (which can remove pollutants by uptake). In this study the potential of FTWs for ammoniacal nitrogen (AN) removal was evaluated using experimental mesocosms operated under steady-state flow conditions with ten different treatments (two water depths, two levels of FTW mat coverage, two different plant densities and a control, all replicated three times). A simple model was constructed as a framework for understanding N dynamics in each treatment. The model was calibrated using data obtained from one treatment and validated independently for the other treatments. Specifically, we hypothesized that the nitrification and volatilization rate constants are inversely proportional to water depth and proportional to mat surface area. This allowed the relative magnitude of different removal mechanisms to be estimated. The model was able to predict steady-state concentrations of AN and total oxidized nitrogen (TON) across the different treatments well (values for correlation in the regression between measured and predicted steady-state concentrations and RMSE were 0.88 and 0.40 mg N L-1 for AN, and 0.63 and 1.75 mg N L-1 for TON). The results confirm that nitrification is the principal AN removal process, with maximum removal occurring in shallow systems with high matrix cover (i.e. a high ratio of biofilm surface area to water volume). Plant uptake was a relatively minor loss process compared to nitrification. Integrated experimental and model-based approach was found to be a useful tool to improve mechanistic understanding AN dynamics in FSCWs and system performance.
Highlights
Constructed wetlands are being increasingly used for water quality improvement during wastewater treatment [1,2,3]
The concentrations of both ammoniacal nitrogen (AN) and total oxidized nitrogen (TON) remain relatively constant over time, confirming that the systems were approximately in steady state
This is consistent with conversion of AN to TON via nitrification and suggests that a competent nitrifier community developed in these vessels between Days 10 and 20
Summary
Constructed wetlands are being increasingly used for water quality improvement during wastewater treatment [1,2,3]. In order to overcome these issues, floating treatment wetlands (FTWs) have been proposed [6, 7] These consist of floating islands constructed from synthetic or natural materials which possess a high submerged surface area that can facilitate the development of microbial biofilms [8]. They provide a platform for the growth of plants which can enhance nutrient removal via uptake and introduce carbon-rich exudates and oxygen via their roots which can facilitate microbially-mediated transformations of pollutants. Uptake of mineral nitrogen (as both NH4+ and NO3-) should be proportional to the number of plants per unit area of wetland
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