The contribution of plant uptake to nutrient removal by floating treatment wetlands

Floating treatment wetlands (FTWs) may provide an appealing alternative to the more conventional (sub) surface flow wetlands to solve problems associated with eutrophication in urban surface waters, because they do not claim additional land area. This study examined the contribution of plant uptake to overall removal capacity of FTWs. A batch mesocosm experiment was performed during the growing season using thirty 84 L polyethylene tanks covered with 0.28 m2 floating Styrofoam mats. Ten tanks served as a control (only Styrofoam cover), 10 tanks were planted with Iris pseudacorus, and 10 with Typha angustifolia. Nutrients were added weekly to keep total nitrogen (TN) and total phosphorous (TP) concentrations at approximately 4 mg N L−1 and 0.25 mg P L−1. Total removal of TN an TP from the treatment with Typha was relatively low, resulting from the limited increase in plant biomass during the experiment. Total removal of TN and TP from the tanks planted with Iris was 277 mg N m−2 d−1 and 9.32 mg P m−2 d−1 during the experiment. These values were significantly higher than the values for total removal from the control tanks, i.e., 54 times higher for TN removal and 10 times higher for TP removal. Plant uptake played a major role in the removal of nitrogen and phosphorous from the water by FTWs, i.e., 74% of TN removal and 60% of TP removal resulted from Iris uptake. These results suggest that FTWs planted with Iris can be applied in a temperate climate to overcome problems with excessive algae growth in surface waters.