Numerous studies have demonstrated the effectiveness of emergent macrophyte-dominated wetlands for removing nitrogen (N) via denitrification. Wetlands containing floating macrophytes have similar capabilities. Anoxic conditions often prevail beneath mats of both large-leaved and small-leaved floating macrophytes. The plant rhizosphere, particularly of the large-leaved species, provides a potential attachment site for denitrifying bacteria, and detritus and exudates produced by the vegetation likely serve as carbon sources for microbial activity. Small-scale studies with floating macrophytes have estimated denitrification rates as high as 3.0 gN/m2-day.

Artificial wetlands dominated by floating vegetation have an advantage in that they can be deployed within stagnant or slow-flowing water bodies. Various construction techniques for floating wetlands have been evaluated, and several are discussed in this presentation.

We deployed and tested a floating wetland system in a small eutrophic urban lake for 18 months. The wetland was contained within a floating boom and flexible skirt that effectively isolated a column of water 262m2 in area and 2.75m deep. The wetland was equipped with a solar-powered pump, which provided approximately one hydraulic exchange per week. Inflow and outflow analyses revealed that total N was reduced from 1.80 to 1.08 mg/L, representing a mass removal rate of 0.3 gN/m2-day. However, because the bulk of the influent N was organic-N (i.e, phytoplankton), rates of algal cell mineralization and subsequent nitrification probably limited the availability of NO3-N for denitrification. During two batch incubations, we injected NO3-N into the water column beneath the floating plant mat at concentrations as high as 8.8 mg/L. NO3-N levels were reduced to below 0.004 mg/L within 11 days, representing a NO3-N removal rate of 2.2 gN/m2-day. These data demonstrate that floating artificial wetlands can be an effective tool for N removal in urban and agricultural settings.