Constructed and natural wetlands are widely used to improve many water quality parameters. Vegetated floating islands (VFIs) placed on the surface of these wetlands significantly enhance the efficiency of natural processes that reduce nutrients, suspended solids, heavy metals and other pollutants.
Pollutant reduction in VFIs, particularly nutrients such as nitrogen and phosphorous, occurs primarily through the actions of bacterial biofilms growing within the island matrix and on plant roots hanging below the islands. Direct uptake of nutrients by plants is minor, although plants are essential as they provide additional substrate for biofilm development while supplying oxygen and carbon for use by the bacteria.
Nitrogen-based nutrients are primarily removed from wetlands as nitrogen gas. Phosphorous is mostly deposited as organic-rich sediment which accumulates within or beneath the floating islands. This material can become anoxic and return its contained phosphorous to the water column, making it biologically available for algal or bacterial blooms that degrade water quality. Physical removal of this P-rich material is an essential wetland management action.
VFIs can remove phosphorous at up to 4.6 g/m2/day and ammonia at up to 8.1 g/m2/day with simultaneous denitrification of nitrate to nitrogen gas.
VFIs can significantly increase the efficiency of pollutant removal from natural and constructed wetlands.