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A constructed wetland is an artificial marsh or swamp, created for human use, such as wastewater or sewage treatment, as habitat to attract wildlife, or for land reclamation after mining or other disturbance. Natural wetlands act as biofilters, removing sediments and pollutants such as heavy metals from the water, and constructed wetlands can be designed to capitalize on this feature.
Vegetation in a wetland provides a substrate (roots, stems, and leaves) upon which microorganisms that break down organic materials can grow. This community of microorganisms is known as the periphyton. The periphyton and natural chemical processes are responsible for approximately 90 percent of pollutant removal and waste breakdown. The plants remove about 7 to 10 percent of pollutants, and act as a carbon source for the microbes when they decay. Different species of aquatic plants have different rates of heavy metal uptake, a consideration for plant selection in a constructed wetland used for water treatment.
Constructed wetlands are of two basic types: subsurface-flow and surface-flow wetlands. Subsurface-flow wetlands move effluent (agricultural or mining runoff, tannery or meat processing wastes, wastewater from sewage or storm drains, or other water to be cleansed) through a gravel or sand medium on which plants are rooted; surface-flow wetlands move effluent above the soil in a planted marsh or swamp. In subsurface-flow systems, the effluent may move either horizontally, parallel to the surface, or vertically, from the planted layer down through the substrate and out. Subsurface horizontal-flow wetlands are less hospitable to mosquitoes, whose populations can be a problem in constructed wetlands. Subsurface-flow systems have the advantage of requiring less land area for water treatment, but are not generally as suitable for wildlife habitat as are surface-flow constructed wetlands.
Plantings of reed beds are popular in European constructed wetlands, and plants such as cattails (Typha ssp.), sedges, and bulrushes are used worldwide. Recent research in use of constructed wetlands for subarctic regions has shown that buckbeans (Manyanthes trifoliata) and pendant grass (Arctophila fulva) are also useful for metals uptake.
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