Constructed wetlands mimic moisture regimes, from moist to flooded, and have been used since the mid-1970’s to clean industrial, agricultural and domestic wastewater. They have the added benefit of creating a habitat for wildlife and human enjoyment.
Typically, a constructed wetland is a series of rectangular plots filled with soil or gravel and lined to prevent waste form leaching into groundwater. The plants grown in these plots, not only offer a root mass for filtration, but also provide oxygen and carbon for water treatment. The roots offer attachment sites for microbes which consume the available oxygen in the process of breaking down pollutants. The plants themselves also take up pollutants.
Natural Marshes are wetlands with soft vegetation, such as catails and reeds. Low marsh and high marsh environments are created in nature by flooding. Low marsh vegetation typically grows next to open water, where it is flooded regularly by the tides or irregualrly by wind and rain, much like marshes along coastal sounds and estuaries. High marsh vegetation is flooded periodically where it grows in the transition area between low marsh and higher upland grounds.
Two types of constructed wetlands are:
- Low marshes (subsurface or horizontal flow wetlands) have a porous fill of coarse sand or stone. The fill and plant roots are the primary filters of pollutants. These wetlands are effective in the anaerobic second step of nitrogen treatment, because the wastewater flows below the surface.
- High marshes (vertical flow wetlands) are layers of sand over gravel, much like sand filters with plants. Wastewater is flooded onto the top of the marsh and allowed to flow down through the sand. Dry periods make oxygen plentiful for the first step of nitrogen treatment. As with subsurface wetlands, plant roots and soil are effective filters, and a microbial treatment area usually forms around the roots.
Constructed wetlands by themselves perform well as a medium for wastewater management (House, C. H., S.W. Broome and M. T. Hoover. 1994). However, hybrid systems formed by the combination of constructed wetlands with other designed ecosystems perform even better.
Excerpted in part from Waterwise, Winter 1995.