National Association of Conservation Districts

National Association of Conservation Districts

NACD's mission is to serve conservation districts by providing national leadership and a unified voice for natural resource conservation.

Constructed Wetlands Put To Work

In several parts of the country, man-made wetlands are being constructed to serve filtration needs of homes, businesses and communities. Natural treatment methods, using aquatic plants and microorganisms to uptake and absorb pollutants and excess nutrients, are proving to be more cost-effective than traditional methods.

Constructed wetlands use a process called phytoremediation, in which plants and microbes in the root systems purify domestic, industrial or agricultural wastewater. Systems must be engineered for adequate size, water depth, flow patterns and vegetation to properly handle the treatment application. When properly designed and maintained, these wetlands become more effective over time in uptake by the vegetation of excess nutrients, chemicals and heavy metals. Systems have been well established in the backyards of homes to handle septic systems, the municipal systems of small towns and the full output of major industries.

One of the first constructed wetland systems in the U.S. to treat industrial wastewater was developed at the NASA John C. Stennis Space Center in southern Mississippi in the early 1970’s in their search for space station applications. It was designed for the treatment of 20,000 – 40,000 gallons a day of laboratory and photographic waste. Scientists determined that certain vascular aquatic plants such as water hyacinth can bioaccumulate heavy metals in their roots in concentrations that are thousands of times greater than is present in the soil or water where they are grown.

One of those scientists, Dr. B. C. Wolverton, established his own private company that installs phytoremediation systems. Gale Martin, Executive Director of the Mississippi Soil and Water Conservation Commission, has a Wolverton Environmental Services system installed in his backyard to serve as the household septic system. Solids are captured in a septic tank and the liquid effluent flows into a beautiful aquatic garden with canna lilies, elephant ears and other plants in a bed of rich topsoil underlain with gravel. "The system does a very efficient job plus it’s an aesthetic feature in my backyard", Gale said.

The same principles are being applied as storm water management systems in urban areas. Called "bio-retention cells" or "rain gardens", a relatively small area engineered and planted with specified vegetation in topsoil under mulch and over layers of sand can uptake and filter the grease, gasoline and heavy metals that wash off a parking lot of several acres.

Bio-retention cells are being used in Prince George’s County, Maryland. Maryland was the first state in the country to require developers to prepare and implement sediment control and storm water management plans and practices by state law. Early storm water management facilities were "dry ponds", grassed basins that filled up with water only during storms and released the water to receiving streams slowly after the storm had passed. These devices focused more on quantity control than quality control. Prince George’s County alone has over 1,000 dry ponds to maintain.

Over the last 30 years, storm water management requirements have added more water quality features. County officials believe that existing technology can use low-impact designs that could function with nearly zero polluted runoff impact to ground water or receiving streams. In some developing areas of Maryland, EPA estimates that urban storm water currently contributes more nitrogen and phosphorus than sewage plants and agriculture.

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