Constructed Wetlands: A Natural Treatment Alternative
This is the first of three articles on constructed wetlands. They are reprinted with some modification from Pipeline, a quarterly publication of the National Small Flows Clearinghouse at West Virginia University, Morgantown, WV.
What type of images first come to mind when you think about wastewater treatment? Chances are you wouldn't immediately picture your wastewater system as a good place to watch a sunrise or catch a glimpse of a moose, a crane, or other exotic wildlife. But for the growing number of small communities and individual homes using constructed wetland systems, effective wastewater treatment can be achieved without disturbing the natural beauty and serenity of rural areas.
It has long been known that natural wetland areas (such as marshes, swamps, and bogs, for example) play an important role in protecting water quality. Constructed or artificial wetland systems mimic the treatment that occurs in natural wetlands by relying on plants and a combination of naturally occurring biological, chemical, and physical processes to remove pollutants from water. In fact, some constructed wetland system designs closely resemble natural wetlands and even provide additional habitat area for the many birds, animals, and insects that thrive in wetland environments. But because constructed wetland systems are designed specifically for wastewater treatment, they work more efficiently than natural wetlands.
Constructed wetlands can treat wastewater generated by a variety of sources. One of their more common uses is to provide additional or advanced treatment of wastewater from homes, business sites, and entire communities. This means they are used by homes and communities to treat wastewater that has already had most solid materials removed from it through some type of primary or secondary treatment (for example, in a septic tank, lagoon, aerobic unit, or treatment plant).
While the exact roles of some of the natural treatment processes in wetlands are still not completely understood, enough information currently is available for experienced wastewater consultants to design systems that perform reliably and meet environmental treatment standards. Communities should hire a consultant or engineering firm that has specific experience designing wetland systems.
Constructed wetland systems have only been used for wastewater treatment since the 1970s, which makes them a relatively new wastewater treatment technology. However, interest in their use has quickly become widespread. For example, in a recent survey of Pipeline readers, constructed wetland technology was recommended most frequently as a topic for future articles. Wetland systems also are a popular subject with the many community leaders, health officials, and homeowners who call the National Small Flows Clearinghouse's (NSFC) technical assistance hotline (800-624-8301 or 304-293-4191).
One of the reasons that people are curious about constructed wetlands is the low cost associated with these systems. Compared to many other treatment methods, wetland systems are inexpensive to construct and maintain, and operating costs are low because energy is not required to provide treatment.
This article, and the two to follow, offer some basic information for community leaders and homeowners and about the two types of constructed wetland systems—surface flow systems and subsurface flow systems—how they work, and some of their advantages and disadvantages. Readers are encouraged to reprint the articles in local newspapers, and include them in flyers, handouts, newsletters, and educational presentations. Please include the name and phone number of the NSFC, given above, on the reprinted information and send us a copy for our files.
How Wetlands are Used to Treat Wastewater
Constructed wetland systems have many uses. When they are applied to treat domestic sewage or wastewater from typical small community sources, they provide additional, secondary, or advanced treatment to wastewater that already has had most solid wastes removed in a septic tank or by some other form of preliminary treatment.
Constructed wetlands can be used as part of onsite wastewater treatment or with larger centralized and decentralized systems that serve residential developments, resort areas, or entire communities.
To find out whether constructed wetlands are an option in your area, contact your local health department or state environmental health agency for permit information. Also, many universities have U.S. Department of Agriculture Extension Service offices on campus and in other locations that provide a variety of services and assistance to small communities and individuals. Call the NSFC or the USDA directly at 202-720-3377 for guidance on this matter.
Homes, businesses, farms, schools, and other individual wastewater sources in rural areas sometimes can add a constructed wetland to a septic system or other onsite system to replace or assist a soil absorption field. Some onsite systems can be specifically designed from the start to use a constructed wetland in addition to a soil absorption field on properties with site constraints, such as soils that are too dense or saturated to work well with a conventional onsite system.
Wetlands also are good at handling intermittent periods of both light and heavy wastewater flows. Therefore, they often work well with wastewater treatment systems that serve hotels, campsites, resorts, and recreational areas.
In environmentally sensitive areas, constructed wetlands can be used with onsite systems to improve the quality of wastewater before it is returned to the environment. They also are used on farms as an inexpensive way to provide extra treatment to animal wastes, and by certain industries, for example, pulp and paper mills. Constructed wetlands also are common in Appalachia and other mining regions of the U.S. where they are used to treat acid mine drainage.
Wetland systems are not practical for treating every type of wastewater, however. Chemicals in some industrial wastewater effluents—for example, pesticides, herbicides, and large amounts of ammonia—can kill the plants in wetlands that contribute to treatment. Additionally, wetland plants may accumulate high concentrations of metals from some wastewater sources. This may affect the habitat value of the wetland.
Many small communities have aging centralized wastewater treatment plants, which have fallen into disrepair or have trouble keeping up with the needs of a growing community and no longer consistently comply with environmental regulations. Communities often can retrofit their existing treatment plants with wetland systems to help them reliably meet environmental discharge requirements. Wetlands also can be used with community systems as added protection for water quality in sensitive areas, such as shellfish beds and estuaries, while at the same time providing additional habitat area for local wildlife species.
Another important way that wetland systems benefit both large and small communities is their usefulness for treating storm water before it is discharged into receiving waters. Garbage, dirt, grit, leaves, and other solid materials that get washed into storm sewers from streets and yards are first removed or allowed to settle out before the storm water is sent to the wetland for treatment. Wetland systems provide an inexpensive means of protecting water resources by removing some of the harmful chemicals and nutrients from fertilizers and other sources that wash into storm sewers when it rains, and which are not normally removed by preliminary treatment.
Again, wetlands may not be a good option for community systems that accept industrial, agricultural, or commercial wastewater streams containing significant amounts of toxic chemicals harmful to biological treatment organisms and wetland vegetation.
Editor's Note: The next article on this topic will appear in approximately two weeks. It will describe how surface flow wastewater treatment systems work very much like natural wetlands.