Prepared by:
Frank Humenik, Extension Specialist-in-Charge
Biological & Agricultural Engineering
Joe Zublena, Extension Specialist-in-Charge
Soil Science
Jim Barker, Extension Specialist
Biological & Agricultural Engineering
Tara Disy, Information Specialist
Published by: North Carolina Cooperative Extension Service
Publication Number: AG-473-13
Last Electronic Revision: March 1996 (JWM)
In transitional areas between land and water, nature forms wetlands. Wetlands are distinguished by wet soils, plants that are adapted to wet soils, and a water table depth that maintains these characteristics. Wetlands provide an attractive home for a variety of plants and animals, guard against flooding, and help protect water quality. The media, special-interest groups, the government, and scientists all have a keen interest in the benefits of wetlands. Recently, many researchers have focused on the effects of wetlands on water quality and the possibility of constructing wetlands for wastewater treatment.
Wetlands help maintain water quality by storing some of the nutrients, such as nitrogen and phosphorus, and removing some of the sediment from water that runs off upland areas, reducing the amount of pollutants that enters water- ways. Natural wetlands also help to reduce shoreline erosion and to maintain the levels of rivers, lakes, and streams by storing and slowly releasing flood waters.
Understanding how natural wetlands work is the first step in trying to create similar man-made areas for wastewater treatment. Interest in constructing wetlands for wastewater treatment is growing rapidly. The U.S. Environmental Protection Agency (EPA) encourages the use of constructed wetlands for the treatment of domestic wastewater through the State Revolving Fund Program. Enthusiastic media reports have gained the public's attention. It is often reported that constructed wetlands are relatively inexpensive to build, need almost no maintenance, are pleasant to look at, become a habitat for wildlife, and effectively remove bacteria and nutrients from water.
These things may be true, but there is some debate as to just how effective constructed wetlands really are. Concentrated wastewaters, such as those from animal production, place a large burden on the wetland's ability to remove pollutants. Whether or not constructed wetlands can handle this burden has not yet been determined.
A constructed wetland for wastewater treatment is planned, designed, built, and operated to simulate a natural wetland in terms of its ability to remove pollutants from water. Advantages of a planned system as compared to a natural system are increased flexibility and control over where the wetlands are located and where the water comes from. Constructed wetlands are being used to treat municipal, industrial, and agricultural wastewaters.
Geographic location, which determines climate, vegetation, and soil types, is critical to the effectiveness of constructed wetlands. Sedimentation, sorption, filtration, biological processes, and biochemical interactions are the principal mechanisms by which wetlands remove pollutants. Any change in wetland characteristics affects pollutant removal efficiency. Because of variations in characteristics, treatment effectiveness differs from one wetland to another.
Plants also play an important role in the functioning of wetlands. Roots and stems in the water serve as a medium for bacterial growth and for filtration and absorption of solids. Stems and leaves at or above the water surface provide shade, which reduces the growth of algae. Many wetland plants can transport oxygen to and from the roots, which allows oxygen- demanding changes in nitrogen to take place in the rotation. These changes convert nitrogen to a form that can be used by the plant, thereby keeping it from entering surface water or groundwater. Choosing the proper plants is an important part of wetland construction.
The age and maturity of a wetland area can also influence its ability to treat wastewater. If the system is not planned properly, increased infiltration in newly exposed soils may allow more contaminants to leave the system and move into surface water or groundwater. In old systems, the opposite may happen. The soils may become saturated with nutrients, thus reducing the wetland's ability to assimilate pollutants and purify wastewater.
Flow characteristics of the area are another factor in wetland performance. Changes in wastewater loading, detention time, water level, and season alter the wetland's ability to remove pollutants. A well-planned water flow is essential for proper functioning of the wetland system.
Constructed wetlands can be a component of an overall animal waste management system. Preliminary studies by the EPA and Tennessee Valley Authority (TVA) suggest that a constructed wetland can provide a high level of wastewater treatment when waste- water has first been treated in a lagoon. The Agricultural Stabilization and Conservation Service (ASCS) has approved cost-share funding for constructed wet lands. ASCS considers constructed wetlands a suitable way to reduce pollution of water, land, and air through proper handling of agricultural wastes.
Any discharge from constructed wetlands that are planned, designed, and constructed according to Soil Conservation Service (SCS) technical requirements (to be explained later) must be stored in a wastewater containment facility. The stored wastewater may be applied to cropland or recycled through the animal waste management system.
In North Carolina, constructed wetlands can be used as part of an animal waste management system with- out discharge to surface waters. For any wastewater treatment system to discharge to surface waters, a National Pollutant Discharge Elimination System (NPDES) permit is needed from the Division of Environmental Management (DEM). Obtaining this permit may be difficult because, given current technology and North Carolina's antidegradation policy, it is unlikely that a direct discharge from a constructed wetland is a better alternative than land application of the effluent. Constructed wetlands, however, can be used as a component of a nondischarge system, particularly in areas where there is an overabundance of nutrients or limited cropland to which the effluent may be applied.
SCS technical requirements for constructed wetlands to be used in treating agricultural wastewater are based primarily on studies of municipal waste treatment systems. These guidelines have been published in SCS National Bulletin No. 210-1-17. The requirements must be met to qualify for ASCS cost-share assistance. They are also useful as a guide to anyone planning to use constructed wetlands. The SCS technical guidelines include five types of requirements:
Figure 1. Amount of land required for a constructed wetland system facility and for a system with direct application to cropland.
Constructed Wetland System Lagoon Land-Application System Source ---> Pretreatment ---> Constructed Source ---> Lagoon Wetland || || || || \/ \/ Land Discharge Storage || || \/ Land
After new technologies are developed and proven to be effective, national conservation standards are published to set forth the minimum requirements for using the technology. Because the technology for constructed wetlands has not yet been developed sufficiently, a national conservation standard (which is different from SCS guidelines) has not been published.
Because constructed wetlands simulate natural methods of wastewater treatment, it would seem that they should be much less expensive to construct, operate, and maintain than mechanical treatment methods. Thus, constructed wetlands may seem like the answer to all wastewater treatment needs. Many questions, however, remain unanswered:
Research is being done to answer these questions, but the results are not yet available.
In addition to the effectiveness of a waste management system, the cost must also be considered. If discharge is not possible, animal waste management systems that include a constructed wetland may require more land than systems in which waste is applied directly to cropland or from a lagoon to land. (See Figure 1.)
Another consideration is that using a wetland for wastewater treatment prevents the valuable nutrients in wastewater from being used for crop production. If not enough cropland is available for direct application of lagoon effluent, however, a wetland system may be a good choice. Effluent that has been treated in a wetland system has a lower nutrient concentration, and therefore less area is needed for land application.
The use of constructed wetlands for agricultural wastewater treatment is still new. Though federal agencies (SCS and ASCS) have published guidelines and provide cost-share funding for constructed wetlands, optimal design plans and standards have not been developed in North Carolina. Although planners and regulators are learning about this new option, they are not yet familiar with the technology. Before the North Carolina Cooperative Extension Service or the Soil Conservation Service can make detailed recommendations, data are needed.
Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
AG-473-13