Agricultural runoff is one of the nonpoint sources of pollution that threatens the water quality in rivers and lakes of the United States. Water that flows off the land after precipitation events picks up fertilizers and animal wastes that have been applied to the soil and deposits them in lakes and rivers.
If the runoff is uncontrolled, it causes soil erosion and increases the presence of suspended solids, which can contain nutrients, pesticides, herbicides, and metals, in the water. Flooding and the degradation of rivers, streams, and lakes are possible consequences. Nonpoint source pollution can also threaten groundwater quality as the same pollutants leach through the soil.
Runoff can be controlled. Best Management Practices (BMPs) can be adopted as part of the poultry grower’s operating procedure. For example, stormwater can be diverted from poultry houses and manure storage areas, and land applications can be made when no storms are predicted. In addition, the arsenal of BMPs now includes the use of constructed wetlands for treating runoff and wastewater.
Functions of Constructed Wetlands
Constructed wetlands are not considered to be waters of the United States, but components of a wastewater treatment system. Therefore, if there is a discharge from a constructed wetlands, a federal or state discharge permit may be required.
A constructed wetlands is a designed structure, or set of structures, that attempts to replicate the functions of a natural wetlands. As with natural wetlands, they support water tolerant or aquatic plants and their soils are saturated (water- logged) or covered with shallow water for some part of the year. However, since constructed wetlands are designed to treat wastewater efficiently and effectively, they generally do not fulfill all the functions provided by a natural wetlands (e.g., they do not recharge groundwater or contribute to the creation of hydric soils).
The constructed wetland is the heart of the treatment system. It cleans wastewater by filtering and settling solids, decomposing organics, and adsorbing/absorbing other pollutants such as phosphorus and trace metals. The dissolved organic pollutants are removed by a complex group of microbes (bacteria, fungi, algae, and protozoa) that live in the wastewater and on plant and sediment surfaces. Since waste materials are food for most of these microbes, pollutants are gradually converted through complex food cycles into environmentally less damaging by-products (gases that escape to the air and inert solids that stay in the system).
The primary purpose of wetland plants is to provide a place for these microbes to attach and grow. Generally, treatment effectiveness increases with plant density, which allows a larger quantity of attached microbes to exist within the system. The density of plants also affects flow hydraulics. Uniform flow is enhanced by uniform plant densities, but variable densities create short-circuiting which reduces the retention time and treatment effectiveness of the wetland. In addition, plants make the system attractive and provide food and shelter for wildlife.
The system remains effective during winter because the microbes are still present on the dead stalks, stems, and roots of the vegetation. Because the biological processes slow down during winter, wetland systems are typically sized to meet treatment objectives during cold weather.
Designing Constructed Wetlands
Constructed wetlands can effectively treat poultry industry wastewaters, including stormwater runoff. These wetlands are designed by engineers and built to restore, enhance, or replace the physical, chemical, and biological processes in natural wetlands. They are typically used as polishing cells following conventional primary treatment facilities such as lagoons, settling basins, or septic tanks. The integrated treatment system provides a higher quality wastewater that may be recycled or discharged to a receiving stream if appropriate permits are obtained.
In addition, the volume of treatable wastewater may be substantially reduced during the growing season because of evapotranspiration by the plants. For example, a poultry producer currently having difficulty with overflowing lagoons during wet weather now has the option of adding constructed wetlands, which can be used to treat a portion of the lagoon wastewater during the growing season. Typically the wastewater in the wetlands will be evapotranspired, but any effluent can also be recycled as process waste or as irrigation water.
Constructed wetlands consist of one or more “cells” of wetland plants in series or parallel. Construction can be easily accomplished. Excavate the area to shape the bottom of the wetlands and build small dikes around it. Line the bottom and sides of the excavated areas with clay or a synthetic material. Use PVC pipe to distribute and collect wastewater and to control water levels in the wetland. Water levels are normally shallow — about 3 to 12 inches. Uncontaminated runoff can be diverted from the system by berms or other buffers or grading.
A lagoon, detention basin, or other type of solids trap is used in front of the constructed wetlands to remove heavy or coarse solids. Some contaminated runoff contains high sediment loads and decomposing organic matter that may settle in bottom deposits. Because these deposits can adversely affect the hydrology and life forms in the wetland, the solids trap is particularly important.
Most wetland systems for treating agricultural- related wastewaters will not be larger than one or two acres. In general, they should not be located in areas with steep topography, shallow topsoil, or limited space. They must be properly constructed to protect groundwater. Federal, state, or local cost-share funds may be available for constructed wetlands.
Management
Wetland plants include mixtures of cattails, reeds, bulrushes, sedges, and grasses that are normally native to the area. The plants provide the right conditions for the microorganisms that live in the wetlands and break down the pollutants.
Pond and wetland systems are particularly effective because ponds can be designed to catch the stormwater and slowly release it to the wetlands following the storm. This technique keeps the wetlands wet for longer time periods, which can be especially important during dry seasons.
The systems need little routine maintenance but should be inspected periodically to detect any loss of plants, leakage through the dikes, clogging of the pipes, mosquitoes, or short-circuiting of the flow. These problems and others are usually easily corrected.
Properly managed constructed wetlands are cost effective, energy efficient, and simple to operate. They accept varying pollutant loads, attract a variety of wildlife, and add diversity to the farm landscape. Above all, constructed wetlands can help achieve clean water. Information on the design and construction of wetlands for managing wastewater is available from USDA Natural Resources Conservation Service local offices, and the U.S. Environmental Protection Agency.
References
Olson, R.K. 1993. Created and Natural Wetlands for Controlling Nonpoint Source Pollution. CRC Press, Boca Raton, FL.
Schueler, T.R. 1992. Design of Stormwater Wetlands Systems. No. 92710. Metropolitan Washington Council of Governments, Washington, DC.
Tennessee Valley Authority. No date. Natural and Constructed Wetlands. Fact Sheet in cooperation with U.S. Environmental Protection Agency.
U.S. Environmental Protection Agency. 1990. Uses of Wetlands in Stormwater Management. Watershed Management Unit, Water Division, Region 5, Chicago, IL.
U.S. Department of Agriculture. 1991. National Bulletin No. 210-1-17. Soil Conservation Service, Washington, DC.