Kenneth D. Simeral
The Ohio State University
Environmentalists have referred to wetlands as nature's kidneys. Much interest has developed in recent years in using constructed wetlands to remove contaminants from water, whether it is effluent from municipal or private waste systems, industrial or agricultural wastewater, or acid mine drainage.
In a practical, low-cost approach, this publication deals with the principles of creating surface flow wetlands for the removal of contaminants from wastewater generated by livestock production operations. Only surface-flow wetlands are referred to in this publication.
To design and develop a wetland for effective wastewater treatment, it is necessary to understand the processes that occur in wetlands. Primary processes include:
(Handbook of Constructed Wetlands, Vol. 1)
Using constructed wetlands to remove pollutants in livestock wastewater is not for everyone but in some instances, it can be a viable alternative to or a component of a larger wastewater treatment system or plan.
Livestock producers must consider both the advantages and limitations of such a system to decide if a constructed wetland is applicable to their operations.
A properly constructed wetland designed to fit the topography has these advantages:
Figure 1 - Summary results of the percentage of pollutants removed from
three (3) demonstrational constructed wetlands. NOTE: In the case of
nitrate nitrogen on Site #1, very little was present in the inflow, so
little could be removed.
Figure 2 - A water-collection ditch of a dairy heifer/dry cow feed lot.
Notice the reed canarygrass for vegetation and the picket fence that
help separate solids.
Even the best design has its limitations. Consider that a constructed wetland:
The better the design, the easier it is to manage a constructed wetland. Wastewater generated by livestock operations, especially feedlots, varies according to the:
The two most important considerations in wetland design are solids removal from wastewater and total water budget. It is also important to keep the design simple. (see Figure 3.) Design the system for minimal maintenance, to use gravity flow, and to fit in with the landscape. The design should also include provisions for extremes in weather and climate, such as floods and drought.
Figure 3 - A small constructed wetland with vegetation not totally
established with a small inexpensive earthen settling basin in the
But First, the Substrates - With a goal in mind of keeping construction costs to a minimum, it is important to study the soils in the proposed wetland area. While a constructed wetland can be made to function on almost any soil, if the soil is porous so an artificial or imported liner must be used, the cost could become prohibitive. An ideal substrate would consist of a large quantity of clay, preferably with enough organic matter to aid plant growth during establishment with a cation exchange capacity (CEC) of 15 or greater. The pH probably should be in the range of 6.5 to 8.5. Natural Resource Conservation Service (NRCS) conservationists and technicians can assist in the determination of the suitability of your soil properties.
Solid Waste Removal - Wetlands can only be used to treat wastewater, not solids. It is extremely important to design a system that will ensure removal of all settable solids from the wastewater before it enters the wetland for treatment. Nothing will kill a wetland faster than the intrusion of solid waste.(see Figure 4.) The design of settling basins, etc. for removal of solids, will depend on the characteristics of the material. NRCS has design guidelines for building settling mechanisms. The design of these structures will be site specific because of topography and existing livestock and manure handling facilities. (see Figure 5.)Do not be afraid to be creative or innovative when designing the solid removal system.
Figure 4 - An example of what happens if solids are not removed. Severe
damage and/or destruction of wetland can occur.
Figure 5 - A simple settling basin for removal of solids.
Water Budget - Water budgets refer to the amount of water going into, flowing out of, and remaining in the wetland. Water going into the wetland includes the inflow (wastewater and supplemental water) and rainfall. Infiltration, evaporation, transpiration, and outflow are ways the wetland loses water. Residence time is the time it takes the water to leave the wetland. Literature indicates that it is necessary to have a minimum of 12 days (Handbook of Constructed Wetlands, Vol. 1) residence time to remove the necessary contaminants. Wetland water levels must remain relatively constant to provide adequate vegitation for the system to facilitate the removal of contaminates. If water levels drop drastically, plant life diminishes and the system does not function correctly. Supplemental water can be obtained from a variety of sources including springs, creeks, storage ponds, etc. depending on the topography and landscape design. If excess water floods the wetland from rain or run off from roof gutters, pavement, etc., the balance in the wetland will be upset. Contaminated wastewater will wash through the system before being treated. The design of the wetland must exclude surface water.
A holding pond, located before the wetland, provides a multipurpose benefit when considering water budgets. It will serve as additional settling basins for removing solids, as well as storage for excess water during high rainfall or water input. It can also store water in the winter when the water treating capacity is less. Water in the holding pond can be a source during dry periods for maintaining the wetland.
Determining Size - Since it is extremely difficult to measure precisely the amount of contaminants and the volume of wastewater, it is better to oversize. The downside to this is that it takes more land area and more water for water budget. When designing wetlands, calculate the size needed to remove the necessary amount of BOD5. Natural Resources Conservation Service (1992) uses two methods to determine the size of a constructed wetland for a specific site. These two methods are the presumptive method and field-test method.
The presumptive method is used when the pretreatment system has not yet been installed or for some other reason, the BOD5 of the wastewater cannot be determined. The field-test method is used when the BOD5 concentration can be adequately measured. If the field-test method is used, multiple tests must be taken over a considerable period to determine the extremes of the concentration of wastewater contaminants. If the wetland is being designed to remove ammonia, it would need a longer retention time since ammonia requires a longer residence time than BOD5 for removal. Specific formulas and calculations for both methods can be obtained from NRCS (USDA SCS Engineering Field Handbook 210-EFH, 1/92).
Physical Characteristics - There are three physical factors to consider when designing a wetland.
Given topographical considerations, a minimum of two parallel cells should be designed to aid management and maintenance. Having more than one cell will allow for shut down and maintenance of one cell without hindering operations of the wetland. This will allow for changing the system or resting a portion of it while the other cell continues to treat wastewater. Install outflow control devices that will allow for easy change of water levels within the wetland. This can be accomplished through weirs, flexible elbow pipes, or commercially manufactured water-level control devices. Commercial devices are the handiest but also the most expensive. If ammonia is present in the wastewater, creating a series of pond, marsh, pond, marsh, etc. will aid in dispersing ammonia through the wind movement across the pond water (Hammer, 1989). This pond, marsh, pond, marsh environment also enhances beneficial wildlife in the wetland (CH2Mhill).
Winter management may create special needs. The function of the wetlands in both plant growth and microbial activity stops or slows during the winter. While cattails whose tops are dead still continue to harbor enough microbes to do a fair job of treating the wastewater, the retention time must be extended significantly to achieve the same level of treatment. It is necessary to increase the water level during the winter to keep the entire system from freezing which would prevent wastewater from entering.
Choosing the specie(s) of vegetation to establish is the first decision to make. The type of vegetation to establish depends on the goals and objectives for the wetland. Ideally, vegetation should include a variety of species, however, constructed wetlands for treating wastewater need to be as versatile and easily maintained as possible. Practicality might dictate limiting plant species to the most hardy, commonly found, and easily managed.
There is literature available about characteristics and requirements of various wetland plants. However, demonstrational research has found that cattails and reed canary grass have proven to be low cost, easy to establish, low maintenance, and tolerant of a wide range of climatic and contamination conditions. (see Figure 6.) Cattails and reed canary grass (Phalaris arundinacea) can both tolerate drought conditions for several weeks. Broadleaf cattails (typha latifolia) can withstand water depths up to 18 inches and narrowleaf cattails (typha angustifolia) up to 12 inches. This makes control of water levels less critical for vegetation. The next most versatile and easily managed plants would be common reed (Phragmites australis) and various species of bulrush (Scirpus).
Figure 6 - Cattails in a nearly mature wetland established by
transplanting of rhizomes.
After determining the type of vegetation desired, consider the method of establishment. The three choices include mechanical seeding; transplantation of rhizomes, stolons or entire plants; and natural evolution. From work done by Mitsch, 1996, there is evidence that over a period of three or more years, a constructed wetland left to evolve on its own will equal or surpass wetlands that were deliberately seeded or planted. The success of this method depends greatly on the source of the establishment water and the proximity of the constructed wetland to naturally occurring wetlands or other aquatic vegetation. Water used to establish a wetland that comes from a stream or pond would evolve more quickly than water from a spring. Natural evolution is the least expensive method. However, most situations do not lend themselves to this method because of the number of years to achieve sufficient vegetative growth. Whatever the source, the water used to initially fill the wetland should be "clean" and not the wastewater that will be treated once the wetland is created.
Mechanical seeding success rates vary because of climatic conditions, water levels, etc. Most plants will not establish from seed in standing water, but do need constantly wet soils. Reed canary grass will establish easily from seed with proper seedbed preparation and a shallow seeding with good seed-soil contact. Reed canary grass makes good cover for berm areas as well as marsh areas of shallow water within the wetland. Only the low alkaloid varieties of reed canary grass should be used. Low alkaloid varieties include Palaton, Venture, and Rival.
Transplanting rhizomes, stolons, and plants is perhaps the fastest and most reliable method of establishing vegetation. It is the most expensive if plants are purchased. Obtaining appropriate plant material from local sources such as road drainage ditches, edges of ponds, natural seepage areas, etc, reduces the cost of establishing the wetland. These local plants also tend to be more vigorous and have a higher survival rate than plants brought in from other areas because they are already accustomed to climatic and other environmental factors. Local plants found close to the site are desirable. Some literature (Handbook of Constructed Wetlands, Vol. 1) suggests plants should be obtained within a 50-mile radius to the wetland site. Plants obtained from seepage areas with a concentration of the type of contaminants to be removed from the wastewater will aid in the function of the wetland. Microorganisms will be present that are already adapted to the pollutant. The microorganisms that are found on a cattail originating from a polluted seepage area would be different from those found on a cattail that is growing in clean water. The microorganisms from the seepage area would then be available to aid in the breakdown, transformation, and uptake of contaminants found in the wastewater treated in the wetland.
Transplanting does not have to be complicated. Cattail rhizomes can simply be dug and spread onto the substrate. When proper conditions exist, they will take root and grow. In one trial, cattails were obtained from the edge of a pond on the property of the producer. They were placed in an old-fashioned rear beater manure spreader and simply spread on the relatively dry substrate. Conditions remained dry for twelve days after spreading. Once the substrate received water, the cattails took root and grew. This vegetation establishment cost the producer very little. Another inexpensive method is to contact local highway officials and arrange for them to deliver the material obtained when cleaning road ditches containing wetland plants. One disadvantage is that this material contains unwanted objects such as bottles, cans, and other trash.
One final note on transplanting. Planting materials should not be taken from naturally occurring wetlands. These areas are protected by government regulations and proper permits must be obtained before taking plants from donor wetlands.
While a properly-designed wetland will not have a great amount of effluent, the disposal of effluent needs to include proper design elements and proper regulatory approval. Some of the options available include:
Grass filter strips and infiltration strips can be designed according to NRCS specifications. These strip designs are advantageous because of ease of installation and low maintenance, but have the disadvantage of wasting nutrients and taking up additional land area. Recycling is the preferred method because it is an efficient way to dilute the influent, decrease the potential for odors, and may enhance nitrification. It also is an efficient way to maintain adequate flows during low flow periods. The biggest disadvantage of recycling is that it increases construction and operating costs. There still needs to be effluent disposal during periods of high inflow and recycling requires the use of pumps. Irrigation has the advantage of using the few remaining nutrients available in the water, but has a major disadvantage in capital and operational costs. Also, some locations and topographies are not suited for irrigation. Direct discharge to surface water, while simple, requires the most attention to regulatory testing and monitoring to ensure that National Pollutant Discharge Elimination System (NPDES) requirements or more stringent local standards are met. Over time, this can be quite expensive (CH2MHill).
For small livestock producers, constructed wetlands are a viable, low-cost, effective means of treating livestock wastewater. A properly constructed wetland that is designed to work with the topography has many advantages including: a high level of treatment, low operational expenses, low construction costs, reduced or eliminated odor problems, handling of varying wastewater loadings, reduced land costs for land application of wastewater, ascetic appeal, and providing a wildlife habitat. The main disadvantage of the system is the continuous water supply requirement. Other disadvantages would be that certain soil features and topographies make wetlands expensive to construct and operate. An overload of solids or ammonia levels will destroy the system. Finally, nutrients removed by the system are not available for land application for crop production.
Elements of good design for a created wetland are substrates, water budget, size determinations, and physical characteristics such as shape, slope, and embankments. Other important factors to consider are solid waste removal from wastewater, effluent disposal, and vegetation establishment.
For the livestock producer with the right topography on the farm combined with the right management skills and scope of operation, constructed wetlands are of great benefit. More information can be obtained from NCRS or any of the publications listed in the bibliography.
Cathcart, T., D. Hammer, and S. Triyono, 1994. Performance of a Constructed Wetland - Vegetative Strip System Used for Swine Waste Treatment. P. DuBowy and R. Reaves, eds. Constructed Wetlands for Animal Waste Management. Proceedings of workshop sponsored by the Conservation Technology Information Center, the US Department of Agriculture (USDA) Soil Conservation Service (SCS), US Environmental Protection Agency (EPA) Region V, and Purdue University Research Program, April 4 to 6, 1994.
CH2M HILL and Payne Engineering, January, 1997. Constructed Wetlands for Livestock Wastewater Management Literature Review, Database, and Research Synthesis. Prepared for the Gulf of Mexico Program, Nutrient Enrichment Committee. Prepared under contract to National council of the Paper Industry for Air and Stream Improvement (NCASI) and Alabama Soil and Water conservation Committee.
Davis, L., A Handbook of Constructed Wetlands A Guide to Creating Wetlands for: Agricultural Wastewater, Domestic Wastewater, Coal Mine Drainage, Stormwater in the Mid-Atlantic Region, Volume 1: General Considerations. Prepared for the United States Department of Agriculture (USDA) Natural Resources Conservation Service and the Environmental Protection Agency (EPA) Region III in cooperation with the Pennsylvania Department of Environmental Resources.
DuBowy, PJ. and R.P. Reaves, eds. Constructed Wetlands for Animal Waste Management. Proceedings of a Workshop. West LaFayette, IN: Purdue University, 1994.
DuBowy, PJ., ed. Proceedings of the Second National Workshop on Constructed Wetlands for animal Waste Management. College Station, TX: Texas A&M University, 1997.
Hammer, D.A., ed. 1989. Constructed Wetlands for Wastewater Treatment: Municipal, Industrial and Agricultural. Chelsea, MI: Lewis Publishers.
Hammer, D.A., B. P. Pullin, T. A. McCaskey, J. Easton and V. W. E. Payne, 1993. Treating Livestock Wastewaters with Constructed Wetlands. G.A. Moshiri, ed. Constructed Wetlands for Water Quality Improvement. Boca Raton, FL: Lewis Publishers.
Kadlec, R.H. and R.L. Knight, 1996. Treatment Wetlands. Boca Raton, FL: Lewis-CRC Press.
Knight, R.L., R.W. Ruble, R.H. Kadlec, and S.C. Reed. Wetlands for Wastewater Treatment Performance Database. G.A Moshiri, ed. Constructed Wetlands for Water Quality Improvement. Boca Raton, FL: Lewis Publishers. 1993.
Mitsch, W.J. Self-design and Wetland Creation: Early Results of a Freshwater Marsh Experiment., Olentangy River Wetland Research Park at The Ohio State University Annual Report, 1996. Columbus: The Ohio State University. May 1997.
Mitsch, W.J., ed. 1994. Global Wetlands: Old World and New. Amsterdam: Elsevier.
Mitsch, W.J., and J.G. Gosselink. 1993. Wetlands. New York: Van Nostrand Reinhold.
Payne Engineering and CH2M Hill. Constructed Wetlands for Animal Waste Treatment. Montgomery, AL: Alabama Soil and Water Conservation Committee, 1997.
U.S. Environmental Protection Agency (EPA). Constructed Wetlands for Wastewater Treatment and Wildlife Habitat: 17 Case Studies. EPA832-R-93-005. September 1993.
U.S. Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS). Agricultural Waste Management Field Handbook. Washington, D.C.: NRCS, 1992.
U.S. Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS). 1991 Constructed Wetlands for Agricultural Waste Treatment, Technical Requirements. Washington, D.C.: SCS.
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