A five-year, on-farm study shows constructed wetlands to be an economical, low-labor manure treatment option.
Through the years there has been a lot of talk about saving the wetlands. Now pork producers are talking about constructing them.
A long-range federal and state research project in North Carolina has taken the technology to the farm. It began in 1992 on the Gerald Knowles farm in Duplin County, where he has a 2,600-pig nursery operation on contract with Murphy Family Farms. Mark Rice, a North Carolina State University agriculture engineer, coordinates the project.
He and others closely monitor the cells that receive waste-water from the farm’s lagoon and transform nitrogen in the waste-water to atmospheric nitrogen.
In a hog operation, Rice says, there are three options to use the water cleaned by wetlands:
1. Recycle it back to the buildings and use it in a flush system.
2. Pump it back into the lagoon to dilute the waste water.
3. Use it for crop irrigation in accordance with your state’s manure-management regulations.
Let’s take a look at the prospects that constructed wetlands have to offer.
Researchers have looked at two wetland loading rates on the Knowles farm.
When nitrogen levels of 3 pounds for each wetland acre per day was released, about 95 percent of the nitrogen was removed from lagoon waste-water by both rush and bulrush and cattail and bur reed plant combinations.
At 10 pounds of nitrogen per wetland acre, both plant combinations showed reduced efficiency. Cattails and bur reed removed 73 percent of nitrogen, and rush-bulrush extracted 63 percent.
Current studies are looking at even higher loading rates of 20 pounds of nitrogen per acre.
Phosphorous reduction has ranged from 20 percent to 80 percent, but long-term phosphorous removal is limited, Rice says. “Although some of the phosphorous is removed from lagoon waste-water, it is simply retained in the wetlands’ soil and plant materials.
Levels of zinc and copper are being measured in lagoon waste-water before it enters the wetlands and again after treatment. Researchers also are tracking heavy metals in the plants and soil.
What is put into the wetlands is extremely important due to the high nitrogen content of swine manure, says Rice.
“Because plants’ tolerance to ammonia is limited,” he explains, “you have to be careful about the concentration going into a constructed wetlands. Unless swine manure is sufficiently diluted with fresh water, it can stunt or kill the plants. That’s why our emphasis has been on loading rates.”
Several plant species found in natural wetlands have been used: bur reed, juncus, cattail and bulrush.
“We have seen little difference in the treatment capacity of various species. Rather than having one kind of plant in a cell, it’s preferable to have a diverse plant community in case there’s a problem with any one species in a given year, says Rice.
“Natural selection will take care of finding the most adapted species for a particular wetlands system,” he explains. “The best advice is to observe plants in natural wetlands found in your area and select those species for your constructed system.”
Climate influences function
Constructed wetlands work best in moderate-to-hot weather when biological activity is high but function even during freezing weather. Freezing does not harm wetlands plants, Rice says.
In eastern North Carolina’s moderate climate, where hard freezes are infrequent, cells can function well for at least nine months of the year. During the first two years at the Knowles farm, researchers tried operating the wetlands year-round. But because pipes were near the ground surface, they froze.
To safeguard against this, the shallow pipes from Knowles’ lagoon are drained each year about Dec. 1. Around March 1, the researchers start letting water back into the wetlands.
This would not be necessary, Rice says, even in the coldest climates if the pipes are buried deep enough or insulated well enough.
Due to lower water temperatures in winter, biological activity slows in constructed wetlands. To compensate for this, design the system with adequate capacity. If sufficient waste-water storage in a lagoon is available, you should not load the wetlands during periods of sustained freezing weather.
“For year-round operation, a system can be designed with larger cells, more waste-water storage capacity or a combination,” says Rice, adding that pipes should be located to avoid freezing.
Cell construction key
At least two parallel cells are necessary at a location, according to Rice. “Should maintenance be required on one cell, this allows you to take it off-line.”
To construct a wetland, you will need 1-foot soil depth for planting. A soil high in clay content works most effectively. It absorbs and retains more phosphorous than a soil with high organic matter or sand content.
You will then need a compacted clay liner in the bottom and on the sides to serve as a seepage-control barrier. It must comply with state regulations. Excavation depth will be site-specific but should allow for accumulation of plant material and provide emergency storage for excess rainfall.
The bottom should have a 0.2 percent slope. Side slopes should be no steeper than a 1-foot rise per 3-foot run to allow for mowing.
In regions prone to burrowing rodents, such as muskrats, take special precautions to protect dikes. Rice says welded, galvanized wire-mesh screen serves as a barrier when laid on top of the clay liner before you add topsoil.
Sizing the cells to your need is site-specific and varies from one location to another. “Cell size depends on the system’s goals for removing nitrogen. How the water will be used after it leaves the wetlands determines how you treat it.
Seedlings should be planted 2 to 3 feet apart. You can purchase them from a commercial nursery or transplant from existing sites, such as ponds or ditches. But do that without disturbing natural wetlands, says Rice. “Cattails will grow from seed and you don’t need to buy them.” You can strip seed from mature cattails along a road or in pondsù with the owner’s permission.
A year’s growth is needed to establish wetlands plants; it’s best not to apply waste-water until root systems are developed. This takes place in about a year.
When the system on the Knowles farm was started, a 10:1 dilution rate was used to reduce the ammonia concentration to about 50 parts per million.
“We now use a 1:1 dilution rate with the ammonia-nitrogen concentration at 200 ppm,” Rice adds.
A maintenance water depth may be between 4 and 10 inches, depending on the plant types. You can recycle the treated water to use as flush water in your hog buildings instead of lagoon water. This should help improve air quality and reduce odors.
Current environmental regulations don’t allow stream discharge from an animal-manure-treatment system.
“Wetlands by themselves,” says Rice, “cannot produce an effluent that meets discharge standards. But as more advanced systems are developed and available, the day may come when constructed wetlands will be a final step before discharging clean water from a hog operation. This is being done successfully by municipal treatment systems in parts of South Carolina and California.”
All you usually must do with a well-designed constructed wetlands is to check for a stopped-up line, remove tree seedlings and check for burrowing rodents.
Removing tree sprouts is especially important to prevent roots from puncturing the clay liner, causing leakage. Weeds have not been a problem on the Knowles farm’s wetlands.
“It’s important to let dead vegetation in a wetlands accumulate on the bottom of the cell, says Rice. “Plant material decomposes and the organic matter builds up. This forms a detritus layer in which the plants’ root systems grow and thrive. The detrital layer also provides surface area for microbial activity.”
Buildup of organic matter in constructed wetlands determines its useful life. “We expect the cells on the Knowles farm to function effectively for at least 20 years,” says Rice.
For more information about constructed wetlands, related regulations, construction and management, contact the these offices in your state: Cooperative Extension Service, National Resource Conservation Service, Soil and Water Conservation District.
Overland Flow Enhances Wetlands
In conjunction with constructed wetlands, overland flow treatment of swine lagoon waste-water is being studied at the Duplin County, N.C. farm of Gerald Knowles. Working with Rice on this research project is USDA soil scientist Ariel Szogi.
A thin film of waste-water piped from the lagoon flows through slightly sloped cells containing grasses. The grasses include: Coastal Bermuda grass, fescue and reed canary grass.
Water processed by overland flow can be further treated by the larger plants in constructed wetlands.