Advancements in constructed wetland technologies are offering real advantages to pork producers with anaerobic waste lagoons.

  In a North Carolina system under review, fast-growing duckweed placed in a pond located between marsh cells further “scrubs” wastewater. Some of the results have revealed the following:

  • As much as 95 percent of nitrogen and 75 percent of phosphorus are removed.
  • A resulting byproduct can be used as a nutritious feed ingredient.
  • Treated water is odorless and almost clear.

Known as the marsh-pond-marsh system, the process has been researched and refined during the past seven years. North Carolina A & T State University researchers G. B. Reddy, Anthony Grubbs and Richard Phillips have directed the studies.

The system treats wastewater from a 150-sow, farrow-to-feeder-pig operation at the university’s swine research unit near Greensboro. The researchers have worked closely with USDA’s Agricultural Research Service scientists at Florence, S.C.

Three types of vegetation “scrub” the water from the farm’s primary and secondary lagoons. The marshes contain cattails and bulrushes. The pond between each pair of marshes contains duckweed. Harvesting it removes nutrients from the waste stream.

Treated water is almost clear and has no discernible odor, but does not quite meet discharge-permit standards. However, due to the reduction in nutrient concentration, the water can be recycled back into hog buildings for flushing or it can be land-applied at a considerably higher rate than non-scrubbed water. Thus, it reduces the acreage needed.

The lagoons have capacity for 12 months worth of wastewater from the five adjoining hog buildings. The wetlands system is operated from March through October. To operate the system in freezing weather, it would require exposed pipes to be winterized.

Effluent that accumulates in the lagoons during the winter is further processed by the marsh-pond-marsh system during spring and summer.

The five buildings have partially-slatted floors and a flush waste-removal system. Pits are flushed automatically twice a day. Effluent drains by gravity to a primary lagoon, then on to an adjacent secondary lagoon. From there, the wastewater is pumped up a slope to a 2,100-gallon tank located on the bank above the first marsh-pond-marsh. It flows into the first 30-foot x 30-foot marsh, where it is maintained around a 3-inch depth. This is repeated when the other five systems are filled.

Next, the water flows into the 30-foot x 60-foot central pond containing duckweed. Water in it is kept at a depth of about 30 inches to allow oxygen to diffuse at the water surface and to prevent encroachment of wetland plants. Water from the pond flows continuously into the adjacent second marsh. Each marsh-pond-marsh system operates independently.

The North Carolina A&T research includes measuring the systems’ efficiency at different loading levels. Twice a day, solar-powered devices automatically sample water in each marsh at the inflow and outflow ends. Composite weekly samples are analyzed to measure “in” and “out” concentrations of nitrogen and phosphorus.

Initial construction cost of the six marsh-pond-marsh systems was about $15,000. Researchers emphasize that a system’s actual cost is site specific. Variable factors that influence the system include the operation’s daily water volume production, slope of the terrain and distance that the water must be pumped.

Research continues at North Carolina A&T on three factors’ influencing efficiency of the marsh-pond-marsh constructed wetlands; concentration of nutrients in the wastewater; nutrient load in the system (pounds per acre per day) and retention time of water in the system.

“This is on-going, long-term research,” says Anthony Grubbs. “Each year, we find out more about what does and does not work to improve the wetland’s performance.” 

Duckweed has Many Attributes
The marsh-pond-marsh wetland system has much to offer, thanks to duckweed’s filtration abilities. Here’s a look at some specific findings from the North Carolina A&T State University research.

  • Duckweed sequesters a high level of nitrogen (about 5 percent of its dry weight) and about 10 times as much phosphorus (about 1 percent of its dry weight) as other plants.    
  • Air-dried duckweed contains nearly 40 percent protein.
  • Investment in duckweed plants for a pond is modest. It’s available from specialty nurseries.
  • At the North Carolina A&T farm, seed was brought in naturally on the feet of ducks and geese.
  • Once duckweed is established, it does not need replanting. 

This summer, researchers will run tests to determine how often duckweed can be harvested and how much of a pond’s surface can be harvested at a time.

In addition to duckweed’s ability to remove nutrients from the waste stream, it shows promise as a livestock feed supplement. Animal performance with air-dried duckweed has been checked in feeding trials at North Carolina State University. After pelleting, it was used in goat and beef cattle rations, successfully replacing up to two-thirds of required soybean meal.

Further research will look at duckweed’s potential use in swine, poultry and fish rations.