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United States Department of Agriculture

Agricultural Research Service

Title: Denitrification Potential in Constructed Wetlands Used for the Treatment of Swine Wastewater

item Hunt, Patrick
item Matheny, Terry
item Szogi, Ariel

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 18, 2002
Publication Date: March 1, 2003

Interpretive Summary: Modern animal production has produced the need for more effective methods of treating animal waste. One of the methods being developed and tested for treatment of swine wastewater is a constructed wetland. These wetlands are constructed on upland landscapes for the express purpose of using their nutrient capture and transformation properties to treat swine wastewater. They have been found to be particularly effective in the treatment of nitrogen. In fact, the treatment has been so effective that it has been attributed to microbial transformation rather than to plant and soil capture. This study investigated the activity of a particular group of microorganisms, denitrifiers, associated with the transformation of nitrogen forms found in swine wastewater to the inert di-nitrogen gas that composes approximate 78% of the atmosphere. This process is called denitrification. The wetlands had different plant communities - bulrushes and cattails. The bulrush wetlands were found to be more effective for denitrification than the cattail wetlands. This was attributed to their ability to transport more oxygen to the plant roots. The bulrush wetlands were found to be more effective at shallow water depths of about 2 inches. They were also found to be very efficient in transforming nitrogen in the nitrate form which is directly converted to di-nitrogen gas. The high rates of denitrification in the bulrush wetlands are consistent with their reported high level of nitrogen treatment - greater than 25 pounds per acre in a day. Our results also indicate that the wetlands can be managed to take even better advantage of the higher treatment with shallow water depth and nitrate forms of nitrogen.

Technical Abstract: Constructed wetland treatment is being considered as an alternative method for treatment of swine wastewater. Wetlands have been found to be effective in the removal of nitrogen, and substantial denitrification is assumed to be part of their function. We investigated denitrification and potential for denitrification in bulrush and cattail wetlands used for the treatment of swine lagoon wastewater. The wetlands were 3.6 m wide and 33. m long. Soil samples and intact cores were collected at the 25.4- and 50.8-mm depth from four quadrants of constructed wetlands on 12 sampling dates over three years (1994-1997) for measurement of denitrification and denitrification enzyme activity, respectively. The acetylene blockage method was used. The bulrush wetlands were found to be more active in denitrification than the cattail wetlands, 0.516 and 0.210 ug N/g/hr, respectively. The mean value for the bulrush wetlands was equivalent to 9.55 kg N/ha/day. Nitrate was found to be a more limiting factor to denitrification than carbon source. One of the most determining factors of denitrification in the wetlands was water depth. In the case of bulrush, denitrification in wetland cores decreased by >60% as the water depth increased from 48 to 88 mm, R2 = 0.88; the slope was -0.128 mg N/m2/h/mm depth. Deeper depth is useful for increased residence time and surface cover. However, it decreased the potential denitrification rates in both the bulrush and cattail wetland systems. Our results suggest that the denitrification potential might be increased even more by internally recycling the water to the shallower depths at the upper end of the wetlands, particularly in the bulrush wetland.

Last Modified: 4/21/2015