Submitted to: Ecological Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/3/2009
Publication Date: 7/27/2009
Citation: Hunt, P.G., Stone, K.C., Matheny, T.A., Poach, M.E., Vanotti, M.B., Ducey, T.F. 2009. Denitrification of nitrified and non-nitrified swine lagoon wastewater in the suspended sludge layer of treatment wetlands. Ecological Engineering. 35(10):1514-1522.
Interpretive Summary: Animal production plays a vital role in USA agriculture, both in terms of economic prosperity and food stability. However, the concentration of animals feeding operations makes the treatment of the resultant wastes a more complex matter than the historical land spreading of manures on croplands. The treatment complexity can be exacerbated by issues such as odors, pathogens, and residential encroachment. In the case of swine wastewater, producers often treat the wastewater via anaerobic lagoons. This practice proves effective as long as sufficient land is available for balanced application of nutrients and reasonably non-offensive odors. If land application rates exceed crop uptake rates, excess nutrients can contaminate surface and ground waters. Thus, there has been interest in practices that can keep the nutrient load in balance with the available cropland. For the past several decades, wetlands have been utilized for the treatment of agricultural, municipal, and residential wastewaters. For the treatment of these wastewaters, wetlands were considered to be natural, operationally passive, relatively cost effective, and simple in design and operation. In regard to animal wastewater, the use of constructed wetlands has focused primarily on reducing nutrient mass load, especially N; the concomitant result being the reduction of cropland necessary to assimilate the remaining nutrients. Constructed wetlands have demonstrated the ability to treat large quantities of nutrients. They have been reported to remove N at 70–95% efficiency when N loading rates were much higher than could be applied to cropland. Most of this loss is via a microbial process call denitrification in which nitrogen is converted to inert di-nitrogen gas. We measure denitrification in the soil, detritus, and sludge layer. In all three layers, there was significant denitrification. However, the sludge layer had the highest rates.
Technical Abstract: The management of N from animal wastes is important to global agriculture. One method for managing this N is the use of constructed wetlands to obtain substantial removal of N via denitrification. The objectives of this study were to 1) evaluate the denitrification enzyme activity (DEA) rates in wetland soil, detritus, and sludge layers; and 2) determine how well wetland and wastewater conditions would predict DEA rates in the sludge layer under different treatment conditions. The study used four wetland cells (3.6 x 33.5 m) with two cells connected in series. The wetlands were planted with species of Juncus, Schoenoplectus, Scirpus, Sparganium, and Typha. Each wetland received either untreated or partially nitrified swine wastewater from a single cell anaerobic lagoon. From July 2000 through August 2001, the DEA of the soil, detritus, and sludge layers of the constructed wetlands was measured by the acetylene inhibition method. The DEA values of all three layers indicated significant denitrification – it was limited by nitrate but not by carbon. The soil layer control treatment had a mean DEA of 0.65 ± 0.31 µg N2O-N/g soil/h. The soil layer was somewhat lower in DEA than the detritus layer. Specifically, the detritus layer control treatment had a DEA rate of 1.50 ± 1.09 µg N2O-N/g detritus/h. The overwhelmingly highest rate of DEA was found in the sludge layer. Its control treatment had a DEA rate of 18.3 ± 10.5 µg N2O-N/g sludge/h. his rate of DEA was consistent with the fact that these wetlands removed very high rates of wastewater N.