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Effect of Substrate Organic Matter Addition on Nitrate Removal Efficiency in Surface-Flow Constructed Wetlands

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  Paper number  022038,  2002 ASAE Annual Meeting . (doi: 10.13031/2013.9835) @2002
Authors:   M.R. Burchell II, R.W. Skaggs, S.W. Broome, and C.R. Lee
Keywords:   Constructed wetlands, nitrate, substrate, organic matter, Scirpus validus

Secondary treatment of nitrified drainage water with constructed wetlands may help reduce nitrogen loads from agricultural facilities. Denitrification is considered a major pathway for nitrate removal in constructed wetlands, occurring mostly in microsites at the substrate-water interface. This process requires, among other conditions, low dissolved oxygen and an adequate carbon source. Soils located at sites where constructed wetlands are to be utilized for nitrate treatment are not always suitable as wetland substrate, because they would fail in providing ideal conditions for denitrification and plant establishment in the first few years after construction. Providing a suitable substrate for these sites is critical for the success of the constructed wetland system. This study evaluates the ability of mesocosm-scale surface-flow constructed wetlands (SFCWs), with six different substrates, to remove nitrate-nitrogen (NO3 --N) from drainage water. The soil available at the study site at the USDA-Tidewater Research Station near Plymouth, NC was amended with wheat straw and biosolids to increase organic matter (OM) from approximately 5%, to approximately 11 % OM and 16% OM. A dredged material (12 % OM) from USACE - Eagle Island Confined Disposal Facility in Wilmington, NC, was also evaluated as a wetland substrate. The OM of this material was increased to approximately 18% and 22% OM by blending with Phragmites compost and biosolids. Two batch studies were conducted in July and August of 2001 with initial NO3 --N concentrations of 25 mg/L (Batch 1) and 50 mg/L (Batch 2). Each of the wetland mesocosms was evaluated on its ability to remove NO3 --N with respect to the OM% of the substrate, and the production of wetland biomass, the wetlands renewable carbon source. All wetland treatments with greater than 11% substrate OM removed on average >90% of the NO3 --N in 7 days in Batch 1 and in 10 days in Batch 2. Statistical analysis of the NO3 --N removal curves and the 2001 biomass production of the site soil based wetlands indicated that increased substrate OM significantly increased the NO3 --N removal and biomass production of these wetlands. However, there was no statistical difference in NO3 --N removal and biomass production between the two amended site soil wetland treatments with 11% and 16% substrate OM. Increases in OM to the dredged material based substrate had no statistical effect on the ability of these wetlands to remove NO3 --N or produce biomass.

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