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United States Department of Agriculture

Agricultural Research Service

Title: Characterization of Oxidation-Reduction Processes in Constructed Wetlands for Swine Wastewater Treatment

Authors
item Szogi, Ariel
item Hunt, Patrick
item Sadler, Edward
item Evans, Dean

Submitted to: Applied Engineering in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 20, 2003
Publication Date: February 1, 2004
Citation: Szogi, A.A., Hunt, P.G., Sadler, E.J., Evans, D.E. 2004. Characterization of oxidation-reduction processes in constructed wetlands for swine wastewater treatment. Applied Engineering In Agriculture. 20(2):189-200.

Interpretive Summary: Constructed wetlands have potential for treatment of animal wastewater before land application and, in this manner, reduce the area of cropland required for wastewater application and nutrient assimilation. Their primary role is to remove nitrogen (N) via biological processes under flooding conditions. Due to flooding, soils rapidly become anaerobic and oxygen depleted. Since oxygen is difficult to measure under field conditions, in situ measurement of oxidation-reduction potential (ORP) using platinum electrodes was used as an indirect method to characterize oxygen status and intensity of biological processes in flooded soils. The objective of this investigation was to characterize soil wetland processes related to nitrogen (N) treatment (nitrification-denitrification) and phosphorus (P) removal using soil oxidation-reduction potentials (ORP) data. We evaluated three surface-flow wetland systems constructed for treatment of swine wastewater in Duplin Co., NC, in 1992. Each system consisted of two 3.6-m x 33.5-m cells connected in series. The three systems were planted to bulrushes, cattails, and agronomic crops (soybean and flooded rice), respectively. Soil aerobic/anaerobic conditions were determined by monitoring soil ORP using platinum (Pt) electrodes. A data logger was used for hourly acquisition of soil ORP and temperature. Frequency analysis of daily ORP showed that bulrush and soybean soils were moderately reduced and anaerobic about 70% of the time. However, cattail and rice cells were anaerobic 100% of the time and had reduced to highly reduced soil conditions. For the three constructed wetlands of this study, ORP and reduced soil conditions indicated that their flooded soils were more prone to release inorganic P than to remove it. From this, it was concluded that wetland soils planted to bulrush and soybean could promote better conditions for N removal than soils planted to cattails or rice.

Technical Abstract: Constructed wetlands designed and properly operated for treatment of swine wastewater treatment may enhance oxidation-reduction processes and nutrient treatment performance. The objective of this investigation was to characterize soil wetland processes related to nitrogen (N) treatment (nitrification-denitrification) and phosphorus (P) removal using soil oxidation-reduction potentials (ORP) data. We evaluated three surface-flow wetland systems constructed for treatment of swine wastewater in Duplin Co., NC, in 1992. Each system consisted of two 3.6-m x 33.5-m cells connected in series. The three systems were planted to bulrushes, cattails, and agronomic crops (soybean in saturated soil culture and flooded rice), respectively. Soil aerobic/anaerobic conditions were determined by monitoring soil ORP at 18 sites using platinum (Pt) electrodes. Three monitoring sites were placed in each wetland cell. Each site consisted of five Pt electrodes at three soil depths (0.02, 0.05, and 0.10 m) and a reference electrode. A data logger was used for hourly acquisition of soil ORP and temperature. Hourly ORP data were averaged on a 24-hour basis and corrected to standard hydrogen electrode readings (Eh). Frequency analysis of daily Eh showed that bulrush and soybean soils were moderately reduced (Eh between +100 and +300 mV) and anaerobic about 70% of the time (Eh less than +300mV). However, cattail and rice cells were anaerobic 100% of the time and had reduced to highly reduced (Eh<+100 mV) soil conditions. These conditions reflected both persistent low oxygen and limited nitrification in cattail and rice soils. It was concluded that wetland soils planted to both bulrush and soybean could promote better conditions for nitrification-denitrification than soils planted to cattails or rice. A strong dependency of Eh on daily temperatures was found for bulrush (r2=0.59) and soybean-rice (r2=0.48), but not significant for cattails (r2=0.001). For the three constructed wetlands of this study, ORP and reduced soil conditions indicated possible reduction of ferric-iron oxides and potential release of associated inorganic P from soil substrate into the water.

Last Modified: 9/2/2014