Characterization of Oxidation-reduction Processes in Constructed Wetlands for Swine Wastewater Treatment

Constructed wetlands designed and properly operated for treatment of swine wastewater 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 potential (ORP) data. We evaluated three surface-flow wetland systems constructed for treatment of swine wastewater in Duplin Co., North Carolina, in 1992. Each system consisted of two 3.6× 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 established 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 records. Hourly ORP data were averaged on a 24-h basis and corrected to standard hydrogen electrode readings (Eh). Frequency analysis of daily soil Eh showed that bulrush and soybean cells were moderately reduced (+100 < Eh < +300 mV) and anaerobic (Eh < +300 mV) about 70% of the time. However, cattail and rice cells were anaerobic 100% of the time and had reduced (-100 < Eh < +100 mV) to highly reduced (Eh <-100 mV) soil conditions. These results indicate that different wetland plant species promote distinct anaerobic and reducing soil conditions. Outflow concentration of ammonia-N (NH3 -N) and soluble P increased with increasing ORP values for bulrush and soybean-rice wetland cells due to lower temperatures during fall and winter, but not for cattails. Denitrification enzyme activities and ORP indicated that soils in bulrush wetlands promoted better conditions for nitrification-denitrification than cattails or rice soils. However, equivalent NH3 -N removal rates (4.8-5.6 kg ha-1 d-1) for cattails and bulrush suggested that treatment occurred mostly in the water column for cattails rather than the wetland soil. Prevalent anaerobic soil conditions and soluble P outflow concentrations determined rather poor P retention capacity for all three wetlands.