CHARACTERIZATION OF OXIDATION-REDUCTION PROCESSES IN CONSTRUCTED WETLANDS FOR SWINE WASTEWATER TREATMENT

Abstract

Constructed wetlands designed and properly operated for treatment of swine wastewater may enhanceoxidation -reduction processes and nutrient treatment performance. The objective of this investigation was to characterizesoil wetland processes related to nitrogen (N) treatment (nitrification-denitrification) and phosphorus (P) removal using soiloxidation -reduction potential (ORP) data. We evaluated three surface-flow wetland systems constructed for treatment ofswine wastewater in Duplin Co., North Carolina, in 1992. Each system consisted of two 3.6- 33.5-m cells connected inseries. The three systems were planted to bulrushes, cattails, and agronomic crops (soybean in saturated soil culture andflooded rice), respectively. Soil aerobic/anaerobic conditions were determined by monitoring soil ORP at 18 sites usingplatinum (Pt) electrodes. Three monitoring sites were established in each wetland cell. Each site consisted of five Pt electrodesat three soil depths (0.02, 0.05, and 0.10 m) and a reference electrode. A data logger was used for hourly acquisition of soilORP and temperature records. Hourly ORP data were averaged on a 24-h basis and corrected to standard hydrogen electrodereadings (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 indicatethat different wetland plant species promote distinct anaerobic and reducing soil conditions. Outflow concentration ofammonia-N (NH3 -N) and soluble P increased with increasing ORP values for bulrush and soybean-rice wetland cells dueto lower temperatures during fall and winter, but not for cattails. Denitrification enzyme activities and ORP indicated thatsoils 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 inthe water column for cattails rather than the wetland soil. Prevalent anaerobic soil conditions and soluble P outflowconcentrations determined rather poor P retention capacity for all three wetlands.