Importance of Denitrification to the Efficiency of Wastewater Treatment inForested Wetlands

摘要

Wastewater, even after secondary treatment, typically contains highconcentrations of nutrients that can cause eutrophication of receiving waters and deterioration of water quality. Therefore, there has been much interest in the use of natural wetlands as a simple and energy-efficient means of removing nutrients from wastewater and improving water quality. The utilization of a wetland for tertiary treatment of wastewater is based on the ability of the wetland to act as a nutrient sink (i.e., outputs are less than inputs). One of the most important processes in wetland ecosystems that influences their capacity as a nitrogen sink is the gaseous exchange of nitrogen with the atmosphere known as denitrification. Since denitrification represents a loss of nitrogen to the atmosphere, the mechanism tends to be most favorable for the removal of nitrogen. The objectives of the research project were to (1) determine the temporal and spatial ambient rates of denitrification and compare these rates to those of sediments amended with increased concentrations of nitrate comparable to concentrations of total nitrogen in the sewage effluent to be discharged; and (2) determine the proportion of total denitrification that can be attributed to direct utilization of nitrate loaded into the wetland, as compared to nitrate produced via nitrification within the wetland. Although nitrate is readily denitrified, short-term incubation rates are relatively low which is attributed to the presently low nitrate concentrations and subsequent reduced denitrifying microbial population in the wetland sediments. Nitrate concentrations varied seasonally associated with increased flooding during spring. Rates of nitrification coupled with denitrification were investigated with nitrogen-15 isotopes. Nitrification is limited in the wetland sedments; therefore, controls the rate of total nitrogen loss from the system. Maximum rates of nitrification depend largely on oxygen availability which may be enhanced by the rooting system of certain aquatic macrophytes which provide oxygen microsites within reduced sediments.