Quantifying Nitrous Oxide Emissions from Wastewater Treatment Plant Effluent

Abstract

Nitrous oxide is one of the most potent greenhouse gases, with a global warming potential of around 265 times that of carbon dioxide. It is emitted from various anthropogenic sources, including wastewater treatment plants. While many of the emissions due to wastewater treatment within a plant have been quantified, there is little understanding of the magnitude and spatial scale of the emissions due to effluent discharged by these plants into nearby rivers and tributaries. When effluent is discharged from the plants, it often still has low concentrations of nitrogen-containing compounds that are transformed into nitrous oxide by microbial activity in the water. The IPCC has devised a simplistic equation to estimate these emissions; however, very few tests have been conducted to determine the accuracy of this equation. For this thesis, a floating flux chamber was constructed that can take measurements of the concentration of nitrous oxide in a fixed volume over time. It was used on the Millstone River, at various points upstream and downstream of an effluent discharge point from a wastewater treatment plant, to determine whether the nitrous oxide flux significantly changed with proximity to the effluent site. From the change in concentration over time at each location, a flux rate from the river was calculated, and then compared to documented flux rates from other bodies of water. It was found that the nitrous oxide emissions are significantly higher at the effluent discharge site, at about 10-15 times times larger than upstream and downstream locations on the same river and up to about 30 times larger than emissions from the literature.