Trace Gas Emission from in-Situ Denitrifying Bioreactors

Abstract:

Despite decades of concerted effort to mitigate nonpoint source nitrate (NO₃^-) pollution from agricultural lands, these efforts have not been sufficient to arrest eutrophication. A primary process for removing excess NO₃^- from water is denitrification, where denitrifying bacteria use NO₃^- for respiration in the absence of oxygen. Denitrification results in reduced forms of nitrogen, often dinitrogen gas (N₂) but also nitrous oxide (N₂O), an aggressive greenhouse gas. A promising solution to NO₃^- pollution is to intercept agricultural discharges with denitrifying bioreactors (DNBRs). DNBRs provide conditions ideal for denitrifiers: an anaerobic environment, sufficient organic matter, and excess NO₃^-. These conditions are also ideal for methanogens, which produce methane (CH₄), another harmful trace gas. While initial results from bioreactor studies show that they can cost-effectively remove NO₃^-, trace gas emissions are an unintended consequence.

This study’s goal was to determine how bioreactor design promotes denitrification while limiting trace gas production. Reactor inflow and outflow water samples were tested for nutrients, including NO₃^-, and dissolved inflow and outflow gas samples were tested for N₂O and CH₄. NO₃^- reduction and trace gas production were evaluated at various residence times, pHs, and inflow NO₃^- concentrations in field and lab-scale reactors. Low NO₃^- reduction indicated conditions that stressed denitrifying bacteria while high reductions indicated designs that optimized pollutant treatment for water quality. Several factors influenced high N₂O, suggesting non-ideal conditions for the final step of complete denitrification. High CH₄ emissions pointed to reactor media choice for discouraging methanogens, which may remove competition with denitrifiers. It is critical to understand all of potential impacts that DNBRs may have, which means identifying processes and design specifications that may affect them.