Effect of operating conditions on N2O emissions from one-stage partial nitritation-anammox reactors

One-stage partial nitration-anammox granular sludge technologies are widely applied to treat nitrogen-rich wastewater. However, the strong greenhouse gas nitrous oxide (N2O) can be released as a by-product. In this study, a mathematical model of a granular sludge reactor was applied to investigate the factors that influence N2O emissions from these systems. Individual changes of operating parameters led to large and often non-monotonic changes in the simulated N2O emissions, which could explain the large variety of emissions factors and some apparent contradictions found in literature. Nitrifier denitrification in the outer layer of granules was found to be the main source of N2O. Heterotrophic denitrification acted as a sink in deeper layers, even though this pathway has been neglected in previous modelling studies. The dissolved oxygen (DO) concentration that allows simultaneous low N2O emissions and high nitrogen removal appears to be very site-specific, depending on the wastewater composition and granule size. The presence of organic substrates makes process optimization easier because it can stimulate consumption of N2O via heterotrophic denitrification.