N2O is a natural intermediate product of the biological treatment of wastewater that takes place every day at WWTPs around the world. During the biological conversion of ammonium (NH4+) to free nitrogen (N2) N2O is formed. Originally, N2O was only described as an intermediate product of the denitrification, the conversion of NO3-/NO2- to free nitrogen. However, today the scientific community has documented that N2O is a significant byproduct from the nitrification as well, formed during the conversion of NH4+ to NO2- and to NO3-.
The main processes and factors for nitrous oxide formation are presented below and the schematic overview is from the Global Water Research Coalitions state of science report “N2O and CH4 emission from wastewater collection and treatment systems”.
Nitrification is the stepwise oxidation of NH4+ to NO2- and to NO3- with the use of O2 as the electron acceptor. The predominant bacteria responsible for the conversion from NH4+ to NO2- are Ammonium Oxidizing Bacteria (AOB) and Nitrite Oxidizing Bacteria (NOB). Denitrification is the stepwise reduction of NO3- to NO2- and further on to N2 via NO and N2O intermediates and with the use of a carbon source and the electron donor. The predominant bacterium responsible for the conversion is Heterotrophic denitrifying bacteria (HET).
Ammonium Oxidizing Bacteria (AOB) can under oxygen limiting condition reduce NO2- to N2O, but not all the way to free N2. This pathway is called the Nitrifier Denitrification pathway and is believed to be one of the major sources to N2O formation and emission, especially stimulated during energy optimization of the aeration strategies. Furthermore, N2O can also be formed as the end-product during the denitrification if trace levels of oxygen are present as oxygen inhibits the reduction process of N2O to N2. Finally, hydroxylamine oxidation can also result in N2O formation via intermediates of the biological hydroxylamine oxidation or chemically.