Nitrous oxide formation during nitritation and nitrification of high-strength wastewater

The purpose of this study was to investigate the formation of nitrous oxide (N₂O) in nitritation and nitrification under stable, comparable and not limiting conditions typical for treatment of high-strength wastewater. A laboratory-scale aerated chemostat was operated with reject water at different sludge retention times, achieving suppression of nitrate formation by wash-out of nitrite-oxidizing bacteria for nitritation. The N₂O formation factor during stable nitritation was higher (2.90% N₂O-N /NH₄⁻-N_ox) than during nitrification (0.74%). The positive correlation of N₂O formation rates and ammonium oxidation rates was linear and thus did not contribute to changes of the N₂O formation factor. The dominant factor for N₂O formation during stable operation was high nitrite concentration, which was positively correlated with N₂O formation rates. The highest formation factors were observed during a transition phase from nitrification to nitritation with unstable process conditions (4.81%) and during a short-term experiment with increased pH of 7 (10.28%). The results indicate that even with operational conditions that are regarded favourable for the process of nitritation N₂O formation can be limited but not avoided.