Nitrogen removal in a wastewater treatment plant through biofilters: nitrous oxide emissions during nitrification and denitrification

In order to estimate N₂O emissions from immersed biofilters during nitrogen removal in tertiary treatments at urban wastewater treatment plants (WWTPs), a fixed culture from the WWTP of “Seine Centre” (Paris conurbation) was subjected to lab-scale batch experiments under various conditions of oxygenation and a gradient of methanol addition. The results show that during nitrification, N₂O emissions are positively related to oxygenation (R ² = 0.99). However, compared to the rates of ammonium oxidation, the percentage of emitted N₂O is greater when oxygenation is low (0.5–1 mgO₂ L⁻¹), representing up to 1% of the oxidized ammonium (0.4% on average). During denitrification, the N₂O emission reaches a significant peak when the quantity of methanol allows denitrification of between 66% and 88%. When methanol concentrations lead to a denitrification of close to 100%, the flows of N₂O are much lower and represent on average 0.2% of the reduced nitrate. By considering these results, we can estimate, the emissions of N₂O during nitrogen removal, at the “Seine Centre” WWTP, to approximately 38 kgN-N₂O day⁻¹.