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The exact triggers for N2O production from activated sludge are not fully understood. However, the predominant emission of N2O comes from the aerated nitrification, mainly from the metabolism of NH4 by ammonium oxidizing bacteria.

Especially, lack of sufficient oxygen during the nitrification process has been shown to result in high levels of N2O production and due to the simultaneous aeration the N2O is stripped into the atmosphere. It is also generally accepted that sudden perturbations of the bioprocesses such as changes in dissolved oxygen, NH4+ shock loads or NO2- spikes lead to immediate increases in N2O emissions. 

In overview, process parameters that promote the formation and emission of N2O:

  • Low oxygen set point or operation conditions, typically below 0.5 mg/L, during nitrification

  • Low COD/N ratio for denitrification, typically below 3.5-4 
  • Chock loading of NH4+, e.g. reject water pulse

  • NO2- accumulation during the nitrification, e.g. due to high NH4+ conversion rate.

  • Oxygen availability during denitrification

  • Short sludge age

  • Low temperatures

Real-time measurements of N2O concentration and emission have been utilized by full-scale plants as an indication of biological nitrification failure owing to toxic shock loads or insufficient aeration.