Unwanted mainstream nitritation-denitritation causing massive N2O emissions in a continuous activated sludge process

Anna Kuokkanen*, K. Blomberg, Anna Mikola, M. Heinonen

*Corresponding author for this work

Research output: Contribution to journalArticleScientificpeer-review

11 Citations (Scopus)
82 Downloads (Pure)


Nitrous oxide emissions can contribute significantly to the carbon footprint of municipal wastewater treatment plants even though emissions from conventional nitrogen removal processes are assumed to be moderate. An increased risk for high emissions can occur in connection with process disturbances and nitrite (NO2-) accumulation. This work describes the findings at a large municipal wastewater treatment plant where the levels of NO2- in the activated sludge process effluent were spontaneously and strongly increased on several activated sludge lines which was suspected to be due to shortcut nitrogen removal that stabilized for several months. The high NO2- levels were linked to a dramatic increase in nitrous oxide (N2O) emissions. As much as over 20% of the daily influent nitrogen load was emitted as N2O. These observations indicate that highly increased NO2- levels can occur in conventional activated sludge processes and result in high nitrous oxide emissions. They also raise questions concerning the risk of increased greenhouse gas (GHG) emissions of the nitritation-denitritation processes - although the uncontrolled nature of the event described here must be taken into consideration - and underline the importance of continuous monitoring and control of N2O emissions.

Original languageEnglish
Pages (from-to)2207-2217
Number of pages11
JournalWater Science and Technology
Issue number9
Early online date31 Mar 2021
Publication statusPublished - 1 May 2021
MoE publication typeA1 Journal article-refereed


  • activated sludge process
  • GHG
  • N2O
  • nitritation-denitritation
  • nitrous oxide
  • NO2-


Dive into the research topics of 'Unwanted mainstream nitritation-denitritation causing massive N2O emissions in a continuous activated sludge process'. Together they form a unique fingerprint.

Cite this