Home National N2O mapping and reduction of N2O-emission through advanced online-control
N2O sensor in water

National N2O mapping and reduction of N2O-emission

Based on data from Danish WWTPs through advanced online-control.

Shared with permission from Krüger Veolia.
Authors: Ellen Marie Drastrup, Krüger A/S and Jakob Fink, Aarhus University


Nitrous oxide (N2O) has been identified as a significant contributor to the water sector's carbon footprint. N2O is produced as a byproduct of nitrogen removal in biological wastewater treatment plants (WWTP), and it is therefore obligatory for the water sector to look into this if they want to reduce their carbon footprint.

The scope of this project is to identify different N2O pathways by looking into examples from Danish WWTP plants. The design, composition of load and control strategy have a big impact on the N2O production of the specific plant, and will impact which N2O pathway which is dominant. Only by data observation and analysis can the correct control method be applied, and N2O reduced. Due to lack of data from Fornæs WWTP, this poster will use an example from Næstved WWTP.

N2O Mechanisms


In order to observe how N2O is produced in relation to NH4, NO3 and O2-levels, it is crucial to have online sensors in the waterphase. Most Danish WWTPs do already have online NH4, NO3 and O2 sensors in order to observe and control the nitrogen removal processes. Next step has been to install N2O-online sensor in the waterphase.

By being part of many N2O measuring-project, we have had the opportunity to gather knowledge from a broad range of Danish WWTP. Here examples from 4 WWTP is shown.

N2O is produced through three different pathway during nitrification (N) and denitrification (DN). It varies a lot from plant to plant which pathway is dominant, and what N2O-pattern we see in the online data. It is not always clear which pathway that is dominant or the reason for a high N2O.

To try to understand the N2O-production at each plant, we have looked into the data, and observed how NH4, NO3, O2 and N2O relates. That requires qualitative analysis of the figures of measured data. When the dominant pathway have been identified it is possible to suggest of measured data. When the dominant pathway have been identified it is possible to suggest a control method.

Based on literature and in-situ experience, three N2O control methods has been selected (see above).

N2O Production Pathways

Results & Discussion

In order to take action about N2O reduction, the following method has been identified:

  • Go through you treatment plant in order to find the relevant process tanks, where nitrogen removal takes place.
  • Install N2O sensor in the process tank
  • Observe the pattern of N2O production
  • Select the control strategy based on the observed N2O mechanism
  • Implement the control strategy through advanced online control.
  • Evaluate the N2O reduction and reconsider

Click the image to the right for more results and for information on location and control method.

N2O at Mariagerfjord
N2O at Avedøre
N2O at Næstved
N2O at Aalborg


N2O plays a major part of wastewater treatment plants climate impact and it is crucial to look into this challenge for all plants in order to tackle it.

In order to reduce N2O, the first step is to get knowledge of the amounts and the pattern of production, in order to interpret the biological pathways.

First when you have this knowledge, you can choose the best control method and start reducing the emission.

  • Fink, J. 2022. Analysis, mitigation and modeling of nitrous oxide emission from Fornæs wastewater treatment plant. Master thesis, Department of Biological and Chemical Engineering, Aarhus University.
  • Ekström, S.E.M., Vangsgaard, A.K., Lemaire, R., Valverde Pérez, B., Benedetti, L., Jensen, M.M., Smeths, B.F. (2017). Simple control strategy for mitigating N2O emissions in phase isolated full-scale WWTPs. In Proceedings of 12th IWA Specialized Conferende on Instrumentation, Control and Automation Quebec, Canada; IWA Publishing.
N2O Reduction through Hubgrade

*Krüger's patented N2O-module in Hubgrade