Home Identifying N2O at the Flensburg wastewater treatment plant
Klärwerk Flensburg_credit-photo-Andreas Große

Identifying N2O at the Flensburg wastewater treatment plant

Photo: Andreas Große

“The biggest change was the knowledge we gained about our nitrous oxide sources at the Flensburg wastewater treatment plant.”

In 2022, the TBZ Flensburg wastewater treatment plant started a series of small measurement campaigns to identify the primary sources of nitrous oxide in their processes. The project is part of the plant’s goal to achieve climate neutrality. Jan Philipp Schnack, Civil Engineer and Innovation Project Manager, was responsible for the project in co-operation with Robin Schauser, student at the Flensburg University of Applied Sciences.

The Flensburg plant contains several steps that deviate from the typical setup. So, the team identified 7 measuring points in the plant sections where nitrogen degradation takes place.

Reactor Main Selection Criterion
1: Upstream denitrification Denitrification
2: Aeration tank Nitrification
3: Trickling filter Nitrification
4: Sludge bed reactor Denitrification
5: Fixed bed reactor mainstream Nitrification
6: Fixed bed reactor sidestream Nitrification, high NH4 concentration
7: Fine filtration denitrification Denitrification


The main N2O formation pathways are during nitrification and denitrification as a by-product of the biological conversion of nitrogen.


“The investment costs had initially deterred us somewhat. However, the knowledge and results we gained made up for it. We see the benefits of using the system in the long term.”

Flexible measurements showed N2O variations

The measuring campaigns of 1-2 weeks at each spot took place from October to February. “The flexible use of the datalogger between the different measuring points was an advantage. We could easily pull the stored data onto a USB stick, for viewing and evaluation”, says Jan Philipp.

The results from the measurement campaigns showed low nitrous oxide formation in the main stream and low to typical nitrous oxide formation in side stream treatment. “Our results show that we are dealing with low nitrous oxide levels. But we could identify two places where nitrous oxide is formed”, explains Jan Philipp. However, N2O production often shows strong seasonal variations, so it is necessary to monitor it over a year to get a comprehensive picture.

WWTP-Flensburg_credit-photo-Andreas Große
Plans for efficient management

In the future, nitrous oxide measurements will provide data for Flensburg wastewater treatment plant’s CO2-equivalent accounting on their road to climate neutrality.

Jan Philipp has further plans for the system: “One of our next steps for the N2O Wastewater System is to integrate it into our process control system. We are also considering a possible use for the control of our C-source dosing. Furthermore, we want to gain knowledge about possible fluctuations in nitrous oxide formation through long-term use at a measuring point. Our goal will then be to take countermeasures to achieve an improvement of the wastewater treatment plant.”


Monitor N2O emissions with the N2O Wastewater System

Climate Goals in Flensburg

Size: 224,000 P.E.
Approx. 10 million m3 wastewater / year
Process combination: Preliminary sedimentation, activated sludge process, advanced treatment (trickling filter, sludge bed reactor, 2-stage filtration) filtration), turbid water treatment.
The TBZ Flensburg wastewater treatment plant has a goal to improve their carbon footprint and to achieve climate neutrality. Through the application of sewage gas, the plant already covers own heating plus 1,000 MWh to Flensburg’s district heating system as well as a large part of its own electricity requirements.
Additionally, the WWTP produces 35,000 kW/h through photovoltaics and 15,000 kW/h via hydropower. The plant also plans to convert the entire vehicle fleet to e-cars and expand the photovoltaics to supply itself up to 100% in the foreseeable future.

Tech Notes & References

Tech Notes
N2O Sensor
Tech Notes References
N2O sensor matches 91% of measured off-gas emissions

Case Study: Full-scale comparison of N2O emissions determined by liquid sensors and off-gas measurement

Tech Notes References
Direct Effect of Activated Sludge Concentration on N2O Emission and CO2-equivalents at Full-scale

Significant 3-fold increase in IPCC2019 wastewater N2O emission factor supported by Danish studies.

Figur 1_featured_700x400
Tech Notes References
Positioning of the N2O Wastewater Sensor

A case study from Kralingseveer WWTP in the Netherlands explores the influence of sensor placement.

N2O mechanisms
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National N2O Mapping and Reduction of N2O Emission through Advanced Online Control

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

Figure 1: Activated Sludge Tank
Tech Notes References
N2O Monitoring Highlights Potential for GHG Emissions Reduction

Learn about the results from N2O monitoring in the activated sludge tanks at Severn Trent’s Spernal sewage treatment plant

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A Tool for Carbon Dosage Control

Monitor the N2O concentration in the liquid and use N2O as a control parameter for carbon dosage in the denitrification process.

N2O Sensor Controls Emissions from Deammonification Processes
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N2O Sensor Controls Emissions from Deammonification Processes

Learn how the water utility Aquafin controls emissions from deammonification processes using the the N2O Wastewater Sensor

Nitrous Oxide Measurement
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Nitrous Oxide Measurement as Key Step towards Climate-Neutral Wastewater Treatment

Greenhouse gas emissions at wastewater treatment plants are coming into focus as the water industry works to reduce its climate footprint

WWTP-Flensburg_credit-photo-Andreas Große
Identifying N2O at the Flensburg wastewater treatment plant

The investment costs had initially deterred us somewhat. However ...

Tech Notes
Characterizing N2O emissions from WWTPs

A study of three very different WWTPs in Denmark: Bjergmarken (125,000 PE), Holbæk (60,000 PE), and Hvalsø (11,570 PE)

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N2O Emissions from Danish WWTPs – a two year monitoring project

The available data show that the nitrous oxide emission varies in time and between wastewater treatment plants.

Trickling Filters Fig. 1
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Nitrous Oxide Emissions from Trickling Filters

Information regarding N2O emissions from trickling filters is limited, partly caused by the difficulties in capturing off-gases.

Tech Notes
IPCC Greenhouse Gas Emission Factors

Significant 3-fold increase in IPCC2019 wastewater N2O emission factor supported by Danish studies.

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N2O Monitoring Puts VCS Denmark at the Technological Forefront

VCS Denmark, one of the largest and oldest water and wastewater companies in Denmark, is actively committed to resource optimization

Frederikshavn Water Utility

Frederikshavn Water Utility contacted Unisense Environment for assistance in determining the actual N2O derived CO2 footprint