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.

Carbon dosage – Fig 1 _1400x800

“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.

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
Nitrous Oxide Monitoring in a Typical Nitrifying Activated Sludge Process

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

N2O Mechanisms
Tech Notes
National N2O mapping and reduction of N2O-emission through advanced online-control

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

Tech Notes
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.

Sensor Nivå_1400x800
Tech Notes
N2O Sensor Controls Emissions from Deammonification Processes

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

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.

Operational experience with the N2O Wastewater Controller

WWTP Pforzheim uses an N2O sensor to determine the formation rate of nitrous oxide in the denitrification and the nitrification zone

Recirculation setup frederikshavn_1400x800
Frederikshavn Water Utility

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

Ejby Mølle bassin_1400x800
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

Controller Biofos_1400x800
Tech Notes
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.

Fig. 1_1400x800
Tech Notes
Nitrous Oxide Emissions from Trickling Filters

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

IPCC 2019 report_1400x800
Tech Notes
IPCC Greenhouse Emission Factors

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

Carbon Dosage Control
Tech Notes
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.

Climate impact from nitrous oxide emission
Tech Notes
Climate impact from nitrous oxide emission

Based on 2-years data from Danish WWTPs, you get an insight into the reduction potential in ton CO2 equivalent.