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IPCC Greenhouse Gas Emission Factors

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

Wastewater treatment plants (WWTPs) contribute significantly to the global greenhouse gas stock through the production and emission of nitrous oxide (N2O). The more than 3-fold increase of the wastewater N2O emission factor by IPCC2019 underlines this.

Nitrous oxide emission constitutes about 42% of the CO2 footprint from Danish WWTPs. This goes to demonstrate that earlier years underestimation of the impact of N2O and the importance of control strategies targeted at understanding and reducing N2O emissions from WWTPs.

Danish studies support the increased IPCC2019 wastewater N2O emission factor. The studies also highlight the large temporal and WWTP site variations. In the VARGA study, N2O emission control has shown a potential CO2 footprint reduction up to 65%. Other WWTP sites show lower N2O emission. Overall, Danish WWTPs expect a to potentially reduce their CO2 footprint by 50% . This number will increase with the new IPCC2019 wastewater N2O emission factor. However, the site and temporal variations make it necessary to implement online monitoring and control strategies to harvest this reduction potential. In this tech note, we will show you data of successful N2O emission monitoring and control strategies.

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IPCC Greenhouse Gas Emission Factors

In May 2019, IPCC refined the greenhouse gas emission factors. As one of the more noticeable changes, IPCC more than tripled the N2O emission factors for WWTPs incl. the factor for direct sewage discharge to nutrient-impacted freshwater, estuarine, and marine environments.

Furthermore, IPCC stated that a shift towards higher bioenergy demand can increase emissions of nitrous oxide. Hence, the ammonia waste from biogas production needs appropriate management to avoid a negative impact on the CO2 footprint.


Real-time N2O monitoring data

More than 1.5 years of online monitoring of the N2O concentration in a 350,000 PE WWTP shows a highly variable and also dynamic seasonal pattern. In this example, the cumulative N2O emission over +365 days was above 2.5% of the daily N-load on the plant (not shown). This N2O emission is 60% higher than the average IPCC2019 factor and indicates the importance of performing on-site monitoring of N2O rather than relying on an average assumption.

Moreover, the majority of the emission is accumulated from March to August. The large seasonal/monthly variations clearly undermine results deducted from typical short-term scientific monitoring campaigns. Especially, when the results are extrapolated to a seasonal or yearly emission result or an N2O emission factor.

N2O Level

Mitigation Strategy

This figure shows a mitigation strategy to decrease the greenhouse gas emission and operation cost of a WWTP, which doses carbon sources to improve the effluent quality. The WWTP used two sensors in two treatment plants in parallel with NOx-N probes. The purpose was to demonstrate the applicability of the N2O wastewater sensors for automation of COD dosing pumps. The effects of operational conditions, such as COD/N ratios, and the correlation between NOx and N2O were followed.

In particular, the results showed that the N2O production was a function of influent nitrogen load and the ratio of COD/N. For optimum dosage automation of external carbon source and reducing N2O, the plant combined a feedforward control algorithm with nitrates as its measured variable and a feedback control algorithm with N2O as input variable. The WWTP used the N2O sensor as a proxy sensor for nitrates in a feedback automation of the dosing pumps, and they were able to completely avoid N2O emissions while the effluent quality was maintained below 1.2 mg N/L NOx.

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VARGA Project

The Danish project VARGA has implemented and tested a number of new controls for minimizing nitrous oxide at the Avedøre WWTP. The project demonstrated a 30% reduction (4,260 ton CO2 equivalent) in the project's early stages. The full minimization potential is expected to be even greater - up to a 50-80% reduction. On a national scale, the reduction potential in Denmark will be 42-152,000 ton CO2 equivalent annually. With the adjusted upward IPCC2019 emission factor, the potential is likely to be significantly greater.

Similarly, many Unisense Environment clients have demonstrated N2O minimization potentials between 25% and 90% by balanced controlling of aeration, loading, and carbon resources with the integration of the N2O Wastewater System.

Sludge tank Varga project full

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.

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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
Tech Notes
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

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.

N2O Sensor Controls Emissions from Deammonification Processes
Tech Notes References
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
Tech Notes
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)

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.

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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.

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