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Pathways of N2O production

A Tool for Carbon Dosage Control

N2O Produced in Denitrification

Full biological nitrogen removal in wastewater is achieved through a combination of nitrification and denitrification. N2O is an obligate intermediate in denitrification where a lack of carbon source will stop the process prematurely.

Optimizing Economy and Environmental Footprint

Incomplete denitrification is a source of N2O emissions, while a well operated denitrification can significantly reduce a WWTP’s emissions.
By using NOx-triggered and N2O-limited carbon dosing (as in schematic above), operators can ensure optimal use of external carbon source and achieve significant financial savings while ensuring tight effluent control.

Carbon Dosage Control

Case Study: N2O Wastewater Sensor as Tool for Carbon Dosage Control

Dosage of external carbon source is a common method to enhance denitrification, especially on sites with very low N-limits in their effluent. In practice the amount of carbon is often not based on real time measurements, leading to regular overdosing.

The amount of carbon available relative to nitrogen to denitrifying bacteria limits their ability to complete the last step of denitrification and to produce N2 from N2O (Fig 1).

Pathways of N2O production
The connection between carbon dosage and nitrous oxide

Applied research has shown that a low COD/N ratio can lead to increased N2O production and that a ratio below 3.5 can lead to significant N2O emissions.

Figure 2 demonstrates the relationship between COD/N and N2O. The bell-shaped relationship between N2O and COD/N makes it possible to avoid excessive N2O emission by aiming for either a high (>3.5) or a low (<1) COD/N ratio.

From a process control standpoint, the slope of the N2O curve in response to the different COD/N-ratios can be used as an indicator for the dominating sludge process. If aiming at complete denitrification to N2, the N2O sensor can be applied to monitor N2O concentration and increase the carbon load when the N2O concentration is increasing to ensure a COD/N ratio above 3.5. Once N2O falls under a given setpoint, carbon dosing can again be stopped, ensuring optimal process efficiency and resource use.

N2O sensor feedback prevents carbon overdosing

Andalib et al. 2017 demonstrated the carbon dosage automation in full scale at four American wastewater treatment plants. The researchers applied real-time control of carbon dosing based on NOx and N2O sensors.

The study showed that nitrogen oxide (NOx) probes of both UV and ISE types alone were less dependable when concentrations fell below 1-1.5mg/L, leading to an overdosing of carbon source.

By using the N2O sensor as feedback parameter, the WWTPs avoided overdosing, resulting in high potential savings – both financially and in terms of greenhouse gas emissions.

COD added pr NOx-N

Andalib et al. 2017. Correlation between nitrous oxide (N2O) emission and carbon to nitrogen (COD/N) ratio in denitrification process: A mitigation strategy to decrease greenhouse gas emission and cost of operation. Water Science & Technology 76. 10.

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
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.

Trickling Filters Fig. 1
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.

N2O Monitoring at Vandcenter Syd bassiner_700x400
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