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

Direct Effect of Activated Sludge Concentration

Over more than 4 years, nitrous oxide emissions from BIOFOS Avedøre’s biological process tanks have been measured to derive the underlying process mechanisms responsible for climate gas emissions. Figure 1 shows the four years of online monitoring of N2O in 4 of the 8 process compartments at Avedøre WRRF (i.e. monitoring 50% of the bioprocess). The data was collected in the SCADA system and the N2O emission is presented as daily averages of KgN N2O /day. The graph highlights the dynamic seasonal and yearly patterns. Although the yearly pattern is repeating, the emission magnitude varies significantly between years.

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Insights from Continuous Monitoring of Nitrous Oxide

During the 4 years of online N2O emission monitoring, the data showed a high and variable EFN2O above the IPCC2019 factor based on daily TNLoad or daily TNTreated. Furthermore, simple process changes have made a significant impact on the cumulative N2O emissions. Based on the IPCC CO2-equivalent factor (CO2-eqv) of 298 kg CO2 per kg N2O, the total CO2 footprint ranged from 18,396 in 2019 and 9,885 Ton CO2-eqv in 2021. On average, in the periods of winter and summer emissions are 6.22 and 49.79 Ton CO2-eqv/day.

The EFN2O of Avedøre WRRF is 2-4 times higher than national Danish emission factor and the IPCC2019 factors. This highlights the importance of performing on-site monitoring of N2O rather than relying on average assumptions. N2O emission and inferred CO2 footprint were highly variable and yet seasonally dependent, emphasizing the need for long-term continuous monitoring.

Figure 1 VARGA
MLSS Control: A Key Factor in Lowering N2O Emissions

Law et al. (2012) found that increased N2O production is due to an increased specific NH3 oxidation rate by AOBs. To decrease this rate, we tested increasing the MLSS (mixed liquor suspended solids) concentration in LT31 and LT32 while keeping LT11 and LT12 as reference tanks. Figure 2 shows a significant reduction in N2O emissions during the spring months of 2020 and 2021, matching with the increased MLSS concentration.

The MLSS controlled tanks exhibited a lower EFN2O of 0.58% and 0.83% of the daily TNLoad for the years 2020 and 2021. The large seasonal and yearly variations demonstrate the need for longterm monitoring campaigns and the present work exemplifies the difficulty in extrapolating even yearly emission results or use of a reference year for a general N2O emission factor.

Figure 2 VARGA

Abstract presented at the IWA World Water Congress 2022.


Baresel, C., Andersson, S., Yang, J., Andersen, M.H. 2016 Comparison of nitrous oxide (N2O) emissions calculations at a Swedish wastewater treatment plant based on water concentrations versus off-gas concentrations. Advances in Climate Change Research, 7(3), p 185-191.

2019 Refinement Report https://www.ipcc-nggip.iges.or.jp/public/2019rf/pdf/19R_V5_advance.zip

Andersen, M.H., Chen, X., Mielczarek, A., Madsen, J.A, G. Sin, G., and Thornberg, D. 2021 Effect of nitrogen loading and control strategies on online N2O emission and CO2 accounting at fullscale. IWA Digital World Water Congress 2021

Law, Y., Ni, B-J., Lant, P., Yuan, Z. 2012 N2O production rate of an enriched ammonia-oxidising bacteria culture exponentially correlates to its ammonia oxidation rate.

IWA World Water Congress
BIOFOS Avedøre WRRF, Copenhagen, Denmark
Avedøre is a 345,000 PE Water Resource Recovery Facility (WRRF) employing the activated sludge process in four parallel tanks (LT1-LT4), each consisting of two alternatingly fed and intermittently aerated compartments regulated by STAR Control®.
What to kN2Ow about N2O emissions?
Wastewater treatment contributes significantly to the global greenhouse gas (GHG) stock through the production and emission of nitrous oxide (N2O).

The increase of the wastewater N2O emission factor (EFN2O) to 1,6% N2O-N/TN by IPCC2019 underlines this. In Denmark, a new national EFN2O of 0.84% N2O-N/TN has been adopted by the Danish EPA based on a 2-year monitoring campaign on 10 different WRRFs in 2021.

Tech Notes & References

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N2O Sensor
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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
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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 ...

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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
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