Nitrous Oxide Levels Monitored For One Year At Wastewater Treatment Plants In Europe And Australia

Wastewater treatment plants in Denmark, Australia and the UK joined forces for one year to monitor emissions of nitrous oxide. This greenhouse gas may be subject to stricter regulation in future and is thus worth learning more about.

Nitrous oxide (N₂O) is a greenhouse gas which is almost 300 times more harmful than CO_2, and makes a substantial contribution to global warming (IPCC report). Consequently, from a CO₂-equivalent perspective, it makes sense to consider reducing nitrous oxide emissions.

"Studies show that nitrous oxide contributes substantially to the CO2 footprint of utilities with biological nitrogen removal." - Professor Bo Thamdrup of the University of Southern Denmark

Together with Melbourne Water and the UK’s  Severn Trent, Aarhus Vand has formed a leading international alliance called the Net Zero Partnership with the goal of collectively reducing carbon emissions by one million tons annually. They agree that a significant proportion of emissions derive from the activated sludge process.

Nitrous oxide monitored in both liquid and gas phase

For one year, each partner thus monitored nitrous oxide emissions in both the gas and the liquid phase in order to learn more about the variations in emissions and how to reduce them. The results of the analysis were presented at the IWA Leading Edge Conference on Water and Wastewater Technologies in South Korea in June 2023, with a keynote talk by Morten Rebsdorf, senior project manager at Aarhus Vand.

"We’re looking at a future with a potential CO2 tax on emissions, which means it will be highly relevant to reduce nitrous oxide, due to its high impact on global warming." - Morten Rebsdorf, senior project manager, Aarhus Vand.

He explained that nitrous oxide occurs, for example, when there is a high nitrogen load, which puts pressure on microbiological mechanisms, leading to the formation of nitrous oxide. This can be reduced through new control strategies for the activated sludge process.

Measurements performed every minute

Wastewater treatment plants in Australia, the United Kingdom and Denmark monitored three full-scale activated sludge processes for one year. Nitrous oxide emissions were measured using both liquid phase sensors and gas analyzers connected to gas collection hoods. The measurements were performed online and recorded once every minute. To enable comparison, as well as understand mechanisms that lead to emissions, similar measurement parameters between the three sites were essential.

“We lack a fundamental understanding of how the contribution varies seasonally, and with different types of operation, and across climate zones. Detailed monitoring of N₂O in different systems is an important step towards reducing emissions,” said Professor Bo Thamdrup.

The key results of the study were as follows:

• On a yearly average, the daily emission factors were essentially the same in the three wastewater treatment plants studied, at approximately 1.5% N₂O/TN

• In spring, emissions of nitrous oxide peaked at all three plants

• Measurements in both water and gas are important to gain an overall impression of formation vs emissions

• Calibrating the frequency of measurement sensors is important to obtain reliable and comparable results

Similar average results in all three countries

Despite the quite different operating conditions between the wastewater treatment plants involved, such as average temperature and sludge age, the average annual results of the measurements were similar in all three countries.

One of the most surprising results was the seasonal variation. The amount of nitrous oxide peaks during spring in all three countries, and there are several hypotheses as to why this is the case.

“The question is whether rising temperatures causing a change in the microbial composition are the reason for an imbalance, which in turn brings about a rise in the level of nitrous oxide. We will need to investigate this further,” says Morten Rebsdorf.

The annual average of daily emission factors also turned out to be quite similar in all three countries, at approximately 1.5% N₂O/TN. This accords quite well with the current IPCC (Intergovernmental Panel on Climate Change) emission factor of 1.6% N₂O/TN, but only in an average perspective.

However, national emission factor guidelines are lower in all three countries. In Denmark the emission factor is 0.84% N₂O/TN, while in the UK and Australia the factors are 0.4 % N₂O/NH₃-N and 0.5% N₂O/TN, respectively.

Standardization needed

At the IWA-LET conference in Korea it was concluded that there is a need to develop well-proven monitoring, calculation and reporting methodologies. It is  very important to have standardized methods to calculate emission factors to ensure reported emissions are reflective of actual emissions.

“I experienced a great deal of interest from the audience in relation to standardizing the methods for measuring and analyzing nitrous oxide emissions. With this international pilot project, we hope to have contributed to creating a pathway towards common ground in this respect,” concludes Morten Rebsdorf.