Why utilities need to prioritize reducing nitrous oxide emissions
Nitrous oxide (N2O) can be a major contributor to greenhouse gas emissions from wastewater treatment, but the water sector still has a long way to go in implementing solutions for monitoring, controlling and reporting these emissions. What are some steps utilities can take now to start managing N2O emissions, and what solutions can they expect in the future? Xylem expert Dr. Oliver Puckering explains.
A 2023 Xylem survey of 100 utilities found that 75% had plans to reduce greenhouse gas emissions by 2040, and many were already reducing emissions by installing more energy-efficient technologies and using digital solutions to optimize operations. Despite their significant impact, utilities ranked process emissions, such as methane and nitrous oxide, close to last in their priorities for action.
To understand why utilities should prioritize reducing nitrous oxide emissions and how to take action, Making Waves spoke with Dr. Oliver Puckering, lead for the Partnership Accelerator program at Xylem Innovation Labs.
Q. Why is it important that wastewater treatment plants take steps to reduce nitrous oxide emissions?
Nitrous oxide has a global warming potential 300 times higher than carbon dioxide. As many utilities have signed up to net-zero targets, reducing nitrous oxide emissions will play a key part in meeting these goals.
Nitrous oxide is generated at wastewater plants during the nitrification and denitrification processes, which are used to remove ammonia and protect receiving waters from harmful nitrogen loads. Suboptimal process operations can produce greater levels of nitrous oxide, which is then stripped from the liquid and released into the atmosphere when additional air is added via blowers.
Xylem’s research shows that N2O emissions can make up between 25-75% of a treatment plant’s total emissions, depending on the process and electricity mix, while other N2O emissions studies have shown N2O emissions amounting to 60 to 80% of total emissions. N2O emissions fluctuate consistently – they are influenced by seasonal patterns and factors like temperature, nitrogen loads, and aeration demand patterns in treatment plants.
Increasing understanding of nitrous oxide emissions at wastewater treatment plants will be necessary for utilities to meet their greenhouse gas emission goals and to establish a firm baseline for total water-sector emissions.
Q: What technology is currently available for measuring N2O emissions?
Compared to technology for measuring standard process variables, such as ammonia, dissolved oxygen and suspended solids, reliable measurement solutions for nitrous oxide are in their infancy. As regulations become more stringent, however, we expect to see significant technological development. A range of monitoring solutions for N2O emissions has already emerged, with some using direct measurement and others relying on modeling to estimate N2O production and release.
Sensors capable of directly measuring N2O in liquid, gas, or both phases are available. While these sensors offer reliable measurements within their specified range, their large-scale implementation across multiple treatment plants remains a challenge. Nonetheless, they prove beneficial for conducting annual campaigns to measure N2O in specific locations within treatment plants, such as activated sludge basins.
Q: How does modeling of N2O emissions differ from direct measurements?
Empirical and risk-based models incorporate parameters like dissolved oxygen, nitrite, nitrate, and computational fluid dynamics. These models offer a comprehensive understanding of N2O emissions and are appealing due to their ease of implementation and scalability. Although they face challenges related to validation and verification, utilities worldwide are increasingly adopting these models in demonstration projects, accelerating the validation process.
Real-time monitoring of N2O emissions, whether through direct measurement or modeling, is crucial for informed mitigation strategies.
Decision support systems with machine learning, for example, can use N2O monitoring data to optimize operational conditions, including oxygen control and the return of activated sludge. This minimizes N2O generation and thus the risk of emissions via gas stripping during nitrification and denitrification processes, while meeting treatment requirements.
Q: What new methods can be used to prevent N2O production?
It's important to understand that while mitigation solutions can reduce N2O emissions, they may not entirely eliminate N2O formation, which is intrinsic to biological treatment processes. Identifying alternative treatment technologies and methodologies is key for utilities to begin a gradual shift away from activated sludge processes and towards resource recovery.
Several utilities have implemented these alternative recovery processes, whether physical, chemical, or enhanced natural (such as algal treatment), to remove the pathways that produce N2O. In order to produce the most efficient and effective treatment train, utilities should evaluate their options based on site-specific needs rather than adopting a one-size-fits-all approach.
Q: What guidelines are available to help utilities estimate their N2O emissions?
In 2019, the Intergovernmental Panel on Climate Change (IPCC) introduced guidelines with a tiered approach for N2O emissions estimation. These guidelines can be a good starting point for raising awareness of N2O emissions, however, they often underestimate actual emissions, making it difficult to precisely plan mitigation efforts.
There are several actions that utilities can take today to begin assessing and reducing actual nitrous oxide emissions.
Utilities should take a strategic approach, focusing on their largest treatment plants with the most complex aeration patterns, coupled with combined energy and emissions audits. This will enable utilities to start implementing targeted actions, which will then form the foundation for broader improvement programs across their treatment plant portfolio.
Q. How is Xylem Innovation Labs working to develop solutions for nitrous oxide monitoring and control?
Xylem Innovation Labs works with start-ups through our Partnership Accelerator program to bring innovative water solutions to market faster. This annual program helps us identify, engage, and assess technologies across the spectrum of customer and global water challenges, with net zero and sustainability being a key focus.
Addressing nitrous oxide emissions forms a major pillar of this work, including precursor and direct nitrous oxide monitoring, modeling and digital twins, and novel treatment options for nutrient recovery.
Since the Accelerator launched in 2022, we have taken 35 companies through the program. An example of three program participants linked to different aspects of emissions reduction are:
- Gross Wen Technologies, which has developed an algal treatment process to capture and recover nutrients.
- Aquamonitrix, offering a nitrite and nitrate sensor to look at precursor build-up.
- Cobalt Water Global, providing software that assesses process risk of nitrous oxide production and emission.
Learn more about how Xylem is helping utilities achieve net zero goals.