The fine balancing act of tackling N2O emissions
While at the Water Environment Federation’s Technical Exhibition and Conference 2024 (WEFTEC 2024), the largest conference of its kind in North America, Anna moderated a conversation around an urgent, emerging environmental issue: nitrous oxide (N2O) emissions.
During her panel, “N2O Unmasked: Understanding and Taming Emissions,” she and industry leaders discussed how wastewater treatment plants are a significant source of N2O — a potent greenhouse gas (GHG) with a global warming potential nearly 300 times greater than carbon dioxide (CO2).
As the discussions unfolded, it became clear that N2O emissions, though still an emerging climate challenge, require urgent action. Below, Anna reflects on learnings from her panel, and how the industry can respond to the crisis of N2O emissions.
Understanding N2O and the interdependencies between greenhouse gases
At the heart of the conversation was our approach to the challenge of N2O in wastewater treatment. N2O, more commonly known as laughing gas, is the largest GHG contributor for treating wastewater biologically, according to emerging science. We’ve reached a point where the industry understands how to measure N2O emissions, but the next step is mitigation.
Those who presented at WEFTEC have been studying mitigation strategies through models and full-scale campaigns. A major focus of the discussion was on the models currently being used to identify potential strategies. During this interactive session, we spent time discussing the most important questions. For instance: Is it better to add external carbon in order to reduce N2O emissions? And are the results of the model reliable enough for us to take action?
The balancing conundrum
Unlike pollutants such as methane and sulfur dioxide, N2O is a stable molecule that does not easily react with other compounds in the atmosphere or break down naturally. Therefore, minimizing its release is imperative.
Looking at the models discussed during the panel, carbon fractions, dissolved oxygen levels and process configurations we choose — whether it’s complete mix systems, plug flow systems, or carousel systems — all have an impact on how much N2O is produced. Each system behaves differently, not only in how it handles nitrogen removal but also in how it affects the release of N2O. In other words, there is a delicate balance we must strike when reducing GHG emissions in our work, as an attempt to reduce one GHG may result in the higher net release of GHGs.
Quantify, understand and mitigate: Reconsidering the order of actions
A particularly resonant message from the panel discussion was that following a strict sequence of quantify, understand, mitigate won’t support our urgent need to tackle this issue. Although, traditionally, these steps are performed in sequence, when it comes to N2O emissions, it is critical to realize that we can — and should — perform these steps in parallel. This shift in thinking is essential for accelerating our response to the climate crisis.
While the models we use to predict N2O emissions still require calibration, the consensus is that they are reliable enough to guide action. This marks a turning point. We don’t need to wait for perfect understanding before we act. There is enough data today to influence design and operational decisions that can significantly reduce N2O emissions.
For example, in New Zealand, AECOM has carried out N2O and CH4 modelling for both operational and embodied emissions to inform client decision-making regarding which new wastewater treatment process they should construct and operate. Working across the global AECOM network to inform a practical emissions monitoring plan, we were able to yield recommendations regarding installation of measurement devices to identify emissions hot spots, then feedback to plant controls to ultimately pre-empt and reduce emissions where feasible.
The role of engineers in environmental health
Beyond N2O emissions, my experience at WEFTEC reminded me of the broader responsibility we have as wastewater engineers. Our work directly impacts the global biogeochemical cycle — the way nutrients like nitrogen and phosphorus move through the environment.
Oftentimes, human activities — even well-intentioned ones like building levees for flood protection — can have unintended negative consequences on ecosystems, such as the destruction of aquatic habitats. It’s a powerful reminder that the choices we make as engineers don’t just affect the immediate challenges we’re solving but also have ripple effects across the entire environment.
On the road to tackling N2O emissions, even with blind spots
Our discussions related to N2O emissions at WEFTEC 2024 underscored the fact that while we still have so many questions without concrete answers, there are a few things we know for sure, as well as specific actions we can take.
First, when our models are not calibrated with detailed measurements, we know they are appropriate for trends. We also know that while our understanding of managing N2O is an iterative process, we can make calculated decisions by carefully balancing trade-offs — should we be accepting higher N2O emissions where the approach reduces overall GHG emissions, for instance, versus adding reactor volume using concrete structures with their own embedded carbon footprint to reduce N2O emissions.
Our ability to balance these factors will determine the health of both our water systems and the planet as a whole.