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, ArchTam 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 ArchTam 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.

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Anna Cleaver is the market sector leader for wastewater in Ontario, Canada and a process engineer who focuses on biological wastewater treatment design and operations. She is a former student of ArchTam’s Water Academy, a global advanced training program that nurtures and develops emerging water technical leaders at every stage of their career with the company. Anna has recently taken on the role of co-director of the Water Academy’s Wastewater Treatment: Liquid Stream program and hopes to inspire budding wastewater process engineers across the globe.

Tell us about what inspired you to join the industry

I grew up on a farm in southwestern Ontario, where my family was fortunate to have a good well on our property, whereas many well sources in the area contained hydrogen sulfide. I understood at an early age how water quality could significantly influence one’s quality of life.

After doing a school project on dichlorodiphenyltrichloroethane (commonly known as DDT), I was inspired to develop alternative biopesticides to lessen farming’s impact on the environment. I pursued chemical and biochemical engineering at university, which led me to hydraulics in the oil and gas industry in Calgary, Alberta. This experience provided me with an opportunity to eventually move back to Ontario and work in water distribution. In this new role, I worked closely with a wastewater engineer who loved these ‘little bugs’ and I immediately became curious about biological wastewater treatment. I was thrilled! I had finally found my way back to improving water quality for the public and protecting the environment. Without any formal education in this area, ArchTam’s Water Academy provided me with the knowledge to launch a career in the field. I am certainly grateful for this training and support in developing my skills and pursuing my passion — utilizing biology to reduce peoples’ impact on the environment.

What is your favorite ArchTam project that you’ve worked on and why?

The upgrade, expansion and optimization of the Greenway Wastewater Treatment Centre in London, Ontario is by far my most memorable work experience. Greenway is in the heart of the city, adjacent to the very popular river path and equally popular off-leash dog park. Although the surrounding area is owned by the City, it was important to the public to contain the treatment plant within its existing boundaries. I was excited by this challenge and to be involved in this project from the start — from its beginnings in the planning phase through to final construction and optimization. The project increased the plant’s treatment capacity within the existing site boundaries while mitigating raw sewage discharges to the Thames River. Our team repurposed poor performing clarifiers with square concrete tanks for chemically enhanced primary treatment of wet weather flows. The increased plant capacity delayed the need for a new wastewater treatment plant in the City’s southeast. After construction was successfully completed, our team continued to work with the City to evaluate the sewershed and satellite plants to best use available capacity at all plants and avoid the need for future large capacity upgrades.  

Tell us a story of how your work positively impacted the community.

I am acutely aware that in Canada, the water infrastructure that ArchTam designs is funded by public money. I work closely with all levels of wastewater operations to ensure the decisions I make provide value. Large capital upgrades are necessary to sustain our infrastructure – and I search hard to find the balance of spending and improving. The highest value isn’t necessarily the highest cost option. I look to provide infrastructure that the client will use to improve the effluent quality, minimize overflows and simplify operational procedures.

Share a piece of career advice.

A mentor once told me, “You can have 20 years of experience, or you can have 10 years twice.” I work hard to ensure I’m on a 20-year career path. Careers are built one day at a time. With that in mind, never be comfortable with your current level of knowledge or consider your work to be routine. Always be curious, ask questions, follow up with operators and find learning opportunities in everyday tasks.

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