What the new Carbon Accounting Workbook means for wastewater emissions

The wastewater carbon challenge has become clearer but so have the opportunities

The latest version of the UK Water Industry Research’s Carbon Accounting Workbook has put wastewater greenhouse gas process emissions more firmly in the spotlight. Mott MacDonald technical director for process engineering in the water sector, Kenny McGibbon, sets out the challenges and opportunities of the new workbook for the water industry in UK and Ireland.

Many wastewater treatment works are about to see reported process emissions rise sharply. This is not because performance has deteriorated, but because the industry now has a better way of accounting for one of its most significant greenhouse gases.

The recently released Version 20 of the UK Water Industry Research’s Carbon Accounting Workbook (CAW) is the industry-standard tool in the UK and Ireland for quantifying and tracking operational greenhouse gas emissions. As the basis for carbon inventories, regulatory reporting and investment decisions, the latest changes to the CAW have implications far beyond the spreadsheet itself.

One of the most significant updates in CAW Version 20 is the increase in the wastewater treatment nitrous oxide (N₂O) emission factor from 0.004 to 0.016kg N₂O-N per kg of nitrogen treated. This means the CAW now estimates four times higher N₂O emissions for the same nitrogen load, even when treatment performance has not altered.

There is another important change that could have a significant impact on reported emissions. The update also increases the default wastewater nitrogen load used in population-based calculations from 8 to 14.4g of nitrogen per population equivalent per day. In simple terms, the calculation starts with the population served by a wastewater treatment works and uses that to estimate how much nitrogen is entering the treatment system. The more nitrogen there is, the more potential there is for nitrous oxide to be produced during treatment.

That means the change is not just about the proportion of incoming nitrogen assumed to be emitted as nitrous oxide, it is also about how much nitrogen is assumed to be available to create it in the first place.

Why this matters

Taken together, these changes will give water companies a revised picture of reported emissions at many wastewater treatment works. They don’t diminish the progress already made through renewables, energy efficiency and asset optimisation. Instead, they highlight what needs to be done to continue delivering meaningful carbon reductions.

The carbon accounting calculations use generic emission factors, which are a necessary approximation, but remain a blunt instrument. Actual nitrous oxide emissions can vary dramatically between treatment processes, sites and operating conditions. Two treatment works receiving similar nitrogen loads may exhibit very different nitrous oxide emissions profiles depending on factors such as process configuration, load variability, dissolved oxygen control, sludge age and nitrite accumulation. Therefore, relying solely on generic emission factors may provide limited visibility of where emissions are concentrated across treatment assets, making it more difficult to identify priority sites and target mitigation measures where they can deliver the greatest impact.

Water companies that invest in improving how their process emissions are measured going forward, may find that their assets produce less nitrous oxide than the generic, country-wide emission factor. This could have a significant impact on an organisation’s journey towards net zero targets, its investment priorities and the development of future carbon reduction programmes.

The real challenge is to avoid viewing this purely as a reporting issue

Responding to higher reported nitrous oxide emissions

The real challenge is to avoid viewing this purely as a reporting issue. Higher reported emissions increase the value of understanding and managing nitrous oxide at source. Historically, many net zero programmes have rightly focused on reducing carbon emissions associated with energy consumption. The update to CAW reinforces the need to place biological process emissions alongside energy as a core component of decarbonisation strategies.

The revised emission factor provides an opportunity for water companies to reassess their reported nitrous oxide emissions. The priority should be to identify high-impact sites, improve the quality of emissions data and build a clearer picture of what is driving nitrous oxide formation.

Mott MacDonald is supporting UK water companies and international utilities to develop process emissions monitoring strategies, establish monitoring programmes, interpret emissions data and identify practical mitigation opportunities. Through this work, we have gained first-hand experience of the challenges associated with measuring nitrous oxide emissions, understanding site-specific drivers and prioritising interventions that deliver the greatest carbon benefit.

This knowledge shows that the best results come from organisations reassessing investment through a wider greenhouse gas lens. Measures such as advanced process control, process modelling, aeration optimisation, real-time monitoring and targeted process improvements should increasingly be considered, not just for their energy optimisation and operational benefits, but for their ability to reduce total emissions.

Designers also have an important role

Mott MacDonald co-founded the Process Emissions Action Community (PEAC) with Jacobs and AquaEnviro in 2024. Recently published PEAC guidance highlights that many of the most effective opportunities to reduce nitrous oxide emissions are established when treatment works are being designed. Decisions around process selection, load balancing, aeration design, process control, instrumentation and monitoring provision can all influence long-term emissions performance. As water companies respond to the evolving carbon accounting landscape, designers have a crucial role in delivering treatment works that are not only lower risk from a process emissions perspective, but are also equipped to support future monitoring, optimisation and emissions reduction programmes.

The right support is vital

As nitrous oxide becomes a more prominent component of reported emissions, water companies will need a better understanding of where emissions are generated, why they occur and which interventions will deliver the greatest benefit.

We have learned that successful process emissions programmes need site-specific monitoring strategies that reflect process type, asset configuration and operational risk, rather than applying a one-size-fits-all approach. Equally important is considering the data journey from the outset, ensuring that emissions data can move seamlessly from site instrumentation into Supervisory Control and Data Acquisition (SCADA) systems or cloud-based data platforms and be transformed into meaningful insights that support operational and investment decisions. Building these foundations early enables organisations to target effort and investment where it will deliver the greatest benefit.