Tackling nitrous oxide in wastewater: new strategies for decarbonising AMP8 and beyond

02 June 2025

Water companies are investing over £300m in AMP8 to cut nitrous oxide emissions from wastewater treatment. From avoidance to destruction, the tools are evolving fast.

Tackling nitrous oxide – possibly better known to readers as ‘laughing gas’ – has for the first time been included in the business plans of English and Welsh water companies. In AMP8, the asset management plan for water companies that began this past April, more than £300 million of funding has been allocated to reduce emissions of this potent greenhouse gas from wastewater treatment. Yet with many of the proposed mitigation solutions yet to be demonstrated, the next five years are set to be an exciting time to be working in water sector decarbonisation.

Nitrous oxide (N2O) and methane (CH4) are potent greenhouses gases produced and emitted during wastewater treatment as ‘process emissions’. Nitrous oxide, in fact has 273 times the global warming potential of carbon dioxide. Globally, the waste sector (including wastewater) is the third largest anthropogenic methane emitter and wastewater is the sixth largest source of anthropogenic nitrous oxide. Nitrous oxide and methane emissions now comprise the most significant proportion of greenhouse gases (GHG) from most wastewater treatment plants (WWTPs), particularly as electricity-related ‘Scope 2’ (ie indirect) GHG emissions are becoming decarbonised.

In recent years, the global effort to measure and reduce process emissions from wastewater treatment has unveiled significant challenges and costs. Methane presents an easier opportunity now. As well as being a potent GHG, methane leaks are a health and safety risk, and a source of lost revenue, because of the gas’s usefulness as a source of biogas. Water companies are therefore beginning to recognise the importance and economic viability of finding and fixing their main source of methane emissions, which are leaks from biogas and biomethane infrastructure.

Mitigating nitrous oxide is a different story. Despite the urgency in climate terms, full-scale, sustained nitrous oxide mitigation remains largely unexplored, with few case studies to guide the way. Examples relevant to wastewater utilities remain limited, which makes developing meaningful baselines, reduction targets and viable business cases very difficult.

Here, we discuss how nitrous oxide reduction at WWTPs should be prioritised via different approaches – what we term an abatement hierarchy – and the potential role for nitrous oxide destruction within this.

How nitrous oxide is produced in wastewater treatment

Nitrous oxide is produced during the biological treatment of wastewater. The micro-organisms or ‘biological infrastructure’ in our WWTPs generate nitrous oxide via multiple recognised pathways in different parts of the plants.

Nitrogen – the root source of nitrous oxide – gets into our wastewater in the first place in various ways, including as a result of food processing, from soaps and detergents, and from industrial and agricultural sources. The majority, however, is excreted by humans as urea in our urine, having come into our bodies in the first place through our consumption of protein (which for most of us is higher than dietary guidelines recommend).

When urine enters the sewer system, the urea converts to ammonium. At the treatment plant, this ammonium is the starting point for microbial processes that convert nitrogen compounds into less harmful forms than ammonium – but under certain conditions, these processes can lead to the production and release of nitrous oxide instead. Whilst nitrous oxide can also be removed in WWTPs, it is typical that more nitrous oxide is produced than removed – and production can be greater when the microbial populations at a WWTP are under stress (such as high loading rates or insufficient sources of ‘food’ or oxygen). Under these conditions, the biological pathways that normally convert nitrogen compounds to harmless nitrogen gas (N₂) may be partially completed only – leading to the accumulation and release of nitrous oxide.

When it comes to mitigating emissions of this nitrous oxide from WWTPs, continuous, long-term measurement is a crucial first step. As utilities around the world start understanding and mitigating their nitrous oxide emissions, the sector is learning that there is a lot of variability within and across WWTPs. Levels of nitrous oxide emissions depend on the type of WWTP process as well as operational conditions such as levels of biomass loading, dissolved oxygen and available carbon (carbon may be required to ‘feed’ the microbial population where total nitrogen reduction is required). Nitrous oxide production and emissions also show significant seasonal variation as well as variation within WWTPs.

Reducing nitrous oxide production and emissions

Despite our incomplete understanding of the full nitrogen cycle and nitrous oxide production and emission mechanisms, there are several viable mitigation strategies already in use in the wastewater treatment sector (see infographic below). The preferred strategy is avoiding nitrogen in the first place; followed by reducing the production of nitrous oxide; then, for residual emissions which cannot be avoided or reduced, removing the nitrous oxide which has been produced.

A nitrous oxide abatement hierarchy. Credit: iChemE.

Nitrogen can be avoided through upstream interventions such as urine source separation in new development. It can be avoided within WWTPs too, by recovering nitrogen from sidestream processes, treatment units that handle specific high-strength waste streams separately from the main flow, and which have been linked to very high nitrous oxide production and emissions.

Recovered nitrogen, whether from dilute ammonium or from concentrated ammonium salts, can be used in industrial installations or as fertiliser, although large volumes make transport costly. Key nitrogen recovery technologies have been demonstrated to recover between 75-97 per cent of nitrogen as usable ammonia compounds, thereby reducing nitrogen and avoiding nitrous oxide production and emission.

Switzerland provides a leading example (the only one to date, in fact) of nitrous oxide offsetting through Klik, an innovative public-private initiative that provides carbon reduction opportunities for fossil fuel importers. This includes a catalogue of carbon reduction schemes at WWTPs, including four nitrous oxide reduction options. Nine nitrous oxide schemes at WWTPs have been funded so far, most of which are still being implemented. An example is at the Altenrhein WWTP, where an innovative scheme is enabling recovery of 75 per cent of nitrogen and reducing nitrous oxide emissions by 50-60 per cent.

Optimising the process

There are various operational strategies we can use to minimise nitrous oxide production, including balancing ammonia loads, adjusting dissolved oxygen levels and ensuring sufficient carbon where denitrification is taking place. These interventions are likely to help maintain stable conditions for biological treatment, avoiding microbial stress.

An increasing number of utilities are gaining full-scale mitigation experience and two were shared at the IWA the World Water Congress last year for the first time (Toronto, 2024). Helsinki Water shared its success with increasing alkalinity and carbon dosing while the Dutch firm Royal HaskoningDHV and Dutch utility Waterschap Vallei en Veluwe have also showed that advanced control systems can help to reduce nitrous oxide by improved control of oxygen levels in treatment zones, preventing conditions that favour nitrous oxide production and supporting its biological removal.

Nitrous oxide destruction

Whilst the priority should be avoiding nitrous oxide and reducing it as much as possible, these alone are not likely to be enough. Mitigation solutions to destroy nitrous oxide are still likely to be required.

Full-scale destruction of nitrous oxide from WWTPs can take place through a process called regenerative thermal oxidation (RTO). Regenerative thermal oxidisers work by heating waste gases to 800-1,000˚C; this converts the nitrous oxide to nitrogen and oxygen, as well as removing potentially harmful volatile organic compounds, carbon monoxide, ammonia and unpleasant odours – improving local air quality and community wellbeing.

This ‘end-of-pipe’ solution is already being successfully used to destroy nitrous oxide in the wastewater industry, as well as in sewage sludge combustion. Notably, a system in Luzern, Switzerland, REAL, has been operating for nearly 10 years, treating off-gas from sewage sludge incineration to remove nitrous oxide. Funded by the KliK programme, the facility reduces nitrous oxide by more than 90 per cent. A second Swiss project, at the wastewater treatment facility ARA Bern, has also received funding under this programme, and will start up later in 2025.

Whilst they still do require a source of fuel, such as biogas, for operation, regenerators use energy very efficiently, with up to 97 per cent of the heat contributing directly to the process. This reduces fuel needs and cuts operating costs. Modern systems use ceramic honeycombs as heat sinks, which are resistant to chemical, thermal and mechanical influences which may otherwise reduce the efficiency of the system. Like many other resource recovery technologies, regenerative thermal oxidisers may result in competing uses for biogas at WWTPs as they do require some fuel – though they can also be heated with electrical power.

There are downsides to the RTO process: thermal destruction of nitrous oxide produces nitric oxide (NO) and nitrogen dioxide (NO₂), gases which are major contributors to air pollution. Where these exceed regulatory limits, additional steps are needed. Pilot units can be deployed for field tests at WWTPs to gather data and optimise the performance and design of the proposed RTO system.

Emerging technologies

Trials are being carried out for systems which use catalysts to enable nitrous oxide destruction to take place at lower temperatures. These are at an early stage of technological development and depend on catalyst performance and lifespan, which vary based on gas composition, moisture and operating temperature.

The NACAT project in Denmark trialled lower temperature (~400˚C) catalytic destruction across three full-scale WWTPs, achieving 70-80 per cent nitrous oxide removal. The project concluded that the longevity of catalysts is affected by hydrogen sulphide and methane, which could mean they have a short life and cost more to implement at some WWTPs. However, for sufficiently high-concentration nitrous oxide off-gas streams, these systems could be cost-effective with legislation around nitrous oxide reduction proposed by the Danish government, though further work on catalyst longevity and system design is required. To date, no published data from utilities or industry demonstrates that catalytic nitrous oxide destruction at ambient temperatures is effective. More research is needed.

Biological removal of nitrous oxide from concentrated off-gas streams is another option, especially for higher strength off-gas streams. Other alternatives, such as capturing nitrous oxide-rich off-gas for combustion with fuels for heat and energy production, or using chemical sieves for recovery, also offer exciting areas for much needed research.

Nitrous oxide abatement is possible today

The journey to mitigate nitrous oxide emissions from wastewater treatment is complex and multifaceted. While significant strides have been made in understanding nitrous oxide emissions, the path to full-scale, sustained reduction remains challenging. Incentives, such as the Swiss Klik programme and proposed Danish regulation, are driving significant progress and showing opportunities, including for nitrous oxide removal through thermal destruction from high concentration off-gas streams.

Sharing knowledge and the ongoing lessons learned from these efforts will be invaluable in supporting global progress in water sector decarbonisation.

This article originally appeared in the Summer 2025 print edition of The Environment. CIWEM members can read the issue in full (and all back issues dating back to 2016) via MyCIWEM. You can also stay up-to-date with our free monthly 'The Environment' newsletter – subscribe here.

Amanda Lake is a specialist in wastewater treatment and resource recovery at Jacobs
Thomas Binninger is a sales manager at CTP Air Pollution Control