Real-time N₂O control helped Rennes Métropole cut wastewater emissions and energy use through AI-driven optimization and smarter aeration.
Cutting Wastewater GHGs: What Real-Time N₂O Control Can Actually Deliver
Nitrous oxide doesn’t get the attention CO₂ does, but for wastewater treatment plants it’s arguably the bigger problem. At some facilities, N₂O accounts for up to 80% of total greenhouse gas emissions; at around 275 times the warming potential of CO₂, it represents a significant liability for any operator with net-zero commitments.
The difficulty is that N₂O is hard to manage because it has historically been hard to see. Most plants have relied on generic emission factors rather than site-specific measurements, making it nearly impossible to identify when and why emissions spike, let alone act on them in time. As national climate targets tighten and scrutiny of the water sector grows, that blind spot is becoming harder to justify.
A recent Trial Reservoirs Initiative demonstration at Rennes Métropole’s Beaurade wastewater treatment plant has shown that this problem is not only manageable, but that substantial energy and GHG savings don’t necessarily require new infrastructure. What made the difference was both down to the technology and also the way the trial was structured: with defined success criteria, a committed end user, and a clear pathway from solution to adoption built in from the outset. Together, this is what separates a promising dataset from a working model that can be replicated at scale.
What the trial set out to do
Rennes Métropole has its own climate plan targeting a 42% reduction in greenhouse gas emissions by 2030. Given that N₂O is the largest direct emissions source at major treatment plants, reducing it is an obvious priority. The demonstration brought together Rennes Métropole as end user, technology integrator Purecontrol, and ClimateTech startup Cobalt Water Global to test a combined measurement and control approach under real-world operating conditions.
Cobalt Water Global and Purecontrol do different but complementary things, and their partnership was designed this way from the start. The two companies began collaborating in early 2022, sharing data from two French wastewater treatment plants to co-design a minimum viable product before Beaurade was on the table. Their pre-trial groundwork was crucial, as it meant the deployment at Beaurade wasn’t two technologies being introduced to each other on site.
Cobalt Water Global’s contribution moves away from the industry’s traditional reliance on broad emission estimates. Instead, sensors installed directly in the aeration lanes generate continuous dissolved N₂O data, feeding a predictive model trained on site-specific conditions. At Beaurade, effluent flow rate, temperature and ammonium concentration emerged as the strongest predictors of emissions, yet this pattern would have been invisible without continuous monitoring.
Purecontrol’s AI platform takes that emissions intelligence and acts on it. The two systems are connected via a purpose-built application programming interface (API), automating the data exchange between Cobalt Water’s N₂O risk modelling and Purecontrol’s real-time control platform. Purecontrol anticipates how key process parameters will evolve and adjusts aeration cycles accordingly, issuing updated control setpoints every minute. During peak electricity tariff periods, aeration is reduced to what is strictly necessary. During off-peak hours, the system aerates more actively to pre-empt the next demand peak, all while keeping effluent quality within compliance limits. One system identifies what is happening and why while the other decides what to do about it.
It is worth noting how Purecontrol approached the question of competing priorities. Rather than treating energy efficiency and N₂O reduction as an inevitable trade-off, they designed a system flexible enough to optimise for either: an economic mode prioritising cost reduction within a defined energy budget, or an environmental impact mode focused on maximum GHG reduction. The choice of which to prioritise at any given time remained with Rennes Métropole, which is an essential reassurance for utilities cautious about handing operational decisions to a third-party platform.
For end users, this does more than optimise plant performance behind the scenes. It gives utilities a practical way to balance cost, carbon and compliance without pretending those priorities always point in the same direction, because they often don’t, despite what glossy innovation decks like to suggest.
It also gives operational teams greater control and confidence. By allowing Rennes Métropole to choose between an economic mode and an environmental impact mode, Purecontrol keeps the utility in the decision-making seat. That matters because wastewater treatment is not a simple “set it and forget it” environment. Conditions change, priorities shift, and operators need tools that support judgement rather than override it.
From an end-user perspective, the benefits include:
- Lowering operational costs, particularly through reduced energy use when economic optimization is the priority.
- Reduced greenhouse gas emissions, especially N₂O, which is a significant contributor to wastewater treatment’s climate impact.
- Greater operational flexibility, as the system can adapt to different priorities depending on budget, regulatory, environmental or seasonal pressures
- More transparent decision-making, because the utility can clearly see what objective is being prioritised and why
- Reduced perceived risk as the platform does not remove human oversight or force a single optimization pathway
- A more resilient operating model, where digital optimization supports staff rather than replacing their expertise.
What the results showed
The combined approach delivered approximately 20% electricity savings at Beaurade, avoiding nearly 25,000 tonnes of CO₂ equivalent emissions – achieved entirely through smarter use of existing infrastructure with no physical upgrades to the plant required. By linking real-time operational data, including dissolved oxygen levels, to N₂O risk, the system could identify not just when emissions were elevated, but why.
As a result, interventions became targeted rather than precautionary, with aeration adjusted in response to actual process conditions rather than conservative default settings. Without continuous process visibility, this kind of precision would not have been possible; instead, the default would have been to aerate more than necessary, at greater cost and with greater emissions.
Why this matters for technology integrators and their clients
The decision to bring in something outside an existing portfolio is not risk free. There are risks to compliance, to the client relationship, to the contract itself. Purecontrol managed that by treating Beaurade as an applied R&D initiative rather than a straight commercial deployment, which gave them room to validate the technology before committing to a full rollout. The investment was in software, sensors and operational expertise: a considerably more manageable risk profile than physical upgrades to a third-party asset.
That risk calculus was shaped, in part, by the clarity of what Rennes Métropole needed. A utility with a defined emissions target and a contractual relationship gives an integrator something concrete to work towards. Rennes Métropole’s climate plan not only set a direction but created the conditions in which an integrator had both the motivation and the framework to innovate. For utilities in a similar position, this is worth noting. Ambitious but specific targets, clearly communicated to operators, are one of the more effective ways to drive genuine innovation within an existing contract rather than waiting for a procurement cycle to force the issue.
The commercial case goes beyond managing risk, however. Beaurade delivered on Rennes Métropole’s targets, and that success has been self-reinforcing on both sides. The relationship has since been extended through 2028, with the technology deployed at Saint-Erblon in 2025 and Betton in 2026. Purecontrol have cited the results in securing new contracts with public and private utilities across France and the UK.
From trial to practice
Successful trials are most valuable when the results hold beyond the original site, and when the structure around them is sufficient to convert proof into adoption. That second condition is where most trials fail. The technology performs, but without a clear post-trial pathway, momentum dissipates before a contract is signed.
Beaurade is a case where both conditions were met. What made replication possible was not simply that the technology worked, but that Beaurade established a standardised six-step methodology: audit, data collection, a three-month learning phase, simulation, real-time control, and performance analysis. That framework has since proved transferable across sites with different biological characteristics, moving what began as a bespoke solution towards a scalable industrial model.
That outcome reflects what happens when a demonstration is designed from the outset with adoption in mind: with defined success criteria, a committed end user, and a commercial structure that gives all parties a clear basis for the decision the trial was designed to reach. Across the demonstrations the Trial Reservoirs Initiative has supported to completion, that kind of structure has delivered an adoption rate above 70%, nearly three times the industry average.
For operators navigating the competing pressures of emissions targets, contract structures and tight margins, Beaurade offers something more useful than a promising dataset. It offers a methodology that has already been tested, refined and replicated, and a trial structure, delivered through the Trial Reservoirs Initiative, designed to convert results into adoption.
Learn more
This demonstration was delivered through our Climate Change Trial Reservoir. If you are an end user considering trialling technologies that reduce emissions, we’d love to hear from you. Trial Reservoirs Initiative offers support for trials with low financial risk and no obligation unless performance targets are met. To learn more, get in touch with Dr Jo Burgess at trialreservoir@isleutilities.com.