In the water sector’s journey to net zero emissions by 2030, the big unknown has always been process emissions from wastewater treatment. As well as carbon dioxide, other greenhouse gases are released when chemical changes are made to raw materials during the sewage treatment process. They make up a significant proportion of the sector’s overall carbon footprint but until now the precise amounts of these atmosphere-changing gases had to be estimated based on 2006 guidance from the United Nations.
However, Severn Trent has developed a way to monitor emissions of two of the most serious of these gases – methane and nitrous oxide – and early results indicate that emissions are likely “substantially” higher than previously believed. The more optimistic flip-side of this is that the gases have scope to be captured and resold as commodities rather than being discarded as waste. Severn Trent has thrown its weight behind innovation at its wastewater processing plants to pilot and scale up novel treatments on its live systems.
Peter Vale, carbon and circular economy architect at Severn Trent will speak on the Net Zero Challenge Stage at Utility Week Live on 16 May. Find out more about the UWL23 Challenge Programme here.
Severn Trent has developed this monitoring capability in partnership with Melbourne Water in Australia and Aarhus Vand in Denmark. The alliance came about as the three organisations each realised they shared similar net-zero targets and that by joining forces they could act as “critical friends” of one another’s methodology, accelerating the gathering of data under a variety of conditions.
With the clock ticking towards that 2030 target, the water companies’ findings reveal a greater than expected presence of po tent greenhouse gases methane and nitrous oxide. The latter has roughly 265 times the global warming impact of CO2 if released into the atmosphere, while methane is 30 times more potent.
Tests undertaken by Seven Trent with its Danish and Australian partners suggest that the level of nitrogen oxide produced from the wastewater treatment is four times higher than expected.
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Peter Vale, carbon and circular economy architect at Severn Trent, tells Utility Week: “[The levels] had been underestimated. We followed an established methodology protocol whereby you make assumptions about how much nitrogen is entering a plant, then convert 4% of it into nitrous oxide.”
This was based upon work carried out in the past, but the efforts made by the company and its international partners to achieve sector net-zero goals by 2030 have challenged these global assumptions.
Vale says: “The monitoring we have done alongside Melbourne Water and Aarhus Vand suggests the amount of nitrogen converted to nitrous oxide is about four times higher.” The monitoring is ongoing to establish richer data sets. “We will find out more over the coming years, but it is very likely that emissions are substantially higher than have been reported historically.”
By pooling their research results the companies were able to better understand how variations in the treatment process under different conditions affected the amount of emissions produced. The work showed that emissions are not steady or constant, due to seasonal variations, with levels higher in the spring and dropping later in the year. The teams discovered variation diurnally as well as spatial variations, even within a single plant, signalling the erratic nature of the emissions, and why they had previously proved so challenging to properly monitor.
“We are learning a lot about how it varies,” Vale says. “And you don’t need much nitrous oxide to have a big impact on the carbon footprint.”
Likewise methane, which is a useful biofuel that can be injected into the grid as biomethane or used for combined heat and power engines to produce renewable energy, was found in much greater quantities than anticipated.
Vale says the light bulb moment came when they realised emissions from nitrous oxide and methane are far worse than previously thought. “Initially we thought, wow, that is quite alarming, but it’s much better to know. Now we can target the mitigation strategy, which is what we’re working out now.”
The three companies are co-creating a white paper on emissions to share with the wider sector. “We thought it would be great to test each other’s methods and methodologies to act as critical friends,” Vale says, describing the findings so far as “eye-opening”.
He adds: “The whole reason to do this is to really face into and tackle the climate emergency.”
Innovation teams working at the three companies will invite colleagues from the broader sector to review the approaches they have taken, to learn from them and apply them to their own treatment plants.
Vale says innovation is essential because there are no standards yet, so the teams are learning as they work and are able to share findings to work from more complex data sets than possible from one company alone.
“We want to be opened up to scrutiny so people can look at the method and understand the scale of what needs to be done,” Vale explains.
Across two of its treatment plants, Severn Trent will pilot and scale up a method of covering the main treatment process using active cover technology, which contains a catalytic media. This is activated by sunlight and removes the nitrous oxide as it percolates through the filter in the cover.
The process is currently being optimised at the group’s test facility in Warwickshire before being scaled up at its Net Zero Hub.
Vale says that as well as scaling up, the goal for the coming year is to identify alternative ways of mitigating emissions now that they are better understood. By tightening controls of the process, the team hopes to understand the triggers for these potent emissions.