SVT warns process emissions far more potent than first thought

Research by Seven Trent Water (SVT) has indicated that emissions from wastewater treatment processes are likely to be “substantially” higher than previously believed.

The water company’s findings show a greater than expected presence of potent greenhouse gases methane and nitrous oxide.

The latter has roughly 265 times the global warming potential of CO2 if released into the atmosphere, while methane is 30 times more potent.

Tests undertaken by SVT with partners in Denmark and Australia have suggested the level of nitrogen oxide produced from the wastewater process is four times higher than expected.

Peter Vale, carbon and circular economy architect, told Utility Week that even a small amount has a significant impact on a plant’s footprint. “(The levels) had been underestimated,” he said. “We followed an established methodology protocol based on assumptions around how much nitrogen is entering a plant, then 4% is converted to nitrous oxide.”

This was based upon work carried out in the past but the efforts made by SVT and its international partners to achieve net zero by 2030 have challenged these global assumptions.

Vale said: “The monitoring we have done alongside Melbourne Water and Aarhus Vand suggested the amount of nitrogen converted to nitrous oxide is abut 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.”

He said the work showed emissions are not steady or constant due to seasonal variations with levels being 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.

“We are learning a lot about how it varies,” Vale said of the collaborative project. “But 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.

“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,” Vale explained.

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 said the aspiration for the coming year is to undertake this at scale as well as other ways of mitigating the emissions now they are better understood. By tightening controls of the process to understand what triggers more and less of the potent emissions to be generated.

It has a separate scheme funded through the Ofwat Water Breakthrough Challenge as part of the innovation fund. This utilises a specific type of bacteria which is able to remove ammonia without producing nitrous oxide, however they do not ordinarily survive in an activated sludge plant.

Vale will explain more about the project at Utility Week Live on 16 May. Register your interest in attending here.