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ENGINEERS at Stanford University, US, have developed a sewage treatment process which produces large quantities of nitrous oxide (N₂O) and methane that can be used to power the treatment plant.

Stanford civil and environmental engineer Craig Criddle worked with Brian Cantwell, a professor of aeronautics and astronautics, who has spent five years developing rocket thrusters powered by N₂O, to develop the method.

In standard wastewater treatment plants, sludge is aerated to encourage the growth of bacteria which break down the nitrogenous components into nitrogen gas whilst feeding on the sugars and other organic matter. By reducing the oxygen available, the researchers instead encouraged the growth of anaerobic bacteria which produce N₂O. This reduces the amount of organic matter consumed by bacteria, leaving two or three times more available for conversion into methane which can be used to power the plant. Not aerating the sludge has additional cost benefits – the aeration process accounts for up to half of a sewage treatment plant’s running costs.

The N₂O produced can be broken down by one of Cantwell’s rocket thrusters. N₂O decomposes into nitrogen and oxygen, both harmless atmospheric gases, unlike N₂O which is a gas with 300 times the global warming potential of  CO₂.

One of Cantwell’s PhD students, Yaniv Scherson, was investigating whether N₂O could be used as an energy source for thrusters, but required a cheap, renewable source. Discussions with Criddle provided him with his answer. Cantwell says that one thruster the size of a basketball could turn all the environmentally-damaging N₂O from a standard-sized treatment plant into harmless N₂ and O₂.

“Normally, we want to discourage these gases from forming, but by encouraging the formation of nitrous oxide, we can remove harmful nitrogen from the water and simultaneously increase methane production for use as fuel,” says Criddle, adding that the technology has the potential to be “a game-changer”.

Cantwell believes the technology could be used in a new generation of sewage treatment plants which would be self-sufficient in energy. The researchers are also looking at ways to recover waste nitrogen fertiliser from contaminated groundwater.