Scientists think outside the bowl and ask if poop can power planes

By Nell Lewis, CNN

Editor's note: This story has been updated to correct inaccuracies about the analysis and qualification of the biocrude oil, and the proportion of jet fuel demand that it could supply.

(CNN) — In the race for alternative, sustainable jet fuels, some companies are getting creative. We’ve heard about planes powered with cooking oil, but what about jet fuel made entirely from human poop? Firefly Green Fuels, an aviation company based in Gloucestershire, UK, has created just that – and, unsurprisingly, the prospect of poop-powered planes is attracting attention.

While sustainable aviation fuel (SAF) is not new, the idea of using sewage – an abundant and unavoidable waste – is a novelty. So, could it really be the future of air travel?

Commercial aviation produces about 2.5% of global carbon emissions, contributing to climate change. Efforts to reduce the sector’s impact are underway, with the development of electric and hydrogen-powered planes. But the technology is still a long way off powering long-haul passenger flights. Instead, the industry is looking to use SAF – with the International Air Transport Association (IATA) estimating that it could contribute up to 65% of the reduction in emissions needed for aviation to reach net-zero in 2050.

SAF burns like normal jet fuel and produces the same amount of emissions while a plane is flying, but it has a lower carbon footprint during its entire production cycle, because it’s usually made from plants that have absorbed carbon dioxide (CO2) from the atmosphere when they were alive. Or, in the case of sewage, it’s made from plants and other food that have been eaten by humans and passed through the digestive system. That absorbed CO2 is released back into the atmosphere when the SAF burns, whereas burning jet fuel made from fossil fuels emits carbon that has been locked away.

So far, sewage has been an untapped resource when it comes to SAF, but James Hygate, CEO of Firefly, thinks this is a missed opportunity. “There’s loads of it, it’s everywhere in the world and there’s not really any good use for it at the moment which makes a very low-value material,” he tells CNN.

That’s why the company, a spin-off from Green Fuels, which has been developing low-carbon fuels since the early 2000s, including biodiesel made from rapeseed oil for cars and trucks, turned its hand to jet fuel – and poop.

Processing poop

To turn human waste into a usable fuel, Firefly uses a method called hydrothermal liquefaction, which is good for wet waste. By combining high pressure and heat, it converts the sewage into carbon-rich biochar (a powder that can be used as a crop fertilizer) and crude oil.

So far, production has been on a small scale in a laboratory. But early results have been promising, with independent analysis by researchers in universities in the EU and US finding it almost identical to standard fossil jet fuel. According to a life cycle analysis carried out by Cranfield University in the UK, it also has a 90% lower carbon footprint than standard jet fuel.

Firefly is looking to scale-up production in the coming years. The company expects to submit an application this year for a fuel qualification process with the standards body ASTM International. Then it will start building a processing facility in the UK, which Hygate hopes will be operational before 2030 and capable of handling 100,000 tons of biocrude oil a year – or producing around 40 million liters of SAF. To put that in perspective that’s enough for 800 flights from London to New York, according to Hygate. He adds that it would be more expensive than conventional kerosene used by planes, but cheaper to produce than other biofuels.

Getting hold of the sewage should be straightforward, he says, adding that Firefly is already in talks with a number of UK water utility companies. But he admits that financing the processing facilities could be a challenge. “These are big infrastructure projects that need money behind them to actually come to fruition,” he says. So far, the company received a £2 million ($2.5 million) research grant from the UK Government and a £5 million ($6.3 million) investment from the European airline Wizz Air.

However, the quantity of sewage is one thing that can’t be scaled up. Hygate estimates that if all usable UK sewage waste was put into making aviation fuel, it would still only meet 5% of the UK’s demand for jet fuel. Therefore, it would have to be used alongside other SAF feedstocks, like rapeseed oil.

A 2023 report from the Royal Society on net zero aviation solutions found that “the scale and availability of feedstock” is a restriction for biofuels, and that producing enough to sustain the UK’s aviation demand would require more than half of the country’s agricultural land.

It also noted that there is some debate over whether agricultural waste is really “waste,” since it is often used for animal bedding or feed. Cait Hewitt, policy director at the Aviation Environment Federation, a UK non-profit that monitors aviation’s environmental impact, asks the same question of sewage.

“One of the important questions you need to ask about any form of feedstock for alternative fuels, including waste, is what would have happened to this stuff otherwise?” she says. In the UK, a large quantity of sewage is currently used by farmers as a fertilizer, she adds. If it’s used to make SAF instead, that fertilizer would need to be replaced.

Hygate says that the biochar by-product could be used as an alternative by farmers, although potentially not at the same scale. He adds that there is a possibility that the UK may follow other countries like the Netherlands in banning the spread of sewage on fields. If this happens, the other most common disposal route is incineration, an energy intensive process.

Not to be sniffed at

Despite their limitations, biofuels are likely to play a big part in the future of aviation. The first commercial transatlantic flight powered by 100% SAF, made from waste cooking oils and animal fat, took off from London to New York in November.

Sewage is an interesting potential solution and is not one to be sniffed at, says Hewitt. But she cautions that, as with all SAF, it will still produce the same amount of carbon emissions when the plane is flying, and it doesn’t solve the problem of contrails, which also contribute significantly to the warming created by aviation.

“To have a chance of getting from where we are today to net zero aviation by 2050, we need to be really focused on genuine, scalable, zero-emission solutions,” she says.

“There might be some limited role for some of these alternative fuels in the short to medium term,” Hewitt adds. “But the big danger is when you hear something like this, it sounds like intuitively such a good idea, and people say, ‘That’s great, we are on the way to sustainable flying, we don’t need to worry about how much we fly.’”

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