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In the race for alternative, sustainable aviation fuels, some companies are getting creative. We’ve heard about airplanes powered by cooking oil, but what about jet fuel made entirely from human poop? Firefly Green Fuels, an aviation company based in Gloucestershire, Great Britain, has created just that – and it’s not surprising that the prospect of poop-powered aircraft is attracting attention.
While sustainable aviation fuel (SAF) is not new, the idea of using sewage – an abundant and unavoidable waste – is novel. So could this really be the future of air travel?
Commercial aviation produces approximately 2.5% of global CO2 emissions and contributes to climate change. Efforts are underway to reduce the sector’s impact, with the development of electric and hydrogen-powered aircraft. But the technology is still far away to enable long-haul passenger flights. Instead, the industry is looking to adopt SAF – with the International Air Transport Association (IATA) estimating it could contribute up to 65% of the emissions reductions needed to take aviation to net zero by 2050.
SAF burns like normal jet fuel and produces the same amount of emissions while a plane flies, but has a lower carbon footprint over the entire production cycle because it is usually made from plants that have absorbed carbon dioxide (CO2) from the atmosphere when they were alive. Or, in the case of sewage, it is made from plants and other foods eaten by humans and passed through the digestive system. That absorbed CO2 is released back into the atmosphere when the SAF burns, while burning jet fuel made from fossil fuels emits carbon that is trapped.
Until now, sewage has been an untapped resource when it comes to SAF, but James Hygate, CEO of Firefly, believes this is a missed opportunity. “There’s a lot of it, it’s all over the world and there’s not really a good use for it at the moment, making it a very low-value material,” he told CNN.
That’s why the company, a spinoff of Green Fuels, which has been developing low-carbon fuels since the early 2000s, including biodiesel made from canola oil for cars and trucks, has turned its hand to jet fuel — and poop.
Processing poop
To convert 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, the sewage is converted into carbon-rich biochar (a powder that can be used as fertilizer for crops) and crude oil.
Until now, production has taken place on a small scale in a laboratory. But early results are promising, with independent analyzes by international aviation regulators showing it is virtually identical to standard fossil jet fuel. It also has a 90% lower carbon footprint than standard jet fuel, according to a life cycle assessment conducted by Cranfield University in Britain.
Firefly wants to scale up production in the coming years. The company is currently undergoing a fuel qualification process with the standards body ASTM International, which is expected to take up to two years. It will then start building a processing facility in Britain, which Hygate hopes will be operational before 2030 and be able to process 100,000 tonnes of biocrude oil per year – or produce around 40 million liters of SAF. To put that into perspective: according to Hygate, that is enough for 800 flights from London to New York. He adds that it would be more expensive than conventional kerosene used by aircraft, but cheaper to produce than other biofuels.
Getting control of the sewage should be easy, he says, adding that Firefly is already in discussions with a number of British water companies. But he admits that financing the processing facilities can be a challenge. “These are major infrastructure projects that need money to actually come to fruition,” he says. So far, the company has received a £2 million ($2.5 million) research grant from the British government and a £5 million ($6.3 million) investment from European airline Wizz Air.
However, the amount of sewage is something that cannot be scaled up. Hygate estimates that if all usable UK sewage waste were used to produce jet fuel, it would still only meet 5% of the UK’s demand for SAF. Therefore, it should be used alongside other SAF feedstocks such as canola oil.
A 2023 report from the Royal Society on net-zero aviation solutions found that “the scale and availability of feedstocks” is a limitation for biofuels, and that producing enough to meet UK aviation demand could account for more than half of would require agricultural land from the country.
It also noted that there is some debate over whether agricultural waste is truly “waste” as it is often used for bedding or animal feed. Cait Hewitt, policy director at the Aviation Environment Federation, a British nonprofit that monitors aviation’s impact on the environment, asks the same question about sewage.
“One of the important questions to ask about any kind of feedstock for alternative fuels, including waste, is: What else would have happened to this stuff?” she says. In Britain, a large amount of sewage is currently used by farmers as fertilizer, she adds. If it is used to make SAF instead, that fertilizer would have to be replaced.
Hygate says the biochar byproduct could be used by farmers as an alternative, although possibly not on the same scale. He adds that there is a possibility that Britain will follow other countries such as the Netherlands in banning the spread of sewage on fields. When 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 major role in the future of aviation. The first commercial transatlantic flight powered by 100% SAF, made from waste oil and animal fat, took off from London to New York in November.
Sewerage is an interesting potential solution and should not be sniffed at, says Hewitt. But she warns that, like all other SAF systems, it will still produce the same amount of CO2 emissions as the aircraft flies, and will not solve the problem of contrails, which also contribute significantly to the warming caused by aviation causes.
“To have any chance of getting from where we are today to net-zero aviation by 2050, we really need to focus on real, scalable, zero-emission solutions,” she says.
“There may be a limited role for some of these alternative fuels in the short to medium term,” Hewitt adds. “But the big danger is that when you hear something like this, it intuitively sounds like such a good idea, and people say, ‘That’s great, we’re on the way to sustainable flying, we don’t have to worry about how much we fly.'”
Jacopo Prisco contributed to this report.
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