Whenever the media covers Firefly Green Fuels, the coverage inevitably includes an element of toilet humour and puns. That’s because the company aims to turn human waste via sewage into jet fuel. But for Firefly CEO James Hygate, it is the logical conclusion of more than two decades spent working on alternative fuels.
“If you want to make fuel at scale, you have to start with the feedstock,” Hygate explains. “That’s where Firefly begins.”
Hygate’s journey into biofuels began in the early 2000s, when he founded a company called Green Fuels after studying climate change in the late 1990s. What started as small-scale biodiesel production quickly became a global business supplying equipment and technology around the world. But over time, one limitation became clear: feedstocks like used cooking oil, the current preferred feedstock for sustainable aviation fuel, are finite.
Here are the key highlights of the conversation:
Why sewage biosolids emerged as the ideal SAF feedstock (4:02)
How hydrothermal liquefaction turns wet biomass into drop-in jet fuel (8:49)
Why Firefly’s fuel could be carbon negative, not just low-carbon (19:05)
ASTM certification timeline and the new SBTJ pathway (12:52)
Wizz Air offtake agreement and what makes SAF projects bankable (23:17)
Can biosolids rival cooking oil volumes globally? (29:47)
The path to cost parity with fossil jet fuel (38:43)
Keep reading for a detailed overview of the episode.
Biosolids as a feedstock: From waste liability to fuel potential
“Aviation is the hardest sector to decarbonise,” Hygate says. “So the question became: what feedstock is abundant enough, consistent enough, and scalable enough to matter?”
The answer, Firefly believes, lies at the end of the wastewater treatment process, in biosolids. These include the treated sewage sludges produced in large volumes wherever people live. Unlike many waste streams, biosolids are highly regulated and remarkably consistent in composition.
Historically, biosolids have been spread on agricultural land or disposed of through costly alternatives such as incineration. But tightening regulations around contaminants like PFAS and microplastics have prompted a search for new disposal routes. Firefly positions itself as part of that solution.
Turning sewage into jet fuel using hydrothermal liquefaction
To produce SAF, the company utilises hydrothermal liquefaction (HTL), a process particularly well-suited for wet materials. Rather than drying biosolids, an energy-intensive step, HTL uses high pressure and moderate heat to convert the material into bio-crude oil and biochar.
“HTL is essentially a pressure cooker,” Hygate explains. “It allows us to make bio-crude oil quickly, without fossil carbon.”
That bio-crude is then upgraded using conventional refining processes to produce fuels. Crucially, Firefly’s SAF is chemically very close to fossil jet fuel, containing the aromatics and hydrocarbons aircraft engines require.
“What we produce is indistinguishable from fossil jet fuel,” Hygate says. “That’s why this pathway matters.”
It is important to note that sewage-to-jet via HTL is not yet an ASTM-approved SAF pathway. Firefly is currently progressing through the ASTM qualification process, where new fuel routes must demonstrate safety, material compatibility, and performance before being cleared for commercial use.
This is typical for second-generation SAF pathways, many of which spend several years in certification before approval. Firefly anticipates that this process will be completed by the end of this decade, allowing its fuel to be used as a true drop-in blend with conventional jet fuel.
An independent lifecycle analysis conducted by Cranfield University shows that Firefly’s fuel delivers over 90% CO2 savings compared to fossil jet fuel. Firefly believes the pathway could prove carbon-negative, once the carbon sequestered in biochar and the emissions displaced by waste disposal are fully accounted for.
Scaling and commercial viability: replicable plants, local feedstock, and bankable economics
Scale is where the model becomes particularly interesting. Firefly’s planned first UK facility could produce around 45,000 tonnes of SAF per year. While that is modest in aviation terms, Hygate argues the real impact comes from replication.
“Wherever there are people, there’s sewage,” he says. “You don’t have to import it.”
Firefly has already secured long-term offtake agreements, including a 15-year deal with Wizz Air, designed to be bankable. The company believes its costs will ultimately rival those of HEFA-based SAF, enabled by low feedstock costs and the avoidance of expensive drying steps.
Looking ahead to 2035, Hygate sees waste-derived fuels as one part of a broader SAF ecosystem. “There isn’t one route that can decarbonise aviation,” he says. “We need all of them to work.”
For Firefly, success will be measured not by novelty, but by tonnes of fuel produced, emissions avoided, and a business that stands on its own feet.
Get our new book
Our new book Sustainability in the Air: Volume Two is now available for purchase on Amazon. Learn more about the startups, strategists, and sceptics working to close the gap between growth and green ambition.
‘Sustainability in the Air’ is the world’s leading podcast dedicated to sustainable aviation. Through in-depth conversations with top aviation leaders, we break through the clutter and provide a clear roadmap for a net-zero future.
