The Waste-to-Jet Initiative: Firefly’s Sewage-Derived SAF Pathway and Its Place in the HTL Industry

By Abdul Ahad NazakatReviewed by Laura ThomsonJul 2 2026

Firefly Green Fuels, a Bristol-based developer, calls its sewage-to-fuel concept a “wet-to-jet” platform. In May 2026, the company completed a technology agreement with Turkish engineering firm Altaca, which supplied the upstream conversion step needed to turn treated sewage sludge (biosolids) into bio-crude oil at commercial scale.¹ The deal places Firefly’s project among the more advanced examples of a broader technology category, hydrothermal liquefaction (HTL) of wet organic waste, that a growing body of academic and industry research has been validating piece by piece over the past several years.

Image Credit: Horst Bingemer/Shutterstock.com

The Underlying Chemistry

HTL subjects wet biomass to high pressure and moderate-to-high heat, typically in the range of 300–350 °C at roughly 10–20 MPa, for residence times of minutes rather than hours. Under these conditions, organic polymers break down and recombine into a viscous bio-crude, alongside an aqueous phase, solid char, and gas.² Because the process works directly on wet feedstock, it skips the energy-intensive drying step required by pyrolysis, one reason researchers keep returning to it for sludge valorization specifically.

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A 2025 systematic review covering more than 600 peer-reviewed studies, published in Renewable and Sustainable Energy Reviews, reports that sewage sludge biocrude has a higher heating value ranging from 8.6 to 39.2 MJ/kg, with an average of 33.1 MJ/kg. This is well below the 42.8 MJ/kg typical of petroleum crude, reflecting the oxygen and nitrogen still locked in the molecule.³ That same nitrogen content, typically 4–6 wt% in sludge-derived biocrude depending on feedstock and extraction conditions, is the central obstacle the next processing stage has to solve.⁴

Bio-crude on its own is not a usable fuel. It needs hydrotreating, a reaction with hydrogen over a catalyst to strip out oxygen, nitrogen, and sulfur, followed by fractionation into the jet-range cut.

A 2022 study from Pacific Northwest National Laboratory, published in Energies, ran sludge and other wet-waste biocrudes through this full sequence and showed that the hydrotreated product met several ASTM tier-α screening properties for SAF, including carbon number distribution, density, viscosity and flash point.⁵

The same paper notes that jet fuel specifications cap nitrogen at roughly 2 ppm, compared with feedstock nitrogen levels that start out at three orders of magnitude higher, which explains why catalyst selection and extraction chemistry, not the HTL step itself, tend to be where commercialization efforts get stuck.⁵

Separate solvent-extraction work has shown that nitrogen distribution in the recovered biocrude can be reduced to around 5%, with energy recovery near 74%, though usually at the cost of lower overall biocrude yield.⁴

More recent research has extended the same upgrading logic to other wet-waste streams. A 2025 Nature Communications paper used a cobalt-molybdenum catalyst to convert food-waste biocrude into SAF via single-stage hydrotreating, evidence that the chemistry developed for one wet-waste feedstock transfers reasonably well to others.⁶

What the Altaca Agreement Actually Adds

Firefly’s own process description follows this two-stage architecture. Sludge goes through HTL to yield bio-crude and biochar, with the char intended for carbon sequestration in construction or as a soil improver, while the bio-crude is refined into SAF using methods adapted from conventional jet fuel synthesis.⁷

Before the Altaca deal, Firefly already had the downstream piece, refining technology supplied by Chevron Lummus Global, and a 15-year offtake agreement with Wizz Air for the finished fuel. What it lacked was a proven, scalable route from sludge to bio-crude.

Altaca’s CatLiq system fills that gap.¹ In a conference paper co-authored by Altaca’s own engineers, CatLiq is described as a catalytic HTL process run at supercritical water conditions, roughly 230–250 bar and 350–420 °C, producing a biocrude the company calls “Altaca oil” from mixed organic waste including sewage sludge.⁸

Those operating pressures sit toward the upper end of the range reported across the broader sludge-HTL literature, consistent with a catalytic, supercritical design rather than the subcritical processes more common in academic pilot studies.² With CatLiq supplying the upstream conversion, Chevron Lummus Global handling the refining, and Wizz Air locked in as buyer, Firefly now has a complete chain rather than three separate pieces awaiting integration.

The company points to a third-party life-cycle assessment from Cranfield University, reporting greenhouse gas emissions over 90% lower than fossil jet fuel and a carbon intensity of 7.97 g CO₂e/MJ.⁷

This is a company-commissioned LCA carried out by an independent academic group, not a peer-reviewed, published study, so the figures sit somewhere between a marketing claim and verified data, directionally consistent with the wider HTL literature but not yet subject to the same scrutiny as the academic papers cited above.

A Market Short on Feedstock

The waste-to-jet model enters a market with a real supply problem. IATA estimates global SAF production reached roughly 1.9 million tons in 2025, approximately 0.6% of total jet fuel consumption, climbing to around 2.4 million tons in 2026, or 0.8% of demand.⁹

Hitting the sector’s net-zero-by-2050 trajectory would require annual SAF supply near 500 million tons, a roughly 250-fold increase from today’s output. Current production still leans heavily on the Hydroprocessed Esters and Fatty Acids (HEFA) pathway, drawing on increasingly tight feedstocks like used cooking oil and tallow.⁹ Sewage sludge sidesteps that constraint: it doesn’t compete with food production, and it’s generated continuously regardless of fuel prices.

The UK feedstock base bears this out. The water industry produces around 3.5 million tons of biosolids a year, roughly 87% of which currently goes to agricultural land, a disposal route under mounting regulatory pressure over PFAS and microplastic contamination.¹⁰ Diverting even a portion of that volume to HTL fuel production wouldn’t close the global SAF gap on its own, but it does explain why water utilities already facing rising sludge disposal costs look like plausible long-term feedstock partners rather than incidental ones.

A few things stand between pilot operations and routine SAF supply at scale. Nitrogen removal remains the toughest chemistry problem, given the gap between feedstock nitrogen content and the fuel specification.^4,5 ASTM certification for new SAF pathways also demands large fuel volumes, over 400 liters, for full qualification testing, which is a meaningful capital hurdle for any producer moving past lab scale.⁵

Policy is shifting too: the EU’s ReFuelEU Aviation mandate and the UK’s SAF mandate have both raised blending requirements since 2025, but EASA’s first annual technical report found 81% of EU SAF supplied in 2024 still came from used cooking oil, with waste-derived alternatives like HTL bio-crude accounting for a comparatively small slice of current volumes.¹¹.

Conclusion

The Firefly–Altaca deal is less a single breakthrough than a commercial assembly of components the research community has been testing separately for years: HTL conversion, catalytic hydrotreating, and secured offtake.

Whether sewage-derived SAF reaches meaningful scale will hinge less on the underlying chemistry, which is reasonably well understood at this point, and more on feedstock logistics, certification throughput, and how quickly utilities and refiners are willing to put capital behind wet-waste infrastructure.

References and Further Reading

1. Firefly Green Fuels. “Firefly forges partnership with Turkish engineering innovator to decarbonise aviation.” 20 May 2026. https://flyfirefly.uk/firefly-altaca/
2. Castello, D., Pedersen, T.H., Rosendahl, L.A. “Continuous Hydrothermal Liquefaction of Biomass: A Critical Review.” Energies, 11(11), 3165 (2018). https://doi.org/10.3390/en11113165
3. Zhenyao Wang, Xuan Li, Huan Liu, Carol Sze Ki Lin, Qilin Wang. “Hydrothermal liquefaction of sewage sludge: A comprehensive review of biocrude oil production, byproducts valorization, and future perspectives.” Renewable and Sustainable Energy Reviews, 224, 116086 (2025). https://doi.org/10.1016/j.rser.2025.116086
4. Usman, M., Cheng, S., Boonyubol, S., Cross, J.S. “Nitrogen Minimization in Hydrothermal Liquefaction Biocrude from Sewage Sludge with Green Extraction Solvents.” ACS Omega, 9(12), 14530–14538 (2024). https://doi.org/10.1021/acsomega.4c00455
5. Cronin, D.J., Subramaniam, S., Brady, C., Cooper, A., Yang, Z., Heyne, J., et al. “Sustainable Aviation Fuel from Hydrothermal Liquefaction of Wet Wastes.” Energies, 15(4), 1306 (2022). https://doi.org/10.3390/en15041306
6. “From food waste to sustainable aviation fuel: cobalt molybdenum catalysis of pretreated hydrothermal liquefaction biocrude.” Nature Communications (2025). https://doi.org/10.1038/s41467-025-64645-y
7. Firefly Green Fuels. “What & How.” https://flyfirefly.uk/what-how/
8. Unsal, M., Livatyali, H., Aksoy, P., Gul, S., Onoglu, A. “CatLiq – Catalytic hydrothermal liquefaction process from pilot scale to demo scale.” Journal of Fundamentals of Renewable Energy and Applications, 5:5 (2015). DOI: https://doi.org/10.4172/2090-4541.S1.002. https://www.longdom.org/proceedings/catliq-catalytic-hydrothermal-liquefaction-process-from-pilot-scale-to-demo-scale-52449.html
9. International Air Transport Association. “Fact Sheet – Sustainable Aviation Fuels (SAF).” June 2026. https://www.iata.org/en/iata-repository/pressroom/fact-sheets/fact-sheet-sustainable-aviation-fuels/
10. Chartered Institution of Water and Environmental Management (CIWEM). “Sewage sludge and biosolids: Briefing and position statement.” August 2025. https://www.ciwem.org/assets/pdf/Policy/Policy%20Position%20Statement/Biosolids%20PPS%20CIWEM%20Aug%202025.pdf
11. European Union Aviation Safety Agency (EASA). “EASA publishes report on Sustainable Aviation Fuel scale-up progress.” 22 October 2025. https://www.easa.europa.eu/en/newsroom-and-events/press-releases/easa-publishes-report-sustainable-aviation-fuel-scale-progress

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