From Sewage to Sky: Firefly Green Fuels Approach to Sustainable Aviation Fuel

Among the growing list of Sustainable Aviation Fuel (SAF) innovators, one company is quite literally plumbing new depths to bring cleaner skies: Firefly Green Fuels (https://flyfirefly.uk/), a British startup converting human sewage into jet fuel. This post explores Firefly’s unique technology, partnerships, and commercial roadmap, and asks: can waste really fuel the future of flight?

Firefly’s Sustainable Aviation Fuel Breakthrough: Sewage-to-Fuel via Hydrothermal Liquefaction

Firefly’s core innovation lies in its hydrothermal liquefaction (HTL) process. It is novel, but it isn't clear if the company has patents for the process or if they have created a functional system based off of other Intellectual Property (IP) available. HTL mimics the natural formation of fossil fuels by applying heat and pressure to organic material—in this case, biosolids (dewatered human waste). This process converts waste into:

• Biocrude: an energy-dense oil that can be upgraded into SAF.

• Biochar: a carbon-rich residue that can be used in agriculture or building materials for carbon sequestration.

  
  

This dual output not only reduces emissions from fuel combustion but also locks carbon away, enhancing the sustainability case. According to a third-party life cycle analysis by Cranfield University, Firefly’s SAF achieves a 91% reduction in CO₂ equivalent emissions compared to fossil jet fuel—7.97 gCO₂e/MJ, among the best in the industry. Certification, as discussed below, remains in progress.

Why Sewage?

It’s a grimly elegant solution: sewage is abundant, global, and underutilized. In the UK alone, millions of tonnes of biosolids are produced annually. Currently, much of this is spread on farmland—a practice facing increasing environmental scrutiny due to contaminants like PFAS and microplastics.

Firefly’s model taps into this problematic waste stream and upgrades it into high-value energy. According to co-founder James Hygate (also the CEO of Green Fuels Research), it’s an opportunity to “turn a waste problem into a climate solution.” The potential is massive: Firefly estimates it can produce 180,000 tonnes of SAF annually just from the UK’s biosolids.

Commercial Backing & Strategic Partnerships

Firefly’s ambitious vision has attracted a growing roster of backers and allies:

• Wizz Air signed a long-term offtake deal in 2023 to purchase 525,000 tonnes of SAF over 15 years, and also invested £5 million. Wizz Air has been an early investor into SAF, especially as a low-cost carrier

• Synagro Technologies, a major US biosolids processor, partnered with Firefly in July 2024 to expand operations into North America. It seems like Firefly wants to be the technology expert and leave operations of facilities to others; that's just my read of the partnership

• Boeing and the Clear Sky consortium joined forces with Firefly in mid-2024 to support fuel qualification, facility development, and regulatory engagement.

• In May 2025, Firefly announced a major investment from Builders Vision, the impact platform of Lukas Walton, to accelerate commercialization. This will help fund the company’s first full-scale SAF production facility in Harwich, UK, slated to begin operations by 2028–2029.

  
  

Fuel Certification & Regulatory Hurdles

One of the biggest obstacles facing SAF startups is the ASTM D7566 certification process, which ensures new fuels are chemically identical to conventional jet fuel. Firefly’s fuel must pass rigorous lab testing, engine trials, and blend ratio validation (usually capped at 50%).

Firefly is currently in the pre-qualification stage and is working with Boeing, UK regulators, and academic labs to advance toward certification.

Concurrently, SAF mandates are tightening. The UK government is pushing for 10% SAF blending by 2030, with similar targets in the EU and US. Firefly’s approach offers a domestic, circular solution that could help meet those goals.

Is It Really Sustainable?

Skeptics have questioned whether sewage should be redirected from fertilizer use, or whether SAF will simply perpetuate high-emission air travel. Firefly contends that biosolids spreading is increasingly problematic, and that their method removes contaminants while delivering a climate-positive fuel.

Importantly, unlike other SAF feedstocks—such as used cooking oil or vegetable oils—biosolids do not compete with food or arable land. This could make Firefly’s model one of the few truly scalable SAF pathways that doesn’t create indirect land use change. Biochar, the carbon-sequestering byproduct, further strengthens their carbon accounting.

Facility Roadmap & Scale Plans

Firefly’s first commercial SAF facility is in Harwich, England, with target production by 2028 or 2029. The plant will be capable of producing 43,000 tonnes of SAF annually from local sewage treatment plants.

The company envisions a modular rollout model: co-locating facilities with major wastewater treatment centers across the UK, EU, and North America. If proven cost-effective, this could unlock a network of SAF hubs in urban regions.

They’re also pursuing government support, including grants through the UK’s Jet Zero Council and the “Green Fuels, Green Skies” program, which awarded Firefly £2 million in 2021.

What Sets Firefly Apart?

Feature	Firefly’s Advantage
Feedstock	Waste biosolids (non-food, abundant)
Tech Pathway	Hydrothermal liquefaction (HTL)
GHG Reduction	>90% CO₂e savings over fossil jet fuel
Strategic Partners	Wizz Air, Synagro, Boeing, Builders Vision
Byproduct	Biochar (carbon negative)
Market Entry	Harwich plant by 2028–29

Final Thoughts: Waste Not, Fly More?

Firefly Green Fuels represents a rare intersection of climate pragmatism and technological ingenuity. By targeting a globally available, underutilized waste stream and converting it into low-emission jet fuel, Firefly offers a compelling blueprint for circular decarbonization in aviation.

The path to scale is still long—and regulatory, technical, and commercial hurdles remain. With strategic partnerships in place and a first facility on the horizon, Firefly will be interesting to watch as utilizing "waste" is never a bad idea