How Sewage Becomes Sustainable Aviation Fuel
Can the waste flushed down millions of toilets every day become the jet fuel that decarbonises commercial aviation with over 90% lower carbon emissions?
The Story Behind Firefly Green Fuels
Aviation accounts for roughly 2 to 3% of global
carbon emissions, and unlike cars or trains, aircraft cannot easily switch to
batteries or electric power. The industry's most viable path to decarbonisation
runs through sustainable aviation fuel, a drop-in alternative to fossil-based
kerosene that works in existing aircraft engines without modification. The
problem is supply. Current SAF production relies heavily on feedstocks like
used cooking oil and agricultural crops, both of which face availability limits
and compete with food production. Firefly Green Fuels, a Bristol-based company
founded by James Hygate, is pursuing a different feedstock entirely: sewage.
Firefly's process, called Sewage Biosolids To
Jet, transforms treated sewage sludge into a drop-in jet fuel. Hygate brings
over two decades of experience in the biofuels industry, having founded Green
Fuels Limited, which has supplied biofuel production equipment to more than 80
countries. He received an OBE for his services to low carbon fuels. Dr. Paul
Hilditch, a Cambridge graduate with a PhD in biochemistry, co-founded Firefly
and provides strategic direction. The company operates from Temple Studios in
Bristol and is currently developing its first commercial production facility at
a site in Harwich, Essex.
How Sustainable Aviation Fuel From Sewage Is Made
The conversion process begins with hydrothermal
liquefaction, a technology that uses heat, pressure, and water to break down
wet biomass into bio-crude oil. Sewage sludge is naturally wet, which is
typically a disadvantage in energy production because most processes require
drying the material first. In hydrothermal liquefaction, however, the water is
part of the reaction rather than an obstacle. This makes sewage a technically
efficient feedstock for this particular technology. Firefly has partnered with
Altaca Energy for the hydrothermal liquefaction stage of the process.
Once the bio-crude oil is produced, it is
upgraded into sustainable aviation fuel through proprietary hydrotreatment
processes derived from conventional jet fuel refining. The resulting fuel is a
drop-in replacement, meaning it can be blended with or substituted for fossil
kerosene in existing aircraft engines without any modifications to the aircraft
or fuelling infrastructure. Independent testing by the German Aerospace Centre
and Washington State University has confirmed that Firefly's bio-kerosene exhibits
a chemical signature almost identical to standard aviation fuel. The process
also produces biochar as a byproduct, which can be used for carbon
sequestration in construction or as a soil improver in agriculture.
Exploring the Practical Benefits of Using Firefly's SAF
The central advantage of sewage as a feedstock
is its abundance and predictability. Unlike used cooking oil, which is limited
in supply and increasingly expensive, or crops, which require land and compete
with food production, sewage is a continuous waste stream generated by every
human settlement on the planet. It does not need to be grown, harvested, or
collected from restaurants. It arrives at treatment plants constantly, in
volumes that scale directly with population.
According to Firefly, the sustainable aviation
fuel from sewage achieves over 90% carbon reduction compared to fossil-based
jet fuel across its full lifecycle. The feedstock-first approach also provides
less exposure to fuel price volatility because sewage sludge is a waste product
that treatment plants already need to dispose of, rather than a commodity
traded on global markets. This combination of carbon reduction, feedstock
reliability, and price stability is what Firefly describes as a pathway to keeping
flight affordable while meeting decarbonisation mandates.
Partnerships and the Path to Commercial Production
Firefly's first commercial facility is under
development in Harwich, Essex, on a site owned by Haltermann Carless that has
existing refinery infrastructure. Using an existing industrial site reduces
construction time, cost, and emissions compared to building a new plant from
scratch. The facility is designed to produce 100,000 tonnes of sustainable
aviation fuel per year, with feedstock supplied by Anglian Water and other UK
water companies.
Wizz Air has committed £5 million in direct
investment and signed a 15-year offtake agreement to purchase up to 525,000
tonnes of SAF, valued at nearly $1 billion. The airline plans to supply its UK
operations with Firefly's fuel as part of its roadmap to net-zero emissions.
Boeing is also a strategic partner through its Clear Sky initiative, which
identified Firefly as its first beneficiary. Additional partners include
Petrofac for engineering services, Chevron Lummus Global for refining
technology, and CEPSA, Neste, OMV, and CleanJoule across various stages of the
production chain. The UK Government has also announced a £43 million investment
to accelerate green aviation projects.
Where Sewage-Based SAF Fits in the Broader Aviation Fuel Landscape
The global aviation industry consumed roughly
350 billion litres of jet fuel annually before the pandemic. SAF currently
represents less than 1% of that total. Governments around the world are
introducing mandates that require airlines to blend increasing percentages of
SAF into their fuel supply over the coming decades. The European Union, the
United Kingdom, and several other jurisdictions have set escalating SAF
blending targets, creating regulatory demand that far exceeds current
production capacity.
Most SAF pathways approved to date rely on
feedstocks with known supply constraints. Sewage-based SAF opens an additional
feedstock pathway that does not compete with food, agriculture, or limited
waste oils. Whether Firefly can scale its process from a first commercial
facility to a global network of plants will depend on securing consistent
feedstock agreements with water utilities, maintaining fuel quality at higher
production volumes, and navigating the regulatory approvals required for each
new SAF pathway. The company has stated that additional facilities are in
development across several other geographies beyond the UK. If the Harwich
plant delivers on its production targets, it would provide one of the first
large-scale demonstrations that sewage can serve as a commercially viable,
globally scalable source of aviation fuel.
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