Is sewage the future of green aviation?

One-liner: Planes could technically run on something other than fossil fuel already — so why don't they? Starting from Richard Branson's 2023 "cooking-oil flight," this episode breaks down what Sustainable Aviation Fuel (SAF) actually is, why it's so expensive, and whether sewage, corn, or thin air can scale it up.

Key findings

• SAF currently makes up only about 0.6% of global jet fuel consumption (Joshua Heyne, director of the Bioproducts Sciences and Engineering Lab at Washington State University).
• 11 SAF conversion processes have been approved by regulators. The most common is HEFA (Hydroprocessed Esters and Fatty Acids), which turns used cooking oil, fats, and greases into hydrocarbons chemically close to jet fuel. Virgin Atlantic's 2023 transatlantic "all-SAF" flight ran mostly on HEFA.
• A Reuters investigation found that of 165 SAF projects announced by airlines in recent years, only 10 reported producing commercial volumes.
• Alaska Airlines managing director of sustainability Ryan Spies says rising fuel prices have put the airline "back hundreds of millions of dollars." Host Regina Barber adds context: the Iran war has left the Strait of Hormuz "effectively closed for months," leaving some airlines running low on fuel and pushing ticket prices up.
• A new $20 million venture in Washington State, the Cascadia Sustainable Accelerator, is a nonprofit backed by Alaska Airlines and Washington State University aiming to build a reference library of SAF production methods; Heyne's lab has already received SAF samples from 40 different institutions.
• The Trump administration cut the Biden-era SAF tax credit "almost in half" last year, weakening federal-level policy momentum for SAF.

Scientific background

SAF (Sustainable Aviation Fuel) isn't one recipe — it's a category of fuels that are chemically close enough to jet fuel to fly on, but don't come from crude oil. The episode covers three main routes:

1. HEFA (Hydroprocessed Esters and Fatty Acids) — the most mature and already-commercial route, made from used cooking oil, fats, and greases. It's chemically similar to renewable diesels already used in land and aquatic vehicles. But the feedstock (leftover cooking oil) is inherently limited — as Heyne puts it, the core problem isn't chemistry, it's economics: "there's not enough."
2. Alcohol-to-jet — starches like corn or cellulose biomass are fermented into ethanol, then converted into jet fuel. It's technically possible, but scaling it takes heavy financial investment and raw material, reviving old land-use and food-price debates (Regina Barber recalls a similar corn-biofuel-vs.-solar land-use debate at Washington State University in the early 2000s — though she notes that debate was mostly about cars, not planes).
3. Synthetic fuel — captured CO2 (e.g., from direct air capture) combined with hydrogen from water, converted into liquid hydrocarbons. This is even more expensive than the bio-based routes and "just not practical right now to scale."

All three routes share the same bottleneck: as Kwong summarizes it, SAF is caught in a chicken-and-egg cycle — it's too expensive and resource-intensive to make, so supply stays low, so planes keep running on regular jet fuel.

People

• Joshua Heyne — director of the Bioproducts Sciences and Engineering Lab at Washington State University, a self-described "fuel nerd" and technical lead for the Cascadia Accelerator, building a cross-institution reference library of SAF production methods.
• Christina Cassotis — CEO of the Allegheny County Airport Authority, which operates Pittsburgh International Airport. In 2021 she made Pittsburgh the first major airport in the world powered by its own microgrid (five natural gas turbines plus 8 acres of solar), and is now pushing the airport toward building its own SAF plant.
• Annie Petsonk — former Assistant Secretary of Transportation for Aviation and International Affairs under the Biden administration, who points out that the Iran-war fuel price shock is pushing airlines and policymakers to see alternative fuel as a national security matter.
• Nafisa Lohawala — a fellow with the think tank Resources for the Future, who frames the core SAF market problem as "derisking": private investors aren't yet comfortable putting money in.

Rich details & trivia

• The episode's title is a direct nod to the sewage angle: Heyne's lab is "working with companies that take post-anaerobic digested human sewage and turn that into jet fuel." Barber's verdict: "sewage seems like the best fuel source... humans are always going to poop. Let's fly on it, you know?"
• Pittsburgh's microgrid isn't just for show. When the main grid went down last year, "the lights flickered for a couple of seconds, and it kicked into gear" — staff didn't even notice until weeks later. Around the same time, London's Heathrow Airport had to shut down for a day over a similar grid issue.
• Cassotis frames the airport's resilience obsession beyond punctuality: Pittsburgh is a health care town, with live organs and transplant teams needing to fly in reliably every day.
• Cassotis told the hosts by email that she doesn't anticipate any land-use changes or impacts to existing commodities from the planned SAF plant — a direct response to the land-use critique leveled at bio-based SAF. She hopes to see it built near the Pittsburgh airport within two years.
• The much-referenced Virgin Atlantic 2023 transatlantic SAF flight received up to £1 million from the UK government — purely to prove the scientific point that a plane could run entirely on something other than jet fuel.

Cross-domain connections

This episode is fundamentally about the chemistry and industrial economics of aviation fuel — the question of what it takes for a technology to go from scientifically feasible to commercially scalable. That question rhymes conceptually with a recurring theme in the acquired business domain (moats, economies of scale, capital intensity), but the episode names no company or industry that directly overlaps with anything already covered there — the airlines and energy ventures here are unique to this episode. So there's no genuine strong cross-link to make yet; this is a note for future reference if aviation- or energy-focused business history gets ingested later.

Pages to build

• sustainable-aviation-fuel — SAF as a cross-episode scientific concept worth its own page once more aviation/energy-focused Short Wave episodes are ingested, consolidating the chemistry routes and the underlying economics.

Source · shortwave