Flying on sewage

Can human waste provide a solution for sustainable aviation? Cranfield University – academic partner to industry lead Green Fuels Research and energy services company Petrofac — has already proven the viability of this approach, as Dr Ying Jiang reports.

Global transport systems are under huge pressure to decarbonise – aviation in particular. So often the aviation sector is held up as the villain of the climate change story when in reality it makes up only around 2-3% of carbon emissions worldwide.

But the negative public perceptions remain, and the industry is one of those most determined to transform itself, for the sake of building long-term business sustainability and resilience. With the third largest aviation network in the world, the UK has a particular need to take action and demonstrate its commitment to change.

The industry and its stakeholders have come to a mutual agreement on carbon reduction that points only one way: low-carbon liquid fuels or Sustainable Aviation Fuels (SAFs), as the short and medium-term answer to making substantial progress on cutting carbon emissions ahead of the 2050 targets for a net zero aviation sector.

Typically, SAFs emit 80% less carbon over their life cycle compared with conventional jet fuels. There’s no need for expensive investment in new aircraft or new infrastructure, SAFs are a a plug-and-play solution for the industry during a period when the radical new world of hydrogen and electric-based technologies, demanding vast investment, is worked out fully.

The only issue with SAFs, as tends to be the case with biofuels generally, is their availability on a large enough scale. They rely on mammoth — and ongoing — supplies of feedstocks. These are either in limited supply (like food waste) or come loaded with other environmental issues (the impact of bioenergy crops on land use, for example).

In this context, using human waste from sewage systems to produce SAFs would provide an ideal, circular solution. Sustainability for aviation. A way to limit feedstock costs. A means of limiting the need for human waste to be disposed of into rivers and seas. An additional revenue stream for water companies. And a real opportunity for biofuels enterprises.

SAFs boom ahead

Last year saw the first commercial flight with a full load of passengers using 100% SAF for one of its two engines. Rolls-Royce has confirmed that all its engines will be SAF-ready by 2023. The US government has taken a lead on incentivising the use of SAFs through its Inflation Reduction Act (IRA) 2022 and has a goal of enabling the production of 3 billion gallons of SAF by 2030, with a trajectory of growing production facilities in order to deliver 35 billion gallons a year by 2050.

Current production rates of approved SAF technologies, however, only meet a small fraction (less than 1%) of the actual fuel demand from commercial airlines. One issue is the relatively low cost of fossil-based fuels for airlines. As an industry still recovering from the financial shocks of the Covid-19 pandemic, businesses are still looking for the lowest cost options and keep on seeing SAFs as a critical option, but an option for the future rather than now. Another is the strict quality standards needed from any alternative aviation fuels.

Feedstock dilemmas

Biofuels in general are in demand. The UK continues to be a net importer of biofuels, with most of its supply (traditionally at least) coming from Ukraine, China, the US and the EU. With government plans to increase the mandate for ‘biofuel blending’ in transport fuels to 14.6% by 2032, there is only going to be more competition over land use internationally: more land being turned over to bioenergy crops rather than food; weakening food security; damage to local economies; and higher prices for biofuels.

There are alternatives in the form of waste products which would provide more of a sustainable option. Food waste, the types collected through municipal collections from households, have potential as an ongoing supply. But this would need a strong intervention from local authorities across the country to ensure food waste was collected separately to deliver the kind of consistency and quality of waste that is needed for conversion into a biofuel.

The technology and processes for converting the world’s mass of waste plastics into SAFs are currently being trialled across several schemes. Virgin Atlantic has a deal with Norwegian chemicals firm Agilyx to turn plastic waste into a crude synthetic oil that will, in turn, be converted into a SAF in a US facility. In the UK, the Clean Planet Energy start-up has a production facility capable of converting 60 tonnes of plastic waste a day into SAFs that are expected to deliver a 75% reduction in carbon emissions. The bigger question here though is over the use of plastics for burning as fuel at all. It shouldn’t be regarded as a ‘sustainable’ process, not when it involves the use of a finite resource: hydrocarbons that have been created over millions of years are a critical element to the chemical engineering industries and in so many different polymers and plastic-based products and materials which are not re-used or recycled but destroyed.

Flying on sewage

Using municipal sewage sludge to produce SAFs will be a genuinely sustainable foundation, addressing both waste management and low-carbon energy demand. As a feedstock, the scale of supply more than works. It is estimated that around 53 million tonnes of sewage sludge are produced every year that require to be treated in the network of around 200 sludge treatment centres in the UK. The UK’s wastewater system is heavily over capacity, meaning storm events (or even, in some locations, just a light rain) regularly lead to raw sewage ending up in rivers and seas. According to Environment Agency figures, there has been a 37% increase year-on-year in incidents: meaning 9 million hours of sewage being pumped into rivers and seas since 2016. Finding alternative uses for sewage sludge will be a huge benefit to businesses, to the environment and society as a whole.

The FIREFLY project, funded by Department for Transport as part of its Green Fuels, Green Skies competition, has shown how this can work; proved that the quality of the resulting biofuel will meet strict industry standards; and led to the engineering design and construction of a demonstrator plant for production. With further industry investment, the project is due to lead to a first-of-a-kind commercial refinery and the development of a network of UK sites close to airports, pipeline terminals and wastewater treatment works. The integrated processes involved will also reduce the processing costs involved and help keep unit costs low. Alongside other development pathways, the FIREFLY technology is expected to become a strategic asset for the UK as part of the broader national SAF portfolio.

Costs for SAFs will remain relatively high, at least for the short-term. Which is why the huge potential for SAFs — and the opportunity for a circular economy approach to their production — will only be realised with a clear mandate from government, stimulating mass uptake of SAFs, more investment from finance institutions and a rolling cycle of lower costs for every stakeholder involved.

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