Jet Fuel from Sewage — is it Feasible?

10 min readMay 12, 2023

There’s been a glut of scientific papers recently exploring the potential of using biomass such as wood, crops, and human waste to generate sustainable aviation fuel (SAF) as a replacement for conventional jet fuel. On paper it’s great: theoretically carbon-neutral, with cheap and highly accessible feedstock.

My previous article concluded the Fischer-Tropsch process is the most promising method for creating SAF — calling out the the Biomass-to-Liquid and Power-to-Liquid pathways. Power-to-Liquid, using feedstock of green hydrogen and carbon captured directly from the air, is a the closest to a zero-carbon solution for creating drop-in jet fuels and hence is seen as the SAF holy grail. It’s not yet commonplace due to the high costs — according to the World Economic Forum it’s currently up to 6x more expensive than producing conventional fuel. The primary cost drivers are carbon capture and green hydrogen production — nascent technologies that need many more years of maturity to have a shot at cost-parity.

Biomass-to-Liquid (BtL) has far lower feedstock costs, and therefore is a better bet for near-term SAF viability. In particular, the use of sewage sludge from wastewater plants has attracted attention as a great way to both recycle water and remove unsightly, toxic waste from our environment, plus avoid issues with other biomass feedstock related to food competition and unsustainable farming practices.

Process design

Feedstock

BtL can take in a variety of feedstock — see below for examples, focussing only on waste products (data collected by Sánchez et al. 2022). The process of converting waste to useful products is known as valorisation.

On the metrics:

HHV (higher heating value) defines the amount of energy released upon combustion relative to weight (higher is better);
Generally less moisture is desired (depending on the liquefaction process);
Less ash is desirable — it’s not combustible so a lower ash content means more valuable matter that can be burned;
A higher bulk density is desirable as it reduces transportation costs.

Distribution also becomes a major factor in the desirability of a certain feedstock. Wood and farm residue is typically dispersed over large regions, meaning gathering the feedstock for processing requires more transportation and ultimately higher costs (and greater carbon emissions). Conversely, municipal waste and sludge are produced in a more concentrated areas, such as landfill sites and treatment plants — the simpler logistics and lower collection costs make these appealing feedstock. Sewage sludge is worth looking closer into as a feedstock that is localised, energy dense, produced globally, and simple to process.

Sewage sludge is a by-product of wastewater treatment, rich in organic matter and heavy metals that are suspended in water. To get an idea of scale: in 2018 the US produced sludge containing 5.82m metric tonnes of biosolids, with 53% going to useful applications (typically agricultural fertiliser) and the remainder incinerated or going to landfill. As of research done in 2015 (Westerhoff et al.), the value of the biosolids is c.$600 per metric tonne. Therefore US water treatment plants fail to make us of some 2.7m tonnes or $1.6bn in raw material value every year, not to mention incurring extra costs through disposal.

Liquefaction process

In order to turn biomass to fuel, the input must first be depolymerised (have its long polymer chains decomposed into smaller ones) and turned into bio-oil (which can later be “upgraded” to SAF). There are two key options to go about this.

The first is pyrolysis — thermal decomposition in the absence of oxygen, which is a well-established, commercialised process. The feedstock is heated to 400–500°C, and through vapour quenching pyrolysis bio-oil is produced with a yield of c.75 wt% and a thermal efficiency of c.70%. The HHV of the pyrolysis bio-oil product is typically in the range of 22–26MJ/kg — roughly half the current standard of conventional fuel at 46MJ/kg. Due to the zero-oxygen requirements, any feedstock must be dried before entering the reactor, which is often energy-intensive and costly.

A less commercially-established method is hydrothermal liquefaction (HTL). Here the biomass is reacted in the presence of water, under moderate temperature (200–400°C) and high pressure (5–25MPa). The bio-oil yield can be anywhere between 20–60 wt% depending on feedstock, while boasting a thermal efficiency of c.85%. HTL has greater flexibility in terms of feedstock input due to the lack of drying requirement, and is also superior to pyrolysis in terms of energy density of product since the reaction with water leaves fewer reactive oxygenated species — resulting in HHV of 30–36MJ/kg, or c.70% of conventional jet fuel. It’s also suited to input slurries of c.5–35 wt% solid concentration — making the previous preference of sewage sludge an ideal input.

Given the better integration with desirable feedstock, lower heating costs, and more energy-rich product, generating bio-oil via HTL is a more attractive option than via pyrolysis. Through further refinement, including ash removal and treatment, this oil can be brought up to the standard to be put directly into jet engines.

Below is a process flow diagram for the HTL process:

The HTL process has been well-understood for years, but there are number of critical focus areas to be optimised:

Superior heat exchanger designs to prevent fouling and charring;
Optimisation of the solid content of HTL feed slurry to create a homogeneous input;
Increasing bio-oil yield through better HTL reactor designs;
Pumping of feedstock into HTL system to get a consistent input flow — there have been studies into substituting pumps for pressurised gas, but a drawback is the flow rate can’t be adjusted easily;
Logistics and transport costs — integration into existing feedstock production plants is by far the most cost-effective solution, followed by a spoke-and-hub design that processes feedstock off-site before transferring to a central facility.

Is there enough waste?

The conversion of sewage sludge into bio-oil via HTL seems promising and technologically feasible — but can it realistically replace conventional aviation fuel?

Here’s back-of-the-envelope math to explore the feasibility of SAF made from sewage sludge via HTL, using values suggested by Lozano et al. in 2022 for wastewater treatment plants (WWTPs) in The Netherlands (which we’ll take as a proxy for more economically developed countries):

1.25 million tonnes / year of sludge produced
Average solid non-ash dry content of 17.4 wt%
320 WWTPs in The Netherlands (with roughly 15% of the plants, producing 55% of the total sludge)

Let’s first assume that HTL process is integrated into directly existing WWTPs (co-location) to minimise costs — utilising existing infrastructure and setting up plants as close as possible to feedstock production are two ways to cut expenditure, covered in my last post. Assuming HTL is economically feasible for only the 15% most productive plants (in line with work done by Seiple et al. suggesting HTL is feasible at plants with flows ≥17 ML/d), this means 120,000 tonnes / year of material is available to be valorised (1.25m * 55% * 17.4%).

A bench-scale HTL reactor was tested using human wastewater by Kong et al. in 2023, resulting in a bio-oil yield of 28 wt% (at 300°C, 9MPa, solid non-ash dry content of 15 wt%) which is in line with other scientific papers. Meaning theoretically, around 33,000 tonnes / year of bio-oil could be produced in The Netherlands with reactors operating at maximum capacity. For reference, The Netherlands in 2019 consumed 83,300 barrels of jet fuel a day — roughly 4m tonnes / year.

So (very loosely) sewage sludge produced at the main plants in The Netherlands could cover about 1% of their yearly jet fuel demand. Even assuming all WWTPs in the country can be leveraged and pyrolysis-level yield of 75% can be achieved, this number only becomes 4%.

How about municipal solid waste? Work done by Seiple et al. in 2022 estimates that 4.4bn litres / year of bio-oil (or over 5% of the US’ jet fuel demand) could be produced from MSW generated in the US. In another analysis, Farooq et al. in 2020 found 22.8% of the current UK jet fuel requirements could be met with HTL technology, when exhausting all organic resources in the UK including sewage sludge, algae and food waste. This however didn’t consider the competition for other uses of the feedstock.

Takeaway

The upshot is that the waste produced each year in Western countries is a fraction of the amount needed to fully rely on for making carbon-neutral SAFs. Even using other sources of feedstock to minimise the deficit, such as agricultural waste and residue, will result in more decentralised feedstock production and therefore added transportation cost and complexity.

Despite this — it’s still a good idea to make productive use of our waste. Using sewage as an agricultural fertiliser is a well-established method — with c.50% of US sludge and over 80% of UK sewage being repurposed like this. However, according to the Marine Conservation Society, this treated sludge still contains microplastics and PFAS which may eventually make its way into the oceans — estimations predict 50% of plastics in untreated sewage are re-released back into the environment. By breaking down these toxic compounds and putting them to new use as fuel, these environmental concerns can be avoided, and a new revenue stream can be generated for the plants.

Does it make economic sense?

There are a number of techno-economic analyses estimating the cost of an HTL plant running on sewage sludge. Farooq et al. 2020 suggested the following:

10,000kg / hour throughput
8000 hours per year
30 year plant life

This means a theoretical 80,000 tonnes / year of sludge processed by a plant, equivalent to 13,920 tonnes of solid material (17.4 wt% from before). Taking the yield of 28 wt% as before, this is 3,900 tonnes of bio-oil produced per plant per year.

The literature analysis resulted in $16.3m (£12.7m) of capex to set up the plant, with opex of $3.8m (£3m) per year. Using the capital recovery factor at a discount rate of 10%, the total annual cost of the plant is roughly $5.5m —resulting in $1.4 / kg bio-oil. This is more than double the cost to produce fossil jet fuel, which is about $0.62 / kg according to the World Economic Forum.

Takeaway

SAF still very much carries a ‘green premium’ — for airlines to buy in, there need to be significant government subsidies.

Despite this, embedding HTL into their processes could make economic sense for WWTPs. The plants are facing a range of challenges from ageing infrastructure, increasing demand for services and strict emissions regulation around incineration. With the average cost to send sewage sludge to landfill at $440 / tonne, plants may also want to avoid millions per year in sunk costs and choose to invest in technologies that could generate revenue. Despite the dearer cost vs. current jet fuel, Boeing has committed to 21.2m litres of SAF to support its US operations through 2023, showing an appetite for airlines to get on board with bio-fuels to cut their carbon emissions.

What next?

Two big questions linger over the future of SAFs. First, how can the technology become viable at scale? It’s clear from the analysis that no one feedstock will be a clear winner due to variances in availability and chemical makeup — building an entire new ecosystem that can accommodate all these different inputs will be a monumental infrastructure challenge.

This brings us to the second question — is it worth it? A strong argument is that SAFs are likely to be a bridging fuel and not the end goal of aviation, since they are not zero-carbon (or truly carbon neutral for that matter, due to emissions during preparation and processing). Creating SAFs in the cleanest possible way via Power-to-Liquid, using CO2 from direct air capture and green hydrogen as feedstock, is still years away due to the high prices as covered previously. As the price of green hydrogen comes down — driven by electrolyser technology and the cost of renewable energy — would it instead make more economic sense to invest in the brand-new infrastructure required to scale hydrogen as a fuel alternative?

With aviation contributing 2% of global GHG emissions, it can be argued that there are more urgent sectors that need to be decarbonised to limit warming to 1.5°C above pre-industrial levels — heating and industrial processes are two standout areas ripe for disruption (each responsible for c.20% of global emissions). The technologies (and main cost drivers) underpinning these sectors are again the likes of hydrogen production, carbon capture, heating technology, and renewable electricity — meaning that progress in these areas is likely to have positive knock-on effects felt by the aviation industry. It’s clear that the future of energy is one connected tapestry, and these new solutions do not exist in silos —by focussing investment on these enabling technologies, rewards will be reaped by many players up and down the value chain.

Jet Fuel from Sewage — is it Feasible?

Process design

Feedstock

Liquefaction process

Is there enough waste?

Takeaway

Does it make economic sense?

Takeaway

What next?

Sources

Written by James Bannon