Crud-to-Crude: The Global Potential of Biofuels Made from Human Waste

May 13, 2023

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by Sean Mowbray (Mongabay) • Creating liquid biofuels from human waste shows promise as a way to meet one of alternative energy’s greatest challenges: reducing the transportation sector’s heavy carbon footprint. The good news is there is a steady supply stream where waste is treated.

Humanity produces millions of tons of sewage sludge annually via wastewater treatment. Existing disposal methods include landfilling, application on agricultural land, and incineration; each with social and environmental consequences.
Harnessing the carbon-rich potential of sludge as a transportation fuel for planes, ships and trucks is part of a drive toward zero waste and creating a circular economy, say experts. A host of projects are underway to prove the effectiveness of various methods of turning all this crud into biocrude.
Some techniques show promise in lab and pilot tests, but large-scale industrial plants have yet to be built. Using pollutant-laden sewage sludge as a biofuel comes with its own environmental concerns, but lacking a silver-bullet solution to the human waste problem, it could be part of a suite of best alternatives.

Last year, a car fueled by human waste toured the European countryside, covering more than 2,000 kilometers (1,200 miles). It was the culmination of To-Syn-Fuel, a pathfinding project using technology developed by Germany’s Fraunhofer Institute UMSICHT to make 50,000 liters (13,200 gallons) of biocrude oil from 500 tons of sewage sludge.

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Next up is the construction of a larger-scale crud-to-crude facility in Germany, built upon lessons learned from a smaller proof-of-concept demonstration plant. The aim is to process up to 400,000 metric tons of sewage sludge into “sustainable aviation fuel” by 2030, Daschner (Robert Daschner, head of the department of renewable energy at Fraunhofer UMSICHT) wrote.

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Wastewater treatment itself is a carbon-intensive process, accounting for as much as 5% of humanity’s greenhouse gas emissions, according to some estimates. Currently just 20% of global wastewater is treated, meaning that as waste treatment service expands, its environmental footprint could grow with it.

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Generating energy (in the form of biogas) and recovering nutrients (such as nitrogen and phosphorous, useful as fertilizers) are two other alternatives for extracting that “value,” with waste treatment operators already creating power from poop to shrink their carbon footprint, says Lillian Zaremba, collaborative innovations program manager for Metro Vancouver’s liquid waste service. Using a process known as anaerobic digestion, sewage sludge is sometimes used as feedstock to produce biogas in the form of methane, which is typically pumped back into treatment facilities to meet their energy generation needs.

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A review paper published last year outlines options including “biological processes, thermochemical technologies, bioelectrochemical processing, biorefineries and others.”

Though small-scale demonstrations of liquid biofuel generation from sludge have seen success, no large-scale industrial facilities are operational yet. “[A]lthough tested and validated at a laboratory or pilot scale, [the various processing techniques] still remain in their infancy from the industrial application aspect,” says the review paper.

Proponents tout the advantages of sludge-based biofuels, noting that they can avoid many of the common criticisms of other liquid biofuel feedstocks, including corn, palm oil or vegetable oil. Sludge as a feedstock offers zero competition with food or agricultural land use, and land conversion is avoided — issues that can drive down the sustainability and up the carbon intensity of various biofuels.

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Hydrothermal liquefaction (HTL), using pressure and heat, is one route for turning municipal waste into biocrude, and it’s considered one of the options with the greatest potential to create fuels for shipping and possibly sustainable aviation fuels. Last year, researchers working on a pilot-scale project at Denmark’s Aarhus University announced a “breakthrough” as biocrude was successfully produced using feedstocks including sewage sludge.

In Canada, an HTL demonstration project under development by Metro Vancouver is planned to go operational by 2025, turning around 10 metric tons of wet sewage sludge into five barrels of biocrude daily. Though that’s a drop in the bucket compared to global fuel needs, starting small is a crucial proof of concept before scaling up further, Zaremba said. Similar pilot projects are underway elsewhere.

Stian Hegdahl, a Ph.D. candidate at Norway’s University of Bergen, also believes HTL shows promise for turning human waste into biofuel. His lab-scale research uses sludge that has already gone through the anaerobic digestion process.

He cautions that biofuel production isn’t without its environmental problems. The conversion of sludge to fuel produces both solid and liquid waste, for which disposal would be required, and you “would have to consider how this is done [safely],” he said.

Another hurdle: Crud-to-crude refineries, if they’re to meet large-scale demand, may need to truck in sludge from numerous waste treatment plants, depending on their geographic location.

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A Canadian project took an alternative approach to HTL, testing instead the capacity of fast-growing willow to treat wastewater as a nature-based solution; a purpose for which it proved effective. Researchers then found that this waste treatment solution could also provide a sustainable source of biofuels. Treating wastewater with willow greatly increased the plant’s biomass, explains Pitre, who was part of the research team. That biomass could then be made into liquid fuel, or other products such as bioplastics, an outcome he describes as a win-win solution.

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“We’re seeing that the carbon intensity of the biocrude is 90% lower than fossil crude,” she said. But that high figure is because benefits are being calculated after the sewage has “already been collected and treated.” Which means that none of the “additional carbon emissions from that part of the process” are counted in the 90% reduction.

Likewise, creating fuel using the HTL process requires hydrogen, and making hydrogen has its own carbon footprint.

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Another conundrum: the toxic contaminants found in wastewater. Sludge isn’t only packed with extractable carbon and agricultural nutrients, it also comes laden with undesirable pollutants. Depending on the method used to make biofuels, these contaminants remain a headache, and may need to be landfilled or burned.

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A paper published last year found that some PFAS forever chemicals are not completely eliminated during HTL conversion, suggesting that “other treatment approaches for PFAS removal from sewage sludge need to be identified.” READ MORE