Charcoal made from human waste could help tackle global fertiliser shortages, cut pollution, and reduce the energy demands of agriculture, according to new research.
The study, published in the journal PNAS by scientists at Cornell University, examined the potential of using biochar, a form of charcoal made from organic matter treated at high heat, as a nutrient-rich soil additive.
Biochar is already used to improve soil health and lock carbon away in the ground. The new research suggests that biochar made from solid human waste could supply up to 7% of the world’s phosphorus needs each year. If nutrients from urine were also captured and added, that figure could rise to 15% for phosphorus, 17% for nitrogen, and 25% for potassium.
Lead author Dr Johannes Lehmann, professor of soil biogeochemistry, said: “Talking about sewage is not as glamorous as renewable energy, but preventing resource wastage by creating a circular economy is also key to the green transition.”
While treated sewage sludge is already spread on farmland, it often contains microplastics, heavy metals, PFAS 'forever chemicals', pathogens, and pharmaceuticals. By separating waste at source and using pyrolysis to convert solids into biochar, most of these contaminants, besides heavy metals, can be removed.
The process also slashes the weight and volume of human waste by up to 90%, making it far easier and cheaper to transport compared with wet sewage sludge. Nutrient proportions in the resulting biochar can be adjusted to match crop needs, reducing the risk of fertiliser overuse that can lead to weed growth or eutrophication.
The production of synthetic fertilisers is highly energy-intensive and environmentally damaging: nitrogen fertilisers made via the Haber process emit an estimated 2.6 billion tonnes of CO₂ a year, more than the combined emissions of global aviation and shipping. Phosphate mining impacts landscapes and produces radioactive byproducts, while potash extraction generates waste salts that can contaminate water and soil.
The researchers argue that recovering nutrients from human waste could reduce dependence on mined fertilisers, especially for countries that lack domestic reserves. Morocco alone holds 70% of the world’s phosphate reserves, creating a potential geopolitical vulnerability for others.
“An alternative future where nutrients are recycled through a circular economy could empower countries to produce food without relying on imported fertilisers,” said Dr Lehmann. “It could also help address environmental justice, particularly in the global south, where agricultural failure drives climate migration.”
The team believes biochar-based fertilisers could become cost-competitive if environmental costs are factored into pricing, for example through carbon credits. They recommend public–private partnerships to develop safe, marketable products that incentivise investment and close the nutrient loop.
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