By Abbas Nazil
Charcoal made from human waste could help address global fertiliser shortages, reduce agricultural pollution, and cut energy use, according to a new study published in the journal PNAS.
Biochar is a form of charcoal produced by heating organic matter at high temperatures, often applied to farmland to enhance soil fertility.
It also acts as a carbon sink, locking away carbon that would otherwise enter the atmosphere.
Researchers estimate that biochar derived from solid human excrement could supply up to 7% of the world’s annual phosphorus needs.
If nutrients from urine are also recovered and added, the process could meet 15% of yearly phosphorus demand, 17% of nitrogen, and as much as 25% of potassium required by crops.
Unlike treated sewage sludge, which is already used on farmland but often contains contaminants like microplastics, heavy metals, and persistent chemicals, biochar production can avoid these risks by separating waste at the source.
The study found that the process can reduce the weight and volume of solid waste by as much as 90%, making transport far more efficient compared with conventional sewage sludge, which is heavy due to high water content.
Biochar production also allows for precise adjustment of nutrient content to match the needs of different crops.
This can help reduce problems such as weed growth and eutrophication — a process in which excess nutrients enter waterways, triggering algal blooms that deplete oxygen and block sunlight, harming aquatic ecosystems.
Lead author Dr Johannes Lehmann of Cornell University emphasised that while sewage may be an unglamorous topic, recovering resources from it supports a circular economy and is essential for the green transition.
Agriculture currently accounts for about a quarter of global greenhouse gas emissions, according to the Intergovernmental Panel on Climate Change.
As global food demand grows, so does the use of synthetic fertilisers, which rely on energy-intensive and environmentally harmful production processes.
Nitrogen fertilisers are made through the Haber process, which captures nitrogen from the air to produce ammonia.
This alone emits around 2.6 billion tonnes of CO2 annually, exceeding the combined emissions from global aviation and shipping.
Phosphorus is obtained by strip-mining phosphate rock, a process that permanently damages landscapes and produces radioactive byproducts, while potassium extraction through potash mining generates waste salts that can degrade soils and contaminate freshwater.
Lehmann noted that reliance on finite mineral resources can create geopolitical vulnerabilities, as countries without reserves become dependent on major suppliers like Morocco, which holds about 70% of global phosphate reserves.
By recycling nutrients through biochar, nations could strengthen food security, reduce dependence on imports, and address environmental justice issues in the global south.
Such changes could even mitigate climate migration, which is often driven by agricultural collapse.
