Biochar made from human excrement can ‘alleviate’ global fertiliser challenges

US RESEARCHERS think they have the answer to developing low-carbon crop fertiliser – and it stinks. Literally. Their secret weapon? Biochar made from human poo.

A team lead by Johannes Lehmann, a professor of soil biochemistry at Cornell University, has developed a type of biochar – a charcoal-like residue – that locks in key nutrients like phosphorus, nitrogen and potassium, offering performance on par with synthetic fertilisers.

Agriculture accounts for 25% of global greenhouse gas emissions, and fertiliser is reported to be responsible for 720m t/y of CO2 emissions. Fertiliser use brings a surge of toxic byproducts, from nitrous oxide (N₂O) emissions to the environmental toll of natural gas. In response, the industry is exploring alternative bio-based and synthetic options.

Circular Approach

Treated sewage is already used as a fertiliser in agriculture but has been found to contain microplastics and PFAS, which can further contaminate soil. The Cornell researchers advocate for a “circular bio-nutrient economy” (CBE), where fertilisers are produced directly from organic waste – sidestepping the need for hazardous chemicals by reclaiming nutrients at source.

Pyrolysis – a well-established method for producing bio-based fertilisers – can operate under “moderate” temperatures, making it both practical and energy efficient. The Cornell team used the process to concentrate nutrients in solid human waste, finding that biochar enriched with urine could supply up to 15% of crops’ annual phosphorus needs, 17% of nitrogen and 25% of potassium.

The study states: “Organic pollutants can be removed by the pyrolysis process itself, whereas heavy metals remain in biochar.”

It continues: “Biochars with recovered nonvolatilizable nutrients (such as P, K, Ca, etc.) can be pyrolyzed again for removal of pathogens and pharmaceuticals while retaining the nutrients.”

Industry incentives

The study also found that drying solid excrement could reduce its weight by up to 90% compared to treated sewage, making transport significantly more efficient and cost-effective.

Lehmann said: “The implications [of biochar resource recovery] go beyond just agriculture, and involve economics and geopolitics. As finite mineral resources become scarcer, countries without significant reserves could become dependent on those with for their agricultural needs and food security. For example, Morocco holds 70% of the entire world’s reserves of phosphates.”

The Cornell researchers hope their innovation will promote nutrient circularity in agriculture and offer a pathway for developing carbon markets that incentivise farmers and food producers to shift from typically cheaper synthetic fertilisers to biochar-based alternatives.

The study states: “Technologies offering CDR typically garner greater prices in the voluntary carbon market than emission reductions would on their own.”

It continues: “Introducing biochar into the fertilizer market using CDR financing mechanisms may provide a suitable testbed for filling the CDR gap to limit warming to 1.5 °C.”