Globally, human excreta are a huge untapped source of nutrients. Now, a new perspective paper makes the case that biochar could help to unlock it.
The PNAS paper shows that human waste contains over 20 billion kilograms of nitrogen, almost 3 billion kilograms of phosphorus, and 6 billion kilograms of potassium annually. Urine alone contains the equivalent to 17% of global annual nitrogen needs, and up to 25% of potassium demand as well.
Extracting these valuable agricultural nutrients from waste products could help to build a powerful circular nutrient bioeconomy, the researchers believe: food becomes waste, which helps to grow food again, instead of being discarded and lost.
But urine especially reveals the challenge of unlocking this incredible resource: getting to the good stuff means finding a way to deal with liquid — a lot of it. This is where the biochar comes in, a dry carbon-rich product made by heating organic matter at very high temperatures in a deoxygenated environment, using pyrolysis.
Under the high heat of pyrolysis, large amounts of moisture gets burned off and so this process could decrease the weight of human waste by 85 to 90%. It can also decrease its volume by up to 90%, the researchers say. Together, this would make the nutrients easier to store and less costly to transport.
Recovery rates are also impressive, according to the perspective, which notes that it’s possible to extract between 96 and 100% of nutrients like phosphorus and potassium from waste. Meanwhile, the pyrolysis process can effectively remove contaminants such as pathogens and pharmaceutical residues from the waste—the only exception being heavy metals.
“Any amendment to soil has to be scrutinized for pollutants, including pathogens, PFAS, heavy metals [like cadmium],” says Johannes Lehmann, a professor at the School of Integrative Plant Science, Soil and Crop Sciences at Cornell University, and lead author on the perspective. Lehmann adds, “Actually cadmium contamination in mined commercial fertilizer is higher than in biochar from excreta.”
Lehmann says that the process of making biochar is not an energy-intensive process. “Pyrolysis is an exothermic process, it generates energy, it does not consume energy…you may need a match to light your fire, but once it is lit, it releases energy, and you feel that as heat.” So it holds the potential to produce fertilizer nutrients with a lower footprint compared to traditional methods.
Over 20 countries have more nutrients available in human excreta than they currently use as mineral fertilizer: this shows the scale of nutrients that are available to be tapped, which could also relieve pressure on other reserves, for instance of phosphorus of which there are declining reserves.
Conventional fertilizer is an unevenly distributed resource around the world, so producing agricultural nutrients from human waste is a way to make this resource ubiquitous and evenly dispersed. A more localised, circular waste bioeconomy also insulates agriculture from global shocks like war and trade that can disrupt supply chains of conventional fertilizer.
Converting human waste to feed for crops still has major challenges to overcome, not least of which is public resistance to the idea. “Taboos are a hurdle not easily overcome,” says Lehmann. But it could be possible with supportive regulation and methods to bring down the cost of bringing biochar to market.
Lehmann hopes that with the help of research perspectives like his, ultimately waste-gathering becomes integrated into society, whether via waste-recycling infrastructure in homes, municipal waste water treatment plants, or even waste-gathering toilets at music festivals.
“I hope that it becomes common practice to recycle nutrient-rich residues, especially human excreta, as a fertilizer for crops.”
Source: Lehmann et. al. “Biochar in the circular bionutrient economy.” PNAS. 2025.
Image:©Anthropocene Magazine
