Recycling our poop to grow food more sustainably

Over the course of a year, the average human produces rather a lot of bodily waste. Around 500 kilograms (more than 1,110 pounds) of urine and 50 kilograms of feces, to be more precise. Multiplied by almost 8 billion humans, that collectively translates to entire mountain ranges of poop and rivers of pee. Yet the vast majority of it is discarded.

A couple of projects around the world are now trying to change that.

One such project is just outside Paris, where researchers at the Laboratoire Eau Environnement et Systemes Urbains (Leesu) are seeing positive results from urine-based fertilizer trials on wheat crops. The yield, farmers have found, is equivalent to the harvest produced using synthetic fertilizers, which are either phosphate-based or made using natural gas — a polluting fossil fuel.

And besides having a lower carbon footprint, waste-based fertilizers provide organic matter that improves soil.

Yet no matter how good our processed flushables might be for the environment, eating food produced with the help of human feces or even urine can be a hard sell. Are we being overly sensitive?

Using our poop for agriculture is not new

Ancient civilizations knew the truth about bodily waste. They understood that the nutrients in our urine and our excreta — nitrogen, phosphorus and potassium — helped to make plants grow.

In her book "The Other Dark Matter," science journalist Lina Zeldovich writes about life in Japan in the 1600-1800s, when people traded in shimogoe or "night soil," that was processed into manure to be spread on less fertile rocky terrain.

In China, it cost more to buy waste produced by richer people on the grounds that a more nutrient-rich diet would result in a better product, while in Mesoamerica, the floating chinampa gardens of the Aztec age were heavily replenished with poop.

But in the 19th century, things started to change. Zeldovich said modern sewers and synthetic fertilizers disrupted nature's cyclical design.

"When we started farming and living in cities, we created this really interesting problem," she said. "We grow food in certain places, then we transport and consume that food in other places."

Nutrients are therefore not being returned to local fields, but end up in wastewater treatment plants or in nearby bodies of water.

And that can be bad news if nutrients end up boosting algae blooms in our lakes and rivers, threatening fish and other water flora. It's a huge problem in mainland US, for example, where about 65% of all estuaries and coastal waters in the lower 48 states are degraded by nutrients like nitrogen, flowing from poor septic systems and stemming from fertilizer running off farmlands.

Where is human waste being reused?

The trials outside Paris are not the only example of our waste being put to good use. Similar initiatives — including those with a poop focus — are in operation from North America to Africa.

The Rich Earth Institute, a research organization in the US state of Vermont, runs a community program that saw 180 people donate their urine for farming in 2021.

In Kenya, the startup Sanivation is taking things a step further by making cooking and industrial fuel out of solid waste, providing an alternative to charcoal made from felled trees. They have clients in the manufacturing, milk processing and textile industries.

The company says it has sold 2,000 tons of its burnable poop pellets since 2018.

Similarly, the Swedish company Sanitation 360 has developed a process to turn urine into pellets in an effort to create a circular economy around human waste.

But Colin McFarlane, a professor of urban sanitation at Durham University in the United Kingdom, said the infrastructure to handle high volumes of waste is lacking.

"We still haven't come close to embracing the possibilities of seeing human waste as a resource," he said. It would help to embrace all possible solutions to recycling and managing our waste, he added.

Poop's got an image problem

And there's also the issue of acceptance. Research suggests there are both cultural and psychological barriers standing in the way of wider bodily waste recycling.

In Ghana, for example, fecophobia — a fear of solid human waste, particularly in its untreated human form — is commonplace, and many perceive growing food with it as unhygienic. Though one study suggested once people understand that feces-based fertilizer is treated and processed, the negative perception is significantly lower.

Health concerns over waste-based fertilizer are also a barrier. Researchers find that people perceive bodily waste as being unhealthy, pointing out that feces in particular contains harmful pathogens. Studies have found that when poop fertilizer isn't properly treated, people could ingest harmful worms.

Yet Jojo Casanova-Linder, co-founder of Swiss compost toilet business Kompotoi, describes poop processing in a straightforward manner. The company exposes solids to high heat that kills off bacteria and in about 12 weeks, compost forms. Liquids, on the other hand, are distilled into concentrates that can be diluted with water on application.

Casanova-Linder predicts it will take some time for the process to become mainstream. His company has so far sold around 300 toilet units, mainly in Switzerland. But perhaps timing shouldn't be the focus, he added.

"The question is how long we can afford to [poop] in clean drinking water and not recover the resources."

Edited by: Tamsin Walker