Liquid gold: could we turn our pee into fertiliser?

Human urine has long been a valuable resource, containing nutrients that support plant growth. Ancient civilizations utilized urine as fertilizer alongside feces, and even today, smallholder farmers in areas where synthetic fertilizers are unaffordable or unsustainable incorporate urine into their farming practices. As synthetic fertilizers have largely replaced natural ones like urine, the reliance on these industrial products has surged, with half the world's population now depending on them for their food supply.

However, synthetic fertilizers can have negative environmental impacts, such as contributing to toxic algal blooms and depleting finite resources like phosphate rock, which is mined in limited locations.

With the ongoing Iran war driving up the costs of synthetic fertilizers, the potential for urine-based fertilizers to make a comeback is emerging. Known as "peecycling," this practice involves collecting and using human urine and feces as fertilizers. Historically, civilizations such as Greece, China, and Rome also used these resources for agriculture. While the modern sewage system has improved sanitation and reduced infectious diseases, it has also shifted the perception of pee and poo from valuable resources to waste.

Throughout history, humans have produced significant amounts of urine, which contains high levels of nitrogen, phosphorus, and potassium – three essential nutrients for plant growth. In ancient times, people stored their waste in cesspits or used chamber pots in cities, and during times of high population density, waste collection workers would gather human waste for agricultural use.

The 19th century saw modern central sewage systems improve sanitation and reduce the spread of infectious diseases, but this also led to a change in attitude towards urine and feces, treating them as waste rather than resources.

During the 19th century, nations vied to access guano, the nutrient-rich droppings of seabirds, as a new source of fertilizer. However, this resource was limited and exhausted over time. With the invention of synthetic fertilizers, crop yields increased, but these products also created new social and environmental issues due to their energy-intensive production and reliance on fossil fuels.

The Haber-Bosch process, used to manufacture synthetic nitrogen fertilizers, is particularly problematic, as it consumes vast amounts of energy and relies on natural gas, a major contributor to greenhouse gas emissions.

Synthetic phosphorus fertilizers are produced by treating phosphate rock, a finite resource mined in a few places, including Western Sahara, a disputed territory occupied by Morocco. As the world grapples with escalating global shocks and the vulnerability of relying on global markets for fertilizers, exploring sustainable local sources of nutrients becomes increasingly attractive.

Urine, which represents less than 1% of all wastewater, contains most of its nitrogen and half of its phosphorus, making it a potentially valuable resource for fertilization. However, incorporating urine into fertilization practices would require separate plumbing in homes, public facilities, and at events to collect and use this resource efficiently.

While the idea of using urine as fertilizer may appear unconventional, it offers a promising solution to the challenges of expensive synthetic fertilizers and the environmental consequences of their production. By adopting urine-based fertilizers, communities could reduce their dependence on expensive external sources and contribute to a more sustainable and environmentally friendly agricultural system.