Fertiliser can be made from human urine in just a few simple steps

A more efficient way to extract fertiliser from human urine could help make better use of wastewater in cities and farms, without exacerbating global warming.

Human urine is rich in compounds useful for growing crops, such as nitrogen, but chemical processes to extract these compounds are less efficient than industrial methods of making fertiliser, such as the Haber-Bosch process, which converts nitrogen in the air to ammonia by adding hydrogen. However, these methods are often energy intensive and produce climate pollutants.

Now, Xinjian Shi at Henan University in Kaifeng, China, and his colleagues have found that adding oxygen from air and a graphite catalyst to urine produces a nitrogen-rich chemical called percarbamide. The process only requires a few steps and produces no waste products.

“Prior to our method, the traditional method of separating [the nitrogen-rich compound] urea from urine was to concentrate urine to precipitate urea and inorganic salts, and then purify urea by exploiting the differences in solubility,” says Shi. “This process is cumbersome and the resulting purity is low.”

Shi and his team placed thin sheets of graphite, which had been modified to have defects in the way their atoms were joined, on an electrode. This was then put in a concentrated urea-rich solution.

When they passed electricity through the solution, solid crystals of percarbamide formed, made from oxygen in the air, hydrogen from water and urea from the urine. These crystals could then be easily separated from the liquid solution.

Next, the team tested how percarbamide works as a fertiliser and found it helped wheat, peanut and lettuce plants grow taller than when they were grown with just water or normal urea fertiliser. Shi says this may be because percarbamide can steadily release oxygen and helps control nitrogen levels in soil.

James McGregor at the University of Sheffield, UK, says the approach could be used for other chemical processes, but it might be difficult to scale it up to city-wide wastewater treatment.

“I would be surprised if we were sitting here in 10 years’ time and this was a major industrial process, but it potentially has applications for decentralised, local, small-scale production, probably particularly in agricultural settings,” he says.