Human waste could help tackle a global shortage of fertiliser

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Tests on cabbage plants suggest fertilisers derived from human urine and faeces are safe and could help bring down food prices

Environment 19 January 2023

By Brian Owens

Two people spreading fertiliser in rows of cabbage plants

Fertilisers derived from recycled human urine and faeces are just as safe and effective as conventional ones, according to tests on cabbage plants. Using human waste in this way could help alleviate the fertiliser shortage that is contributing to rising food prices – if people can be convinced to use them.

Nitrogen-based fertilisers are manufactured in an energy-intensive process using natural gas as a raw material. Human waste can be a good source of plant nutrients like nitrogen and phosphorus, but can also carry disease-causing pathogens and parasites, so needs to be carefully treated to make it safe. It is still used – sometimes untreated – as a fertiliser in some low-income countries, but has been largely abandoned in high-income nations.

Franziska Häfner at Agroscope in Zurich, Switzerland, and her colleagues compared cabbages grown using organic fertiliser derived from vinasse, a by-product of ethanol production, with fertilisers made from treated human urine and faeces.

The yield for cabbages grown with nitrified urine fertilisers (NUFs) was comparable to those grown with vinasse. Cabbages grown with faecal compost, or compost and NUFs together, had lower yields, but this fertiliser may increase soil carbon content in the long term, the study found.

The researchers also tested for more than 300 chemicals in the faecal compost, including pharmaceuticals, flame retardants and insect repellents. Just 6.5 per cent of these were detected, all at very low concentrations. Of the 11 pharmaceuticals detected in the compost, just two were found in the edible parts of the cabbage: the painkiller ibuprofen and the anticonvulsant and mood-stabilising drug carbamazepine. But the concentration of the latter was so low you would need to eat half a million cabbages to get a single dose.

“The products derived from recycling human urine and feces are viable and safe nitrogen fertilizers for cabbage cultivation,” Häfner said in a statement. “They gave similar yields as a conventional fertilizer product, and did not show any risk regarding transmission of pathogens or pharmaceuticals.”

The researchers estimate that, if correctly prepared and quality controlled, up to 25 per cent of conventional synthetic mineral fertilisers in Germany could be replaced by ones recycled from human urine and faeces. In some places, that trend is already under way. One of the NUFs they tested, called Aurin, has already been approved for agricultural use in Austria, Switzerland and Liechtenstein.

Benjamin Wilde at ETH Zurich obtained similar results in yield and safety when he tested NUFs in field trials in South Africa. But getting people to use them can take some convincing. The Zulu farmers he worked with, like those from many cultures, have strong social taboos around human waste. However, long discussions about the process of making such fertiliser and a field trip to where this happens helped them overcome those. “Farmers are very practical people once they see that something works,” he says, although the farmers pointed out they might have a harder time convincing their customers.

If people can be convinced to overcome their squeamishness, fertilisers from recycled human excreta might make a serious dent in the fertiliser shortage. There are billions of people in the world and that is a lot of available nitrogen, says Wilde.