Astronauts on a mission to Mars or beyond may be able to survive on plants watered with their own urine. Our liquid waste is 95 per cent water. The other five per cent is composed of nutrients, such as nitrogen, potassium, and phosphorous, which may pose harm to humans over the long term — but not to plants.
Using computer-generated test crops of dwarf wheat and soybeans, a team led by at the University of Sydney in Australia simulated how these plants take up nutrients from human urine. They simulated plants growing in natural soil – as opposed to artificial – in an isolated chamber with its own ventilation system.
This “cropping unit” would have both urine and water injection systems. The team analysed different injection modes – both periodic and continuous – of untreated, unfiltered urine to determine how they would affect the plants’ growth.
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Over a simulated 20 years, urine “largely met” the plants’ nutritional demands without high levels of harmful byproducts or emissions, such as carbon dioxide or ammonia. It’s not just the urine that makes this system work – soil is key. Thanks to long-lived microbes, soil provides what Maggi calls “forgiveness of neglect”, which is an ability to adapt to a surrounding environment in a way that hydroponic and aeroponic systems may not be able to.
Faecal add-on
A twenty-year mission is longer than any human off-planet endeavor is likely to be in the near future, but no matter when a deep space mission happens, the crew will have to be somewhat self-sufficient. They’ll have to use everything on board to get by – even their own urine.
“If you’re trying to operate independently and grow food, plants need fertiliser. And the only fertiliser that would be available would be [human] wastes,” says at the Rich Earth Institute in Guilford, Vermont.
Because these elements aren’t destroyed between eating them and passing them, they could indeed be recycled indefinitely. However, Noe-Hays notes that urine may not provide enough nutrients because the bulk are expelled in faeces. While urine does have the necessary nutrients to support this kind of regenerative system, there simply wouldn’t be enough of them.
“Because urine contains most, but not all, of the nutrients, you wouldn’t be able to maintain a plant agroecosystem indefinitely with only recycling the urine. You’d have to supplement that with nutrients to make up the difference,” Noe-Hayes says. He concludes that astronauts would likely have to get comfortable with recycling their own faeces as well, which comes with bacteria risks that urine doesn’t have.
There are a few factors this study doesn’t take into account, namely the challenges of growing plants in microgravity. Soil consistency matters much more in microgravity, as astronauts have learned– roots don’t grow downward and water and nutrients float. However, an experiment aboard the ISS has helped mitigate some of these problems with “plant pillows” full of soil, nutrients, and water. Perhaps in the future, astronauts can try using urine to nourish these plants.
Life Science in Space Research
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