Manure No More: Study Finds ‘Aged’ Urine Can Safely Be Used As Fertilizer

ANN ARBOR, Mich. — Certain things only get better with time — a fine wine, certain types of cheese, or works of art are just a few immediate examples that come to mind. Now, a new study is adding urine to that list, at least from a fertilizer perspective. Researchers at the University of Michigan have concluded that recycled and “aged” human urine can safely be used as fertilizer, with minimal risk of transferring antibiotic resistant DNA to the environment.

These findings may seem bizarre at first consideration, but scientists all over the world have been hard at work trying to formulate more sustainable fertilizer options. Most widely used modern fertilizers result in water pollution. The large quantities of nitrogen and phosphorus present in these artificial fertilizers cause algae growth, which in turn threatens drinking water sources. Furthermore, manufacturing these modern fertilizers is costly and requires lots of energy.

Urine, on the other hand, naturally contains the nutrients (nitrogen, phosphorus and potassium) that plants need to grow. While urine has actually been used as fertilizer for thousands of year, it is rarely used in contemporary times in developed nations to grow crops over concerns of bacteria, viruses, and pharmaceuticals being present in urine and making their way into crops.

The University of Michigan has taken charge of the largest consortium of researchers working on ways to develop urine that can be safely used as fertilizer, as well as exploring if the general public would even be open to such an idea.

This new set of research has found that the practice of “aging” urine in sealed containers for several months at a time effectively deactivates 99% of any antibiotic-resistant genes that were present in bacteria within the urine.

“Based on our results, we think that microorganisms in the urine break down the extracellular DNA in the urine very quickly,” says corresponding author Krista Wigginton, U-M associate professor of civil and environmental engineering, in a release.

“That means that if bacteria in the collected urine are resistant to antibiotics and the bacteria die, as they do when they are stored in urine, the released DNA won’t pose a risk of transferring resistance to bacteria in the environment when the fertilizer is applied,” she adds.

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Urine was collected from over 100 men and women, then stored for 12-16 months. Over that long time period, ammonia levels in the urine increased, causing a decrease in the urine’s acidity and killing nearly all bacteria in the samples. Once the bacteria expired, any DNA that was residing within the bacteria entered the urine. Researchers observed to see how long it took that DNA to break down, signaling the urine would be perfectly safe to use as fertilizer.

“There are two main reasons we think urine fertilizer is the way of the future,” Wigginton says. “Our current agricultural system is not sustainable, and the way we address nutrients in our wastewater can be much more efficient.”

“We are doing field experiments to assess technologies that process urine into a safe and sustainable fertilizer for food crops and other plants, like flowers. So far, our experimental results are quite promising,” concludes Nancy Love, the Borchardt and Glysson Collegiate Professor and professor of civil and environmental engineering at U-M.

The study is published in Environmental Science and Technology.

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