Most of us don’t think too much about what’s in our pee. We go, we flush, we forget. But that wastes a golden opportunity, some scientists say. They’re finding new — and quite valuable — uses for the chemicals in urine.

A recent study shows that some yeast can turn ingredients from urine into hydroxyapatite (Hy-DROX-ee-AP-eh-tyte). This calcium-based mineral makes bone hard.

Surgeons and dentists sometimes use it to repair bones and teeth. It also could help make sturdy building materials. And it’s worth more than past pee-based products.

“We’re taking a waste product and turning it into something high-value,” says Jeremy Guest.

He hopes his team’s findings may inspire cities to “manage waste in a new way.” Guest is an environmental engineer at the University of Illinois Urbana–Champaign. His team shared its new work May 6 in Nature Communications.

Turning tinkle into treasure

Cities today view pee as simply a problem to be solved. Sewage-treatment plants filter germs from water to stop the spread of disease. This cleaning also strips out elements in pee that could harm the environment. Nitrogen is one such contaminant. So is phosphorus. 

But those elements aren’t necessarily bad, says Treavor Boyer. Most “contaminants are just chemicals in the wrong place at the wrong time.” Boyer is an environmental engineer at Arizona State University in Tempe. He did not take part in the new research. But he does study how we might recover resources from wastewater.

The nitrogen and phosphorus from urine can be used to make fertilizer, Boyer says. Urine’s breakdown also produces ammonia and urea. Many companies use these chemicals to make their products.

Changing how society views urine won’t be easy, though. Fertilizer, for instance, is not very costly. It’s hard to get nutrients from wastewater “and have the sale of the nutrient products [repay the costs of doing that],” explains Guest.

That’s why his team turned to hydroxyapatite. It’s made from the same stuff as fertilizer, but sells for more. That might motivate cities to revamp their water processing to harvest the mineral.

Tiny hydroxyapatite factories

Inspiration struck when a scientist in Peter Ercius’ lab noticed something strange. Ercius works at the Lawrence Berkeley National Laboratory (LBNL) in California. There, he looks at how materials change in liquids, such as inside living cells. To do that, he uses an electron microscope.

His team had been looking at a yeast called Saccharomyces boulardii. “Yeast has been used for thousands of years,” notes Ercius. “It’s kind of been almost domesticated by humans.” People use it to make bread, for instance. “We know how to grow yeast, how to keep them alive, how to keep them happy.”

But his team wanted to know more about how this yeast works.

One day, a scientist in the lab spotted what looked like odd mineral buildup inside the yeast cells. “It’s probably not actually inside [the cells],” Ercius recalls thinking. But it was.

His team imaged the yeast from different angles. To work out what was going on, they also used colored dyes to track how materials moved in and out of the yeast.

They found the yeast were taking up minerals from the liquid around them.

Yeast can absorb and release metal ions, says Behzad Rad. A biophysicist at LBNL, he worked on the yeast project. The yeast were cramming those materials into vacuoles — storage tanks inside their cells. Inside the vacuoles, the yeast crafted different chemical ingredients. When released from the cell, these chemicals crystallized into hydroxyapatite.

This process mimics how human osteoblasts, a type of bone cell, create this material. In honor of that, the scientists named their yeast cells “osteoyeast.”

Kai Udert is a systems engineer at the Swiss Federal Institute of Aquatic Science and Technology. It’s in Dübendorf. Though he did not take part in the new work, he does study how cities might recycle urine. Bacteria in toilets make hydroxyapatite from urine too, he notes. But the yeast’s process yields more of this mineral than bacteria do. And unlike bacteria, it doesn’t produce a bunch of other unwanted minerals on the side.

The yeast produced about 1 gram of the mineral from each liter (quart) of urine. Urine doesn’t naturally contain enough calcium to support this process, so the researchers added a bit extra.

Flush with possibilities

Guest and his coworkers calculated how much their hydroxyapatite could sell for. Their estimate: between $19 and $138 per cubic meter (roughly 264 gallons) of urine. That’s about the volume of a refrigerator. The same pool of pee makes only about $12 to $33 worth of fertilizer. So cities could get a bigger profit from turning pee into hydroxyapatite.

Producing this product would require separating urine from other wastes in the toilet. The collected pee would go through a few treatment stages. Guest imagines that the urine and yeast might enter a stainless-steel tank. There, the yeast would use minerals from the pee to build hydroxyapatite.

That mineral could then be drawn from the tank. Everything else would go on to be treated like normal sewage. A processing center would sterilize — remove any germs from — the hydroxyapatite, Guest says. The purified mineral could then be turned into something useful.

Udert thinks these yeasts could have more outlandish — even out of this world — applications. On the International Space Station, for instance, astronauts already use a toilet that splits pee from poo. Wastewater is then recycled to make clean drinking water. Currently, a harsh chemical process achieves this. Perhaps, says Udert, the yeast could offer a safe, greener way to break down astronaut pee.