A new way to get P from pee

The world’s food system is heavily reliant on fertilizers rich in phosphorus, which is mostly mined from phosphorus deposits in Morocco and Western Sahara. Experts are concerned, however, that these once-vast mines could be tapped out within decades.

Urine and feces are prevalent sources of phosphorus, and for years researchers have tried to identify a cost-effective way to extract the element from this otherwise unwanted waste. Now, researchers show they can extract phosphorus from synthetic urine with a simple electrochemical approach instead of with techniques that rely on costly chemical additives. In their new paper, the authors say that they ultimately hope to scale up their concept and test it with real urine in order to tackle the phosphorus (ACS ES&T Water 2020 DOI: 10.1021/acsestwater.0c00065).

“Estimates vary, but we will run out of phosphorus one day if we don’t start recycling it at scale,” says Surendra K. Pradhan, a researcher at the University of Eastern Finland who works on phosphorus recycling but wasn’t involved with the study. “This is a new technique that could help to solve these problems.”

Traditional techniques for isolating phosphorus rely on introducing other chemicals to the mix, such as magnesium, which precipitate a phosphorous-containing mineral called struvite from the waste. However, the cost of the chemical additives often outweighs the value of the extracted phosphorus.

To test their new electrochemical system, Deli Wu of Tongji University and colleagues pumped synthetic urine containing negatively charged phosphorus and chloride ions into an anode chamber, which is separated from another chamber by an anion exchange membrane. When the researchers applied a 10 A/m² current density to the urine, the phosphorus ions reacted to form neutral H₃PO₄ compounds. Through trial and error, the scientists found the optimum conditions including current strength and charging duration to ensure that the phosphorous reacted but the chloride ions remained negatively charged.

The researchers then reversed the electrode polarity with a current density of -15 A/m², driving the chloride ions across the membrane into the other chamber. The H₃PO₄ stays in the anode chamber, forming a phosphorus-rich solution directly suited for fertilizer production.

Although the new system has the potential to be cheaper than previous methods of isolating phosphorus, some experts doubt it could be easily integrated with existing wastewater treatment infrastructure. To do so, it would need to work on sludge, explains chemist Markus Antonietti, director of the Max Planck Institute of Colloids and Interfaces. “This technique is only good for urine, and sludge is a solid.”

Kurt Möller, a chemist at the University of Hohenheim, agrees. “For successful implementation, there needs to be a strong separation of urine and feces streams in our wastewater systems, and that would be very challenging,” he says.

The process also needs to work on the more varied chemistry of real urine. Assuming this can be done, the technique could be deployed at urinals in public spaces instead of centralized wastewater treatment plants, Antonietti says. That’s one way to get around the sludge-solid problem.