Electrochemical prototype recovers phosphorus from wastewater

July 10, 2023
A pilot-scale prototype electrochemical device was able to leach phosphorus from digestate with over 93% efficiency, precipitating approximately 99% of that leachate into solid form.

New research has developed an effective electrochemical phosphorus recovery cell that is capable of recovering phosphorus from wastewater digestate without added chemicals, according to a press release by Washington University in St. Louis.

Phosphorus, which today comes mostly from nonrenewable reserves of phosphate rock, typically winds up in municipal waste streams. In the best cases, wastewater treatment plants sequester about 90% of that phosphorus in “sludge” and decompose that sludge into something known as digestate. Engineers hope to establish a more sustainable cycle for reusing phosphorus, but toxic compounds in digestate limit the possibility of recycling it as fertilizer — it’s hard to recover phosphorus from solid waste like digestate.

“Existing phosphorus recovery technologies mostly target the liquid,” said Zhen He, a professor in the Department of Energy, Environmental & Chemical Engineering at the McKelvey School of Engineering at Washington University in St. Louis. “Sludge contains a lot more phosphorus compared to the liquid. How can we recover that?”

He’s lab develops technologies to recapture resources like phosphorus from wastewater.

The team, which also included doctoral candidate Zixuan Wang and undergraduate Daran Anand, published details of their experiment online June 26 in Environmental Science & Technology.

Recently, the team explored using electrochemical devices to split water molecules and pull solid, usable phosphorus out from digestate. Through this process, an electrical current flowing through the anode side of an electrode can acidify a slurry surrounding it.

The idea was to have two chambers that operate at different pH. Sludge enters the anode chamber, in which current separates phosphate into liquid solution. The team transferred the phosphorus-rich liquid to the less acidic cathode chamber. And here, free-floating phosphates, calcium and iron from the original sludge should combine into whitish solids that precipitate to the bottom.

To test their prototype, He’s team obtained real digestate from the Missouri River Wastewater Treatment Plant in St. Louis. After tinkering with the electrical current, they identified conditions where the device leached phosphorus with over 93% efficiency and precipitated approximately 99% of it into solid form. Over 42 cycles, the device averaged nearly 80% efficiency in recovering phosphorus.

The harvestable end-products have a phosphorus content on par with high-grade phosphate rock. “You can either apply directly, or you can use them as a feedstock to produce fertilizer,” He said. The solids could feasibly feed directly into existing industrial operations designed to turn phosphate rock into fertilizer.

Now, He imagines taking this beyond the lab bench.

“Going from here to pilot scale to the full scale, that is going to involve a lot of engineering,” He said, adding that energy consumption will likely be the biggest hurdle.

Still, He is optimistic enough that he expects more applications than just treating human waste, thanks to a resource that’s readily available.