Second Life for Phosphorus

Phosphorus, a nutrient required for growing crops, finds its way from farm fields to our food and eventually to our wastewater treatment plants. At the plants, the nutrient causes major problems, building up in pipes or going on to pollute surface waters.

Brushite bounty: Phil Barak displays brushite produced during trials at the Nine Springs Wastewater Treatment Plant of the Madison Metropolitan Sewerage District. Each jar contains brushite harvested from 30 gallons of anaerobic digest. Photo courtesy of Phil Barak

Brushite bounty: Phil Barak displays brushite produced during trials at the Nine Springs Wastewater Treatment Plant of the Madison Metropolitan Sewerage District. Each jar contains brushite harvested from 30 gallons of anaerobic digest.
Photo by Rick Wayne

But soil science professor Phil Barak has an idea about how to retrieve the nutrient from wastewater in a valuable form—and it started from a basic lab experiment. “I was doing some work on crystallizing phosphorus, just out of pure academic interest,” explains Barak. “That led me to crystallize a mineral called struvite. Then I realized it was forming in wastewater treatment plants as a nuisance.”

If he could form crystals in the lab, he reasoned, why couldn’t it be done in the wastewater treatment plants in a controlled way? It could. And, even better, if he collected the phosphorus early on in the treatment process in the form of a mineral called brushite, he could harvest even more of it.

Beyond removing phosphorus from wastewater, brushite can serve as a nutrient source for growers. While Barak will do further testing to prove its utility, brushite is a phosphate mineral that’s actually been found in agricultural fields for years.

“When conventional phosphorus fertilizers are added to soil, brushite forms. I maintain that we’ve been fertilizing with brushite for decades, but nobody’s been paying attention to it,” says Barak.

Being able to remove phosphorus from wastewater and supply it back to growers is a win-win situation, Barak notes. “We’re collecting phosphorus where it’s localized, at really high concentrations, which is the most economical place to collect it,” says Barak. “This works out in just about every dimension you can consider, from the treatment plants to the cost of recycling phosphorus as opposed to mining it new.”

Graduate students in Barak’s lab suggested that he commercialize the technology and start a company. After the Wisconsin Alumni Research Foundation (WARF) passed on the patent, Barak and his students sought help from the UW Law and Entrepreneurship Clinic. They received two federal Small Business Innovative Research grants, and, with some additional funds from the state, including the Wisconsin Economic Development Corporation, their efforts have turned into a spinoff company: Nutrient Recovery & Upcycling, LLC (NRU).

The company’s next step was a big one. This summer, a phosphorus recovery pilot plant is being implemented in a wastewater treatment plant in Illinois. The pilot project will test the research ideas on a larger scale.

Additionally, the NRU team will participate in the Milwaukee Metropolitan Sewerage District’s granting system to determine if a pilot project would be a good fit in Milwaukee. They hope to start collecting and analyzing data from Illinois by September, using that pilot system to lay the groundwork for others in Milwaukee and beyond.

Crystal Clear

Under a microscope, its crystals gleam like tiny gems. But when the phosphate mineral struvite starts clinging to the guts of a sewage treatment plant, it quickly loses its charm.
Fed by the copious phosphorus in wastewater, struvite crystals form by the billions, amassing in cement-like chunks that clog pipes and valves and block water flow. “It’s (like) hardening of the arteries, that’s for sure,” laughs Steve Reusser, operations engineer for Madison’s sewage treatment plant, who routinely has to chip blocks of struvite out of pipes. “It’s one (problem) that just won’t go away. We keep juggling things, but we haven’t come up with a great solution yet.”

CALS soil scientist Phil Barak thinks he may now have one-and it’s surprising for its counterintuitive logic: Why not grow the pesky mineral on purpose?
The point is to bring the natural crystal formation under control, Barak says. One of his graduate students, Merin Abraham, has done this by dosing a sewage mix called acid digestate with limestone. The rise in pH causes the sewage to shed more than two-thirds of its phosphorus as crystals of struvite and brushite, a related mineral. Because those minerals are denser than water, they settle out of solution on their own, making them easy to remove before they congeal into hardened masses.

Limestone is cheaper than the iron salts that treatment plants now use to try to keep struvite at bay. And the new technology may have other benefits, such as reducing the amount of phosphorous in leftover bio-solids, which are often used as fertilizer. Reusser is working with the researchers to set up a pilot project at the Madison plant in the next year.

For Barak-who first encountered struvite 25 years ago, while a medic in the Israeli army-the project adds an unexpected chapter to his long fascination with the mineral. His first experience with it came while treating a tough sergeant who was reduced to tears by a bladder infection. When he examined the soldier’s urine under a microscope, Barak saw a collection of razor-sharp struvite crystals, which had formed due to the infection.

Years later, Barak read about a method for crystallizing minerals on a slick of fatty acid molecules, which scientists had used to grow a rare mineral known as vaterite. He was curious if he could do the same with struvite, and he tucked the idea away as something to try one day.

It took a high-school student-Menachem Tabanpour-to jumpstart the project. Now a senior majoring in biology and French, Tabanpour landed in Barak’s lab in the summer of 2004 as part of a NASA program for gifted high schoolers. Tabanpour handled Barak’s pet struvite project with ease, but he didn’t stop there. While waiting for experiments to finish, Tabanpour searched the literature and learned about the problems with struvite in sewage treatment plants. It was his suggestion that helped turn Barak’s curiosity into a potentially cost-saving solution.

“I’m not so stodgy to have forgotten that you need to listen,” says Barak. “The professor doesn’t know everything out of the gate.”