Fall 2008

On Henry Mall

Chunks of crystallized struvite cling to the walls of a heat-exchange pipe at Madison's sewage treatment plant. Bryce Richter/UW Communications

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.”

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