From flush to fertilizer? Researchers search for benefits in ash from human waste

A giant fire is burning in St. Paul, fueled by our flushing.

The Metropolitan Council incinerates the sludge left behind at the end of the wastewater treatment process at its biggest facility, the Metro Plant, converting the waste into heat and power. The end result is a lot of ash — about 40 tons a day.

It's usually hauled to a landfill in Rosemount. But last week, researchers instead sprinkled bags of ash on a farm field down the road, in hopes that the phosphorus-rich powder could have a future as fertilizer.

"You'd be getting a benefit from it," said Dr. Carl Rosen, who leads the University of Minnesota's Department of Soil, Water, and Climate. "You'd actually be able to potentially even sell it."

It's not uncommon for farmers in Minnesota and elsewhere in the country to use sludge in some form as a fertilizer on crops. But the Met Council does not have a similar reuse for its ash, which lacks common sludge components nitrogen and carbon, so it pays upward of $21 a ton to bury it.

Rosen and his team recently began a three-year study funded by the Met Council, which treats the region's wastewater, to see how the ash works as a fertilizer on corn and soybeans. Researchers divided a field at the U's UMore campus into dozens of segments, applying varying levels of ash and other fertilizers to each.

"You smell that kind of a sweet smell? That's the dried biosolids," Rosen said of one of the other products, called MinneGrow, which is made from wastewater sludge at the Met Council's Blue Lake plant. That plant serves the area including Shakopee, Chanhassen, Chaska, Eden Prairie and cities around Lake Minnetonka.

"I always tell people, 'If you're concerned, a handful of this is probably cleaner than your bathroom at home,' " said Harold Dessner, operations manager for SFS Fertilizer, which sells MinneGrow.

"Sludge and stuff has always kind of been around," Dessner said. "And [MinneGrow] is kind of taking it to a high-end area."

The Met Council says incineration makes more sense at the Metro Plant, which handles about 10 times the volume of sludge each day as Blue Lake. Burning is cost-effective, produces energy and significantly cuts down on the volume of material, but still results in ash that requires disposal.

If the ash proves to be an effective fertilizer, it would cut the Met Council's landfill costs and make better use of the phosphorus, a limited resource largely mined domestically in Florida.

"Depending on estimates, there's anywhere from 100 to 500 years of phosphorus — easily minable phosphorus — in the world," Rosen said. "And so efforts really need to be concentrated now on trying to recycle the phosphorus that we're using."

"All of these [are] things that in people's mind raise a red flag, so we want to make sure we can track all of those," Rosen said. A recent study of the ash in a greenhouse found it did not result in large metal accumulations.

State agencies have been studying what to do with the leftover wastewater ash since at least the 1980s, when it had a much higher metal content before more stringent regulations on industrial waste.

The Metropolitan Waste Control Commission, later merged into the Met Council, paid a Nevada-based firm $7 million to take 275,000 tons of it off their hands in the 1980s. The company had planned to mine it for gold, silver and other precious metals, but the venture went bust by the time the ash had been hauled by train to a former munitions depot in South Dakota. South Dakota officials subsequently had to find a way to bury it.

A 1982 Met Council report concluded that the ash could be used to make asphalt or concrete for building roads, though it's unclear how often that happened. Met Council spokeswoman Bonnie Kollodge said it was used as an ingredient in cement in the 1990s and early 2000s, though the council is unsure where it was used. New incinerator technology in the mid-2000s ended that cement-making.

"In part of the world we're digging phosphorus up and we're paying to dig it up," Met Council Principal Engineer Christine Voigt told a committee in February. "And then here we're paying to bury it. It doesn't make sense and it's not sustainable."

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