Northern Minnesota researchers close in on sulfate pollution solution 

Aug 23, 2023

Outside the wastewater treatment plant in the Iron Range town of Aurora, a small trailer could hold clues to solving a big environmental problem facing northern Minnesota — how to protect wild rice from sulfate, a pollutant released by iron ore mines, wastewater treatment plants and other industries.

Mei Cai, an environmental engineer with the Natural Resources Research Institute at the University of Minnesota Duluth, points to a series of tanks where a chemical called barium chloride reacts with dissolved sulfate in the water to form particles.

Then another chemical is added that clumps those particles together into bigger pieces, forming a sludge that can be removed from the water, which then goes through a final filtration process.

The water that comes out of the Aurora treatment plant has high levels of sulfate; about 250 parts per million. The technology Cai is demonstrating in the mobile trailer has successfully reduced sulfate levels in the water from the plant to below 10 parts per million, low enough to meet the state's strict sulfate rule for water that's released into lakes and rivers where wild rice grows.

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That could be especially important here in Aurora, because treated water discharged from the city water plant eventually flows into the Partridge River, which was recently added by the U.S. Environmental Protection Agency to Minnesota's list of waters that do not meet the state's wild rice sulfate standard.

"What we're trying to deal with here is show that whatever is coming out of the plant, we can reduce the sulfate enough so that when the effluent from the plant goes into the environment, it doesn't pose a significant impact on sulfate concentration in the region," said Rolf Weberg, executive director of the Natural Resources Research Institute (NRRI).

Minnesota's wild rice sulfate standard is globally unique — state lawmakers adopted it in the 1970s after research found that wild rice didn't grow well in waters high in sulfate.

But the standard has rarely been enforced. In 2011, following a lawsuit by the Minnesota Chamber of Commerce to throw out the rule, and pressure from environmental groups and Indian tribes to start enforcing it, the state legislature asked the Minnesota Pollution Control Agency to study the rule and see if it needed updating.

The MPCA found that the rule wasn't wrong, but said it was imprecise. So in 2017, the agency proposed a complex, flexible formula that would determine what standard would be appropriate for each specific lake or stream. But a judge nixed that proposal the following year.

That means the state's wild rice standard of 10 parts per million remains in effect.

"The U.S. Environmental Protection Agency has basically said the MPCA has to apply the law. Period," said Paula Maccabee, advocacy director with the group WaterLegacy.

But for years, most mining companies and wastewater treatment plants have argued that the only proven technology to meet the standard — reverse osmosis, or nanofiltration — is prohibitively expensive.

Only PolyMet Mining, which is seeking to open the state's first copper-nickel mine, has agreed to install reverse osmosis technology to treat sulfate.

"You're talking about capital costs in the range of hundreds of millions of dollars," said Daniel Marx, an attorney who represents a group called the Minnesota Environmental Science and Economic Review Board, made up of municipal wastewater treatment facilities around the state.

Marx said it would also cost water plants millions of dollars a year to operate and maintain the energy-intensive technology, and dispose of the salty byproduct that's left over.

"It's the type of situation where it's not technologically or economically feasible to do sulfate treatment."

That's why NRRI has worked for the past several years to identify sulfate reduction technologies that are different from reverse osmosis, and that could work at a fraction of the cost, including technologies that use microbes and even peat.

Marx said municipal utilities are excited about those new possibilities. But he said they have lots of questions — how much cheaper will alternatives be to reverse osmosis? And what can be done with the salty sludge that's left over?

Jeff Hanson thinks he has an answer to that last question. He started a company called Clearwater BioLogic in his hometown of Babbitt on the Iron Range that developed a bioreactor designed to float in mine pit lakes and clean the water of sulfate.

After years of research, he said his company recently devised a way to effectively treat the byproduct that's left over. He's partnered with a different company that has identified a beneficial use for what previously was a waste product.

"And having a value for that waste material, nothing to put in the landfill, nothing to throw away, dramatically improves the cost scenario on this as well," Hanson said.

Hanson hopes to have field trials in place on the Iron Range this summer.

Paula Maccabee with the environmental group WaterLegacy said it makes sense to pursue new technologies to clean up old mining pollution. But she said state officials shouldn't rely on them when deciding whether to grant permits to new projects.

"That is the biggest concern we have, is that if people start saying, 'Well, maybe at some point in the future, we'll be able to remediate it,' that might be an excuse for failing to use the technologies, whether it's the liners or the nanofiltration that are necessary right now, to prevent more pollution."

Meanwhile the federal Environmental Protection Agency has made it clear to Minnesota officials that they need to address excessive sulfate in water. The question is how.

"Right now there is a lot of uncertainty from our perspective about how MPCA will do that," said Marx, who represents municipal water plants.

One of the questions the group has is how the MPCA will choose to proceed "in light of its findings that there are issues with the current standard on the books that may be outdated," said Marx.

Hanson, who is developing the floating bioreactor technology in Babbitt, said addressing the sulfate issue is going to require that the state play a role.

"We do need to hold [mining companies] feet to the fire sometimes," Hanson said. "And that's why we have agencies that set standards and enforce them."

MPCA spokesperson Darin Broton said the agency continues to have conversations with tribal nations, communities and industrial facilities on the sulfate issue.

"There are no easy solutions or a one-size-fits-all approach," Broton said. "But we’re hopeful that new technologies can help address some of the concerns."

And those technologies may not be that far away from being deployed. NRRI's Weberg said the next step is to engage with water treatment plants to see what additional data they need, and to partner with a plant to build a pilot facility. He doesn't think that future is that far away.

"It could be a year or two away, but it's certainly not 10 years away," Weberg said.