Saying goodbye to sulfate

With help from a MnDrive Environment Demonstration Grant, UMD researcher Chan Lan Chun helps Minnesota Power reduce sulfate in wastewater from the utility’s last coal-burning power plant.

Isaac Conrad

Beginning energy production in 1958, Minnesota Power’s Boswell Energy Center quickly became a bedrock for communities and industries across Northern Minnesota. Sawmills, pit mines, and the small towns they supported, relied on the energy produced at the coal-fired power plant. As the climate crisis intensifies, Minnesota Power is looking toward renewable energy to meet the Northland’s energy needs. As a result, the utility will cease operations at its final coal-burning power plant by 2035. But before shutting down the plant, Minnesota Power must confront a different challenge: neutralizing the sulfate wastewater destined to enter the Mississippi River. 

If released as is, this high-sulfate wastewater poses a threat to manoomin (wild rice) that grows annually in the shallow, slow-moving waters near the Boswell Energy Center. Wild rice, the state grain of Minnesota, is sacred to the Ojibwe. As a result, state regulators place stringent limits on the amount of sulfate that industries like Minnesota Power can release into local waterways—especially those that harbor wild rice. The current technologies designed to remove sulfate from wastewater have a hefty price tag. According to University of Minnesota Duluth researcher Chan Lan Chun, “If we want to keep our wild rice and freshwater safe, we need an alternative sulfate-treatment system.” 

Sulfate enters the environment from natural and human-made sources. In the case of Minnesota Power, when coal is burned, it releases sulfur-containing gas, which can also lead to acid rain if let into the atmosphere. To prevent this, Minnesota Power traps that gas in water using a process known as scrubbing. The sulfur-laden water is then stored in a retention pond on site. When the Boswell Energy Center officially closes, this pond water will be released into the Mississippi River. If left untreated, the sulfate in the wastewater would have detrimental impacts on the ecosystem.

No matter its origins, sulfate serves as a stressor that poses a threat to plants and animals. How? Naturally occurring microbes convert sulfate to sulfite, explains Chun. “Every organism needs a food source and a breathing source. In our case, humans eat Cheetos and breathe oxygen. In the process, we gain energy. Sulfate-reducing microbes use [lactic acid and hydrogen] as a food source and sulfate as a breathing source.” The byproduct, sulfite, is a form of sulfur toxic to plants like manoomin because it can suppress photosynthesis, damage plant cells, and stunt growth. As a result, companies like Minnesota Power must decrease the amount of sulfate in their wastewater to limits that regulators deem safe enough to avoid sulfite contamination. 

Sulfate treatment technologies, like reverse osmosis and ultrafiltration membranes, are available but expensive. They also produce a salty brine solution as a byproduct. “Near the coasts, they can dump [brine] into the ocean because seawater is brine,” says Chun. Minnesota is land-locked, so you can’t just dump this brine into waterways.” Managing brine waste is also energy intensive. Chun and her collaborators believe their biological sulfate treatment system can be part of the solution reducing both cost and energy consumption.

To address the problem, Chun has partnered with Minnesota Power and other University of Minnesota researchers to develop a new technology to neutralize sulfate waste using microbes already present in the surrounding ecosystem. Chun, along with Nathan Johnson at the University of Minnesota Duluth and Lee Penn from the Department of Chemistry at the University of Minnesota, monitors different aspects of the project. The system has many moving parts, but it can be understood as three major steps.

Biosulfate Reduction System Process Diagram

Illustration by Alvina Salim and Issac Conrad

First, sulfate-rich water flows into the bioreactor and interacts with microbes. During this interaction, the microbes convert sulfate to sulfite. Next, the sulfite-rich water flows through columns with iron particles that bind with the sulfite and form solid waste. Finally, once the solid waste settles, the treated water can leave through the other end of the system.

Including microbes that come directly from the pond and surrounding soil makes this biological sulfate treatment system unique. Successfully removing sulfate from the water requires less energy because the microbes do most of the work as they “breathe.” As long as Chun provides the microbes with a food source (lactic acid and hydrogen), they can survive in the bioreactor by “breathing” sulfate and reducing it to sulfite.

Chun’s team isn’t the first group to study biological sulfate reduction, but funding from MnDrive Environment allowed the team to test the technology at an industrial scale. Chun shares that “Our system is 150 gallons; bigger than I am. I could swim in there!” 

She is cautiously optimistic about the scalability of this system. “We treat about 0.1 gallons of water per minute. The proper pilot test is about 5 to 10 gallons per minute, so we are not there yet.”

But not every industrial application requires the same level of sulfite removal as Minnesota Power, which needs to achieve sulfate levels down to 10 parts per million, so the team can manipulate different parameters for clients to reach their respective targets.

“I don’t claim that biological sulfate treatment system is the silver bullet. But we are contributing to the suite of technologies. Our system can combine with others to lengthen their lives,” says Chun. Right now, there is no system like it on the market, but Chun aims to fill that gap, and several industry partners have expressed interest in the project: “Mining companies are curious about what we’re doing, and clearly, so is Minnesota Power.” 

For Chun, the collaboration at the heart of the project is the key to its success. She works closely with all stakeholders—tribal governments, state regulators, Northland communities, power companies, and the mining companies who stand to benefit from the technology. “[But] the biggest stakeholders in this project are tribal members. This is their land, and we are walking it together. They care about wild rice. It’s a special resource in Minnesota that means so much to indigenous people.” Through careful collaboration, Chun has managed partnerships with stakeholders as large as Minnesota Power and as small as the microbes in the bioreactor, to protect something that all Minnesotans care for—wild rice.

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