Beyond Scrubbing: Reducing Sulfate Stress from Wastewater at Minnesota Power’s Last Coal-burning Power Plant
When Minnesota Power ceases coal operations at its last coal-fired power plant, the company must also manage the sulfate-laden wastewater in its retention pond. However, current sulfate-treatment technologies are expensive and energy-intensive. With help from a MnDRIVE Environment Demonstration Grant, University of Minnesota researchers have partnered with Minnesota Power to develop an integrated biological treatment system that can contribute to life-cycle management options of wastewater.
By Isaac Conrad
Current technologies designed to remove sulfate from wastewater have a hefty price tag. According to Chan Lan Chun, research lead at the Natural Resources Research Institute and the UMD Department of Civil Engineering, “If we want to protect freshwater resources from difficult-to-manage pollutants like sulfate, 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 generates sulfur-containing gas that can lead to acid rain if released into the atmosphere. To prevent this outcome, Minnesota Power traps that gas in water using a process known as wet scrubbing. The sulfur-laden water is then stored in a retention pond on site. This pond water must be treated before it is released into the Mississippi River or managed via evaporative processes at the plant.
No matter its origins, excess sulfate serves as a biogeochemical stressor that poses a threat to freshwater plants and animals. How? “Naturally occurring microbes convert sulfate to sulfide,” 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 carbon substrates or hydrogen as a food source and sulfate as a breathing source.” The byproduct, sulfide, 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 generating sulfate-rich wastewater must decrease the amount of sulfate in the discharge to limits that regulators deem safe enough to avoid sulfide contamination.
Illustration by Alvina Salim and Issac Conrad
Existing sulfate treatment technologies, like reverse osmosis and ultrafiltration membranes, are available but expensive. They also produce a salty brine solution as a byproduct. However, 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 when the system becomes mature on an industrial scale.
To address the problem, Chun and her collaborators have partnered with Minnesota Power, Minnesota Department of Health, and Yawkey Minerals Management LLC to develop a new technology to neutralize sulfate wastewater using microbes already present in the surrounding ecosystem. Chun, along with Nathan Johnson from the Department of Civil Engineering at the University of Minnesota Duluth and Lee Penn from the Department of Chemistry at the University of Minnesota Twin Cities, have conducted key aspects of the project. The system has many moving parts, but it can be understood as three major steps.
First, sulfate-rich water flows into the bioreactor and interacts with microbes. During this interaction, the microbes convert sulfate to sulfide. Next, the sulfide-rich water flows through columns with iron mineral particles that bind with the sulfide and form solid waste. Finally, once the solid waste forms, the treated water can leave through the other end of the system, separate from the stable solid waste.
Using 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’s team provides the microbes with a food source (organic carbon, hydrogen or both), they can survive in the bioreactor by “breathing” sulfate and reducing it to sulfide.
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 a pilot scale on-site. Chun shares that “Our system is 150 gallons; bigger than I am. I could dive in there (but will not for safety)!”
She is cautiously optimistic about the scalability of this system. “We treat about 0.1 gallons of water per minute. A next step pilot test (that would prove the process works at larger scales) is about 5 gallons per minute, so we are not there yet.” Also, not every industrial application requires the same level of sulfate removal. The level of sulfate removal depends on water reuse goals and the discharge point, so the team can manipulate different parameters to help clients reach their respective targets.
“I don’t claim that biological sulfate treatment is the silver bullet. But we are contributing to a suite of technologies. Our system can combine with others to achieve sulfate removal to very low levels, resulting in less energy and waste,” says Chun. Right now, there is no system like it on the market, but Chun’s team aims to fill that gap, and several industry partners have expressed interest in the project.
For Chun, the collaboration at the heart of the project is the key to its success. Her team works closely with diverse stakeholders through sharing the findings and listening. “[But] the biggest stakeholders are the freshwater ecosystems we are seeking to protect from sulfate. They provide clean water; they grow manoomin and fish. We all value clean water. It’s a special resource in Minnesota.” Through careful collaboration, the project team has managed partnerships with stakeholders as large as Minnesota Power and as small as the microbes in the bioreactor to protect clean water and healthy ecosystems that benfits all Minnesotans.