Freeze-thaw water desalination goes through tests in North Dakota

Researchers with the University of North Dakota Energy & Environmental Research Centre (EERC) are testing technology that improves water quality by removing salt, a process known as freeze-thaw desalination.

Freeze-thaw water desalination technologies, developed by B.C. Technologies of Laramie, Wyo. and the EERC, uses natural freezing and thawing cycles to do much of the work. This low-energy process relies on the fact that clean water freezes before salty water, a phenomenon that enables salt to be removed from water.

At a demonstration site south of the town of Devils Lake, N.D., freeze-thaw technology is being put to its most rigorous test yet.

Researchers set out to answer three primary questions: Could freeze–thaw technology work in the extreme winter climate of north-eastern North Dakota? Could it clean up water that had a much lower salt content than water used in previous demonstrations? And is the process economical enough to produce large quantities of high-quality water for municipal, agricultural or industrial use?

With winter nearly over, EERC researchers believe they have the answers to two of the questions. Yes, freeze-thaw desal works in harsh winter conditions. And, yes, it can be used to clean up water relatively low in salt. The answer to the third question about freeze-thaw’s economic feasibility won’t be known until the project is completed later this year.

However, John Boysen of B.C. Technologies, who originally envisioned freeze-thaw technology as a method to clean extremely salty groundwater produced during oil and gas drilling operations, is optimistic that the process will prove economical.

“When I started this, I said that I would never get involved in using the technology to produce municipal water because I wasn’t certain it could be done economically. But here I am,” Boysen said.

The situation at Devils Lake is far different from oil and gas field operations. Ed Steadman, a geologist and an EERC associate director, explains that even though water in Devils Lake is currently considered fresh, the amount of salt it contains is about three times greater than municipal drinking water standards. Occasionally, the lake becomes so salty that the only things that can live in it are brine shrimp.

“It’s an interesting lake because it’s in a closed basin,” he says. “The water will always be salty because when it evaporates, there’s no place for the salt to go and it’s left behind. When the lake level is low, the water can become just as salty as seawater.”

The city of Devils Lake, located on the lake’s north side, became interested in freeze-thaw technology when the rising lake began to threaten its municipal water supply. The EERC proposed a demonstration of the technology as a potential means of producing cleaner water that could be further treated for municipal use.

As a result, a project was funded in partnership with the North Dakota Office of Intergovernmental Assistance ($405,000), the city of Devils Lake in conjunction with the North-Central Planning Council (NCPC), the North Dakota Department of Health ($250,000 and analytical services) and the US Bureau of Reclamation ($304,000).

The freeze-thaw site consists of six ponds and three temporary buildings that house pumps and instruments amid a network of pipes. Water from the lake is pumped and held in one of the ponds. Two ponds are used as freezing pads, while another pond is reserved for holding the highly concentrated brine solution. The two remaining ponds are used to hold treated water.

On the pads, water is sprayed into the air to promote rapid freezing. A layer of ice up to 30 feet deep is formed, and 12-foot-high towers of ice rise up around the pipes from which the water is sprayed.

The water is repeatedly circulated, eventually resulting in a small amount of highly concentrated brine solution and a large volume of ice composed of water containing less than a third of the salt it originally held.

In contrast, the brine solution contains 8 to 10 times more salt than normal lake water. When the ice melts in the spring, the project will have produced approximately 4 million gallons of water suitable for treatment in a municipal water system.

From a distance, the freeze pads look like ice castles on the flat prairie. Initially, the salt content of the ice gave it a yellowish appearance. However, as the salts leached out of the ice and concentrated beneath it, the ice became pure white. Researchers say the concentrated brine could be evaporated into a solid for use as road salt during the winter.

Those associated with the Devils Lake freeze-thaw demonstration project say that it has provided valuable information and operating experience.

“There’s no such thing as a normal North Dakota winter,” Steadman says. “We’ve encountered all sorts of weather conditions from 30 below zero to 40 above. This project has given us what we wanted – a demonstration under a broad range of weather and temperature conditions.”

Rick Anderson, NCPC executive director, says, “When I was first told about this technology, I didn’t think it was possible. But now I’m impressed. They’re showing that it works. I’m convinced that it will separate good water from brine.”

EERC research engineer Brad Stevens says the system became easier to operate as experience was gained. “It’s not as hands-on as it was at the beginning. We’ve gotten a lot smarter and more comfortable with the operation,” he says.

EERC director Gerald Groenewold sees the application of freeze-thaw technology as an opportunity with great potential for the northern plains region. “This technology opens the door to change an abundant brackish water resource into a freshwater resource. In this part of the country, we have many sources of water, but we don’t have enough good water. The freeze-thaw process gives us the potential to cost-effectively turn brackish water into useable water.”

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