GRAPHICS: Anas AlBounni-KUST Review

Life on planet Earth needs water. Fresh water.

Water scarcity affects one-third of the world’s population, approximately 2.3 billion people, with this water crisis tipped to become more acute over the next 50 years as the global population increases.

Yet water covers nearly three-quarters of the planet. It’s salty and unpotable, but a practical, economically viable desalination process could be the answer to our collective thirst.

Credit: Anas AlBounni-KUST Review

The most popular method of desalination is reverse osmosis, where large quantities of seawater are pushed through a semipermeable membrane to remove the salt from the water. Although an effective means to sieve through and catch the salt and other impurities, this is a high-pressure, high-cost process requiring robust pumping and expensive pretreatment. It’s energy-hungry, and while the process has steadily improved and evolved, there are systemic problems, including polluting chemicals, membrane fouling, capacity limitations and expensive construction materials.

Freeze desalination, on the other hand, is a natural process: Ice made from saltwater is salt-free.

Isam Janajreh, professor of mechanical engineering at Khalifa University’s Center for Membranes and Advanced Water Technology, says freeze desalination technology has the potential to avoid common desalination challenges: “Desalination is the solution for water security in regions with insufficient resources but this comes at a high energy cost. Freeze desalination is emerging as an attractive low-energy and less corrosive alternative to providing fresh water.”

Freeze and repeat

The process is simple: Partially freeze the water, during which ice crystals form and grow, displacing impurities into the remaining brine solution. Separate the ice blocks from the brine, wash them off, and melt them back down again to provide clean water.

The salty brine can then be frozen again, forming more ice and another more concentrated brine solution. As the salinity increases, the freezing point dips until it reaches the point where salt crystallizes simultaneously with ice.

So far, however, the process has been limited to laboratories and small pilot plants. Janajreh says this is due to the incurred capital cost and the complex operation of separating the ice and melting it.

“The process of salt rejection during the process is still far from being completely understood, especially when the parameters change. One big challenge is salt entrapment between the ice crystals making a super salty saltwater pocket in the ice. This requires further treatment and recrystallizing, which just raises operating costs.”

Abdul Najim, of the Indian Institute of Technology Bombay, says this is the essential requirement for developing the technology — understanding the process of crystal growth to avoid the saltwater pockets.

“This is a process difficult to study analytically. Numerical models can enable the analysis and visualization of different transport processes and computational fluid dynamics (CFD) modeling can also be a valuable tool,” he says.

Najim also says efforts should be directed toward hybrid models where crystallization, ice separation and thawing are carried out in a single unit. This requires novel crystallizers that can be scaled to industrial capacity — quite the engineering challenge right now. Moving from a batch-mode model to uninterrupted potable-water production should also be the focus of research in this area.

Freeze desalination may be a process seen in nature, but converting it to a manmade industry will still require a lot of energy. Janajreh says freeze desalination needs just half the energy conventional reverse osmosis consumes, but that’s still a tall order.

Pair the process with the cold energy from regasification of liquefied natural gas (LNG) and you could have a solution. Or, stick with batch production and use your home fridge/freezer as Fekadu Melak did.

Removing other contaminants

Melak, assistant professor of environmental biotechnology at Bahir Dar University, Ethiopia, investigated freeze desalination as a water-treatment technology for the tanning industry using home-use freezer units.

“Wastewater generated by leather tanning is one of the major contributing sources of chromium pollution in water. Among the various tanning methods used around the world, more than 90 percent of the leathers tanned globally contain chromium, with 30 to 50 percent of the chromium used in the process leaching into the environment. Freeze-melting and removing the contaminants is an alternative physical process which can be used for desalting.”

It worked. Melak’s study saw efficiency as high as 85 percent for cleaning chromium-spiked tap water, and while technical challenges remain — including washing off the chromium adhered to the ice surface after freezing — the cost of freeze desalination was 50 percent lower than other wastewater-cleaning methods.

“In terms of the water quality produced and how cost-effective it is, freeze desalination is a pertinent option for a desalination technology,” Melak said.
Freeze desalination is gaining traction as a research interest but until projects can scale up to industrial levels, it’s doable at home.

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