10th May 2019

Radical new desalination technique may disrupt the water industry

Researchers at Columbia University report a new desalination method for hypersaline brines, known as "temperature swing solvent extraction (TSSE)", which is low-cost and efficient.

Hypersaline brines – water that contains high concentrations of dissolved salts and whose saline levels are higher than ocean water – are a growing environmental concern around the world. Very challenging and costly to treat, they result from water produced during oil and gas production, inland desalination concentrate, landfill leachate (a major problem for municipal solid waste landfills), flue gas desulfurisation in fossil-fuel power plants, and effluent from various industrial processes.

If hypersaline brines are poorly managed, they can pollute both surface and groundwater resources. But a simple, low-cost way to desalinate them could help to clean up the environment, and make vast quantities of water available for all kinds of uses – from agriculture to industrial applications and possibly even for human consumption.

A Columbia Engineering team led by Ngai Yin Yip, Assistant Professor of Earth and Environmental Engineering, reports that they have developed a radically new desalination approach – "Temperature Swing Solvent Extraction (TSSE)" – for hypersaline brines. Their study, published online in Environmental Science & Technology Letters, demonstrates that TSSE can desalinate very high-salinity brines, up to seven times the concentration of seawater. This is a good deal more than reverse osmosis, the "gold standard" for seawater desalination, that can handle twice the seawater salt concentrations.

Currently, hypersaline brines are either filtered by membrane (reverse osmosis) or undergo evaporation (distillation). Each has limitations. Reverse osmosis is ineffective for high-saline brines, because the pressures applied scale with the amount of salt: hypersaline brines require prohibitively high pressurisations. Distillation techniques, which evaporate the brine, are very energy-intensive.

Yip has been working on solvent extraction, a separation method widely employed for chemical engineering processes. The relatively inexpensive, simple, and effective separation technique is used in a wide range of industries, including production of fine organic compounds, purification of natural products, and extraction of valuable metal complexes.

"I thought solvent extraction could be a good alternative desalination approach that is radically different from conventional methods, because it is membrane-less and not based on evaporative phase-change," Yip explains. "Our results show that TSSE could be a disruptive technology – it's effective, efficient, scalable, and can be sustainably powered."

Even with a low-grade heat (< 70°C), the TSSE method removed up to 98.4% of salt, which is comparable to reverse osmosis. The findings also demonstrated a high water recovery (>50%) for the hypersaline brines, comparable to current seawater desalination operations. But reverse osmosis cannot handle hypersalinity – unlike TSEE.

"We think TSSE will be transformational for the water industry. It can displace the prevailing practice of costly distillation for desalination of high-salinity brines and tackle higher salinities that reverse osmosis cannot handle," explains Yip. "This will radically improve the sustainability in the treatment of produced water, inland desalination concentrate, landfill leachate, and other hypersaline streams of emerging importance. We can eliminate the pollution problems from these brines and create cleaner, more useable water for our planet."

TSSE has a clear path to commercialisation, according to Yip. The heat input can be cheaply and sustainably supplied by low-grade thermal sources, such as industrial waste heat, shallow-well geothermal, and low-concentration solar collectors. He is now working to engineer further improvements in performance and plans to test it with real-world samples in the field.

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