Engineers at Columbia University in New York City have developed a desalination method that they claim is more effective and efficient than existing processes
The process has been developed specifically to treat the so-called hypersaline brines, produced through a range of industrial process such as oil and gas production.
These brines, which contain far higher concentrations of dissolved salts than ocean water, are challenging to treat, and present a growing environmental concern around the world.
Currently, hypersaline brines are desalinated either by membrane (reverse osmosis) or water evaporation (distillation). However, reverse osmosis methods are ineffective for high-saline brines because the pressures applied in reverse osmosis scale with the amount of salt: hypersaline brines require prohibitively high pressurisations.
Distillation techniques, which evaporate the brine, are very energy-intensive.
The process developed by the Columbia team, an approach described as temperature swing solvent extraction (TSSE) is claimed to be able to desalinate brines with up to seven times the concentration of seawater far more efficiency than existing methods.
Based on a separation method widely employed for chemical engineering processes, TSSE utilises a low-polarity solvent with temperature-dependent water solubility for the selective extraction of water over salt from saline feeds. Because it is membrane-less and not based on evaporation of water, it can sidestep the technical constraints that limit the more traditional methods. Importantly, TSSE is powered by low-grade heat (< 70 deg C) that is inexpensive and sometimes even free.
A study published in Environmental Science & Technology Letters describes how TSSE removed up to 98.4 per cent of the salt, which is comparable to reverse osmosis.
The findings also demonstrated high water recovery >50 per cent for the hypersaline brines, also comparable to current seawater desalination operations. But, unlike TSSE, reverse osmosis cannot handle hypersaline brines.
“Our results show that TSSE could be a disruptive technology – it’s effective, efficient, scalable, and can be sustainably powered,” said project leader Ngai Yin Yip, assistant professor of earth and environmental engineering at Columbia.
“We think TSSE will be transformational for the water industry,” he added. “It can displace the prevailing practice of costly distillation for desalination of high-salinity brines and tackle higher salinities that RO cannot handle. 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.”
Yip is now working on further refining how TSSE works as a desalination method so that he can engineer further improvements in performance and test it with real-world samples in the field.
I’m not sure why they say that RO cannot handle highly concentrated brines. We have used RO to treat some dreadful effluent streams. The biggest problem is that the concentrate stream is substantial and can be very difficult to dispose of. Evaporation also produces a difficult residue stream, but needs less electrical energy.
At some point above ocean salinity, and below salt saturation, the osmotic pressure become prohibitively high for the mechanical system employed (pumps, membranes, vessels, etc.).
The authors totally overlooked another interesting technique called membrane capacitive deionization (although scaling has to be monitored, this is typically cleaned off with citric acid in this case). Not sure this has been attempted on such waters, but I see no particular reason why not, and it has a very low specific energy input per volume purified water.
This desalination process may still require a significant amount of energy, both to heat the amine:water solution to release water as well as remove heat from the separated amine solvent after fresh water removal so that the solvent can be re-used. The cooling requirement could possibly be mitigated with an evaporative cooling tower that can operate with saline feed water instead of fresh water for cooling tower make-up.
It would be interesting to see a total mass and energy balance around the process.
The ability to use low temperature waste heat is potentially a very significant advantage, either a barometric condenser in a power plant or engine exhaust in the oil field.
One concern-how much of the amine remains in solution in the water phases, both the concentrate and the desalinated water after the heat separation step? If significant, secondary amine removal and recovery steps will add cost and complexity.
Nonetheless, congratulations on your findings and best of luck with continued development.