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A better way to desalinate water?

7 Feb 2018 - 11:00

 

UCT students have commissioned a working water desalination plant that adds a technique called Eutectic Freeze Crystallization (EFC) to the conventional but energy-intensive Reverse Osmosis (RO) process.

The students, Fendi Lin and Anthony Mchendrie, commissioned the RO/EFC system for their final-year thesis. Mr Jemitias Chivavava of the Crystallization and Precipitation Research Unit and Professor Alison Lewis, dean of the Faculty of Engineering & the Built Environment, supervised the project.

Eutectic Freeze Crystallization (EFC) is a novel method that purifies the salty brines that are the by-product of conventional desalination processes. EFC treats the brines by cooling them down to the eutectic temperature. At this temperature, both ice and salts crystallize out of the brine. The ice, being less dense than water, floats to the top, where it can be recovered as pure water. The salt, being more dense, sinks, and can also be recovered as a pure product.

Why Eutectic Freeze Crystallization?

Adding EFC to the desalination process instead of only Reverse Osmosis comes with significant benefits.

Up to now, seawater has been subjected to Reverse Osmosis, which produces a concentrated stream of very salty brine that has to be further treated or disposed of. Typically, only 40% of the seawater becomes drinkable water.

The UCT students used EFC to treat the Reverse Osmosis brine, testing whether EFC was a feasible option to do so.

The results were promising.

Fendi Lin, one of the students who worked on the project, found that Reverse Osmosis managed to yield only 24% of pure, drinkable water.

The brine that remained after Reverse Osmosis was put into the crystalliser, and put through a five-stage process that formed either just ice, or both ice and salt. All the stages have the potential to yield drinkable water.

And so it was: the four EFC stages yielded 47%, 43.092%, 33.1% and 27.8% of ice/water respectively. This amounted to recovering 85.6% water from the brine. It could get even better than that, though.

“Although the data obtained tells us that 85.6% of water is recoverable by EFC, this is not the maximum amount,” Lin wrote in the research report. “However, this is a good start and the EFC process proved to reduce what would have been regarded as waste by a significant amount.”

In total, the RO-EFC process yielded an 89.1% pure water recovery rate.

“Purity of salt was not analysed because very small quantities were formed, such that they were hard to detect by the analytical lab,” Lin added.