A Breakthrough in Desalination

by Mikel Saralegui

“Ocean” Captured by Symbiont Team Member, Natalia Fernandez

With the ongoing water crisis around the world, scientists are scrambling to find a solution to the problem at hand. While 71% of the earth is covered by water, only 3% of that water is freshwater. However, with the majority of that 3% being trapped underground or in glaciers, only 0.5% of water is accessible to humans. It is for this reason that there is an intense urgency to find a solution.

While desalination has been used for centuries, dating back to the Romans using clay filters to trap salt, the beginning of present-day processes of desalination began in 1957 when Professor Robert. S. Silver discovered the process of multi-stage flash distillation. Multi-stage flash distillation is performed by flash evaporating (rapidly vaporizing) a saltwater supply multiple times in order to purify water. While the result of multi-stage flash distillation is pure water, the energy draw per cubic meter can be very costly, at times reaching 25kWh or 25000 watts per hour. To put that into perspective, 25kWh can power 25 microwaves, or 125 office computers at the same time. Regardless of the power draw, many countries in the Middle East such as Saudi Arabia rely on converting seawater into pure water every day, with the Middle East containing 70% of the desalination plants in the world. Due to desalination providing over 50% of Saudi Arabia’s water supply, multi-stage flash distillation plants still constitute 12 of the 27 desalination plants currently in the country. While the Middle East has struggled to part ways with many of their energy-intensive plants, other countries have phased them out, with multi-stage flash distillation composing only 26% of all desalination plants on Earth today.

Today’s desalination plants use the process of reverse osmosis, which incorporates a semi-permeable barrier, only letting through pure water. While energy consumption is much more efficient, performing at wattages between 3-6kWh, another problem lies in the process of reverse osmosis. One would think that after performing reverse osmosis, you can simply dump the remaining salt-liquid back into the ocean, but the reality is quite the opposite. After the pure water has gone through the barrier, the byproduct left from reverse osmosis is brine, a liquid with an incredibly high salinity level that cannot be simply dumped back into the water. If pockets of brine accumulate in the water, they sink to the bottom, decreasing oxygen levels and effectively choking animals on the ocean floor. A current method for dealing with brine is to spread the brine across an area of the ocean to prevent the poisoning of the wildlife. This method, however, can be very costly and challenging to keep track of. Another option that is currently being explored is to convert brine into a manufacturable product, a method in which MIT students have made a breakthrough. The brine solution would be converted into sodium hydroxide, a chemical otherwise known as caustic soda. Sodium hydroxide can actually be used directly inside of the desalination process, as a chemical which treats the incoming seawater and changes its acidity. If the seawater’s acidity was not changed before treatment, over time the acidity would wear down the water filters, causing a vast amount of failures otherwise. Not only would manufacturing sodium hydroxide help control the amount of brine output into the ocean, but it would cut down the costs of the entire desalination process, edging the cost of desalination into a feasible territory as a main source of water. Desalination plants would also be able to sell sodium hydroxide since the amount of brine created in the plants vastly outweigh their needs for the product. The process to manufacture sodium hydroxide is relatively simple in that the brine only needs to go through a cycle of nanofiltration, accompanied by another cycle of electrodialysis to create the final product. While desalination sounds like the be all end all of the water crisis, we have only just started finding a use for all byproducts. Perhaps in the future, we will be able to create a desalination cycle that will go full circle.

MIKEL SARALEGUI is a high school student from New York City. He wants to focus on sustainable entrepreneurship in college.