Desalination: Easing the burden of thirst

From the 1900s onwards, about 11 million people perished as a result of the widespread effects of drought. Drought – a period of lower than average amount of precipitation in a region – is a growing global problem. Consequences include freshwater levels decreasing, famine, and diseases.

Significance of desalination on a global scale

To sustain a growing population, research is bent on developing a solution to these issues. Ground water drilling and recycling of wastewater are examples of temporary solutions. Among these solutions is desalination. Desalination is the process of forcing salted water through a membrane by reverse osmosis, separating freshwater from impurities. Although it is used in places like Israel and California, desalination is yet to be utilized by the rest of the world due to its reputation for massive energy consumption.

An approach to reduce cost is substituting the primary material used in the constructing membrane with a relatively inexpensive material called polyamide. Unfortunately, this substitution comes with another price. It is known that chlorine is a chemical present in the purification of water to destroy bacteria, but contact with polyamide degrades the membrane. To avoid degeneration, the extraction of chlorine becomes an additional step in the desalination process. However, when chlorine is absent, microbes can occur and obstruct the flow of water.

A possible solution is to replace polyamide with graphene oxide. The compound graphene has a structure similar to the honeycomb. It is predicted that this material will be more permeable to water and therefore reduce the pressure required to dictate the flow of water.

MIT materials scientists, Jeff Grossman, Shreya Dave, and colleagues, utilize this compound in their research. Graphene flakes, which are stripped from pieces of graphite, are placed in water. The liquid is then sucked out via vacuum filtration, leaving sheets as remnants. Residues are put together to create chunks by bonding carbon and oxygen atoms. This fusion is altered to make spaces between the flakes large enough to permit the flow of water molecules while hindering salt and other impurities. It was proven that the water molecules travel easier through the graphite membrane than the polyamide. It is also inferred that this material can further reduce the energy demands due to less resistance to water molecules, although this hypothesis is yet to be tested. Additionally, the cost of graphene oxide is not hugely divergent from the price of polyamide.

Application of desalination in Israel

A couple of years ago, Israel was found dealing with a severe drought problem – the worst in 900 years. To combat the drier lands, Israel explored national campaign to ensure water conservation. In 2007, low-flow toilets and showerheads came in use, and water from drainage systems was recycled for irrigation. However, the greatest improvement came after implementing desalination plants. As an example, Sorek desalination plant began operation in October 2013. It is located ten miles south of Tel Aviv and is the largest reverse-osmosis desalination facility in the world.

Following the pressurized flow of water, a common problem in the desalination process is the cost of cleaning blocked pores from molecules left behind. Edo Bar-Zeev and colleagues, from Israel’s Zuckerberg Institute for Water Research, had a remarkable discovery to improve separation between water and contaminants. They brought into use porous lava stone by which microorganisms are prevented from having contact with the membranes. This technology improved the performance of desalination plants. Now, 55 percent of domestic water traces its sources from desalination plants.

Aluminum disks – supplying to developing countries

Further research leans toward alternative materials like carbon nanotubes as the membrane. The underlying issue for integrating such findings is cost. The application of such processes must be considered on a global level. There are rural areas around the globe that are less developed and may not have the resources for developing desalination plants to attend to other areas.

To counter such challenge, Jia Zhu of Nanjing University in China and colleagues worked on alternative sources of energy, such as the sun. Yet depending on direct contact alone from the sun is limiting. Research is looking into the use of absorbable materials to increase the amount of energy from sunlight. A possible solution is the use of aluminum disks that absorb more than 96 percent of sunlight – 90 percent of which is used in forming water vapor. Drinking standards are also met this way. If implemented, aluminum is a low-cost material and can produce water at the same rate as desalination plants. However, due to the pure distilled water after evaporation, the absence of minerals like magnesium and calcium is a consequence. Thus, this serves as a temporary solution but should not be used in the long run.