THERE’S a lot of water on Earth. In fact, some 71 per cent of the Earth’s surface is water but about 96.5 per cent of that is ocean water, which is salty. That means it’s not drinkable water.
Of course it’s possible to desalinate the water. Traditional desalination plants have been around for a long time and the technology for that is well-established but it’s still a very expensive and power-intensive process.
If we want to make cheap potable water universally accessible throughout the planet, we will have to find a better way to desalinate ocean water. But before we look at what scientists have been working on, let’s look at how it’s done currently.
TRADITIONAL WAY
Desalination plants typically use either thermal distillation or reverse osmosis (or a combination of both). In the former, water is heated so that vapour is created. What’s left behind are the salts. In the latter, water is pushed through a filter to remove the salt.
Perhaps not surprisingly, the Middle East is where much of the desalination activities are going on and it’s there that the big desalination plants are located, such as Saudi Arabia’s Ras Al Khair (which uses both thermal distillation and reverse osmosis) and Israel’s Sorek (which uses reverse osmosis).
As mentioned earlier, the traditional approach to desalination works fine. It’s just expensive and uses a lot of energy. Now, let’s look at three new ways researchers are approaching desalination.
GRAPHENE
Just one atom thick, graphene is a wonder material made of carbon atoms that is hailed for its tensile strength. Theoretically, it could act as a molecular sieve that can be used for filtering water. It is more permeable than current membranes being used in water filtration. This is important because a faster flow of water means less energy needs to be used.
The problem is that graphene membranes, as strong as they may be, are still prone to damage. Recently however, a team of researchers from the US, China and Japan revealed their breakthrough in reinforcing graphene membranes for filtration purposes.
The team created a graphene-nanomesh nanotube hybrid membrane that acts as a microscopic framework to support the graphene and increase its structural integrity. The result was a mesh with a honeycomb appearance that was able to keep out up to 97 per cent of the salt from saltwater.
SOLAR POWER
A team of researchers in Saudi Arabia has released a study that says they’ve created a solar power device that not only produces electricity but also purifies water in the process. The device consists of a solar panel positioned horizontally with several tiers below it. When salty water is run through those tiers, the heat from the solar cells warms the water causing it to evaporate through a membrane. The condensed water is then collected in a container.
Seawater usually contains a lot of heavy metals. However, according to the researchers, the water that is produced has levels of lead, copper, sodium, calcium, and magnesium that all fall below the World Health Organisation’s safe-for-drinking maximums.
“Having a significant amount of freshwater produced continuously on a daily basis means many challenging tasks can then be easily achievable,” said Peng Wang, a co-author of the research from King Abdullah University of Science and Technology in Saudi Arabia. “The generated clean water can be used for cleaning solar panels to remove dust particles; it can be used to irrigate plants and crops, making desert agriculture possible.”
The uniqueness of the device is that it kills two birds with one stone. It produces electricity and water as well. This is in stark contrast to common desalination techniques that require a lot of electricity to produce the water.
SOLVENT EXTRACTION
A team from Columbia University has come up with a radically different way to produce potable water that is not dependent on either filtration or distillation. Earlier this year, the team published a study detailing a process called “Temperature Swing Solvent Extraction” (TSSE) whereby a special solvent is added to saltwater. At room temperature the solvent is able to draw in water molecules but not salt. The solvent and water are then drawn off and heated. The heat causes the solvent to separate from the water (which is now salt-free), which can then be collected.
“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,” said Ngai Yin Yip, the lead researcher for the study.
Adding, she said: “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.”
According to the study, this approach is able to remove up to 98.4 per cent of salt, which is quite a good result. Not only that, the researchers say TSSE is able to handle twice the seawater salt concentrations as reverse osmosis. It also requires far less heat than thermal distillation, which means lower power would be required.
Oon Yeoh is a consultant with experiences in print, online and mobile media. Reach him at oonyeoh@gmail.com.