Desalination — can it help us survive water scarcity?

Holly Young, Kira Schacht

From Ancient Greek sailors boiling seawater to Romans using clay pipes to filter salt, making saltwater drinkable through desalination has a long history.

But modern forms of this millennia-old technology are now the "present and future of coping with water scarcity," said Manzoor Qadir, deputy director of the United Nations University Institute for Water, Environment and Health. While 70% of the Earth's surface is covered in water, less than 1% of the planet's 326 million trillion gallons (1260 million trillion liters) of water is drinkable.

And these finite resources are increasingly squeezed. Growing populations, unequally distributed freshwater and increasing drought linked to climate change are leaving many parts of the world dry and thirsty.

A quarter of the global population lives in countries facing "extreme water stress," defined as a place using at least 80% of their entire available water supply every year. That can put communities in danger of running out of water and sometimes forces governments to severely restrict supplies.

Even under optimistic climate scenarios for limiting planetary heating, an additional 1 billion people are expected to be in this situation by 2050. Despite criticisms regarding cost, high-energy use, and environmental impact, experts say desalination "is a growing industry." It's been steadily expanding over the last two decades.

"And the reason is that water scarcity is pushing it very hard," said Qadir. "More and more desalination plants are coming up and being commissioned."

Desalination plants today remove salt from water either using thermal distillation, which involves heating the liquid and collecting the vapor, or through reverse osmosis, where water is filtered as it is pushed through a semi-permeable membrane.

Alternative ways of sourcing freshwater, such as artificially inducing rain through cloud seeding, harvesting water from fog, transporting icebergs to arid regions and water reuse, can play an important role. But they cannot yet be scaled to fully answer the world's water demands, said Qadir.

Today 56 billion liters of desalinated water can be produced every day. That equates to around 7 liters for every person on earth. Of the estimated 16,000 plants in operation around the world, 39% of them are in the Middle East, which alongside North Africa, is the most water-stressed region in the world.

A future without desalination for many of these countries would be "almost impossible," said Qadir, given, for instance, how many countries withdraw more water than is replenished through rainfall. Globally, only around half a percent of all water used comes from desalination. But in countries such as Qatar and Bahrain, it makes up 76% and 56%, respectively.

Desalination uses lots of energy, most of which comes from power plants run on greenhouse gas-emitting fossil fuels. A 2021 study found that the four desalination plants operating in Cyprus, the EU's hottest and driest country accounted for 5% of its total electricity consumption and 2% of greenhouse gas emissions. The plants supply the small island nation with an estimated 80% of its drinking water.

However, increased energy efficiency is one of the factors driving the industry's growth, explained Hugo Birch, desalination and water reuse editor at Global Water Intelligence, an industry information platform.

Birch said that most new desalination plants use reverse osmosis rather than thermal processes and this is much more energy efficient. The switch can potentially half electricity costs.

According to one estimate, the energy required for reverse osmosis desalination dropped by almost 90% between 1970 and 2020. Some predictions say technological advances may reduce the costs of desalinated water by 60% in the next 20 years.

While the costs of producing desalinated water have decreased significantly — to around $0. 50 per cubic meter today — it is "still a business of rich countries," said Qadir. "The key issue is that it is still not affordable for low-income countries."

Over 90% of desalination happens in upper-middle- and high-income countries around the world, even though poorer countries, such as those in sub-Saharan Africa, are predicted to become water scarcity "hotspots" by 2050.

While smaller solar or wind-powered off-grid desalination plants are being developed, Qadir doesn't believe these products are reaching the marginalized communities that need them most.

One of the key environmental concerns with desalination is brine discharge into the natural environment which can cause "marine pollution, underground water pollution, and soil salinization," said Argyris Panagopoulos, a chemical engineer from the National Technical University of Athens.

An estimated 70% of the global brine output comes from the Middle East, and the region also produces a high amount in proportion to desalinated water.

Birch says this can largely be explained by the fact Middle East desalination plants mostly use seawater, which is more saline than the brackish water often used, for example, in the US. He added that plants usually have a built-in diffusion mechanism to ensure brine isn't dumped in one area.

Some new brine treatment technologies are emerging, which could help reduce the volume of pollution and recover valuable materials such as minerals, salts and metals, noted Panagopoulos.

However, there is room for improvement in brine management and switching to green energy sources, according to Panagopoulos. "Desalination has made significant steps in recent years, but there are still challenges to overcome before it can be considered fully environmentally sustainable," said the chemical engineer.

Still, Manzoor Qadir of the UN University believes desalination will play a vital role in tackling future water scarcity because climate change does not affect it.

"Regardless of whether there is rainfall, whether there is a drought … there is seawater … So that's actually the best part of desalination," said Qadir.