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More and more people in water-scarce countries rely on desalinated water for drinking, cooking and washing. The process involves removing salt from seawater and filtering it to produce drinking quality water. But the fossil fuels normally used in the energy-intensive desalination process contribute to global warming, and the toxic brine it produces pollutes coastal ecosystems.

While shifting towards low-carbon energy sources to power desalination plants can help reduce carbon dioxide emissions, the discharge of toxic brine from desalination plants into the ocean is a more challenging problem.

“New technologies are being developed to tackle these issues, but in the meantime it’s important to raise awareness of the trade-offs with desalination,” says Birguy Lamizana, a wastewater expert with the United Nations Environment Programme (UNEP).

Here are five things to know about desalination:

1. It’s a booming business. A 2018 United Nations study says there are now almost 16,000 desalination plants operating in 177 countries, producing a volume of freshwater equivalent to almost half the average flow over the Niagara Falls.
2. Several countries, such as Bahamas, Maldives and Malta, meet all their water needs through the desalination process. Saudi Arabia (population 34 million) gets about 50 per cent of its drinking water from desalination.
3. In most desalination processes, for every litre of potable water produced, about 1.5 litres of liquid polluted with chlorine and copper are created. This wastewater (“concentrate”) is twice as saline as ocean water. If not properly diluted and dispersed, it may form a dense plume of toxic brine which can degrade coastal and marine ecosystems unless treated. Increased salinity and temperature can cause a decrease in the dissolved oxygen content and contribute to the formation of “dead zones”, where very few marine animals can live.
4. Unconventional water resources, such as those resulting from desalination, are key to support Sustainable Development Goal 6 (to ensure availability and sustainable management of water and sanitation for all). Seawater desalination can extend water supplies beyond what is available from the hydrological cycle, but innovation in brine management and disposal is required. Research suggests there are also economic opportunities associated with brine, such as commercial salt, metal recovery and the use of brine in fish production systems.
5. The last decade has seen increased academic interest in recovering resources from brine, according to one study. Seawater contains various minerals, some of which are rare and expensive to mine on land. While extracting materials from brine is possible, its high cost restricts commercialization.

Globally, 80 per cent of wastewater – whether it is the toxic brine generated by desalination or other types of waste - ends up in seas, rivers, lakes, and wetlands. Under the Global Programme of Action for the Protection of the Marine Environment from Land-based Activities, UNEP is working to prevent degradation from land-based activities, such as the operation of desalination plants. The Global Programme also hosts and acts as the secretariat for the Global Wastewater Initiative.

In March 2019, the United Nations Environment Assembly adopted a resolution on the protection of the marine environment from land-based activities.

Protecting and restoring ecosystems from the impact of water, air and other types of pollution is a key tenet of the UN Decade on Ecosystem Restoration 2021-2030 and the UN Decade of Ocean Science for Sustainable Development (2021-2030).