Desalination

Desalination is the artificial process of converting saltwater into fresh water.

More broadly, it refers to removing salts and minerals from any substance. This process is essential for creating water for human consumption and for irrigation. Because it does not depend on rainfall, desalination is a vital tool for global water security. As freshwater shortages intensify, this technology helps close the gap in the world's water supply. However, the process is often more expensive than using groundwater or surface water. This cost is largely due to the high amount of energy required to separate salt from water.

There are two primary mechanical approaches to desalination: thermal methods and membrane-based methods. Thermal methods, such as distillation, rely on heat to separate substances. In these systems, saltwater is heated until it turns into vapor. Because salt does not turn into vapor, the steam remains fresh. When this steam cools, it condenses back into liquid potable water.

Membrane-based methods, specifically reverse osmosis, use a different physical mechanism. In reverse osmosis, water is pushed through a very thin membrane. This membrane acts as a filter that allows water molecules to pass through while blocking salt and minerals. This method is often more efficient because it does not require a phase change, meaning the water does not need to be boiled.

History shows that humans have observed these principles for millennia. The ancient Greek philosopher Aristotle noted in his work *Meteorology* that saltwater becomes "sweet" when it turns into vapor. He also observed that a fine wax vessel could act as a membrane to filter salt. In ancient China, records show that bamboo mats used for steaming rice developed a film that could adsorb salt.

During the Middle Ages, sailors used distillation to survive at sea. Leonardo da Vinci later realized that a still adapted to a cookstove could produce distilled water cheaply. By 1560, a captain in Tunisia built a still during a siege to produce 40 barrels of fresh water per day.

The Industrial Revolution and the age of steam changed everything for desalination. The development of the steam engine provided the power needed for more complex systems. Knowledge of thermodynamics helped engineers design better distilling systems for boilers. In 1852, Alphonse René le Mire de Normandy patented a vertical tube seawater distilling unit. This design was simple and became very popular for use on ships. By the 1860s, the US Army was using Normandy evaporators on islands like Key West. These early land-based units proved that desalination could support permanent populations in remote areas.

Modern desalination has reached a massive global scale. As of 2020, there were roughly 16,800 operating plants worldwide. These plants had a total capacity of about 97 million cubic meters per day.

By 2021, that number grew to approximately 22,000 plants. The largest single facility is the Ras Al Khair plant in Saudi Arabia, which produces 1,401,000 cubic meters of water every day. The industry is also becoming much more energy-efficient. In 1970, desalination required 20 to 30 kWh per cubic meter. By 2018, this energy intensity had dropped to about 3 kWh per cubic meter.

Technological advancements continue to drive the industry forward. In the 1960s, research into reverse osmosis began at universities and companies like DuPont. The first commercial reverse osmosis plant, the Coalinga plant in California, opened in 1965. This was a major milestone because it proved the technology was scalable and efficient.

Today, scientists are developing next-generation membranes. These include graphene-based membranes and aquaporin-inspired biomimetic membranes. These new materials improve water permeability and help prevent fouling, which is the buildup of unwanted material on the membrane.

Desalination is deeply connected to global energy and environmental systems. Because the process is energy-intensive, it represents about 25% of the energy used by the water sector as of 2016.

To make the process more sustainable, many plants are now integrating renewable energy like solar and wind power. Additionally, the process produces a byproduct called brine, which is highly concentrated salt water. As cities face severe droughts, governments are investing heavily in new infrastructure. For example, Spain recently announced a €340 million investment to build a large plant in Casablanca. This highlights how desalination remains a critical component of modern engineering and survival.