Desalination

Desalination is the process of turning salt water, like sea water, into fresh water that people can use for drinking and watering crops. This is done by removing the salts and minerals from the water, which leaves behind a salty liquid called brine.

We need fresh water more than ever, and desalination is becoming very important. It does not depend on rain, unlike many other ways we get water. Desalination is useful in places where other water sources are not available or are running out. There are two main ways to desalinate water: by using heat to evaporate the water, or by using special membranes to separate the salt.

History

See also: Distillation § History, and Distilled water § History

Long ago, people noticed that when seawater turns to vapor, it leaves the salt behind, creating fresh water. Ancient thinkers like Aristotle saw this and found ways to keep fresh water using simple tools.

For many years, people used heat on seawater to make fresh water, especially on ships where clean water was hard to find. Important thinkers and inventors kept improving these ideas. By the 1800s, new machines helped make the process better.

In the last hundred years, new methods like using special filters have made desalination more common and useful, especially in places where fresh water is scarce. Today, many countries use these methods to get clean water from the sea.

Applications

There are about 21,000 places around the world that turn salty water into fresh water. The biggest ones are in the United Arab Emirates, Saudi Arabia, and Israel. The largest one is in Saudi Arabia, and it can make almost 1.4 million cubic meters of fresh water each day.

Desalination is more expensive than most other ways to get water, so it is mostly used in places where water is hard to find, like in dry areas. It is often used on ships and submarines, and in busy places near the ocean, such as Singapore or California. The cost of making fresh water from the sea changes depending on how salty the water is, how big the plant is, and what method is used. Costs are going down as technology improves.

Some people think desalination might not be the best choice for everyone, especially for places far from the ocean or high up. In some areas, it might be cheaper to bring water from somewhere else instead of making it from salt water.

Technologies

The most common ways to turn salty water into fresh water are called distillation and reverse osmosis. There are several methods to do this, each with its own good points and challenges. These methods can be grouped into two types: those that use membranes and those that use heat.

The traditional way to remove salt from water is distillation. This means boiling the water and then catching the steam, which leaves the salt behind. Right now, two methods are used most around the world: multi-stage flash distillation and reverse osmosis.

Distillation

Solar distillation

Solar distillation works like the natural water cycle. The sun heats up sea water, causing it to evaporate. The water vapor then cools and turns back into liquid on a cold surface. There are two types of solar distillation. One uses special panels to turn sunlight into electricity to power the process. The other uses sunlight directly to heat the water, called solar thermal powered desalination.

Natural evaporation

Water can also evaporate through other natural effects besides sunlight. These effects are used in special greenhouses called the IBTS Greenhouse. This place combines making water and growing plants in a way that works well with nature, especially in dry areas.

Vacuum distillation

In vacuum distillation, the air pressure is lowered so that water boils at a lower temperature. This means we can use heat that is not very hot, like waste heat from power plants, to evaporate the water.

Multi-stage flash distillation

Water is turned into vapor and separated from sea water through multi-stage flash distillation. This process uses a series of steps where water vapor from one step helps heat the next step.

Multiple-effect distillation

Multiple-effect distillation (MED) uses a series of heated steps to turn sea water into steam. The steam from one step is used to heat the next batch of water. This method can use steam from power plants to work better. However, it has some limits, like the highest temperature it can reach and the most steps it can use.

Vapor-compression distillation

Vapor-compression evaporation uses a compressor to squeeze the vapor above the water. This heated vapor then helps evaporate more water. This method is best for small-scale uses because it only needs electricity.

Membrane distillation

Membrane distillation uses a special material to let water vapor pass through while keeping the salt out. The design of this material is very important for how well it works. Studies show that some special materials can remove almost all the salt from water.

Osmosis

Reverse osmosis

The most common way to remove salt from water today is reverse osmosis (RO). This method uses special membranes and pressure to push water through while keeping the salts out. Reverse osmosis plant systems usually need less energy than heating methods.

Reverse osmosis uses a very thin layer that lets water pass through but keeps salts out. This layer is very important for how well the process works. However, things like minerals, tiny particles, and living things can make it harder for the process to work well. To prevent problems, special treatments are used to clean the water before it goes through the membranes and to clean the membranes when needed. Some places use solar power to run reverse osmosis without needing electricity from the grid. There are also new ideas like putting the equipment deep in the ocean to use natural pressure to help remove salt.

The reverse osmosis plant in Tampa Bay makes about 25 million gallons of clean water every day, showing how useful this method can be.

Forward osmosis

Forward osmosis uses a special membrane and a difference in water concentration to separate clean water from salts.

Freeze–thaw

Freeze–thaw desalination uses cold temperatures to turn sea water into ice, leaving the salt behind. When the ice melts, it becomes fresh water. This can happen naturally during cold weather or using special machines that freeze the water in a vacuum.

Electrodialysis

Electrodialysis uses an electric current to move salts through special membranes that trap the salt. This can also remove carbonic acid from sea water, and the clean water made can help pay for removing the carbon.

Microbial desalination

Microbial desalination cells use special bacteria to help remove salt from water. These tiny living things create a natural power source that can help clean the water. This method is still being tested and has not been used in large plants yet.

Wave-powered desalination

Some systems use the movement of ocean waves to power desalination. These systems turn wave motion into water pressure to clean sea water. Plants using this method have started operating in places like Australia and Chile.

Wind-powered desalination

Wind energy can also be used to power desalination. Turning wind into water pressure can help clean sea water without many extra parts.

Desalination by thermophoresis

In 2024, scientists tested a new way to remove salt from water using heat differences. By passing salty water through a channel with different temperatures, they were able to reduce the salt content. By 2025, they improved this method and showed it could be good for cleaning very salty waste water.

Design aspects

Turning salt water into fresh water needs energy. The saltier the water, the more energy it needs. Water from rivers or lakes, called brackish water, needs less energy to clean than ocean water. We have found ways to use less energy over time. Today, cleaning ocean water needs about 3 kWh for every cubic meter, which is much less than before.

We can also make heat and electricity at the same time as cleaning water. This helps save energy. Some places use special power plants just for cleaning water. This can make the whole process better and cheaper.

There are other ways to get clean water besides cleaning salt water, like saving more water, reusing wastewater, and catching rainwater. These options can sometimes be better and cost less than cleaning salt water.

Costs

Desalination costs change based on many things like the size of the plant, where it is, the type of water used, labor, energy needed, and how the leftover salty water is handled. Desalination is usually more expensive than getting water from rivers, groundwater, or water recycling, but sometimes those options aren't available.

Getting clean water without treatment in some parts of the world can cost up to US$5 for every cubic meter. Technology has improved a lot since 1975, making desalination cheaper. Even so, desalination uses a lot of energy, which can be a problem, especially in places with limited energy.

The world-wide market for desalination was worth $20 billion in 2023 and is expected to grow because more people live in dry areas near the coast. Desalination systems control pressure, temperature, and the amount of salty water to work better. Some places made water for less than US$0.40 per cubic meter. Singapore made water for about US$0.49 per cubic meter as of 2006. In 2025, a new plant in Rizhao, China, made water for about US$0.28 per cubic meter using waste heat.

Environmental concerns

Desalination plants can affect the environment in several ways. When water is taken in, small sea creatures might get caught and harmed. Some plants are designed to take in water slowly so fish can escape.

These plants also produce a salty mix called brine, which is returned to the ocean. This can change the ocean's salt levels and harm sea life. To reduce this, the brine is often mixed with other water before being released. Even so, there are worries about how this affects marine life.

Desalination also uses a lot of energy, especially in places where fresh water is scarce. This energy often comes from fossil fuels, which can affect the environment.

Health aspects

Iodine deficiency

Desalination can take iodine out of water. This might cause health problems because iodine is important for our bodies. Researchers in Israel noticed that people who drank water from desalination plants often had lower levels of iodine. This is because much of Israel's drinking water comes from desalination, and the country does not add iodine to its salt widely. As a result, more people needed medicine for their thyroids. Thyroid glands need iodine to work well.

Experimental techniques

Other ways to clean salt water use heat that is not needed for other things. For example, heat from engine exhaust can help turn salt water into fresh water. This saves energy and helps the engine work better.

Some methods use natural temperature differences in ocean water. Cold water from deep in the ocean helps make fresh water. Tests of these ideas have been done in places like the United States, Japan, and India.

New ideas keep being invented to clean salt water. Some use sunlight to power the cleaning process. Others use special materials that soak up water and then release it when squeezed. These new methods are being tested to make them work better and cost less. They could help places that do not have clean water or power.

Researchers are also looking at ways to use nature, like special membranes, to clean water. Some methods use electricity or shock waves to remove salt without needing high pressure. These ideas are still being studied and may help in the future.

Other methods use waves from the ocean to power the cleaning process. Small systems made from recycled plastic bottles can make fresh water without needing extra power. These ideas show that many different ways can be used to turn salt water into clean water.

Use around the world

Saudi Arabia makes the most fresh water using desalination. It creates about 7.9 million cubic meters of water each day. Many places use desalination when they need more water.

Cities like Perth and Sydney have desalination plants. Some of these plants use energy from wind farms. In the United States, a plant in Carlsbad helps give people drinking water. Countries such as Israel, Singapore, China, and India also use desalination. The large plant at Jebel Ali in the United Arab Emirates makes over 2 million cubic meters of water each day. Even ships, like U.S. aircraft carriers, can make their own fresh water with special systems.

In nature

In nature, water loses salt in a few interesting ways. When water evaporates from the oceans as part of the water cycle, it leaves the salt behind. When sea ice forms, it has much less salt than the water around it.

Some birds that live near the ocean, like pelicans, petrels, albatrosses, gulls, and terns, have a special gland that helps them drink salty water. This gland collects extra salt, and the bird gets rid of it by "sneezing" it out through its nose.

Mangrove trees, which grow in salty water, also have ways to handle salt. They trap salt in their roots, where small animals like crabs eat it, and they drop salt from their leaves. Some mangrove leaves even have tiny glands that push salt out as crystals that fall off the leaf.

Certain plants like willow trees and reeds can absorb salt and other unwanted materials from water. This natural process is used in special man-made wetlands to clean up dirty water, such as sewage.

Society and culture

Many people want to build desalination plants, even though there are some problems. In one community in Southern California, a survey found that most people supported building a desalination plant there. Places where fresh water is hard to find often support desalination more, while areas with plenty of water support it less.