Desalination Plants: The Future of a Sustainable Water Supply

Water covers at least 70 percent of the world's surface. But 97 percent of it is too salty to drink [source: NatGeo]. This is where desalination comes in.

There's more than one way to separate salt from water, but nearly 90 percent of the time, only one of two methods is used: reverse osmosis and multistage flash. Let's take a closer look at both.

Reverse Osmosis

As we mentioned, drinking salt water can adversely affect our health due to the cellular imbalances it can cause. Within our cells, there's a natural process called osmosis that tries to balance the salt concentration inside and outside the cell. Reverse osmosis takes this principle and applies it to water purification.

It uses a special membrane, similar to a very fine filter, that allows only water to pass through, blocking the salt. To push the salt water through this membrane, strong pumps apply pressure, ensuring that only pure water comes out on the other side, leaving the salt behind. This process effectively transforms salt water into fresh, drinkable water at a reverse osmosis desalination plant.

Multistage Flash

Unlike reverse osmosis, which uses a semipermeable membrane to filter out salt and other impurities, the multistage flash method relies on heat and pressure variations to convert salt water into fresh water.

The term "flash" refers to the rapid evaporation due to sudden pressure drop, and this process is repeated in various stages, hence the name.

In each stage of the conversion unit, salt water is exposed to steam heat and reduced pressure. This causes a portion of the water to rapidly evaporate or "flash" into water vapor, which is essentially fresh water. The remaining salty solution is termed "brine."

This method, like reverse osmosis, typically doesn't require added chemicals or softeners [source: Organization of American States]. However, despite the potential of desalination, large-scale plants currently produce only a small fraction of the world's daily freshwater consumption.

As of 2022, there are over 20,000 desalination plants in operation worldwide, producing more than 95 million cubic meters of water daily. The Middle East continues to be a major hub for desalination, given its water scarcity issues and technological advancements in the field.

The Sorek Desalination Plant in Israel, which began operations in 2013, was once one of the world's largest seawater reverse osmosis (SWRO) desalination facilities. However, the rapid expansion of desalination technology has led to the construction of even larger plants in various parts of the world.

While the desalination process has advanced and become more efficient, public perceptions vary. In places like Israel, there have been concerns raised about the taste or potential health impacts of desalinated water.

However, research and studies, including those from Saudi Arabia, show that when desalination is done correctly, the resulting water is safe for consumption and comparable in quality to bottled water. A notable environmental advantage of using desalinated tap water over bottled water is the reduction in plastic waste.

The concept of portable desalination has also seen developments. In recent years, a variety of portable desalination devices have become available, designed for emergency relief, hiking or personal use. These tools can convert salty or contaminated water sources into drinkable water, offering innovative solutions in areas with limited freshwater access.

Salt and So Much More

Sure, you might expect the desalination process to remove salt from ocean water. But did you know the cleanup doesn't stop there? Desalination — whatever the method — also removes organic or biological chemical compounds, in the end producing high-quality drinking water that doesn't transmit diarrheal or other diseases.

This is important because nearly 4 million people die each year "due to inadequate water supply, sanitation and hygiene," according to the United Nations.

Lewis Carlsbad Desalination Plant

The Claude "Bud" Lewis Carlsbad Desalination Plant, located in Carlsbad, California, is the largest seawater desalination facility in the Western Hemisphere.

Opened in December 2015, the plant utilizes reverse osmosis technology to convert seawater from the Pacific Ocean into potable water. With a production capacity of approximately 50 million gallons of drinking water per day, it provides about 10 percent of the water supply for San Diego County.

The construction of this seawater desalination plant was a response to recurring droughts and the need for a reliable, drought-proof water source. The project includes an energy recovery system to reduce energy consumption and environmental enhancements to protect marine life.

While it offers a solution to water scarcity, concerns include energy consumption and environmental impact, particularly the disposal of brine byproduct. Nonetheless, the Carlsbad plant serves as a model for other regions considering desalination to address water shortages.

As the number of desalination plants worldwide continue to grow, so do concerns about developing new technology to power the plants. Currently, large-scale desalination efforts require a lot of energy to operate and often are high-maintenance affairs, thanks to lots of working parts like membranes that tend to foul frequently [source: Schirber].

Costs are another concern: During the past five decades, public and private investment in developing desalination technology has reached more than a billion dollars worldwide. And even with the progress that's been made, the idea that desalination would do away with water scarcity is far from reality.

That's because it's still really expensive to plan, build and manage desalination plants.

In fact, the average cost to turn one acre-foot — about 325,000 gallons — of salt water into fresh water ranges from $2,000 to $3,000 (and requires a significant amount of energy [source: ABC News]. When comparing reverse osmosis and multistage flash methods for turning seawater into fresh water, the former is more efficient.

With reverse osmosis, you need only one-third of the seawater to get the same amount of fresh water as you would with multistage flash. This means less energy and other costs are needed to pump seawater to the plant and get rid of the leftover salty water.

Unfortunately, both processes — as with all desalination techniques — create brine. This by-product of desalinated water contains high concentrations of salt and, when released back into a natural body of water, poses a significant threat to aquatic ecosystems.

Due to its density, brine tends to settle above the seabed, forming a layer that restricts oxygen circulation. This can lead to oxygen-deprived zones, negatively impacting marine organisms, particularly those living in benthic habitats. [source: University of Texas at Austin].

Scientists are exploring several methods to mitigate the environmental impact of brine from desalination processes. They are researching advanced technologies like electrodialysis and forward osmosis to improve water recovery rates, consequently producing less brine.

Others are studying diluting brine with treated wastewater before discharge, reducing its salinity. The cultivation of halophyte plants, a type of marine vegetable that thrives in high-salinity conditions, is another potential avenue.

A portable desalination device is an innovative solution tailored to turning saline water into potable water, especially crucial in isolated or crisis-hit environments. In 2022, researchers from MIT developed an efficient model, compactly sized like a suitcase. Its energy efficiency is so optimized that a budget-friendly solar panel can drive it.

This device is different from traditional models because it bypasses the need for filters. Instead, it harnesses electrical power for water purification, drastically cutting down on upkeep.

Such advancements make it invaluable for regions like remote islands, ships on extended voyages, refugees impacted by natural calamities or soldiers in remote military outposts.

“This is really the culmination of a 10-year journey that I and my group have been on. We worked for years on the physics behind individual desalination processes, but pushing all those advances into a box, building a system, and demonstrating it in the ocean, that was a really meaningful and rewarding experience for me,” says senior author Jongyoon Han, a professor of electrical engineering and computer science and of biological engineering, and a member of the Research Laboratory of Electronics (RLE).

In addition to government funding in the United States and abroad, private funding sources are beginning to pay attention to developing more efficient desalination efforts. One thing is certain: If the desalination process improves, it could have the potential to change entire water-poor regions for the better.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.