What is Desalination & How Does It Work?

Simply put, desalination is a method of removing salt from water. Sounds simple, right? Well, it’s actually pretty complicated and also vital for the survival of the human race. 

So today, we want to share why desalination is so important, how it works, and what makes it so expensive. We’ll also cover the most common desalination methods.

What Is Desalination?

Roughly 96.5% of the water on Earth is salty, and most of the remaining 3.5% is locked away in glaciers, ice caps, and deep aquifers we can't easily reach. That leaves a surprisingly thin slice of fresh water to support every person, crop, and ecosystem on the planet — and demand keeps climbing.

Desalination, the process of removing salt from seawater to make it drinkable, is one of the few technologies that can meaningfully expand that slice. The oceans are effectively limitless. The challenge is making the process affordable enough to scale. As freshwater reserves shrink, desalination is shifting from a regional workaround to a serious part of how the world plans to stay hydrated.

How Does Desalination Work?

Desalination separates fresh water from the salt dissolved in it. The oldest and most intuitive method is evaporation: heat seawater, capture the vapor, and let it condense back into liquid. The salt, which doesn't evaporate, stays behind. When the sun supplies the heat, the process is called solar desalination, and it's the same mechanism that drives the natural water cycle, where ocean water rises into the clouds and returns as rain.

Humans have been borrowing this trick for thousands of years. Ancient sailors built simple solar stills to make seawater drinkable on long voyages, and the basic principle hasn't changed since.

What's changed is the scale and sophistication. Modern desalination plants stretch across acres, combining solar evaporation with high-pressure membrane filtration to turn seawater into drinking water at industrial volumes.

What Are the Most Common Desalination Methods?

There are two main methods for desalination: solar desalination and reverse osmosis desalination. While solar desalination depends on the power of the sun, reverse osmosis forces salt water through a filter membrane to produce fresh water.

Solar Desalination

Solar desalination is the most common method and replicates the natural rain cycle. Scientists can separate the salt molecules from the water by heating and evaporating salt water to create clean water.

Usually, a process called flash evaporation is used at the end of the desalination cycle. This is where a flash of heat is applied to the water, turning it into gas, and leaving most of the salt behind. The gas is then trapped in a new container and allowed to cool down.

This process must be repeated a few times to desalinate the water thoroughly. Otherwise, small salt molecules will remain, and the water will not be drinkable. Flash evaporation is similar to vapor compression, which also moves the gasses to a different container to separate the steam from salt.

Solar desalination can also occur through a distillation process, which involves heating the water through multiple filters until only the cleanest, salt-free water is available.

Each of these methods releases the salt from the water and makes it potable. However, desalination at scale is still very, very expensive and energy-intensive. Unfortunately, the growing need for water is at odds with the massive amounts of money and energy necessary to run a desalination plant.

Reverse Osmosis Desalination

Osmosis is a naturally occurring process that has been studied extensively in science. However, reverse osmosis (RO) is a human invention that involves forcing a liquid through a membrane to separate the heavier particles from the lighter ones.

The process removes chemical and biological contaminants from water, such as minerals and bacteria.

In the case of salt water, reverse osmosis works by forcing the saltwater through a semi-permeable membrane. When it’s pulled through, the heavier salt particles remain on one side of the membrane while the lighter water molecules cross through.

Once the process is complete, the salt is effectively separated from the water.

How Efficient Is Reverse Osmosis in Desalination?

Reverse osmosis is the most recent development in desalination science and the more efficient method of the two. Instead of having to be filtered and boiled repeatedly, water only needs to pass through an RO filter once to be desalinated.

Plus, many RO filters have a total dissolved solids (TDS) removal rate of over 99%, so they can greatly reduce the amount of salt present in water. However, they’re usually unable to remove 100% of the salt, and they also strip the water of beneficial minerals along with the contaminants.

When it comes to efficiency, another important factor to consider is the water recovery rate. This refers to the percentage of water that enters the RO system and is effectively filtered, rather than being wasted.

The optimal range for this is between 75-85%. If any higher, the lifespan of the filter and the system’s salt removal capacity may decrease.

Why Is Desalination So Expensive?

If desalination is the answer to global water shortages, then why aren’t there more desalination plants? The short answer is money.

Desalination is expensive, and it’s more than just a startup cost as the plants are very expensive to run once they’ve been built.

Desalination plants consume a huge amount of energy, which is the main reason they’re costly to operate. Other factors that increase the costs include frequent membrane replacements, water pre-treatment and post-treatment, and brine disposal.

How Much Does a Desalination Plant Cost?

The cost of a desalination plant depends on the size of the plant. A smaller, 2.5 MGD (million gallons per day) plant can cost more than $32 million to build. However, a larger plant that can clean up to 100 million gallons daily will often cost at least $700 million.

For a technology that is relatively new and not as efficient as we would like, it’s difficult to convince government officials and local benefactors to fund new desalination plants.

However, as the science of desalination evolves and advances, prices should continue to come down.