WHAT IS DESALINATION & HOW DOES IT WORK?

Simply put, desalination is a way of removing salt from water. Sounds simple, right? Well, it’s actually pretty complicated. It’s 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?

Desalination is a process that removes salt from salt water, making it drinkable for humans. This process is vital for getting drinking water on a large scale—96.5% of the earth’s water is salty, which means that most water on earth can’t be drunk by humans.

The other 3.5% is available for human consumption, but much of it remains trapped in polar ice caps or huge lakes.

Of course, fresh water is essential to human life. No one can survive without it for more than a few days, and dehydration is one of the most significant health issues on the planet. The freshwater crisis is a humanitarian issue and something that desalination can help solve.

Ocean water only contains 2.5% salt, but that’s enough to dehydrate a human and cause eventual death if it’s consumed instead of fresh water.

While access to the existing fresh water is an issue, the more significant problem is what humans will do if we ever run out. Desalination may be the only solution.

There is nearly unlimited water in the ocean—and although it’s currently very expensive, desalination might be the future of a planet with dwindling freshwater resources.

How Does Desalination Work?

Desalination works by separating salt molecules from water molecules. Because salt is heavier than water, the most common method is to evaporate the water molecules using the heat of the sun or another source. This system is called solar desalination.

Solar desalination is nature’s way of removing salt from water. This process naturally occurs in the rain cycle, when the sun’s heat pulls water from oceans and lakes into clouds for rain. The salt is left behind, and freshwater is produced as rain water.

Of course, the process doesn’t only occur in nature. Scientists have successfully replicated it in massive desalination plants worldwide. What started as simple, sun-powered bubbles have become huge plants dedicated to creating clean water out of salty water.

Humans have been desalinating water for thousands of years. Many ancient civilizations used solar desalination to procure fresh drinking water while sailing on the ocean. The process hasn’t changed too much—but what has changed are the size and capacity of the methods used for desalination.

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.

This was originally an H2 titled “How Is Reverse Osmosis Used in Desalination?” I changed it to an H3 so the structure of the article makes more sense, but happy to change it back if it's important that it stays an H2 for SEO reasons.