Desalination

Podcast Transcript

Seventy percent of the Earth’s surface is covered with water… and the vast majority of it is useless for consumption or agriculture.

This problem has been known for thousands of years, and for thousands of years, humans have recognized that it is possible to turn seawater into drinking water; it was just difficult to do so.

In the last few decades, however, the ability to get clean drinking water from the sea has gotten easier and might get even easier still.

Learn more about desalination, how it works, and how it has evolved on this episode of Everything Everywhere Daily.0

I’m sure most of us have had the exact same thought when we’ve heard about the crisis of fresh water that afflicts parts of the world. Why don’t we just crank up some desalination plants and get fresh water from the sea?

Problem solved.

In theory, that isn’t wrong; however, as I often like to say on this show, there’s more to it than that.

Removing salt from seawater isn’t conceptually difficult; however, it becomes very difficult to do at scale.

The story of desalination goes back thousands of years.

The story begins with our earliest ancestors, who likely observed that when seawater evaporated in tidal pools, it left behind salt crystals. This natural phenomenon planted the seed of an idea: what if we could capture the pure water that escaped as vapor?

The ancient Greeks were among the first to really think about this observation. Around the 4th century BC, Aristotle wrote about distillation, describing how seawater could be heated to produce vapor that, when cooled, would condense into fresh water.

Ancient sailors discovered they could boil seawater in pots and capture the steam on cloth or metal surfaces. When the steam condensed, they had drinking water. This was labor-intensive, fuel-intensive, and didn’t produce much water, but it could mean the difference between life and death on long voyages.

During the medieval period, Islamic scholars and alchemists significantly advanced distillation techniques. They weren’t primarily focused on desalination, but their work on perfecting distillation apparatus laid crucial groundwork. These innovations would later prove essential for scaling up seawater treatment.

As European exploration expanded in the 15th and 16th centuries, the need for reliable freshwater at sea became critical. Ships began carrying primitive solar stills—essentially glass-covered boxes where seawater would evaporate under the sun’s heat and condense on the cooler glass surface.

Again, this could only produce a few cups of water a day, but it was better than nothing.

The first known land-based seawater distillation plant was established by Spanish forces in Tunisia in 1560, who were besieged by the Ottoman Empire. Facing acute shortages of potable water while stationed near the arid coast, Spanish engineers constructed a rudimentary yet effective desalination apparatus onshore. This plant boiled seawater in metal vessels over open fires and captured the steam in rudimentary condensation coils to collect fresh water.

In the 17th century, Robert Boyle and other early chemists began experimenting with distillation more formally, improving the theoretical understanding of phase changes and condensation. However, the practicality of desalination was limited by the energy demands and complexity of the equipment.

The 19th century marked a turning point. Steam engines weren’t just revolutionizing transportation and manufacturing; they were making large-scale desalination theoretically possible. Ships could now dedicate steam power specifically to distillation, producing more freshwater than ever before.

The first industrial land-based desalination plant was built in 1869 in Aden in what is today Yemen, by the British, who needed to supply fresh water to ships traveling to India. This plant used steam distillation and could produce about 5,000 gallons per day—a significant achievement for its time, though tiny by today’s standards.

Both World Wars accelerated desalination research. Submarines needed compact, efficient systems to produce drinking water during long underwater voyages. The military’s willingness to invest heavily in research, combined with the life-or-death necessity, pushed the technology forward rapidly.

During this period, engineers began experimenting with different approaches beyond simple distillation. They developed multi-stage flash distillation, where water is heated under pressure and then released into chambers at lower pressure, causing it to “flash” into steam. This was more efficient than simple boiling because it could reuse heat energy multiple times.

In the 1950s and 1960s, multi-stage flash distillation became the dominant desalination method, especially in oil-rich but water-poor nations like Saudi Arabia, Kuwait, and the United Arab Emirates. These countries had access to cheap fossil fuels and could build large-scale plants along the coast.

Everything I’ve described up until this point, from ancient times to about the mid-20th century, is all using variations of what is called thermal desalination. Whether it is simple condensation, distilling, or flash distillation, all of these involve the use of heat to separate water from salt.

Thermal desalination is something that you could do on your kitchen stove, although it isn’t that efficient.

It was around the 1960s that a second type of desalination became practical. Generally, this category is referred to as membrane desalination.

Scientists discovered that certain materials could act as selective barriers, allowing water molecules to pass through while blocking salt ions. This led to the development of reverse osmosis, a process fundamentally different from distillation.

In 1965, the first reverse osmosis membranes were developed at UCLA by Sidney Loeb and Srinivasa Sourirajan. These membranes used semi-permeable materials that could separate water from dissolved salts under pressure—a radically different and more energy-efficient approach than distillation.

To describe how this works, I have to explain a few things.

First, a semipermeable membrane has microscopic pores, typically around 0.0001 microns wide, that allow water molecules to pass but block dissolved salts, bacteria, and larger molecules.

Second, to know how reverse osmosis works, you need to know how osmosis works.

Let’s say you have a container with a semipermeable membrane separating it into two. On one side, you put seawater and on the other side, you put fresh water.

What would happen?

Via osmosis, water from the freshwater side will migrate to the saltwater side. This is because the salinity levels on the two sides are out of balance and water will move to the salty side to dilute it to put the two sides in balance.

The water has to move because the salt can’t.

Osmosis, however, is the exact opposite of what you want if you want to make fresh water out of seawater.

This is where reverse osmosis comes in.

In reverse osmosis, you put pressure on the salty side to push water across the semipermeable membrane to separate it from the salt in the seawater.

This isn’t just dumping water on a membrane to filter out the salt like you would use cheese cloth to filter out particulate matter. To get the water through the semipermeable membrane you need pressure……a lot of pressure.

Creating that much pressure takes a lot of energy. It is much less energy than thermal desalination, but it still takes energy.

From the 1980s onward, rapid improvements in membrane technology, particularly polyamide composite membranes, greatly increased the efficiency and viability of reverse osmosis. These membranes could operate at lower pressures, resist fouling, and recover more freshwater from input seawater. Reverse osmosis became the dominant desalination method globally by the early 2000s.

Large-scale reverse osmosis plants have been constructed in Spain, Israel, Australia, Singapore, Chile, and the United States, notably in California and Texas. Israel, in particular, became a global leader, utilizing reverse osmosis to supply over 60% of its domestic freshwater by the 2010s.

Today’s reverse osmosis filters are constructed like a paper towel roll. Instead of paper towels, there are layers and layers of membranes. High-pressure saltwater is on the outside of the layers, and in the core is a pipe where the fresh water flows.

The average pressure used in modern reverse osmosis systems is about 55–70 bar or 800–1,000 pounds per square inch.

Today, there are approximately 21,000 seawater desalination facilities worldwide, spanning approximately 150–177 countries.

These plants combined produce around 100 million?m³/day which translates to 26 billion gallons or almost 100 million liters every day.

That’s a lot of water, but it is only a fraction of the total water used by humans every day.

What would be necessary to increase the amount of desalinated water produced on the planet?

The biggest thing would be to devote significantly more energy to desalination.

There has been talk of building nuclear reactors, especially dedicated to desalination. Likewise, there has been talk of fields of solar panels in deserts and equatorial regions, which would be used for running desalination facilities.

Passive thermal systems have also been proposed, which would be giant glass domes in the desert, where saltwater could evaporate, condense on the glass, and be collected.

Basically, just using current technology, the more energy we throw at the problem, we more desalinated water we can get.

However, there are other new methods that are promising.

One solution would be to just make better membranes.

Graphene-based membranes represent a cutting-edge development in desalination technology, offering the potential for faster, more energy-efficient water purification. These membranes are typically made from graphene oxide or single-layer perforated graphene sheets engineered with nanopores precisely sized to allow water molecules to pass while blocking salts and other contaminants.

Because graphene is just one atom thick, water can flow through it orders of magnitude faster than through traditional polymer membranes used in reverse osmosis. This ultra-thin structure could drastically reduce the energy required to pressurize water, a major cost factor in desalination. Additionally, graphene membranes show high resistance to fouling and chemical degradation, increasing their durability and reducing maintenance.

Another proposal, and one that I personally think is rather brilliant, is to use the natural high pressures of the ocean floor.

If you put a reverse osmosis filter far enough below the surface of the ocean, you can reach pressures that are the same as those required for reverse osmosis systems.

Do you know what else is on the bottom of the ocean? Seawater.

Of course, there needs to be a pressure difference to move the fresh water along. You can create that by creating suction to make a pressure differential. The amount of energy to create suction to suck high pressure water out is much less than the energy required to create high pressure water on land.

To be sure, while this would reduce the amount of energy required, it would also create its own headaches, including performing maintenance on filters sitting on the ocean floor.

There are other potential technologies as well that could be used, including nanofilters and forward osmosis.

The ability to create freshwater from seawater is a vital technology in the 21st century. Depending on where you live, it might not be something that you ever encounter.

However, millions of people every day rely on it to get water for drinking, bathing, and washing. Without it, ships and submarines would find it much more difficult to spend lengthy amounts of time at sea.

Assuming trends continue, the amount of usable water that we get from the sea should be increasing for years to come.