How to Make Ocean Water Drinkable: Methods & Science

Removing salt from ocean water to make it drinkable is entirely achievable and has been practiced for thousands of years, from crude boiling methods used by ancient sailors to the industrial-scale reverse osmosis plants that today supply drinking water to hundreds of millions of people. The core challenge is energy: seawater contains roughly 35 grams of dissolved salts per liter, and separating those salts requires overcoming powerful thermodynamic forces. Modern engineering has found several distinct ways to do that, each with its own tradeoffs in cost, scale, and environmental footprint.

Reverse Osmosis and Why It Dominates

Reverse osmosis, or RO, is the most widely used desalination method worldwide. It works by pushing seawater through a semi-permeable membrane at high pressure. The membrane’s pores are small enough to let water molecules through while blocking dissolved salts. Industrial plants typically operate at pressures between 55 and 70 bar for seawater, and the resulting freshwater, called permeate, emerges with well over 99 percent of the original salt removed.

The reason RO overtook older thermal methods comes down to energy. A modern seawater RO plant consumes roughly 3 to 4 kilowatt-hours per cubic meter of drinking water produced, a figure that has dropped substantially thanks to energy recovery devices. These devices capture the pressure still locked in the concentrated brine leaving the membranes and transfer it back to incoming seawater. Isobaric energy recovery devices achieve a net transfer efficiency of up to 97 percent and can cut a plant’s energy consumption by as much as 60 percent compared to systems without recovery.¹ That efficiency gain is what made large-scale seawater RO economically competitive.

Thermal Desalination Still Has a Role

Before membranes took over, thermal desalination was the standard. The basic idea is simple: heat seawater until it evaporates, then collect and condense the steam. The salt stays behind. The most common industrial version is multi-stage flash distillation, where seawater passes through a series of chambers at progressively lower pressures. Because water boils at a lower temperature under reduced pressure, each stage “flashes” a portion of the brine into steam without needing to reheat it to 100 °C. A 16-stage flash system, for example, requires superheated steam at a top brine temperature of about 130 °C, with the temperature dropping roughly 2.3 °C per stage.²

Thermal desalination uses more energy than RO, but it handles very salty or heavily polluted feedwater better. It also pairs naturally with power plants and industrial facilities that produce waste heat. In the Persian Gulf, where seawater is warmer and saltier than the global average, thermal plants remain common because conditions favor them. Nuclear energy has also been explored as a heat source, since reactors produce large amounts of recoverable thermal energy that can drive both electricity generation and distillation simultaneously.³

What Happens Before the Water Hits the Membrane

Seawater is not just salty. It contains sediment, algae, organic matter, and microorganisms that would quickly clog a reverse osmosis membrane. Pretreatment removes these contaminants before the water reaches the main desalination stage, and the quality of pretreatment largely determines how long membranes last and how reliably a plant operates.

Conventional pretreatment uses sand filters or polymer-based ultrafiltration membranes. These work well under normal conditions but struggle during harmful algal blooms, when the water is loaded with organic compounds. Ceramic ultrafiltration membranes are emerging as a more robust alternative. During algal bloom season, ceramic systems maintained full capacity while conventional polymer systems had to reduce throughput by 30 to 40 percent. The ceramic membranes also removed more than five times as much dissolved organic carbon and about 1.5 times more of the sticky organic particles that promote fouling downstream.⁴ Good pretreatment is often the difference between a plant that runs smoothly and one that constantly shuts down for membrane cleaning.

Membrane Fouling and Scaling

Even with excellent pretreatment, RO membranes gradually degrade. Two enemies are responsible: biofouling, where microorganisms form slimy biofilms on the membrane surface, and mineral scaling, where salts like calcium sulfate crystallize out of the increasingly concentrated brine and coat the membrane. Both reduce water flow and force plants to use more energy or shut down for cleaning.⁵

The interaction between biofouling and scaling is more complicated than researchers initially assumed. Mineral deposits can actually change the membrane surface in ways that temporarily repel microbes, while microorganisms and the organic goo they secrete can alter how mineral crystals grow, sometimes making the crystals smaller and slower to form.⁶ One promising countermeasure is coating membranes with graphene oxide, which in laboratory tests reduced bacterial growth by about 82 percent and cut flux decline from scaling by roughly a third compared to unmodified membranes.⁷

Making the Freshwater Safe to Drink

Desalinated water that comes off a membrane or out of a distillation system is not immediately suitable for your tap. It is nearly pure water, which sounds good but is actually a problem. Water without dissolved minerals is slightly acidic and aggressively corrosive. If you pumped it straight into a city’s pipe network, it would eat through metal and concrete infrastructure.⁸

The fix is remineralization, the last treatment step before distribution. Plants add calcium, magnesium, and bicarbonate back into the water and adjust the pH so the water reaches a stable chemical balance. The goal is a water that neither dissolves nor deposits calcium carbonate in the pipes. Getting this balance right involves managing not just individual mineral concentrations but the ratios between them, since both human health and infrastructure longevity depend on it.⁹ If you have ever heard that desalinated water tastes “flat,” that is what it tastes like before remineralization: water with nothing in it.

What Brine Does to the Ocean

For every liter of freshwater a seawater RO plant produces, it also produces roughly 1.5 liters of concentrated brine. This leftover is about twice as salty as the ocean and often carries traces of chemical additives used during treatment, like antiscalants and coagulants. Because it is denser than seawater, discharged brine sinks to the seafloor and can creep along the bottom for surprisingly long distances. Modeling work has shown brine spreading over the seabed for up to several tens of kilometers from the discharge point, potentially altering nutrient cycling in the sediment.¹⁰

The ecological effects are real but also more nuanced than the headline version suggests. A large-scale study of a desalination outfall found that high-pressure diffusers successfully diluted salinity to within about 1 practical salinity unit of ambient seawater within 100 meters. But bottom-dwelling organisms like polychaetes, sponges, and bryozoans still declined near the outfall, even though the salinity change was tiny. The researchers concluded that the damage came not from salt but from altered water flow patterns created by the diffusers themselves, a tradeoff that plant designers are still working to resolve.¹¹

Solar-Powered and Off-Grid Options

Not every desalination system needs a power plant behind it. Researchers have been pursuing solar thermal desalination for years, and the field has narrowed in on a core bottleneck. Converting sunlight to heat and using that heat to evaporate water are both fairly efficient already. The real performance limiter is recovering the energy released when the steam condenses back into liquid water. Without recapturing that latent heat, the system wastes most of the solar energy it collects.¹²

Some of the most creative recent work involves biomaterial-based solar evaporators. One design uses structures derived from a floating aquatic fern, exploiting the plant’s natural hierarchical surface textures to transport water efficiently and resist salt buildup. Under standard sunlight conditions, this evaporator achieved an evaporation rate of about 2.24 kilograms of water per square meter per hour, which is high for a passive solar system.¹³ These systems are not going to supply a city, but for off-grid communities or disaster response, they offer a way to produce clean water with essentially zero energy input beyond sunlight.

At a slightly larger scale, hybrid renewable systems combining solar panels, wind turbines, and a small diesel backup have been modeled to produce desalinated water at a cost of about $0.89 per cubic meter, with over 93 percent of the electricity coming from renewable sources.¹⁴ Those numbers are getting close to competitive with grid-powered plants in regions where energy is expensive.

Hand-Powered and Portable Desalination

At the smallest end of the scale, hand-powered reverse osmosis units now exist for use in remote villages and emergencies. A field-tested system designed for off-grid rural India used a manual piston mechanism to generate enough hydraulic pressure to drive RO at very low pressures. The units rejected 88 to 95 percent of dissolved solids depending on the membrane used, and their energy consumption ranged from about 0.18 to 0.20 kilowatt-hours per cubic meter of water, which is a fraction of what a full-scale plant uses.¹⁵ The salt rejection is lower than industrial RO, but for turning brackish or moderately salty water into something safe to drink, it works.

Emerging Approaches Worth Watching

Several alternative desalination technologies are in various stages of development, though none yet rival RO at scale.

Forward osmosis flips the RO concept. Instead of pushing water through a membrane with applied pressure, it uses a concentrated “draw” solution on the other side of the membrane to pull water across naturally. The draw solution then needs to be separated from the freshwater in a second step, which is where the energy cost sneaks back in. Researchers are experimenting with draw solutes that can be recovered cheaply using heat, magnetic fields, or electrical stimulation.¹⁶ The technology shows promise for treating very dirty or highly saline water where conventional RO membranes would foul rapidly, but the second separation step remains an obstacle to efficiency.

Freeze desalination takes a completely different route. When seawater freezes, the ice crystals that form are largely salt-free. If you can freeze the water slowly and carefully enough, separate the ice from the remaining brine, and then melt it, you get freshwater. The trick is controlling crystal growth so the salt does not get trapped inside the ice. Researchers have produced drinkable water from both brackish water and seawater by seeding nucleation, keeping crystal growth slow, and maintaining liquid subcooling at about 4 degrees below the freezing point.¹⁷ The appeal is that freezing requires less energy than boiling and is less prone to the scaling and corrosion problems that plague thermal systems. In cold climates with access to cheap refrigeration, freeze desalination could eventually find a niche.

Capacitive deionization takes yet another approach, using electrically charged carbon electrodes to pull salt ions directly out of the water. When a voltage is applied, positive sodium ions migrate toward the negative electrode and negative chloride ions migrate toward the positive one, effectively stripping the water of salt. The performance depends heavily on electrode design, and research suggests that pores smaller than one nanometer are particularly effective at trapping ions.¹⁸ Current CDI systems work best on water that is already only lightly salty, like brackish groundwater, rather than full-strength seawater.

Next-Generation Membranes

The membranes used in conventional RO have not changed fundamentally in decades. They are made of thin-film polyamide, and while incremental improvements continue, there is a theoretical ceiling to how fast water can pass through while still blocking salt. The next leap may come from biomimetic membranes that incorporate aquaporins, the molecular water channels found in biological cell membranes. A graphene-oxide membrane embedded with aquaporins achieved 99.1 percent salt rejection and a water permeance of about 7.8 liters per square meter per hour per bar of pressure, performance that exceeds the upper bound of current state-of-the-art membranes.¹⁹ These membranes are still in the laboratory, but they point toward a future where desalination requires meaningfully less pressure and therefore less energy.

Why Desalination Costs Vary So Widely

You will sometimes see desalination described as costing anywhere from under a dollar to several dollars per cubic meter of water. The range is enormous because almost everything about a plant’s context changes the number. Feed water salinity matters: brackish water is far cheaper to desalinate than ocean water because less pressure is needed. The energy source matters: cheap natural gas or waste heat from a power plant cuts costs dramatically compared to diesel generators on a remote island. Plant size matters: a facility producing hundreds of thousands of cubic meters per day benefits from economies of scale that a small community system cannot match. Local labor, permitting, intake infrastructure, and environmental mitigation all add their own costs, making every plant essentially unique.²⁰

The trend, though, is clearly downward. Energy recovery devices, better membranes, and larger plant sizes have driven seawater RO costs from well over $2 per cubic meter in the 1990s to under $1 in the most efficient modern plants. For context, municipal tap water in many developed countries costs between $1 and $3 per cubic meter, so desalination is no longer in a completely different price universe.

How Seabirds Solved It First

Humans needed thousands of years of engineering to desalinate seawater at scale. Marine birds have been doing it effortlessly for millions of years. Seabirds like albatrosses and petrels drink ocean water freely because they have specialized salt glands, located above the eyes, that excrete a sodium chloride solution more concentrated than seawater itself. When a bird drinks seawater, sodium enters its bloodstream and raises plasma osmolality, triggering the salt glands to secrete. The result is osmotically free water that the bird’s body can use for everything else.²¹ It is a biological desalination system that runs on metabolic energy and requires no membranes, pumps, or pretreatment. Marine reptiles like sea turtles and saltwater crocodiles have analogous glands. The underlying principle is the same one industrial plants use: concentrate and expel salt from a dilute solution. Evolution just got there first.

A Technology With Deep Roots

Desalination often sounds like a modern, high-tech endeavor, but the underlying idea is ancient. Historians believe Minoan sailors in the Bronze Age, as early as 3200 BC, practiced some form of crude seawater distillation during ocean voyages. The industrial era arrived much later: the world’s first land-based desalination plant was built in 1928 in Curaçao, in the Netherlands Antilles, producing about 60 cubic meters of drinking water per day.²² That is roughly enough to fill a backyard swimming pool. Today, the largest seawater RO plants produce over a million cubic meters per day. The jump from 60 to over a million in under a century is a measure of how urgently growing populations in arid regions have needed this technology and how rapidly the engineering has responded.