Removing salt from water comes down to two broad strategies: either you push water molecules through a barrier that blocks salt, or you evaporate the water and collect the steam, leaving the salt behind. The first approach, reverse osmosis, dominates the global desalination market because it generally uses less energy than boiling water. But thermal methods remain important in regions with cheap heat, and newer technologies are starting to carve out niches of their own. Which method makes sense depends on the salt concentration of the source water, available energy, budget, and scale.
Reverse Osmosis
Reverse osmosis (RO) works by applying pressure to saltwater on one side of a semi-permeable membrane. The membrane has pores small enough to let water molecules pass through while holding back dissolved salts and most other contaminants. The key insight is that you are working against osmosis, the natural tendency of water to flow from a dilute solution toward a concentrated one. By applying pressure above the natural osmotic pressure of the saltwater, you force water in the opposite direction, from salty to fresh.
This approach uses less energy than most thermal desalination systems because you are moving water through a membrane rather than boiling it.1Desalination and Water Treatment. A comprehensive review of reverse osmosis desalination: Technology, water sources, membrane processes, fouling, and cleaning A typical large-scale seawater RO plant consumes somewhere in the range of 3 to 6 kilowatt-hours per cubic meter of freshwater produced, depending on the system’s design and the salinity of the feed water. That number drops significantly for brackish water, which has far less salt to overcome. One laboratory study of electrodialysis on brackish water recorded a total energy consumption of about 1.65 kWh per cubic meter, illustrating how much easier it is to desalinate water that is only mildly salty to begin with.2MDPI (Membranes). Brackish Water Desalination Using Electrodialysis: Influence of Operating Parameters on Energy Consumption and Scalability
Modern RO plants owe much of their efficiency gains to energy recovery devices. These capture the residual pressure in the concentrated brine that exits the membrane and transfer it back to the incoming feed water. Isobaric energy recovery devices, which use a direct positive-displacement mechanism, can achieve transfer efficiencies up to 97%, reducing a plant’s total energy consumption by as much as 60% compared to running without any recovery.3Desalination. Seawater reverse osmosis with isobaric energy recovery devices That single innovation has done more to bring down the cost of desalinated seawater than almost any membrane improvement.
Thermal Desalination
Before membranes took over, desalination meant heat. Thermal methods mimic the natural water cycle: heat saltwater until it evaporates, then cool the vapor so it condenses into freshwater. The salt stays behind in the liquid. Two main thermal approaches still operate at industrial scale.
Multi-stage flash (MSF) distillation heats seawater and then passes it through a series of chambers, each at a slightly lower pressure. At each stage, a portion of the water “flashes” into steam without needing additional heat, because lower pressure lowers the boiling point. MSF is one of the oldest and most established desalination technologies and remains a workhorse in the Middle East, where waste heat from power plants can be piped directly into the process. Freshwater recovery rates typically range from about 30% to 80%, depending on the design, and the output water is remarkably pure, often in the range of 5 to 50 parts per million of dissolved solids.4ScienceDirect (Elsevier / Solar Energy). A state-of-art overview of multi-stage flash desalination and water treatment: Principles, challenges, and heat recovery in hybrid systems
Multi-effect distillation (MED) works on a similar principle but arranges its stages so that the steam produced in one effect serves as the heat source for the next. This cascading reuse of energy makes MED somewhat more efficient than MSF in terms of thermal energy per liter of freshwater. Both methods share a practical advantage over RO: they handle extremely salty or dirty feed water without the same risk of membrane damage, which is why they remain the go-to choice for some industrial applications and in areas with very high-salinity source water.
Electrodialysis and Capacitive Deionization
Not all desalination requires either membranes working under high pressure or enormous amounts of heat. Electrodialysis (ED) takes a different tack: it uses an electric field to pull dissolved salt ions out of the water. Charged membranes alternate in a stack, with some allowing only positive ions through and others only negative ions. When voltage is applied, sodium migrates one way and chloride the other, leaving a stream of fresher water in between. ED works best on brackish water, where the salt load is low enough that the electrical energy required stays reasonable.
Capacitive deionization (CDI) is a related approach that uses porous carbon electrodes to adsorb salt ions when a voltage is applied. Once the electrodes are saturated, you reverse or cut the voltage and the salt releases into a waste stream. Research has found that electrodes with very small pores, under one nanometer, are particularly effective at grabbing ions, which suggests that the geometry of the carbon material matters more than its total surface area.5PubMed. Water desalination using capacitive deionization with microporous carbon electrodes CDI is still mostly in the pilot and niche-application phase, but it holds promise for small-scale or low-salinity situations where conventional RO would be overkill.
Keeping the Membranes Working
If you install a reverse osmosis system and simply run raw seawater through it, the membranes will clog and degrade far faster than their design life. Pretreatment is not optional; it is a core part of the process. The goal is to remove particles, organic matter, and potential scale-forming minerals before they ever reach the RO membrane.6Desalination. A short review on reverse osmosis pretreatment technologies
Pretreatment generally falls into two categories. Conventional methods include media filtration (sand or multimedia filters), coagulation to clump fine particles together, and disinfection to kill microorganisms. Non-conventional methods lean on ultrafiltration or microfiltration membranes to do a more thorough job of stripping out colloids and bacteria before the RO step.7Desalination. Pre-treatment strategies for seawater desalination by reverse osmosis system Reducing the microbial load matters because biofouling, where microorganisms form a slimy layer on the membrane surface, is one of the most persistent headaches in RO operations. That biofilm increases the pressure needed to push water through and can permanently damage the membrane if left unchecked.8npj Clean Water. Reverse osmosis membrane biofouling: causes, consequences and countermeasures
Mineral scaling is the other big threat. As water passes through the membrane and salts concentrate on the feed side, minerals like calcium carbonate and silica can precipitate out and coat the membrane surface. Chemical antiscalants are routinely dosed into the feed water to prevent this. Recent research has shown that an antiscalant’s effectiveness against silica scaling depends on its molecular structure and weight, with compounds that are strong hydrogen-bond acceptors or donors both able to hinder the polymerization of silicic acid.9Water Research. Antiscalants for mitigating silica scaling in membrane desalination: Effects of molecular structure and membrane process Getting the chemistry right here means the difference between a membrane that lasts years and one that needs replacing in months.
What Happens to the Leftover Brine
Every desalination plant produces two outputs: freshwater and concentrated brine. For every liter of freshwater a seawater RO plant produces, roughly one to two liters of brine go back out, carrying all the salt that was removed plus whatever chemicals were added during pretreatment. When this dense, salty effluent is discharged into the ocean, it sinks and can spread along the seabed for several kilometers from the discharge point.10PubMed. Impacts of Desalination Brine Discharge on Benthic Ecosystems
The ecological effects are real but complicated. Heavy metals and residual chemicals in the brine can degrade local water quality and interfere with physiological processes of marine organisms, from photosynthesis in seagrass to reproduction in invertebrates.11Frontiers in Marine Science. Characteristics of Desalination Brine and Its Impacts on Marine Chemistry and Health, With Emphasis on the Persian/Arabian Gulf: A Review Studies of organisms near brine diffusers have found signs of oxidative and osmotic stress after initial exposure, though in some cases antioxidant parameters in affected organisms recovered within days once the brine was diluted or removed.12Journal of Hazardous Materials. Evaluating physico-chemical and biological impacts of brine discharges for a sustainable desalination development on South America’s Pacific coast The severity depends heavily on dilution. Plants that use well-designed diffuser systems to mix the brine rapidly with surrounding seawater cause far less local damage than those that simply pipe it out into shallow, enclosed bays.
A growing number of researchers argue that brine should be treated as a resource rather than a waste product. Seawater contains commercially useful minerals beyond sodium chloride, including magnesium, lithium, and potassium. Desalination concentrate is already enriched in these elements, making it a more attractive feedstock than raw seawater for mineral extraction. The most promising approaches use nanofiltration to separate brine into streams enriched in different mineral classes, and osmotically-assisted RO to concentrate the brine further before extraction. In practice, though, the economics still depend heavily on the market for ordinary salt, since the costs of pulling out trace minerals only pencil out once the bulk NaCl has been dealt with.13npj Clean Water. Seawater desalination concentrate—a new frontier for sustainable mining of valuable minerals
Why Desalinated Water Needs Minerals Added Back
Water that comes out of an RO membrane or a distillation process is almost too pure. It is very low in dissolved minerals, which makes it corrosive to pipes and unpleasant to drink. It can also leach metals from distribution infrastructure on its way to the tap. Post-treatment remineralization, usually by dissolving limestone or dosing calcium and bicarbonate, is a necessary step to make the water stable and safe for distribution.14ResearchGate. Post‐Treatment of Desalinated Water If you have ever tasted distilled water and found it flat or slightly metallic, that is essentially what untreated desalinated water would be like.
Powering Desalination With Renewables and Nuclear
Desalination is energy-hungry, and running it on fossil fuels undermines its appeal as a climate adaptation strategy. The push to pair desalination with renewable energy is accelerating. One global modeling study estimated that by 2030, the cost of water from seawater RO plants powered by hybrid solar, wind, and battery systems could fall to roughly 0.59 to 2.81 euros per cubic meter, a range that overlaps with the current cost of fossil-fuel-powered desalination.15Desalination. Local cost of seawater RO desalination based on solar PV and wind energy: A global estimate The wide spread mostly reflects differences in water transportation costs and local solar and wind resources rather than the desalination process itself.
Nuclear power is another option that has attracted serious attention. Nuclear reactors provide steady, round-the-clock heat and electricity, which suits desalination well because plants run most efficiently when they operate continuously. Analyses of nuclear-powered desalination have found it competitive with fossil-fuel-based systems on cost, while producing lifecycle carbon emissions two to three orders of magnitude lower.16ScienceDirect (Elsevier / Desalination). Nuclear desalination: A sustainable route to water security Several countries, including India and Russia, have already coupled nuclear plants with desalination capacity.
Emerging Membrane Technologies
Conventional RO membranes are made from thin-film composite polymers. They work well, but researchers keep looking for materials that can move water faster, reject salt more completely, or resist fouling better. Two frontiers stand out.
Graphene membranes use atomically thin sheets of carbon with precisely sized nanopores. Because the material is so thin, water molecules pass through very quickly. Early experiments showed that single-layer porous graphene achieved a salt rejection rate of nearly 100% with rapid water transport.17PubMed. Water desalination using nanoporous single-layer graphene More recent work has explored multilayer graphene membranes with conical channels, where stacking several layers with slightly offset pores creates funnel-shaped pathways that boost water flow well beyond what a single layer can achieve.18Journal of Membrane Science. Beyond the single-layer limit for desalination based on conical channels of multilayer nanoporous graphene membranes In membrane distillation mode, where a temperature difference drives water vapor across the membrane, ultrathin nanoporous graphene has demonstrated water fluxes an order of magnitude higher than any reported polymeric membrane, with salt rejection above 99.8%.19PubMed. An Ultrahigh-Flux Nanoporous Graphene Membrane for Sustainable Seawater Desalination using Low-Grade Heat The challenge, as with most nanomaterial breakthroughs, is scaling production from lab-sized patches to the thousands of square meters a real plant would need.
Aquaporin biomimetic membranes take a different kind of inspiration. Aquaporins are the protein channels that living cells use to shuttle water molecules while blocking dissolved salts and other solutes. Researchers have been working to incorporate these proteins into synthetic membranes, essentially borrowing biology’s water filter. Recent designs embedded aquaporin-containing liposomes on an anodic aluminum oxide substrate and achieved high water permeability with very low reverse solute flux, demonstrating that the biological channels actively transport water while rejecting salt.20npj Clean Water. Highly permeable and shelf-stable aquaporin biomimetic membrane based on an anodic aluminum oxide substrate The broader appeal of aquaporin-based membranes lies in their potential to overcome the usual trade-off in conventional membranes, where improving water flux tends to sacrifice salt rejection and vice versa.21Advanced Functional Materials. Aquaporin‐Based Biomimetic Membranes for Low Energy Water Desalination and Separation Applications Making these membranes robust enough to survive industrial operating pressures and biofouling is the main hurdle, but progress has been steady.22PubMed Central. Aquaporin-Based Biomimetic Polymeric Membranes: Approaches and Challenges
Forward Osmosis
Most desalination pushes or pulls water in the “wrong” direction relative to natural osmosis. Forward osmosis (FO) tries a different trick: instead of fighting osmosis with high pressure, it uses a “draw solution” on the other side of the membrane that has an even higher osmotic pressure than the seawater. Water naturally migrates across the membrane toward this draw solution. You then need a second step to separate the freshwater from the draw solute and recycle the solute for reuse.
The draw solute is the crux of the whole approach. Sodium chloride is still used in many research studies, but more creative options include thermally recoverable gases (ammonia-carbon dioxide mixtures that can be driven off with gentle heating), magnetic nanoparticles that can be pulled out with a magnet, and thermally responsive hydrogels that release water when heated.23PubMed Central. A Comprehensive Review on Forward osmosis Water Treatment: Recent Advances and Prospects of Membranes and Draw Solutes One team developed a draw solute based on a carbon nanofiber composite that could be regenerated using direct solar absorption, essentially letting sunlight do the work of separating the draw solute from the product water.24Applied Energy. Novel draw solution for forward osmosis based solar desalination FO’s theoretical appeal is that the first step, moving water across the membrane, requires no applied pressure and therefore little energy. The catch is that the second step, recovering the draw solute, usually does require energy, so the total energy budget has not yet beaten optimized RO for seawater. Where FO shines is in treating fouling-prone or high-salinity waste streams that would be punishing for an RO membrane.
Portable and Emergency Desalination
Not every desalination challenge is a million-gallon-a-day municipal plant. Sailors, disaster-relief teams, and remote communities sometimes need a handful of liters per hour from a device that fits in a backpack. Small-scale RO units have been built for exactly this. One recent prototype, about the size of a large book, uses a hand-operated lever to pressurize seawater through an RO membrane. A built-in hydraulic energy recovery system redirects the rejected brine to partially offset the pumping effort, achieving about 49.5% energy recovery. In testing, the device produced roughly 1.15 liters of freshwater per hour from seawater at about 3.6% salinity, with a salt rejection rate above 95%.25Water. A Portable Hand-Operated Reverse Osmosis Desalination Device with Integrated Hydraulic Brine Energy Recovery That is enough to keep one person hydrated in an emergency, and the whole unit measures 200 by 128 by 63 millimeters. Similar hand-pump and solar-powered portable desalinators are commercially available, though output rates and salt rejection vary widely.
How Seabirds Solved the Problem First
Humans have been tinkering with desalination for centuries, but marine birds figured it out on an evolutionary timescale without membranes or electricity. Seabirds like albatrosses and petrels drink seawater freely because they have specialized salt glands located near their eyes. These glands secrete a sodium chloride solution more concentrated than the seawater the bird drank, effectively pulling salt out of the bloodstream and excreting it through the nostrils as a briny drip. The process generates osmotically free water that sustains the bird’s other physiological needs.26PubMed. Regulation of salt gland, gut and kidney interactions Marine reptiles, including sea turtles and marine iguanas, have analogous glands. The underlying principle is the same one that drives all desalination: create a zone where salt is concentrated and expelled while water is retained. Biology just does it with ion-pumping cells rather than 60-bar pressure vessels.