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What Is Desalination?

Desalination is the process of removing dissolved salts and other minerals from seawater or brackish water to produce fresh water suitable for human consumption, irrigation, or industrial use. With 97.5% of Earth’s water being saltwater and freshwater supplies under increasing pressure from population growth, agriculture, and climate change, desalination has grown from a niche technology into a critical water infrastructure serving over 300 million people in more than 150 countries.

The Water Problem Desalination Solves

The numbers are stark. The United Nations estimates that 2.2 billion people lack access to safely managed drinking water. By 2025, half the world’s population is projected to live in water-stressed areas. Climate change is making it worse—shifting rainfall patterns, shrinking glaciers, and increased evaporation are reducing freshwater availability in many regions that are already struggling.

Meanwhile, the ocean contains roughly 1.335 billion cubic kilometers of water. That’s 96.5% of all water on Earth. If you could efficiently turn saltwater into freshwater, you’d solve the planet’s water crisis overnight.

The catch—and there’s always a catch—is that separating salt from water requires significant energy. Seawater contains about 35 grams of dissolved salts per liter (3.5% salinity). Removing those salts, particularly sodium chloride but also magnesium, calcium, and sulfate compounds, takes either intense heat or intense pressure. Neither comes cheap.

But for coastal regions with limited freshwater options—the Middle East, North Africa, island nations, parts of Australia and California—desalination isn’t a luxury. It’s survival.

How Desalination Works: The Two Main Approaches

Reverse Osmosis (RO)

Reverse osmosis accounts for about 69% of global desalination capacity and growing. It’s the dominant technology for new installations, and understanding why requires a quick detour into osmosis itself.

Normal osmosis: If you separate freshwater and saltwater with a semipermeable membrane (one that lets water through but blocks salt), water naturally moves from the fresh side to the salty side. This is osmosis—water moves toward higher salt concentration to equalize the difference. Your cells do this constantly.

Reverse osmosis: Apply enough pressure to the salty side, and you force water to move in the opposite direction—through the membrane, leaving salt behind. For seawater, you need about 55-80 bar of pressure (roughly 55-80 times atmospheric pressure) to overcome the osmotic pressure and push water through.

The membranes are engineering marvels. Modern RO membranes are thin-film composite structures—a polyamide layer just 200 nanometers thick on a polysulfone support—rolled into spiral-wound modules about a meter long. Each module contains about 40 square meters of membrane surface. A large desalination plant might contain tens of thousands of these modules.

The process flow:

Intake: Seawater is drawn from the ocean through screened intakes that exclude marine life
Pretreatment: Filtration removes suspended solids. Chemical treatment prevents biological growth and scaling on membranes. This step is critical—membrane fouling is the primary operational challenge in RO plants.
High-pressure pumping: Pumps pressurize the pretreated water to 55-80 bar
Membrane separation: Water passes through membranes; salt is rejected. Typical recovery is 40-50% of incoming water becomes fresh; the rest is concentrated brine.
Energy recovery: Modern plants use energy recovery devices that capture pressure from the brine stream, reducing energy consumption by up to 60%. This single innovation has been the biggest factor in making RO economically viable.
Post-treatment: Minerals are added back (desalinated water is too pure for pleasant taste and can be corrosive to pipes). pH is adjusted. Chlorine is added for distribution system protection.

A state-of-the-art RO plant consumes about 3-4 kilowatt-hours per cubic meter of freshwater produced. That’s down from 8+ kWh/m³ in the 1990s—a dramatic improvement driven primarily by better membranes and energy recovery.

Thermal Desalination

Thermal methods boil seawater, capture the steam (which is salt-free), and condense it into freshwater. It’s conceptually simple—humans have been distilling water for centuries—but doing it efficiently at scale requires clever engineering.

Multi-Stage Flash (MSF) distillation heats seawater and then introduces it into a series of chambers (stages) at progressively lower pressures. At each stage, some water “flashes” into steam because the lower pressure reduces the boiling point. Steam is condensed and collected. Modern MSF plants have 20-40 stages, each extracting a bit more freshwater.

MSF is the oldest modern desalination technology, developed in the 1960s. It’s less efficient than RO (consuming 10-16 kWh/m³ equivalent) but extremely reliable and well-suited to the Middle East, where cheap natural gas and oil provide abundant heat energy. Saudi Arabia’s Ras Al Khair plant, the world’s largest desalination facility, uses both MSF and RO and produces 1.025 million cubic meters per day.

Multi-Effect Distillation (MED) uses a similar principle but operates at lower temperatures with multiple evaporation chambers (effects). Steam from one effect provides heat for the next. MED is more thermally efficient than MSF and produces higher-quality water, but with lower maximum capacity per unit.

Vapor Compression distillation uses mechanical (MVC) or thermal (TVC) compression to recycle the heat of evaporation. It’s efficient for small-scale applications—ships, military installations, remote locations.

Why RO Is Winning

The economics are clear. RO consumes roughly 3-4 kWh/m³. Thermal methods consume 10-16 kWh/m³ equivalent. As energy costs rise and membrane technology improves, RO’s advantage grows.

Additionally, RO scales more flexibly. You can build small RO plants for a village or massive ones for a city. Thermal plants generally only make economic sense at very large scales where waste heat is available.

The one area where thermal still has advantages: extremely salty feed water (like Persian Gulf water at 45+ g/L) and situations where waste heat from power plants or industrial processes is available for free. In those cases, thermal desalination’s higher energy input is offset by essentially free heat energy.

The Global Desalination Field

As of 2025, there are approximately 21,000 desalination plants worldwide with a combined capacity of over 130 million cubic meters per day. That’s enough to fill about 52,000 Olympic swimming pools—daily.

Middle East and North Africa dominate, with about 48% of global capacity. Saudi Arabia, the UAE, Kuwait, and Israel are the heaviest users. Israel’s story is particularly remarkable—the country went from severe water scarcity to a water surplus in about a decade by building five large RO plants along its Mediterranean coast. Israel now desalinates about 585 million cubic meters annually, covering approximately 80% of domestic water needs.

The United States has the largest single RO plant in the Western Hemisphere—the Carlsbad Desalination Plant in San Diego County, producing 190,000 cubic meters per day. It cost $1 billion to build and supplies about 10% of San Diego County’s water.

Australia built six large desalination plants during the Millennium Drought (2001-2009), including the Sydney Desalination Plant and the Victorian Desalination Plant. These serve as drought insurance—they can ramp up during dry periods and scale down when reservoir levels are healthy.

China and India are rapidly expanding desalination capacity to meet industrial and municipal demand in water-stressed coastal regions.

The Cost Question

How much does desalinated water actually cost? The answer matters because it determines who can afford this technology and who can’t.

Capital costs: A large RO plant costs $1,000-$2,500 per cubic meter of daily capacity. The Carlsbad plant’s $1 billion price tag for 190,000 m³/day works out to about $5,260 per daily cubic meter of capacity.

Operating costs: Energy accounts for 30-50% of operating costs. Membrane replacement adds 10-15%. Chemical treatment, labor, and maintenance make up the rest.

Water cost: All-in, desalinated seawater typically costs $0.50-$2.00 per cubic meter, depending on local energy prices, plant size, and feed water quality. Brackish water desalination is cheaper ($0.20-$0.50/m³) because lower salinity means lower pressure and less energy.

For comparison: conventional water treatment from surface or groundwater sources costs $0.10-$0.50/m³ in most developed countries. Desalinated water is 2-10 times more expensive.

This cost gap is narrowing. In the early 2000s, desalinated water cost $1.50-$5.00/m³. Improvements in membrane efficiency, energy recovery, and plant design have driven costs down by 50-70% in two decades. The Taweelah RO plant in Abu Dhabi (commissioned 2022, 909,000 m³/day capacity) reportedly produces water at about $0.42/m³—approaching parity with some conventional sources.

Energy and Environment: The Hard Tradeoffs

Energy Consumption

Desalination is energy-hungry. Global desalination consumes roughly 75 terawatt-hours of electricity annually—comparable to the total electricity consumption of a country like Chile. As capacity grows, so does the energy demand.

The source of that energy matters enormously. Desalination powered by coal-fired electricity has a very different carbon footprint than desalination powered by solar panels. The Middle East’s reliance on natural gas for desalination energy produces significant greenhouse gas emissions. Conversely, solar-powered desalination—increasingly viable as photovoltaic costs drop—could provide nearly carbon-free water.

The theoretical minimum energy for desalinating seawater is about 1.06 kWh/m³ (set by thermodynamics). Current best practice is 3-4 kWh/m³. There’s still room for improvement, but we’re getting closer to the physical limit.

Brine Disposal

For every liter of freshwater a seawater RO plant produces, it also produces roughly 1-1.5 liters of brine—seawater concentrated to about 65-70 g/L salinity, nearly double normal seawater. Globally, desalination plants produce about 142 million cubic meters of brine daily.

Most brine is discharged into the ocean through diffuser systems designed to mix it rapidly with ambient seawater. In open ocean environments with strong currents, this dilution happens quickly and impacts are minimal. In enclosed or shallow waters with poor circulation, concentrated brine can settle on the seafloor, creating hypersaline zones that harm bottom-dwelling organisms.

The chemicals used in pretreatment (anti-scalants, biocides, coagulants) also end up in the brine, adding chemical pollution to the salinity concern.

Emerging alternatives to ocean discharge include:

Zero-liquid discharge (ZLD): Evaporating brine completely, leaving only solid salt. Technically possible but extremely energy-intensive—currently only economical when combined with mineral extraction.
Brine mining: Extracting valuable minerals (magnesium, lithium, potassium) from concentrated brine. A promising concept, particularly for lithium, which is in massive demand for batteries. Research is active but commercial-scale brine mining from desalination concentrate is still nascent.
Evaporation ponds: Used in arid inland locations. Simple but requires large land areas.
Deep well injection: Pumping brine into deep geological formations. Viable in some locations but raises groundwater contamination concerns.

Marine Life Impacts

Intake systems can trap and kill marine organisms. Open ocean intakes are the worst—they can entrain fish larvae, plankton, and small organisms. Subsurface intakes (drawing water from below the seabed) are much gentler but not always feasible depending on local geology.

Modern plants increasingly use wedge-wire screens with small openings (1-2mm) and low intake velocities to minimize marine life impacts. Some plants use subsurface galleries that filter water through natural sand—essentially copying nature’s filtration.

Emerging Technologies

Forward Osmosis

Forward osmosis uses a concentrated “draw solution” to pull water through a membrane without high pressure. The draw solution is then separated from the water using less energy than conventional RO. It’s still experimental for large-scale desalination but shows promise for specific applications like treating high-salinity wastewater.

Electrodialysis

Electrodialysis uses electric fields to pull salt ions through ion-selective membranes, leaving freshwater behind. It’s more efficient than RO for brackish water (lower salinity) but less efficient for seawater. Electrodialysis reversal (EDR) periodically switches polarity to self-clean the membranes.

Capacitive Deionization

This newer approach uses electrically charged electrodes to attract and hold salt ions, like a battery that absorbs salt instead of storing energy. It’s promising for low-salinity applications and is being explored for household and small-community scale water treatment.

Solar Desalination

Direct solar desalination uses solar heat to evaporate water without electricity. Simple solar stills have existed for centuries, but modern versions using nanotechnology and engineered surfaces achieve much higher efficiency. MIT researchers have developed solar-powered desalination systems that produce freshwater at costs competitive with tap water in some settings, though scaling these systems remains challenging.

Biomimetic Membranes

Inspired by aquaporins—the protein channels that cells use to transport water—researchers are developing biomimetic membranes that could be more selective and energy-efficient than current synthetic membranes. Nature solved the desalination problem billions of years ago in cell biology; the challenge is replicating that solution at industrial scale.

The Geopolitics of Desalination

Water scarcity creates geopolitical tension. Rivers crossing national boundaries—the Nile, the Jordan, the Tigris and Euphrates—are constant sources of diplomatic friction. Desalination changes this active by creating water supplies independent of shared natural sources.

Israel’s massive desalination investment has reduced its dependence on the Sea of Galilee and the Jordan River system, easing one dimension of the Arab-Israeli water dispute. Saudi Arabia’s desalination capacity makes it independent of any transboundary water source—a strategic advantage in a region where water rights are fiercely contested.

But desalination creates new dependencies. Countries relying heavily on desalination need reliable energy supplies to keep the plants running. A disruption to energy supply becomes a disruption to water supply—a vulnerability that military planners take seriously.

Where Desalination Is Heading

The trajectory is clear: more capacity, lower costs, and increasing integration with renewable energy.

Global desalination capacity is projected to grow at 7-9% annually through 2030. The fastest growth is in Asia (China, India, Southeast Asia) and Africa, where water stress is most acute and population growth is highest.

Renewable-powered desalination is becoming economically viable. Solar-powered RO plants in the Middle East and Australia are already operating. Wind-powered desalination is being piloted in island communities. As renewable energy costs continue falling, the energy cost barrier to desalination falls with it.

Smaller, modular systems are making desalination accessible beyond large coastal cities. Container-sized RO units can provide freshwater for communities of 10,000-50,000 people, making the technology viable for small islands, disaster relief, and rural coastal communities.

Hybrid systems combining desalination with water recycling, rainwater harvesting, and aquifer recharge are becoming the standard approach for water-scarce regions. Desalination isn’t a standalone solution—it’s one component of an integrated water strategy.

Key Takeaways

Desalination converts saltwater into freshwater, primarily through reverse osmosis (pushing water through membranes under high pressure) or thermal distillation (boiling and condensing). It currently serves over 300 million people worldwide and is growing rapidly as freshwater scarcity intensifies.

The technology works. The water quality is excellent. The costs have dropped dramatically and continue falling. But desalination carries real tradeoffs: high energy consumption, brine disposal challenges, and marine ecosystem impacts that require careful management.

For coastal communities facing water scarcity, desalination is increasingly not a question of whether but when. The ocean contains more water than humanity could ever use—the engineering challenge is accessing it sustainably, affordably, and at the scale the world needs. That challenge is being met, one membrane at a time.

Frequently Asked Questions

Is desalinated water safe to drink?

Yes. Modern desalination plants produce water that meets or exceeds World Health Organization drinking water standards. In fact, desalinated water is often purer than conventional tap water because the process removes virtually all dissolved solids, bacteria, and viruses. Minerals like calcium and magnesium are typically added back after desalination for taste and health purposes.

Why don't we just desalinate all the water we need?

Cost and energy. Desalination requires 3-10 times more energy than treating conventional freshwater sources. At $0.50-$2.00 per cubic meter, desalinated water costs significantly more than water from rivers, aquifers, or reservoirs. For coastal cities with no alternatives, it makes sense. For inland communities with available freshwater, traditional sources remain far more economical.

What happens to the salt removed during desalination?

The concentrated salt solution (brine) is typically discharged back into the ocean. This is the biggest environmental concern with desalination—brine is roughly twice as salty as seawater and can harm marine ecosystems near the discharge point. Modern plants use diffuser systems to dilute brine before discharge, and research into brine mining (extracting valuable minerals from the concentrate) is advancing.

How much of the world's drinking water comes from desalination?

About 1% of the world's drinking water currently comes from desalination, serving approximately 300 million people daily. While that percentage seems small, it's growing rapidly—global desalination capacity has roughly tripled since 2005. In some regions, the dependence is extreme: Israel gets about 80% of its domestic water from desalination, and several Gulf states exceed 90%.