How Desalination Works: Reverse Osmosis, Distillation, and Global Use

What Is Desalination?

Desalination is the process of removing dissolved salts and other minerals from saline water — primarily seawater (approximately 35,000 mg/L of total dissolved solids, or TDS) or brackish water (1,000–10,000 mg/L TDS) — to produce fresh water suitable for drinking, agriculture, or industrial use. Freshwater scarcity affects over 2 billion people worldwide, and as climate change intensifies drought conditions and population growth increases demand, desalination has emerged as a critical technology for supplementing natural freshwater resources in arid and water-stressed regions.

Global desalination capacity exceeded 100 million cubic meters per day by 2023, supplying fresh water to hundreds of millions of people primarily in the Middle East (Saudi Arabia, UAE, Kuwait, Qatar, Israel), North Africa, and increasingly in the United States, Australia, Spain, and China. The global desalination market is projected to continue growing at approximately 9% annually as water stress intensifies across more regions.

Why Desalination Is Challenging

The salt in seawater is bound to water molecules through the thermodynamic process of hydration — separating them requires overcoming this bond, which demands substantial energy input. Seawater desalination is inherently energy-intensive: while natural precipitation and the hydrological cycle desalinate ocean water for free using solar energy, engineered desalination must accomplish the same thermodynamic work artificially. The minimum theoretical energy required to desalinate seawater is approximately 1 kWh per cubic meter; in practice, commercial plants consume 3–15 kWh/m³ depending on the technology and feed water salinity.

This energy intensity makes desalination significantly more expensive than utilizing natural freshwater sources when those are available, and it means that the economic and environmental viability of desalination depends heavily on local energy costs, freshwater availability, and the energy mix of the grid supplying the plant. Increasing the proportion of renewable energy powering desalination — particularly solar, given the geographic overlap between arid regions and high solar irradiance — is a major focus of current research and deployment.

Reverse Osmosis (RO): The Dominant Technology

Reverse osmosis is the most widely used desalination technology globally, accounting for approximately 65% of total installed desalination capacity. It operates by applying high hydraulic pressure to saline water, forcing it through semi-permeable membranes that allow water molecules to pass but block dissolved salt ions. The term "reverse" osmosis distinguishes it from natural osmosis, in which water moves spontaneously from a region of low solute concentration to a region of high solute concentration across a semi-permeable membrane; RO uses pressure to drive water movement in the opposite direction.

A modern seawater RO (SWRO) plant operates as follows:

• Intake and pretreatment: Seawater is drawn from the ocean through open intakes or subsurface wells. Pretreatment — which may include screening, coagulation, sedimentation, filtration, and pH/chemical adjustment — removes suspended solids, biological matter, and scaling precursors that would foul or damage the membranes.
• High-pressure pumping: Pretreated water is pressurized to 55–70 bar (800–1,000 psi) for seawater desalination using high-pressure pumps.
• Membrane separation: The pressurized water passes through spiral-wound polyamide thin-film composite membranes, which reject 99.5–99.8% of dissolved salts. Approximately 30–50% of the feed water passes through the membrane as "permeate" (product water); the remainder is rejected as concentrated brine.
• Energy recovery: The reject brine exits at high pressure; modern plants recover 40–60% of this pressure energy using pressure exchangers or turbines, dramatically reducing net energy consumption.
• Post-treatment: Product water is remineralized (to prevent corrosion of distribution systems and meet drinking water standards), disinfected, and adjusted for pH before distribution.

Thermal Desalination Technologies

Thermal desalination processes produce fresh water by evaporating saline water and condensing the resulting steam. They dominated the industry before the rise of membrane technologies and remain significant, particularly in the Persian Gulf where abundant waste heat from power generation and high seawater salinity make them cost-competitive.

Multi-Stage Flash (MSF)

In MSF desalination, seawater is heated to approximately 90–120°C in a brine heater, then introduced into a series of chambers (stages) maintained at progressively lower pressures. At each reduced pressure, a portion of the hot brine instantaneously flashes to steam; this steam condenses on tubes carrying incoming cooler seawater, transferring heat and producing distillate (pure water). The brine progressively cools and concentrates as it moves through the stages. MSF plants can have 15–25 stages, with each stage recovering a small fraction of distillate. MSF is highly reliable and tolerant of variable feed water quality, which explains its popularity in the Persian Gulf despite its higher energy consumption compared to RO.

Brine Disposal: A Key Environmental Challenge

Every desalination process produces a concentrated brine stream — typically containing roughly double the salt concentration of the feed water — that must be disposed of responsibly. Brine disposal is one of the most significant environmental challenges in desalination. Discharge of hot, hypersaline brine back to coastal marine environments can affect seagrass beds, invertebrate populations, and fish habitats near the discharge point if not properly diluted. Best practices include high-velocity diffuser discharge designs that rapidly mix brine with receiving water, careful site selection, and continuous environmental monitoring.

Inland desalination facilities face even greater challenges, as they cannot discharge to the sea; they must manage brine through evaporation ponds, deep well injection, or increasingly through zero liquid discharge (ZLD) systems that crystallize salt from the brine for beneficial use or disposal. ZLD significantly increases capital and operating costs but eliminates liquid discharge entirely.

Emerging Technologies

Research is actively advancing several next-generation desalination approaches:

• Forward osmosis (FO): Uses a highly concentrated draw solution to pull water through a membrane osmotically, potentially reducing energy requirements relative to RO — though recovery of the draw solute remains technically challenging.
• Capacitive deionization (CDI): Uses electrically charged electrodes to adsorb and remove ions from water; potentially energy-efficient for brackish water but not yet competitive for seawater at scale.
• Graphene and advanced membranes: Membranes with precisely engineered nanopores at atomic scale could theoretically achieve dramatically higher water permeability while maintaining salt rejection, reducing the pressure (and therefore energy) required for RO.
• Solar desalination: Systems that directly use solar thermal energy for distillation — ranging from simple solar stills to sophisticated solar-powered MED — offer potential for low-cost, off-grid water production in sunny, water-stressed regions.

As water scarcity intensifies globally, desalination technology will play an increasingly critical role in ensuring water security for billions of people. The key challenge for the coming decades is reducing the energy intensity and environmental footprint of desalination while expanding its deployment — particularly in developing nations where water stress is most acute and energy costs are highest.