What Is Desalination?

Desalination is the process of removing dissolved salts and other minerals from seawater or brackish water to produce freshwater suitable for human consumption, agriculture, or industrial use. As global water scarcity intensifies and conventional freshwater sources become insufficient to meet rising demand, desalination has grown from a niche technology used in arid regions into a significant component of municipal water supply systems worldwide. The International Desalination Association estimates that more than 21,000 desalination plants now operate globally, producing over 100 million cubic meters of freshwater per day.

Desalination draws on chemical engineering, membrane science, thermodynamics, and materials research. The two principal technology families are membrane-based processes, dominated by reverse osmosis, and thermal processes, which rely on evaporation and condensation. Hybrid plants combining both approaches are common in large-scale installations. Energy consumption is the central constraint that drives engineering innovation in the field, since separating dissolved salt from water requires work against the osmotic pressure of the feed solution.

Reverse Osmosis

Reverse osmosis (RO) is the leading desalination technology, accounting for roughly 65 to 70 percent of global installed capacity. The process forces high-pressure feedwater through a semipermeable membrane that passes water molecules while rejecting dissolved ions. Applied pressures for seawater desalination typically range from 55 to 70 bar, well above the osmotic pressure of seawater at approximately 27 bar. Membranes are fabricated from thin-film composite polyamide layers coated onto polysulfone support structures, achieving salt rejection rates above 99 percent. Energy recovery devices that capture the hydraulic energy of the concentrated brine stream before it exits the system have reduced the specific energy consumption of modern seawater RO plants to approximately 2 to 4 kilowatt-hours per cubic meter. A detailed review of reverse osmosis desalination technology published in the journal Water Research and available through PubMed covers membrane chemistry, pretreatment requirements, and fouling mechanisms.

Thermal Distillation Processes

Thermal desalination technologies use heat to evaporate saline water and then condense the resulting vapor as freshwater. Multi-stage flash (MSF) distillation passes heated brine through a series of chambers at progressively lower pressures, causing controlled flash evaporation at each stage. Multiple-effect distillation (MED) applies heat to the first of a series of evaporator vessels and uses the vapor produced at each stage to heat the next, improving thermal efficiency. Thermal processes have historically been preferred in the Persian Gulf, where low-cost fuel and elevated seawater temperatures favor their economics over membrane systems. Both MSF and MED produce distillate with very low total dissolved solids, typically below 10 milligrams per liter, which is lower than RO permeate. The Texas Water Development Board report on fundamentals of membranes and alternative technologies for water treatment provides a comparative analysis of thermal and membrane processes.

Emerging and Hybrid Technologies

Forward osmosis, electrodialysis, and solar-driven humidification-dehumidification systems represent active research frontiers in desalination engineering. Forward osmosis uses a draw solution with higher osmotic pressure than seawater to draw water across a membrane without applied hydraulic pressure, potentially reducing energy costs. Electrodialysis uses ion-exchange membranes and an applied electric field to move salt ions out of the feedwater, making it particularly effective for brackish water with lower salt concentrations. Solar desalination systems, ranging from simple greenhouse stills to concentrated solar-powered MED units, are being developed for off-grid and humanitarian applications. The ScienceDirect comprehensive review of reverse osmosis desalination also covers these emerging alternatives and discusses integration with renewable energy sources.

Applications

Desalination has applications in a wide range of sectors, including:

• Municipal water supply, providing drinking water to coastal cities and island communities
• Agricultural irrigation, in arid regions where saline groundwater is the only available source
• Industrial process water, supplying high-purity water for power plant cooling and semiconductor fabrication
• Humanitarian and disaster relief, through portable or solar-powered units deployed in water-scarce regions
• Offshore oil and gas operations, for onsite freshwater production on platforms