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Science & Technology25 Essential Exam Concepts
Desalination: Reverse Osmosis, Thermal Distillation & Potable Water Production
Desalination is an industrial separation process that extracts dissolved mineral salts, suspended particles, and biological impurities from saline seawater or brackish groundwater to produce potable freshwater suitable for human consumption, agriculture, and industrial manufacturing. Earth holds an estimated 1.4 billion cubic kilometers of water, but approximately 97.5 percent of this planetary reserve is saline ocean water, containing an average total dissolved solids concentration of roughly 35,000 parts per million (ppm), predominantly sodium and chloride ions. Because the human physiological tolerance for drinking water generally requires total dissolved solids below 500 ppm under World Health Organization guidelines, desalination bridges the gap between abundant marine waters and acute freshwater scarcity.
The global desalination industry relies on two primary technological methodologies: membrane separation and thermal distillation. Membrane processes, dominated overwhelmingly by Seawater Reverse Osmosis, account for over seventy percent of modern worldwide desalination capacity. Under natural osmosis, water molecules diffuse across a semi-permeable membrane from a region of low solute concentration into a region of high solute concentration until osmotic equilibrium is reached. Reverse osmosis mechanically reverses this natural tendency by applying intense hydraulic pressure—typically 55 to 80 bar, far exceeding seawater's natural osmotic pressure of about 27 bar—forcing water molecules backward through synthetic thin-film composite polyamide membranes with pore diameters smaller than one nanometer. The microscopic membrane pores permit pure water molecules to pass while rejecting more than 99.4 percent of dissolved salt ions and microorganisms.
Thermal distillation techniques, including Multi-Stage Flash distillation and Multi-Effect Distillation, rely on phase change thermodynamics. In these systems, seawater is heated and introduced into vacuum chambers where reduced ambient pressure lowers the boiling point of water, inducing rapid boiling and flash evaporation. The rising steam condenses against cooler pipes carrying incoming feed seawater, producing distilled freshwater while preheating the intake water. While thermal plants have historically dominated energy-rich Gulf economies, modern reverse osmosis facilities operate far more efficiently due to Energy Recovery Devices, such as isobaric pressure exchangers, which transfer the hydraulic energy of pressurized reject brine directly into incoming feed streams, slashing energy consumption to approximately 3 to 4 kilowatt-hours per cubic meter. However, the sustainable management of hypersaline brine discharge remains an environmental priority, requiring multiport diffusers to prevent damage to coastal marine ecosystems.
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Desalination is the industrial separation process that removes dissolved mineral salts from seawater or brackish water to produce potable freshwater.
Seawater contains an average salinity of approximately 35 parts per thousand (ppt) or 35,000 parts per million (ppm), primarily sodium (Na⁺) and chloride (Cl⁻) ions.
The World Health Organization (WHO) and Bureau of Indian Standards (BIS) recommend that drinking water maintain total dissolved solids (TDS) below 500 mg/L.
Seawater Reverse Osmosis (SWRO) is the leading global desalination technology, accounting for more than 70% of installed worldwide production capacity.
Natural osmosis is the spontaneous movement of solvent water molecules across a semi-permeable membrane from a lower-solute solution into a higher-solute solution.
Reverse osmosis applies external hydraulic pressure exceeding the solution's natural osmotic pressure (~27 bar for seawater) to force water through the membrane against the concentration gradient.
Typical operating pressures in SWRO plants range between 55 and 80 bar (800 to 1,160 psi) to overcome osmotic resistance and membrane friction.
SWRO membranes are typically manufactured from Thin-Film Composite (TFC) aromatic polyamides with pore sizes under 0.2 nanometers, smaller than hydrated sodium and chloride ions.
Multi-Stage Flash (MSF) distillation is a thermal process where heated seawater passes through sequential vacuum chambers of decreasing pressure, flashing into steam.
Multi-Effect Distillation (MED) uses steam generated in one stage (effect) to heat evaporating seawater in the subsequent lower-pressure stage, improving thermodynamic efficiency.
Electrodialysis (ED) and Electrodialysis Reversal (EDR) use electrical potential differences to drive salt ions across alternating cation and anion exchange membranes, suited for brackish water.
Modern SWRO plants utilize Energy Recovery Devices (ERDs), such as isobaric pressure exchangers, recovering up to 98% of the hydraulic pressure from the rejected concentrate brine stream.
State-of-the-art SWRO plants consume approximately 3.0 to 3.5 kilowatt-hours (kWh) of electricity per cubic meter (1,000 liters) of freshwater produced.
The primary byproduct of desalination is hypersaline brine, which has roughly double the salinity of natural seawater (65,000 to 75,000 ppm) along with residual pre-treatment chemicals.
Improper brine disposal can create dense, oxygen-depleted plumes on the seafloor, requiring engineered multiport diffusers in open coastal waters to facilitate rapid dilution.
Saudi Arabia, the United Arab Emirates, Kuwait, and Israel lead the world in per capita desalination capacity, with the Gulf region producing over 40% of global desalinated water.
In India, coastal cities facing water stress have adopted large-scale SWRO, notably Chennai with the Minjur (100 MLD) and Nemmeli (100 MLD and 150 MLD) plants.
The National Institute of Ocean Technology (NIOT), Chennai, developed indigenous Low Temperature Thermal Desalination (LTTD) plants deployed across the Lakshadweep islands.
LTTD utilizes the temperature gradient between warm surface seawater (~28°C) and cold deep seawater (~7°C) pumped from 600 meters depth to flash evaporate and condense water.
Coupling desalination facilities with renewable solar photovoltaic and offshore wind generation is the primary global strategy to achieve carbon-neutral freshwater production.