Membrane desalination entered the market in 1960, about a decade before reverse osmosis became widespread. The shared idea is simple: a selective surface holds back salt ions or molecules and lets water through.
Electrodialysis and EDR
Three principles shape electrodialysis: most dissolved salts are ions, ions move toward an electrode of opposite charge, and membranes can be built to pass only cations or only anions. Brackish water flows through channels between alternating anion and cation membranes. One cell dilutes the water (the product); the next concentrates the ions (the brine).
A plant typically includes pretreatment, a membrane stack, a low-pressure circulation pump, a DC rectifier and post-treatment to adjust pH and strip gases such as hydrogen sulphide. In 1970, EDR added polarity reversal so membranes could be cleaned automatically.
Strengths include 85–94% recovery, energy use that tracks the mass of salt removed rather than the volume of water, a 7–10 year membrane life, and little bacterial growth on EDR membranes. Weaknesses are just as clear: periodic chemical cleaning is required, leaks occur, and bacteria and non-ionic matter remain in the product.
Reverse osmosis
Osmosis is the movement of solvent through a semipermeable membrane toward a more concentrated solution. Reverse osmosis applies pressure on the saline side and reverses that flow: pure water passes, salts stay behind. Modern membranes reject 95–99% of dissolved solids and up to 99% of bacteria.
Plants split by feed quality. SWRO treats seawater or beach wells above 10,000 µS/cm. BWRO treats brackish groundwater or river water below that. Feed pressure is 15–25 bar for brackish water and 25–80 bar for seawater; high-pressure pumps take about 70% of a plant’s electricity. An energy-recovery turbo-pump on the brine line cuts that load.
Reid built the first cellulose-acetate membranes at the University of Florida in 1954. UCLA’s 1959 work and thin-film composite (TFC) membranes opened the industrial path. The first membrane desalination plant started in California in 1965.
RO installs quickly, scales from litres to millions of cubic metres a day, starts and stops fast, and can take one train offline without stopping the rest of the plant. Limits include a 2–5 year membrane life, sensitivity to storm-driven suspended solids, heavy pretreatment and high operating pressure.
Forward osmosis, membrane distillation and centrifugal RO
Forward osmosis replaces the high-pressure pump with a concentrated draw solution, usually ammonia and carbon dioxide. Water moves naturally toward the higher concentration; the draw is then heated to about 58°C so NH3 and CO2 can be stripped and reused. Electricity use is very low, but the process is not yet common at city scale.
Membrane distillation was introduced commercially in 1980. Brine is heated to the verge of vaporization, vapor crosses a hydrophobic membrane and condenses as freshwater. The hardware is simple and needs only a small temperature difference, but it needs more space and energy.
Centrifugal RO mounts the membrane on a rotor and uses centrifugal force to create pressure. The patent dates to 1988, yet the process remains largely experimental. A 10 m³/day unit in Canada has been reported at about 20% recovery.
