Membrane separation is an important separation technique in which a semipermeable membrane selectively permits certain components of a mixture to pass through while retaining others. The separation is generally based on differences in particle size, molecular size, charge, solubility, diffusivity, or pressure-driven transport. Membrane processes have gained considerable importance because they can operate continuously, require relatively low energy compared with many thermal separation methods, and can provide high-quality products. The major membrane processes include microfiltration, ultrafiltration, nanofiltration, reverse osmosis, electrodialysis and membrane distillation. Their applications in water treatment and the production of table salt are particularly significant. In water treatment, membrane separation is used for the removal of suspended particles, colloidal matter, microorganisms, organic substances and dissolved salts. The selection of a particular membrane process depends mainly on the nature and size of the contaminants. Microfiltration is one of the simplest membrane processes used in water purification. It employs membranes with relatively large pores, generally in the micrometre range. Suspended solids, algae, bacteria and other particulate contaminants are retained, while water and dissolved substances pass through the membrane. Microfiltration is therefore useful as a pretreatment step before more advanced membrane processes. It reduces turbidity and prevents the rapid fouling of downstream ultrafiltration or reverse-osmosis membranes. Ultrafiltration uses membranes with smaller pores than microfiltration. It is particularly effective for removing colloids, proteins, macromolecules, microorganisms and high-molecular-weight organic substances. Bacteria and many viruses can be removed effectively, although the efficiency depends on membrane characteristics and operating conditions. Ultrafiltration is widely employed in drinking-water treatment, wastewater treatment and the pretreatment of water before reverse osmosis. Nanofiltration occupies an intermediate position between ultrafiltration and reverse osmosis. Nanofiltration membranes can remove very small organic molecules and multivalent ions while allowing a substantial fraction of monovalent ions to pass through. This makes nanofiltration useful for water softening because calcium and magnesium ions responsible for hardness are preferentially rejected. It is also applied to remove colour, natural organic matter, pesticides and certain contaminants from drinking water. Reverse osmosis is one of the most important membrane processes