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Membrane separation is a technique that is used in science and industry to separate substances from a mixture by filtering that mixture through a porous membrane. The substance being filtered can be either a liquid or a gas. Membrane separation has many practical applications, including water filtration and the creation of concentrated solutions. The major types of membrane separation processes are reverse osmosis, nanofiltration, ultrafiltration and microfiltration.
When a feed solution or gas is passed across a porous membrane under high pressure, certain substances from that mixture will permeate the membrane to the area of lower pressure, but other substances will be unable to do so. The consistency and chemical composition of the membrane determines which materials pass through it. For example, a membrane that has larger pores will tend to let in large and small particles, and a membrane that has smaller pores will exclude the larger particles. Similarly, a membrane that has a particular chemical composition might be designed to chemically filter out certain compounds.
In reverse osmosis, two solutions are contained on opposite sides of the membrane — one that is more concentrated and the other that is more watery, or less concentrated. The natural tendency of water to diffuse from an area of low concentration to an area of high concentration is reversed by pressure applied to the highly concentrated solution. This causes water to flow out of the highly concentrated solution across the membrane, making that solution even more concentrated by water removal. Evaporated milk and other condensed products can be made through this process.
Nanofiltration typically is used in processing water to remove its salty taste or undesirable mineral content. This method removes salts, such as sodium and potassium, from the liquid. Membranes that are used in nanofiltration have a slightly larger pore size than those that are used in reverse osmosis.
Ultrafiltration and microfiltration are used to separate out larger particles while letting smaller particles through the membrane. Viruses, bacteria and suspended solid matter are excluded by these larger-pore membranes. Often, ultrafiltration or microfiltration will be used in conjunction with a finer filtration method as part of an overall industrial process.
Many industries use membrane separation as part of their routine operations. For example, removing damaging salts and minerals that could create scale or corrosion in source water is important in the operation of boilers. Membrane separation also can be used for reclaiming chemical compounds from a waste stream. Desalination is another method in which membrane separation is widely used to produce high-quality drinking water.
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