What is Reverse Osmosis? How Does it Really Works?

Reverse Osmosis (RO) is a water purification process that uses a partially permeable membrane to remove ions. The membrane allow certain molecules or ions to pass through it by diffusion, or occasionally by more specialized processes of facilitated diffusion, passive transport or active transport. In reverse osmosis, an applied pressure is applied to overcome osmotic pressure, a collective property, that is driven by chemical potential differences of the solvent, a thermodynamic parameter. This process can remove many types of dissolved and suspended chemical species as well as biological ones from the water. This RO process is used in both industrial processes and also in the production of portable water. The result is that the solute is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side. This membrane should not allow to pass large molecules or ions through the holes, but should allow to pass small components of the solution. In most cases, the membrane is designed to allow only water to pass through this dense layer while preventing the passage of solutes that include salt ions. This process requires that a high pressure be exerted on the high concentration side of the membrane, usually 2-17 bar (30-250 psi) for fresh and brackish water, and 40-82 bar (600-1200 psi) for seawater, which has around 27 bar (390 psi) natural osmotic pressure that must be overcome. This process is best for sea water as in order to remove salt and other minerals from the sea water and to convert it into fresh water, but since 1970s, RO is also been used to purify fresh water for medication, industrial and domestic applications.

How does Reverse Osmosis Work?

In the normal osmosis process, the solvent naturally moves from an area of low solute concentration through a membrane to an area of high solute concentration. The driving force for the movement of the solvent is the reduction in the free energy of the system when the difference in solvent concentration on the either side of a membrane is reduced, generating osmotic pressure due to the solvent moving into the more concentrated solution. The RO process is same as other membrane technology applications. Reverse osmosis is different from filtration as in RO process the mechanism of fluid flow is by osmosis by a membrane. The predominant removal mechanism in membrane filtration is straining, or size exclusion, where the pores are 0.01 micrometers or larger, so the process can theoretically achieve perfect efficiency regardless of parameters such as the solution’s pressure and concentration.

Uses of Reverse Osmosis

Reverse osmosis is commonly used for the purification of drinking water from seawater, removing the salt and other materials from the water molecules. Around the world RO is being used for improving water for drinking and cooking. A solar powered desalination unit produces portable water from saline water by using a photovoltaic system that converts solar power into the required energy of the reverse osmosis. Due to extensive availability of sunlight across different geographies solar powered reverse osmosis lends itself well to drinking water purification in remote settings where no availability of electricity. It is a portable, self-contained water treatment plant. It can provide portable water from nearly any water source. There are many models that are being used by United states armed forces and the Canadian forces. Rain water can also get purified by using reverse osmosis process and then used for landscape irrigation. In industry, RO removes minerals from boiler water or power plants. Some mape syrup producers start using this process to remove water from sap before the sap is boiled down to syrup. The use of RO allows 75-90% of the water to be removed from the sap, reducing energy consumption and exposure of the syrup to high temperatures. It is also used in small scale hydrogen production, as it used to prevent formation of minerals on the surface of electrodes. Many reef aquarium keepers also use reverse osmosis process for their artificial mixture of seawater.

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