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Reverse osmosis: how it works, what it is for, examples

Water flow in reverse osmosis is achieved by supplying pressure to the solution with the highest concentration of solute. Meanwhile, in osmosis the water flows from the less concentrated solution to the more concentrated solution, thanks to a difference in the hydrostatic presence between both compartments.

As seen above, the applied pressure leads the water molecules to pass through the asymmetric membrane, however retaining the dissolved ions on the more concentrated behavior side.

The establishment of the reverse osmosis procedure required the creation of asymmetric membranes, a fact achieved by Sidney Loeb and Srinivasa Sourirajan.

Reverse osmosis has numerous applications such as: seawater desalination, water purification, food processing, hydrogen production, etc.

How does reverse osmosis work?

Osmotic pressure and water flow

In osmosis there is a flow of water through a semi-permeable membrane: from the most dilute solution to the most concentrated solution. This determines an increase in the pressure exerted by the water in the compartment with the highest concentration of solute. This pressure is known as osmotic pressure.

The osmotic pressure opposes the flow of water from the compartment with the lowest concentration of solutes to the compartment with the highest concentration of solutes. Therefore, the re-establishment of an osmotic equilibrium between the compartments is favored.

In reverse osmosis the water flow is oriented from the most concentrated solution to the most dilute. The concentrated solution is the aqueous solution that is treated for dilution or purification. The osmotic pressure is oriented in the direction of this flow and, therefore, favors it.

In reverse osmosis, pressure is supplied by a pump to the treated water. This drives a flow of water from the compartment containing the treated solution into the compartment with the purified water.

The concentration of the treated water increases as the reverse osmosis process time passes. The power of the pump used will depend on the use that is going to be given to it.

Asymmetric membrane

The flow of water from the solution treated for purification occurs through an asymmetric membrane. This membrane is very permeable to water and not very permeable to salts, being necessary that it be resistant to the pressure exerted on the treated water and the substances present in it.

Asymmetric membranes consist of two layers: a thin, skin-like one, and a thicker and more resistant, highly porous one. Like the pump used, the type of membrane to be used in reverse osmosis depends on its intended use.

Components of a reverse osmosis water purification plant

Reverse osmosis production plant. Source: Twhair / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

Feed pump

In schematic form, this plant consists of a feed pump that supplies the necessary pressure for the reverse osmosis system. In some cases, an additional pump is used that is placed before the reverse osmosis system

Filter system

A filter system whose function is to eliminate polluting substances from the water. Among the filters, one of activated carbon is used to eliminate organic compounds that produce bad odors to the water, and another filter to eliminate chlorine, as well as a system that absorbs calcium and magnesium to soften the water.

Antiscalant dosing system

An antiscalant dosing system that removes substances that can damage the membrane of the reverse osmosis system.

It also has a reverse osmosis system and a product storage tank.

What is reverse osmosis for?

Generally speaking, reverse osmosis has the following applications:

-Purification of liquids

-Concentration of substances dissolved in liquids

Both will be addressed in the next and last section with examples.

Examples

Household water purification

In many homes, plants have been installed that use the reverse osmosis system to obtain water for drinking and cooking. This water is characterized by being crystalline, with a good taste and free of bacteria or other contaminating organisms.

The water processing plants, in addition to the reverse osmosis system, are equipped with a filter system and even an ultraviolet light source that guarantee the quality of the water.

Seawater desalination

Many populations are located in arid areas on the seashore, being paradoxical that they suffer from the shortage of drinking water in the presence of large masses of seawater, which cannot be used due to their high salt content.

This produced the idea of ​​desalinating seawater, reverse osmosis being a procedure used in thousands of desalination plants for the production of drinking water.

The problem of the energy necessary for the operation of reverse osmosis plants has been solved using solar energy. Solar energy is used to charge photovoltaic cells that serve as a source of electrical energy for the operation of desalination plants.

The use of reverse osmosis in water desalination has been used in Australian populations, for example: Perth.

Food processing

The only application of reverse osmosis was thought to be in obtaining potable water. However, a very important application of it is the concentration of food liquids, such as juices, milk, beer, etc. Reverse osmosis allows the extraction of water from these foods.

In the case of milk, its submission to reverse osmosis allows the concentration of its proteins to be increased, and it even facilitates the obtaining of whey proteins. Tomato juice can be concentrated by a similar procedure for the production of tomato paste and ketchup.

Reverse osmosis has been used in Bordeaux, France, to concentrate wines, using numerous reverse osmosis plants.

Aquarium care

In many reef aquariums, reverse osmosis is used for the production of the water used in them, since running water contains many elements and compounds that are harmful to the growth and reproduction of the fish that inhabit these aquariums.

Reverse osmosis has the advantage of producing high quality water at low cost. Therefore, it is the preferred procedure for obtaining the water to use in these aquariums.

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