Osmosis and reverse osmosis
Osmosis and reverse osmosis
The process of reverse osmosis as a method of water purification has been used since the beginning of the 60s. Initially, it was used for desalination of sea water. Today, hundreds of thousands of tons of drinking water per day are produced in the world using the principle of reverse osmosis.
Improvements in technology have made it possible to use reverse osmosis systems at home. Today, thousands of such systems have already been installed in the world. Water obtained by reverse osmosis has a unique degree of purification. In terms of its properties, it is close to the melt water of ancient glaciers, which is recognized as the most ecologically clean and useful for humans.
The phenomenon of osmosis is the basis of the metabolism of all living organisms. Thanks to it, every living cell receives nutrients and, conversely, slags are removed.
The phenomenon of osmosis is observed when two salt solutions with different concentrations are separated by a semipermeable membrane.
This membrane allows molecules and ions of a certain size to pass through, but is a barrier to substances with larger molecules. Thus, water molecules can penetrate the membrane, but molecules of salts dissolved in water cannot.
If there are saline solutions with different concentrations on different sides of the semipermeable membrane, water molecules will move through the membrane from a weakly concentrated solution to a more concentrated one, causing the latter to rise in the liquid level. Due to the phenomena of osmosis, the process of water penetration through the membrane is observed even when both solutions are under the same external pressure.
The difference in the height of the levels of two solutions of different concentrations is proportional to the force under which water passes through the membrane. This force is called "osmotic pressure".
In the case when a solution with a higher concentration is affected by an external pressure exceeding the osmotic pressure, water molecules will begin to move through the semipermeable membrane in the opposite direction, that is, from a more concentrated solution to a less concentrated one.
This process is called "reverse osmosis". All reverse osmosis membranes work according to this principle.
In the process of reverse osmosis, water and substances dissolved in it are separated at the molecular level, almost perfectly clean water accumulates on one side of the membrane, but impurities remain on the other side. Thus, reverse osmosis provides a much higher degree of purification than most traditional filtration methods based on mechanical particle filtration and adsorption of a number of substances using activated carbon.
In domestic reverse osmosis systems, the pressure of the incoming water on the membrane corresponds to the pressure of the water in the pipeline. If the pressure increases, the water flow through the membrane also increases.
In fact, the membrane completely retains substances dissolved in drinking water. They penetrate the membrane, but in very small quantities. Therefore, purified water still contains a small amount of dissolved substances. It is important that increasing the pressure at the inlet does not lead to an increase in the content of salts in the water after the membrane. On the contrary, higher water pressure not only increases the performance of the membrane, but also improves the cleaning quality. In other words, the higher the water pressure on the membrane, the more pure water of better quality can be obtained.
In the process of water purification, the concentration of salts on the inlet side increases, due to which the membrane can become clogged and stop working. To prevent this, a forced flow of water is created along the membrane, which washes the "brine" into the drain.
The efficiency of the reverse osmosis process with regard to various impurities and dissolved substances depends on a number of factors. Pressure, temperature, pH level, material from which the membrane is made, and the chemical composition of the incoming water affect the efficiency of reverse osmosis systems.
Inorganic substances are very well separated by a reverse osmosis membrane. Depending on the type of membrane used (acetate cellulose or thin-film composite), the degree of purification is mainly inorganic elements of 85%-98%.
The reverse osmosis membrane also removes organic matter from the water. Organic substances with a molecular weight of more than 100-200 are removed completely; and with less - they can penetrate the membrane in small quantities. The large size of viruses and bacteria practically excludes the possibility of their penetration through the membrane.
At the same time, the membrane allows oxygen dissolved in water and other gases that determine its taste. As a result, at the output of the reverse osmosis system, fresh, tasty water is obtained, so pure that, strictly speaking, it does not even require boiling.
