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Reverse Osmosis in Water Treatment

Reverse osmosis has been expanded in water purification by expanding studies of ideal physical conditions, in which a pure solvent and a solution were separated by a semi-permeable membrane. The difference in fugacity between the solvent and the force solution is the mass transfer in the osmotic process. By reversing this phenomenon in the process of reverse osmosis, water molecules pass through the membrane (membranes), and the ions and impurities of the water are separated and refined.

Reverse osmosis as a cross-flow membrane process is superior to salinity and solvent extraction compared with other filtration methods such as microfiltration, ultrafiltration and nanofiltration, due to the diameter of the reverse osmosis membranes, which is less than 1 nm.

What is reverse osmosis?

For an explanation of what is the reverse osmosis, consider the ideal physical state in the figure shown. A chamber with a semi-permeable wall is divided into two parts. The left part contains a soluble two-component liquid mixture (1) and solvent (2), and the right side contains pure solvent (2). In the case of reverse osmosis water purification, it can be said that the left part contains saline water The right part contains fresh water (refined) and the semi-permeable wall of the reverse osmosis membrane (RO). The wall is only permeable to the solvent (2) or basically water. The temperature is constant, but with the help of pistons, the pressure of the two parts can be adjusted independently.
 reverse osmosis principle

If the pressure is P’ = P, the solvent fugacity in the left part will be smaller than the right. The difference in solvent fugacity’s is the same as the mass propagation force that makes the solvent penetrate the right-to-left wall. The equilibrium is established when the pressure P’ increases to a proportional value P * and the solvent fugacity’s are equal in both sides. The differential pressure P’ * -P is called the osmotic pressure of the solvent. By expanding the relationships to determine the reverse osmosis pressure, If the diluent 1 dissolved in solution is high enough, we arrive at the following relation, which is known as the Hoff vanity equation: π= (x1 RT)/V2

If the water contains a molar fraction x1 = 0.001 of a soluble component, then its osmotic pressure is 1.36 atm. In the reverse osmosis process, this means that, for a pure solvent pressure, P = 1atm, the pressure P’ on the solution should be 2.63 atm in order to prevent the penetration of the solvent from right to left, ie, the osmotic balance is created.

This observation has been tested as an incentive for a process called reverse osmosis, in which the solvent that is usually water gets separated from the solution, by applying sufficient pressure to create the driving force necessary to transfer the solvent from the membrane, which is practically only permeable to the solvent. The difference in the pressure of the liquid against the pure solvent pressure is the same as osmotic pressure.

In practice, the pressure difference is much greater than osmotic pressure. For example, sea water has a osmotic pressure of 25 bar, but the operating pressure of 50-80 bar is used to increase the rate of drinking water recycling. The characteristic feature of reverse osmosis is that, unlike distillation, which is usually a steam source of energy, it only requires mechanical power to pump the solution.

Relevant definitions in reverse osmosis

Reverse osmosis is a cross-sectional filtration method. This means that the water entering the reverse osmosis water purifier is divided into two sections of the permeate concentrate, as shown in the image. The reverse osmosis is simply divided into two sections of pre-treatment units and the reverse osmosis section, and some of its most important definitions include:

Recovery percentage: The ratio of desalinated water to that of initial flow is in percent.

The percentage of recycling or recovery is an important parameter in design, which, in addition to directly affecting the input pressure to membranes, affects the quality of desalinated water as well as the beneficial life of reverse osmosis membranes.

Percentage of impurities: the ratio of impurities in desalinated water to impurities in the intake water is in percent.

The percentage of salt rejection is obtained by subtracting 100 from the percentage of impurity passing.

The percentage of salt excretion in the reverse osmosis system is estimated by the membrane makers. On this basis, it can be estimated that the use of a reverse osmosis water purifier can be used to treat saline or seawater.

Comparison of reverse osmosis with filtration

Filtration is a physical way to remove suspended solids from water. Advanced filtration methods include removing soluble solvents and colloidal particles. In general, industrial water treatment devices can be classified according to their separation capacity as follows.
comparison of reverse osmosis and filteration
As seen in the figure, in reverse osmosis, only water and gas molecules pass through the semiconductor membrane. But in other methods of filtration, there is no ability to remove salts and water salinity. This is due to the diameter of the membrane against particles. This is well illustrated in the image below.
Pore size reverse osmosis
Reverse osmosis membrane with very high effective level and diameter of cavities smaller than 1 nm can separate all solute, suspended particles and organisms from water.

What is reverse osmosis membrane made of?

In industrial reverse osmosis water treatment, the most important part of the system is the membrane, or the semi-permeable membrane. Reverse osmosis membranes are commonly made of polyamide and are made as a spiral wound. Polyamide membranes are subject to a wide range of pH-resistant coatings, and a membrane with hole diameters of less than 1 nm can be produced. Reverse osmosis membranes of polyamide are strongly sensitive to oxidizing substances, such as free chlorine and hence will be destroyed when exposed to them.

The use of spiral wound membranes in reverse osmosis will result in more effective levels and thus more water. On the other hand, ease of guidance and enhancement of reverse osmosis design techniques is another benefit of such membranes.

In a membrane of crude water, the input is divided into two parts after collision with the membrane surface. A refined water section is where water molecules pass through the membrane of the interconnected plates and reach the central tube of reverse osmosis membranes. In the other part, all solutes, suspended particles and coarse molecules, along with the flow of effluent from membranes, are removed. The reverse osmosis membranes are shown below.

Types of reverse osmosis membranes

Reverse osmosis membranes are classified based on the amount of water, size and application in saline water or sea water. The larger the diameter of the membranes, the higher the effective surface and the discharge. In reverse osmosis technology, the higher the salinity of the water entering the membrane, the higher its osmotic pressure. Membrane makers will provide classifications of their membranes of production, discharge conditions, maximum pressure tolerance, amount of solvent rejection and maximum recovery, so designers and manufacturers of reverse osmosis water purification machines will design their units accordingly.