Reverse Osmosis Process

The reverse osmosis process is one of the most widespread separation methods, which has various applications. Desalination of seawater, separation of organic and toxic substances from industrial wastewater are two important uses of this technology. Reverse osmosis is one of the main methods of desalination of water, along with methods such as distillation, electrodialysis, deionization, etc., which is used on laboratory, urban and industrial scales.

The history of the use of the reverse osmosis method dates back to the 1950s, when experiments were conducted by Reed and Burton at the University of Florida, USA, and the first reverse osmosis membrane was made of cellulose acetate, and in the late 1960s this phenomenon was used commercially.

Theoretical principles of reverse osmosis

Understanding the phenomenon of reverse osmosis requires identifying the osmotic property. In general, mass transfer from one environment to another occurs due to a difference in concentration, or better said, due to a difference in chemical potential.

How Reverse Osmosis Works

For example, consider a container of water into which a piece of table salt crystal is dropped. In the part of the container where the salt crystal is present, the salt concentration is high and the water concentration is negligible, and in the other part of the container, the salt concentration is zero and the water concentration is very high. Due to the difference in concentration, the salt crystal molecules begin to penetrate into the other part, which is water, and on the other hand, the water molecules also penetrate into the part rich in salt. This process continues until the concentrations of water and salt in the two parts are equal. Because after that, the driving force, which is the concentration difference, does not exist between the two environments and, in other words, a state of equilibrium has been established.

What is reverse osmosis?

The property of osmosis is the passage of a solvent from the dilute solution part to the concentrated solution part through a semi-permeable membrane. To explain this property, consider the following two figures (normal osmosis and reverse osmosis) which represent two parts and a membrane. Water or a dilute salt solution in one part, a concentrated salt solution in the other, and a membrane that only allows water molecules to pass through. The membrane prevents the ions that make up the salt from passing through because they are too large.

Initially, the liquid levels in the two compartments are equal. Due to the tendency to equalize the concentrations in the two compartments, water molecules from the dilute solution pass through the membrane and penetrate into the concentrated solution. The liquid level in the concentrated compartment increases (natural osmosis). The flow of water molecules through the membrane occurs until the difference in liquid level (in other words, the pressure difference) in the two compartments prevents the movement of water molecules. The pressure difference in the compartment separated by the membrane, where it prevents the net flow of water molecules, is called osmotic pressure.

Osmotic pressure causes equilibrium in the two compartments. Now, if a pressure greater than the osmotic pressure is applied to the concentrated solution compartment, the opposite of the osmotic phenomenon occurs (reverse osmosis), in other words, water molecules flow from the more concentrated solution compartment to the more dilute solution compartment. This phenomenon is called reverse osmosis.

What is osmotic pressure?

The pressure difference in the compartment separated by the membrane, where it prevents the net flow of water molecules, is called osmotic pressure.

Reverse Osmosis Application

The phenomenon of reverse osmosis is used for water purification and, more generally, for the process of concentration and separation. Osmotic pressure is directly proportional to the concentration of salts in the solution. For example, in an aqueous solution of table salt, increasing the concentration by one milligram per liter increases the osmotic pressure by 0.01 Psi. Note that as water molecules pass through the membrane, the concentration of ions in the solution behind the membrane gradually increases, and as a result, the osmotic pressure also increases. Also, as water passes over the membrane surface, colloidal particles, sediments, microbial contaminants, and dissolved salts in the water cause clogging, which requires periodic cleaning of the reverse osmosis membrane (CIP). To remove these contaminants, you can use specialized membrane cleaning solutions produced by Abrizan Company.