Reverse Osmosis Process

The working of the reverse osmosis process is explained in the following steps:

Step 1: The two solutions of different concentrations must be placed in contact through a semipermeable membrane which creates a concentration gradient between them or chemical potential between them.

Step 2: This concentration gradient results in the flow of solution from the side of the low-concentration region to the high-concentration region. This process is called osmosis.

Step 3: During osmosis, hydrostatic pressure is exerted by the solution on the higher concentration side. To counter this pressure an external pressure is applied on the region of the high-concentration side. 

Step 4: When this external pressure reaches a certain level then the process of osmosis stops i.e. there is a flow of solution from the low-concentration side to the high-concentration side. At this stage, the applied external pressure is called osmotic pressure.

Step 5: When the applied external pressure exceeds this osmotic pressure level then the solvent particles start moving from the region of high concentration to the low concentration region of solution which is actually the reverse of osmosis. This movement of solvent opposite to the direction of osmosis is called reverse osmosis

To understand the concept of Reverse Osmosis, we need to understand the concept of Osmosis and Osmotic Pressure.

Osmosis Definition

Osmosis is the process in which particles of solvent move from a region of pure solvent or low concentration of solute to a region of high concentration of solute through a semipermeable membrane.

• Suppose two solutions with different concentrations are taken and separated by a semi-permeable membrane (SPM).
• In that case, Gibb’s free energy per mole is different for the two regions.
• This difference in the energy of the two regions establishes a chemical potential between the two regions.
• This chemical potential acts as a driving force for the flow of pure solvent to the region of high concentration of solute.
• The molecule of solvent flows through the semipermeable membrane (SPM), until the two sides become the same in concentration i.e. they attain equilibrium.
• This movement of solvent exerts hydrostatic pressure on the other side continuously.
• This movement of solvent can be stopped before attaining equilibrium by applying pressure on the region of high concentration of solute called osmotic pressure.

Osmotic Pressure

Osmotic pressure is the minimum pressure needed to stop solvent molecules from flowing through the semipermeable membrane (SPM), from the region of pure solvent or lower concentration of solute to the higher concentrated side.

• It is denoted by π.
• It is a colligative property that is dependent on the number of particles of solute dissolved in the solution.

Reverse osmosis refers to the movement of solvent from a region of high concentration of solute to a region of low concentration of solute or pure solvent through a semipermeable membrane. Commonly called RO, it is one of the oldest techniques used in the purification of seawater.

Let’s learn about reverse osmosis in detail, including its principle, advantages, and applications.