The process that uses a semipermeable membrane to remove dissolved salts and contaminants from water by applying pressure is:

Osmosis is the natural movement of solvent molecules, usually water, through a semipermeable membrane from a region of lower solute concentration (a dilute solution) to a region of higher solute concentration (a concentrated solution). A semipermeable membrane allows water molecules to pass but blocks most dissolved substances. Osmosis is important in living things; for example, plant roots absorb water from the soil by osmosis.

The pressure that must be applied to a solution to stop the flow of water into it by osmosis is called osmotic pressure.

Reverse osmosis (RO) is the opposite process. When a pressure greater than the osmotic pressure is applied to the concentrated solution (such as salty or contaminated water), water molecules are forced to move through the semipermeable membrane from the concentrated side to the dilute side. The dissolved salts, metals, microorganisms and many other impurities are left behind and flushed away as concentrated "reject water".

Uses of reverse osmosis:

Household water purifiers, which remove dissolved salts, hardness and heavy metals such as arsenic and lead.

Desalination of sea water to produce drinking water. Countries in the Middle East depend heavily on RO desalination; in India, there are large desalination plants at Chennai (Minjur and Nemmeli) and in Gujarat.

Industrial uses, such as producing pure water for boilers, pharmaceuticals and electronics.

Concentrating fruit juices and in the dairy industry.

Disadvantages: RO purifiers waste a large amount of water as reject water, and they also remove useful minerals such as calcium and magnesium. In 2019, the National Green Tribunal (NGT) directed the government to regulate RO use where the total dissolved solids (TDS) in water are below 500 mg per litre, to avoid unnecessary wastage.

Other water treatment methods:

• Sedimentation and filtration remove suspended particles.
• Chlorination kills germs using chlorine or bleaching powder.
• Ultraviolet (UV) treatment kills microorganisms.
• Distillation produces pure water by boiling and condensing it.

Why not the other options?

• ✗Distillation purifies water by evaporation and condensation.
• ✗Chlorination disinfects water using chlorine.
• ✗Sedimentation allows suspended particles to settle down.