ANS:B - Reverse osmosis

Reverse osmosis (RO) is a water purification process that utilizes a semi-permeable membrane to remove ions, molecules, and larger particles from water. It is one of the most commonly used methods for desalination of seawater and also for producing purified drinking water from various sources like brackish water, groundwater, and wastewater. Here's how reverse osmosis works:

1. Semi-Permeable Membrane: The heart of the reverse osmosis system is the semi-permeable membrane. This membrane allows the passage of water molecules while blocking the passage of most dissolved salts, minerals, organic compounds, and other contaminants. The membrane is designed to have very fine pores that only water molecules can pass through, while larger molecules and ions are rejected.
2. Pressure: Reverse osmosis requires the application of pressure to overcome the osmotic pressure of the solution being treated. The feed water, whether it's seawater or brackish water, is pressurized using a pump before entering the membrane module. This pressure is essential to force water molecules through the membrane against the concentration gradient, from an area of higher solute concentration to an area of lower solute concentration.
3. Separation: As the pressurized feed water passes through the semi-permeable membrane, water molecules are able to penetrate the membrane pores and permeate through to the other side, leaving behind most of the dissolved salts, minerals, and contaminants. The rejected salts and contaminants, also known as brine or reject stream, are concentrated and discharged from the system, while the purified water, known as permeate, is collected.
4. Post-Treatment: Although reverse osmosis effectively removes a wide range of contaminants, some dissolved gases and certain smaller molecules may still pass through the membrane. Therefore, the permeate may undergo additional treatment steps such as remineralization, pH adjustment, or disinfection to meet specific quality standards and improve taste.

Reverse osmosis offers several advantages for desalination and water purification, including high efficiency, low energy consumption compared to thermal desalination methods, and the ability to remove a wide range of contaminants. It is widely used in both residential and industrial applications for producing high-quality drinking water, as well as in large-scale desalination plants to meet the growing demand for freshwater in arid regions and coastal areas.