What principle underlies reverse osmosis (RO) in water treatment?

Reverse osmosis (RO) is a water treatment process that operates on the principle of using pressure to force water through a semipermeable membrane. This membrane selectively allows water molecules to pass while effectively blocking a wide range of contaminants, including salts, organic molecules, and larger particles. The application of pressure is crucial as it helps overcome the natural osmotic pressure that would otherwise prevent the movement of water from a concentrated solution to a diluted one.

The unique design of the semipermeable membrane is what makes RO efficient in purifying water. It ensures that only the desired molecules (water) can pass through, while impurities are left behind, thus enabling the production of clean water. This mechanism is essential in various applications, including drinking water purification and wastewater treatment, where contaminants need to be removed to meet health and safety standards.

In contrast to other options, such as evaporating contaminants or relying on gravity, reverse osmosis is distinctly characterized by its dependence on pressure and the selective nature of the semipermeable membrane. This specificity highlights the effectiveness of RO in removing even very small contaminants from water, making it a widely used method in the field of water treatment.