What type of filtration process uses pressure against a semipermeable membrane?

The correct choice is reverse osmosis, which is a filtration process that uses pressure applied to a solution against a semipermeable membrane. This type of filtration is essential in separating contaminants or impurities from water, leaving behind a cleaner product. The semipermeable membrane allows certain molecules, particularly water, to pass through while blocking others, such as salts or larger particles.

In reverse osmosis, the pressure must be sufficient to overcome the natural osmotic pressure, effectively reversing the flow of solvent from a region of higher concentration to a lower concentration, hence the name "reverse" osmosis. This process is widely utilized in water purification systems, desalination plants, and other applications requiring the removal of ions, bacteria, and larger molecules from water.

Other filtration methods, like microfiltration and ultrafiltration, also involve membranes but operate under different principles and usually do not require the same level of pressure as reverse osmosis. Regular filtration does not involve the use of a semipermeable membrane and typically utilizes physical barriers such as screens or filters.