What is the major difference between reverse osmosis and ultra-filtration processes?

• A. The pore size in the membrane
• B. The membrane material
• C. The operating pressure
• D. The flow path direction

Answer: (A)

The main factor distinguishing these two processes is the pore size of the membrane, which governs what is retained. Ultra-filtration uses membranes with relatively larger pores that block suspended solids, colloids, and larger organic molecules, while allowing most dissolved salts and small ions to pass through. Reverse osmosis uses a much denser barrier with extremely small openings, so dissolved salts and small molecules are rejected, and water is forced through under higher pressure. Because of this pore-size difference, RO can desalinate water, whereas UF mainly removes particulates and larger molecules but not the dissolved salts.