Which pairing of membrane processes is effective for removing hardness from water?

• A. Nanofiltration and ultrafiltration
• B. Nanofiltration and reverse osmosis
• C. Ultrafiltration and microfiltration
• D. Reverse osmosis and ultrafiltration

Answer: (B)

Hardness comes from dissolved calcium and magnesium ions. To remove hardness, you need membranes that reject ions, not just particles. Ultrafiltration and microfiltration excel at removing suspended solids and large molecules, but dissolved salts pass through them, so they don’t meaningfully reduce hardness.

Nanofiltration and reverse osmosis are selective membranes that reject ions. Nanofiltration already removes a significant portion of hardness by blocking divalent ions, and reverse osmosis provides very high removal of most dissolved minerals, yielding water with very low hardness. Using them together leverages NF’s partial hardness removal and RO’s almost complete mineral rejection, giving the strongest overall reduction of hardness.

Pairing RO with ultrafiltration doesn’t improve hardness removal beyond what RO achieves, since ultrafiltration mainly tackles particulates and macromolecules, not dissolved minerals. The other combinations fail to provide effective ion removal needed to address hardness.