Which membrane process combination removes hardness?

Which membrane process combination removes hardness?

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

Answer: (D)

Hardness in water comes from dissolved calcium and magnesium ions. Membrane processes remove contaminants based on size and charge. Ultrafiltration and microfiltration mainly tackle particles, colloids, and microbes, not dissolved salts, so they don’t effectively remove hardness. Nanofiltration, on the other hand, can reject many multivalent ions like Ca2+ and Mg2+, reducing hardness, while reverse osmosis has very high ion rejection and can remove nearly all dissolved salts. Using nanofiltration first lowers the hardness and helps prevent scaling, then reverse osmosis finishes removing the remaining ions to achieve very low hardness in the treated water. The other paired combinations won’t remove dissolved hardness as effectively because they either don’t target dissolved ions or don’t remove them to a meaningful extent.

Hardness in water comes from dissolved calcium and magnesium ions. Membrane processes remove contaminants based on size and charge. Ultrafiltration and microfiltration mainly tackle particles, colloids, and microbes, not dissolved salts, so they don’t effectively remove hardness. Nanofiltration, on the other hand, can reject many multivalent ions like Ca2+ and Mg2+, reducing hardness, while reverse osmosis has very high ion rejection and can remove nearly all dissolved salts. Using nanofiltration first lowers the hardness and helps prevent scaling, then reverse osmosis finishes removing the remaining ions to achieve very low hardness in the treated water. The other paired combinations won’t remove dissolved hardness as effectively because they either don’t target dissolved ions or don’t remove them to a meaningful extent.