What membrane process is primarily referred to as "softening" in water treatment?

The membrane process primarily referred to as "softening" in water treatment is nanofiltration. This process is particularly effective in reducing hardness by removing divalent and multivalent ions, such as calcium and magnesium, which are responsible for water hardness. During nanofiltration, the membrane allows monovalent ions, such as sodium and chloride, to pass through while effectively rejecting larger, polyvalent ions.

This selective permeability is what makes nanofiltration distinct and suitable for water softening applications, as it can target the specific ions that lead to scale formation and other issues associated with hard water. In contrast, microfiltration and ultrafiltration utilize membranes with larger pore sizes that do not retain divalent ions effectively, and while reverse osmosis can remove hardness, its primary focus is on removing a broader range of contaminants, including larger molecules and salt, which may be more energy-intensive and costly for merely softening water.