25 May 2005
Electrodialysis. The term "electrodialysis" describes a process that
separates all materials and minerals that are ionized from water by attracting the ions
dissolved in the water through membranes that are oppositely charged. When the water
is high in dissolved minerals, as in brackish water that contains more than 2500 ppm
TDS, its use may be more economical than the ion exchange methods. Under some
circumstances, it may remove enough minerals to make seawater usable in industrial
water systems. This process does not remove un-ionized or poorly ionized materials
such as some organics and soluble silica. As with RO, membranes must be kept clean.
Nanofiltration. This is a process that, in terms of the size of materials
removed, is intermediate between ultrafiltration and RO. The molecular weight cut-off
properties of nanofiltration membranes are in the range of < 1 x 10-3 m (400 to 800
Daltons or 10 angstroms). Ionic rejections vary widely depending on the valence of the
salts. Multivalent salts such as magnesium sulfate (MgSO4) are rejected as much as
99%, while monovalent salts such as sodium chloride (NaCl) may have rejections as
low as 20%.