RO and ED are membrane processes that have been traditionally used in desalination plants. While their application in cooling water systems is generally very limited, the following is a brief description of each.

ED is an advanced membrane process which is carried out in a series of cells separated by alternate membranes of different selectivity, each membrane allowing the passage of either anions or cations only. When a direct electric current is passed through a cell in which the water is flowing, Ions are driven through the appropriate membrane leaving deionized water behind.

As the name reverse osmosis implies, the separation of salt from water is achieved by applying a hydrostatic pressure across a suitably supported semi-permeable membrane separating the feed from the product water. When this is done, water with a substantially reduced salt content passes through the membrane in the reverse direction to that in which osmotic pressure would normally cause freshwater to flow, I.e. from the freshwater side to the brine side. The effective driving force is the net difference in pressure between the applied hydrostatic head and the osmotic pressure exerted by the fresh water on the other side of the membrane.