Ion exchange with synthetic resins and adsorption onto activated alumina are water treatment processes in which a presaturant ion on the solid phase, the "adsorbent" is exchanged for an unwanted ion in the water.

To accomplish the exchange reaction, a packed bed of ion exchange resin beads or alumina granules is used. Source water is continually passed through the bed in a downflow or upflow mode until the adsorbent is exhausted, witnessed by the breakthrough of unwanted contaminants at an unacceptable level, in the bed effluent.

The most useful ion exchange reactions are reversible. In the simplest cases, the exhausted bed is regenerated by using an excess of the presaturant ion. Ideally, no permanent structural change takes place during the exchange/regeneration cycle. Resins do swell and shrink, however, and alumina is partially dissolved during regeneration.

In reversible reactions the media can be reused many times before it must be replaced because of irreversible fouling or, in the case of alumina, attrition losses. In a typical water supply application, from 300 to as many as 60,000 bed volumes (BV) of contaminated water may be treated before exhaustion.

Regeneration typically requires from 1 to 5 BV of regenerant followed by 2 to 20 BV of rinse water. These wastewaters generally amount to less than 2 per cent of the product water; nevertheless, their ultimate disposal is a major consideration in modern design practice. Disposal of the spent medium may also pose a problem if it contains a toxic or radioactive substance such as arsenic or radium.

The largest application of ion exchange to water treatment is in the area of water softening. This is the process of removing calcium, magnesium and other polyvalent cations in exchange for sodium ions. Common uses for water softening are in residential and commercial applications for ease of cleaning, improved laundry and dishwashing and a reduction in scaling leading to more efficient use of energy in hot water heating. The solution that is used to regenerate the water softeners is concentrated salt water called brine.

The second application is deionisation. In deionisation, the hardness and other ions that are initially in the water are removed and replaced with H+ and OH- ions, which can combine to form water. This is used in applications where extremely pure water is required. There are two solutions that are used to regenerate a deioniser. One is a concentrated acid, and the other is a concentrated base.