Beds of ion exchange resins can efficiently remove ionised species from water by exchanging them for H⁺ and OH^- ions. Ion exchange resins are sub-1 mm porous beads made of highly cross-linked insoluble polymers with large numbers of strongly ionic exchange sites. Ions in solution migrate into the beads; where, as a function of their relative charge densities (charge per hydrated volume), they compete for the exchange sites. Beads are either cationic or anionic. Strong cation resins are usually polysulfonic acid derivatives of polystyrene cross-linked with divinylbenzene. Strong anion resins are benzyltrimethyl quaternary ammonium hydroxide (Type 1) or benzyldimethlyethyl quaternary ammonium hydroxide (Type 2) derivatives of polysytrene cross-linked with divinylbenzene.

Beds of ion exchange resins are available either in cartridges or cylinders, which are replaced /removed from site for remote regeneration, or as an arrangement of tanks, vessels, valves and pumps, which allows on site regeneration of the ion exchange resins. Positively charged ions (e.g. calcium, magnesium) are removed by the cation resin by exchanging hydrogen ions for the heavier more highly charged cations. Once "exhausted" the cation resin is regenerated by exposing the resin to an excess of strong acid, usually hydrochloric (HCl).

Similarly, negatively charged ions (e.g.sulphate, chloride) exchange with hydroxyl ions on the anion resin. Anion resin is regenerated using strong sodium hydroxide solution (NaOH).

The very large surface areas of ion exchange resins makes them a potential breeding place for micro-organisms and can lead to the release of fines and soluble components. For these reasons, good quality resins should be used and bed volumes kept as small as reasonably possible. Filters are typically installed after the beds to trap fines and other particulate matter. Bacterial build up can be minimised by frequent recirculation of the water and by regular cartridge replacement.

Modern ion exchange plant design uses relatively small resin beds and frequent regeneration - this minimises the opportunity for microbial growth.

With suitable choice of resin, pretreatment and system design, ion exchange enables the lowest levels of ionic contamination to be achieved.

Continuous electrodeionization is a technology combining ion exchange resins and ion-selective membranes with direct current to remove ionised species from water. It was developed to overcome the limitations of ion exchange resin beds, notably the release of ions as the beds exhaust and the associated need to change or regenerate the resins.

Reverse osmosis permeate passes through one or more chambers filled with ion exchange resins held between cation or anion selective membranes. Ions that become bound to the ion exchange resins migrate from the dilute chamber to a separate chamber (concentrate) under the influence of an externally applied electric field, which also produces the H⁺ and OH^- necessary to maintain the resins in their regenerated state. Ions in the concentrate chamber are recirculated to a break tank or flushed to waste.

The ion exchange beds in continuous electrodeionisaton (CEDI) systems are regenerated continuously, so they do not exhaust in the manner of ion exchange beds that are operated in batch mode (with chemical regeneration). CEDI beds are typically also smaller and remain in service for much longer periods.

CEDI is preferred for many purified water generation applications in Pharma, because of its "clean" non-chemical nature and constant high quality water produced. 

The resins used in CEDI systems can either be separate chambers of anion or cation beads, layers of each type within a single chamber or an intimate mixture of cation and anion beads.

Veolia Water Solutions & Technologies' pharmaceutical CEDI process utilizes cation beads in the concentrate stream and layered beds of cation and anion resins in dilute stream.

The resins are housed in wide cells that provide a flow path for the ions in transit. This offers advantages in the flexibility of design and mechanical simplicity on an industrial scale. The ion migration from dilute to concentrate is enhanced by the layered resin bed in the dilute.

Reverse osmosis (and sometimes membrane degassing) is typically used before CEDI to ensure that the CEDI "stack" is not overloaded with high levels of salts. The small volume of resins in the stack results in low bleed of organic molecules. Typically, RO removes about 95% of ions; CEDI will remove 99% of the remaining ions as well as carbon dioxide, organics and silica.

Typically, CEDI product water has a resistivity of 1 to 18.2 MΩ-cm (at 25°C) and a total organic carbon content below 20 ppb. Bacterial levels are minimised because the electrical conditions within the system inhibit the growth of micro-organisms. Current CEDI stacks development allow the user to carry out hot water sanitisation at 85°C, for a period of 1 to 4 hours.