The ion exchange technology is used for different water treatment applications:
- Softening (removal of hardness)
- De-alkalization (removal of bicarbonate)
- Decationization (removal of all cations)
- Demineralization (removal of all ions)
- Mixed bed polishing
- Nitrate removal
- Selective removal of various contaminants
You will find here a description of the above processes, the exchange reactions and the changes in water. Resin types are described in another page, as well as regeneration methods. See also the general introduction to ion exchange, and an overview of ion exchange column designs in other pages.
Natural water contains calcium and magnesium ions (see water analysis) which form salts that are not very soluble. These cations, together with the less common and even less soluble strontium and barium cations, are called together hardness ions. When the water evaporates even a little, these cations precipitate. This is what you see when you let water evaporate in a boiling kettle on the kitchen stove.
Hard water also forms scale in water pipes and in boilers, both domestic and industrial. It may create cloudiness in beer and soft drinks. Calcium salts deposit on the glasses in your dishwasher if the city water is hard and you have forgotten to add salt.
This particular process uses a weakly acidic cation resin. This resin type is capable of removing hardness from water when it also contains alkalinity. After treatment, the water contains carbon dioxide, that can be eliminated with a degasifier tower. The cation resin is very efficiently regenerated with an acid, usually hydrochloric acid.
The removal of all cations is seldom practiced, except as a first stage of the demineralisation process, or sometimes in condensate polishing where the decationiser precedes a mixed bed unit. A strongly acidic cation exchange resin (SAC) is used in the H+ form.
For many applications, all ions in the water must be removed. In particular, when water is heated to produce steam, any impurity can precipitate and cause damage. As there are cations and anions in the water, we must use two different types of resins: a cation exchanger and an anion exchanger. This combined arrangement produces pure water, as presented in the general introduction. Demineralisation is also called deionisation. The cation resin is used in the hydrogen form (H+) and the anion resin in the hydroxyl form (OH–), so that the cation resin must be regenerated with an acid and the anion resin with an alkali.
A degasifier is used to remove the carbon dioxide created after cation exchange when the water contains a significant concentration of bicarbonate.
Mixed bed polishing:
The last traces of salinity and silica can be removed on a resin bed where highly regenerated strong acid cation and strong base anion resins are mixed.
Mixed bed units deliver an excellent treated water quality, but are complicated to regenerate, as the resins must first be separated by back washing before regeneration. Additionally, they require large amounts of chemicals, and the hydraulic conditions for regeneration are not optimal. Therefore, mixed beds are usually only used to treat pre-demineralised water, when the service run is long.
Nitrate can be removed selectively from drinking water using strong base anion resins in the chloride cycle, i.e. regenerated with a NaCl brine. The reaction is:
RSBA-Cl + NO3– RSBA-NO3 + Cl–
Selective Removal of Various Other Contaminants
Selective removal of metals and other contaminants is mainly used for drinking water and for waste. Many of these applications require special resins: cheating resin making stable metal complexes, for instance.
- Removal of boron (boric acid) from drinking water.
- Removal of nitrate from drinking water (shown above).