A simple definition - the process of removing ions from water
Deionization
How is water deionized?
The most common method for deionizing water is ion-exchange. This typically involves passing the feed water through two different types of resin. The cation resin converts the dissolved salts to acids, which are then removed by the anion resin.
Cation Resin (in the Hydrogen form)
When impure water containing cations (positively charged ions such as sodium, calcium, magnesium, etc.) passes through a cation resin bed the cationic impurities are removed due to a chemical reaction and replaced with hydrogen.
Chemical reactions:
RH2 + Ca(HCO3)2 -----> R Ca + 2H2CO3
RH2 + MgSO4 -----> R Mg + H2SO4
RH + NaCl -----> R Na + HCl
RH + KNO3 -----> RK + HNO3
Anion Resin (in the Hydroxyl form)
When impure water containing anions (negatively charged ions such as chlorides, sulphates, carbonates, etc.) passes through an anion resin bed the anionic impurities are removed due to a chemical reaction and replaced with the chemical equivalent of hydroxyl ions.
Chemical reactions:
R(OH)2 + H2CO3 -----> RCO3 + 2 H2O
R(OH)2 + H2SO4 -----> RSO4 + 2 H2O
ROH + HCl -----> RCl + H2O
ROH + HNO3 -----> RNO3 + H2O
Mixed bed Deionizers and Twin-bed Deionizers
Mixed bed deionizers combine cation resins and anion resins within the same bed.
Twin-bed deionizers consist of two separate beds with the water first passing through the cation and then through the anion.
Deionized Water
The hydrogen ions from the cation bed combine with the hydroxyl ions from the anion bed to form high purity water.
The quality of deionized water may be assessed by measuring the water's ability to conduct or resist electricity. Deionized water is a poor conductor and therefore has a high resistivity
Resistivity is measured in MΩ-cm (18.2 at 25°C indicating highly purified water). Conductivity is measured in µS/cm (0.055 at 25°C indicating highly purified water).
Twin-bed deionizers typically produce water of 1 MΩ-cm.
Mixed-bed deionizers produce higher quality water. They are often used to polish water from twin-bed deionizers to produce water of up to 18 MΩ-cm.
MΩ-cm and µS/cm are linked. One is the inverse of the other. It is therefore very simple to convert from micro siemens to megohms and vice-versa.
Convert from micro siemens to megohms:
1 [divided by] MΩ-cm = µS/cm
Convert from megohms to micro siemens:
1 [divided by] µS/cm = MΩ-cm
Regeneration
Regeneration is achieved by passing a base and/or acid solution through the resin beds. This removes the ions (respectively cations and/or anions) trapped by the resins in order to allow a new cycle of ion exchange.
Deionizers from Veolia Water Solutions & Technologies
| Deionizer | Max. Flow Rate | Water Quality at 25°C | |
|---|---|---|---|
µS/cm |
MΩ-cm | ||
| DUFN™ | 60 m3/h |
<25 |
>0.04 |
| Rapide™ Strata | 15 m3/h |
<2 |
>0.5 |
| Rapide™ Strata Plus | 15 m3/h |
0.055 |
18.2 |
| Duo Eclipse™ | 3 m3/h |
<20 |
>0.05 |
| 24r | 400 l/hr |
<25 |
>0.04 |
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