 Partial demineralisation of drinking water can be achieved in a very unusual and environment-friendly process, where a mixture of ion exchange resins is regenerated with carbon dioxide only, without addition of any acid, alkali, or salt.
CARIX stands for "CArbon dioxide Regeneration of Ion Exchange". The process developed in the 1980's in the Nuclear Research Institute of Karlsruhe (Germany) is a partial demineralisation for drinking water using resins that are regenerated with carbon dioxide under pressure. These are a weak acid cation resin and a strong base anion resin used as a mixed bed. A large part of the carbon dioxide is recycled.
This process is particularly useful to treat water with a high hardness and bicarbonate content, a feature extremely common in Western Europe and several other regions of the world. At least ten plants are in operation (January 2008), with flow rates between 50 and 600 m3/h (220 to 2650 gpm). Typical performance of the CARIX process: - Hardness ca. 60% removal
- Sulphate ca. 80% removal
- Nitrate ca. 50% removal
- Chloride: no change
In addition, many heavy metals such as lead or cadmium are removed. The CARIX™ process
A simplified picture of the system is shown here: 
The resins to use in the CARIX process are two acrylic types specially developed by Rohm and Haas for this purpose: AMBERLITE™ PWC11, an acrylic weakly acidic cation (WAC) exchange resin, and AMBERLITE™ PWA12, an acrylic strongly basic anion (SBA) resin. Those are used in a mixed bed column, and unlike in common demineralisation units, the resins never have to be separated: they are regenerated in situ with carbon dioxide under moderate pressure.
Features and advantages of the Carix process
The unused regenerant can be recycled. The carbon dioxide produced in the exhaustion phase can even be recovered. Due to the properties of carbonic acid, a partial conversion of the resin is obtained during regeneration, which means that only partial demineralisation of the water is achieved. However, divalent cations (hardness) and heavy metals are removed preferentially, as well as sulphate and nitrate, which makes the process ideal for the treatment of drinking water in municipal plants.
The process does not produce additional salinity, or very little. | 
