CATION AND ANION EXCHANGE SYSTEMS FOR COMPLETE DESALINATION
Depending on the composition of the raw water and the target amount of residual salt content in the product water, the addition of a reverse osmosis process may be necessary. This is a secondary treatment step that ensures complete demineralisation of the water. Ion-exchange is one of the most common approaches to complete demineralisation of the water. Highly acidic cations and highly alkaline resins are used for this technique.
Both ion exchange resins can be used in separated containers or mixed in so-called “mixed bed systems”. Although a better product quality can be achieved (low conductivity of water) by means of mixed bed systems, the maintenance and regeneration of these systems is more complex and implies an increased consumption of chemicals.
Ion-exchange resins can be used as alternative to reverse osmosis systems, however, continuous operation is not possible due to the regular regeneration cycles. This means that the system needs a redundant design for continuous operation.
If the requirements on the product water are low, it frequently only requires partial desalination. Softening and water decarbonisation are classic use cases. In these cases, weakly acidic cation-exchange resins are used. The disadvantage of the partial desalination is the formation of carbon acid, which has to be removed by an additional process step.