Mixed Bed Filters
A mixed bed filter serves for fine purification of demineralised water, or for demineralisation of smaller amounts of water. This is achieved by ion exchange, with the resin bed consisting of a mixture of acidic and alkaline resins. Regeneration is conducted with acid and lye.
The ion exchange process in a mixed bed filter does chemically work as described under demineralization. However, the mixed bed filter is filled with both strongly acidic and strongly alkaline ion exchange resin at the same time. During operation, the different resin types are mixed with each other. This basically works like a series of many small connected cation and anion exchangers, and thus results in a very high demineralisation effect.
For regeneration, the resins are first separated with water, by making use of their different specific weights. The strongly acidic cation exchange resin is regenerated upstream with acid, the strongly alkaline anion exchange resin is regenerated downstream with acid. The regeneration effluent is drained from the transitional region between the two resin types. Often, the effluent contains an excess of acid or lye, and accordingly needs to be neutralised before being discharged into the sewer system. After regeneration, air is used to intermix the two resin types with each other again.
Compared to other ion exchange processes, the amount of excess chemicals required for regeneration of a mixed bed filter is significantly higher. That is why a mixed bed filter is usually only used for polishing, for example of already demineralised water or condensate, with an accordingly long service life between regenerations.