No sludge is formed during this process.
Water of nearly zero hardness is obtained.
This method is very cheap because the generated permutit can be used again.
The equipment used is compact and occupies a small space.
Its operation is also easy.
The process can be made automatic and continuous.
This process cannot be used for turbid and acidic water as they will destroy the
This treatment replaces only the cations, leaving all the anions like (HCO3)– and
(CO3)2– in the soft water.
When such water is boiled in boilers, CO2 is liberated. Free CO2 is weakly acidic in
nature and extremely corrosive to boiler metal.
Na2CO3 + H2O→2NaOH + CO2
Due to the formation of sodium hydroxide, the water becomes alkaline and can
cause cause caustic embrittlement.
Water containing Fe, Mn cannot be treated, because regeneration is very difficult.
This process cannot be used for softening brackish water. Because brackish water
also contains Na+ ions. So, the ions exchange reaction will not take place.
1.8.1 Ion exchange or Demineralisation process
Ion exchange or demineralisation process removes almost all the ions (both anions and
cations) present in the hard water.
The soft water, produced by lime-soda and zeolite processes, does not contain hardness
producing Ca2+ and Mg2+ ions, but it will contain other ions like Na+, K+, SO42–, Cl– etc.,
On the other hand demineralised (DM) water does not contain both anions and cations.
Thus a soft water is not demineralised water whereas demineralised water is soft water.
This process is carried out by using ion exchange resins, which are long chain, cross
linked, insoluble organic polymers with a micro process structure. The functional groups
attached to the chains are responsible for the ion exchanging properties.