Engineering Chemistry S Datta

362 ENGINEERING CHEMISTRY
Regeneration: The inactivated or exhausted cation exchange resin is regenerated by
dil. H 2
SO 4
/HCl.
R 2
Ca 2+ + 2H + ⎯⎯→ 2RH + + Ca 2+
Similarly, the exhausted anion exchange resin is regenerated by dil. NaOH
R 2
SO 4
2–
+ 2OH – ⎯⎯→ 2ROH – + SO 4
2+
The columns are finally washed with deionized water and the washings are discarded.
Mixed Bed Deionizer
It is actually a single cylinder containing and intimate mixture of cation exchanger and
strongly basic anion exchanger. Hence, water while passing through the column comes in
contact with the two exchangers for a large number of times and the hardness is reduced to a
very low level (1 ppm). Thus, it is actually equivalent to a series of cation and aninon exchangers.
Advantages of ion exchangers include (i) Easy regeneration; (ii) both acidic and alkaline
water can be softened; (iii) residual hardness is very low and hence the water is suitable for
high pressure boilers also.
Disadvantages are (i) the equipment and the process is costly and (ii) turbid water cannot
be directly charged for softening. It needs prior filtration.
Treatment of Water for Domestic Use
Municipalities supply potable water, which is suitable for drinking and it must satisfy
the following requirements:
Requirements for Drinking Water:
• It should be clear and odourless.
• It should be devoid of any unpleasant taste.
• Suspended matter should not exceed 10 ppm.
• It should be devoid of heavy metals and arsenic.
• pH should be about 8.
• Content of dissolved salts should not exceed 500 ppm.
• Lastly, it should be free from pathogenic (disease causing) microorganisms.
Natural water from river, lakes, canals are treated for the domestic purpose. The steps
are as follows:
• Aeration of the raw water by passing compressed air to remove mainly the obnoxious
odour and iron as Fe(OH) 3
.
• Settling in a large tank to remove suspended impurities.
• Sedimentation through coagulation of the colloidal particles by adding a suitable
coagulant. Chemical coagulants are generally (i) Alum [K 2
SO 4
, Al 2
(SO 4
) 3
.24H 2
O]
(ii) Green vitriol [FeSO 4
.7H 2
O].
• Coagulants or flocculants are generally added in the form of solution and mixing is
carried out by stirring.
• The gelatinous precipitates mainly contain aluminium hydroxide, ferrous and ferric
hydroxides (ferrous hydroxides are converted to ferric hydroxide by the dissolved oxygen
of water).
• These heavy flocculant precipitates contain also some bacteria making the water free
of some of the microorganisms.
• These partially clarified water is filtered through sand gravity filters.