To reduce hardness to 80 or 100 mg/l, the lime or limesoda ash softening processes may be used. These processes are used when some hardness can be tolerated, as in domestic water supplies. The operational cost of these processes is much less than for the ion exchange process. In lime softening, calcium is removed as follows:
Calcium Lime Calcium bicarbonate carbonate
The calcium carbonate is insoluble and precipitates out. If noncarbonate hardness such as calcium sulfate is also present, soda ash must be added:
Ca2+ + SOf" + Na2CO3 ® CaCO3^ + 2Na+ + SOf"
Similar reactions are involved in magnesium precipitation.
Since lime is used in excess, the softened water still contains Ca2+ and OH~ ions that must be stabilized (Figure 8.3.3). This can be done by bubbling carbon dioxide through the water (recarbonation). Water softening operations are usually followed by flocculation, settling, and filtration.
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