Image Place Holder right
 |
1 | 2 |
To remove the magnesium, additional lime is used. The reaction for this process is:
Ca(OH) 2 + Mg --> Mg(OH) 2 ¥ + Ca++
Calcium hydroxide plus magnesium ions react to form magnesium hydroxide plus calcium ions
This step has simply replaced the magnesium with calcium. If soda ash is then fed into the water, the calcium will precipitate as calcium carbonate:
Ca++ + Na2CO3 --> CaCO3 ¥ + Na+
Calcium ions plus sodium carbonate react to form calcium carbonate plus sodium ions
There are many variants possible under this general heading. Their discussion here, however, is not essential to our course of study.
Lime-soda ash treatment becomes increasingly costly when the hardness of the water must be reduced to less than 5 grains. Municipally, the complete elimination of hardness is rarely attemped as less than 5% of a municipality's water is used for home consumption. The use of soda ash for the reduction of non-carbonate hardness increases the sodium in the effluent water in the same proportion as ion exchange softening.
The use of the lime-soda ash treatment is impractical for individual home softening of supplies. For one thing, there are difficulties in feeding lime and soda ash into raw water. Further, close control of the operation is required both while the settling and filtering occurs.
An additional deterrent to home use of the lime-soda ash treatment is the size of the equipment necessary, together with the high cost of this method of treatment.
 |
1 | 2 |
|