LIME-SODA ASH WATER TREATMENT METHOD

Lime-soda ash treatment for the reduction of hardness involves the addition of slake lime [Ca(OH)₂] to a hard water supply to remove the carbonate hardness by precipitation with the precipitation being removed by filtration. Non-­carbonate hardness is in turn reduced by the ad­dition of soda ash (Na₂C0₃) to form insoluble precipitate which is also removed by filtration.

This particular method of removing hardness a sometimes used by municipal water plants to reduce the amount of calcium and magnesium in a water supply. While it is quite effective in reducing hardness, it is not a complete removal treatment.

Often when a city has a raw water source that has 35 to 40 grain hard water, the local water system will use the lime-soda ash treatment to reduce hardness to between 5 and 10 grains.

Lime-soda ash treatment is especially effective if a water contains bicarbonate (temporary) hardness. Where calcium and magnesium are primarily in chloride or sulfate compounds, this treatment is noticeably less effective.

Slaked lime is used to remove calcium bicarbonate from water. In the water to be treated, the slaked lime ions react with the calcium bicarbonate to form the very slightly soluble calcium carbonate. This precipitated material is usually removed by first settling and then filtering.

Ca(OH) ₂+ Ca(HC0₃ ) ₂ --> 2 CaCO₃ ¥ + 2 H₂0

Calcium hydroxide plus calcium bicarbonate reacts to form calcium carbonate plus water

NOTE: The arrow pointing down (¥) indicates the formation of an insoluble compound.

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++  + Na ₂CO₃ --> 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