Water Treatment "Part 27": Water softening with quicklime.

9 months ago

Only carbonate hardness (temporary hardness), particularly those linked to calcium and magnesium, can be eliminated using the quicklime treatment method; permanent hardness (non-carbonate hardness) is unaffected.

Limescale buildup in a PVC pipe

Long ago and still in use today is the procedure for removing carbonates, often known as the removal of temporary hardness of water. Development activity is being done on the tools employed, on the one hand, and on the reagents, particularly the components that help in agglomeration and sedimentation, on the other.

Calcium hydroxide, often known as slaked lime (used in the sedimentation process), is produced when quicklime is added to water and reacts swiftly with heat diffusion:

CaO + H2O ======== Ca(OH)2

Acidic carbonate ions and hydroxyl ions combine to form carbonates that are only marginally soluble in water, where it precipitates in the sedimentation basin. The following equations summarise the basic sedimentation reactions:

Ca(HCO3)2 + Ca(OH)2 ====== 2H2O + 2CaCO3

Mg(HCO3)2 + Ca(OH)2 ====== 2H2O + MgCO3+ CaCO3

MgCO3 + Ca(OH)2 ====== Mg(OH)2 + CaCO3

These steps eliminate the water's temporary hardness but have no impact on the water's permanent hardness. However, calcium hydroxide continues to interact with other dissolved salts in the water, and some of it precipitates, it also reacts with free carbon dioxide dissolved in water:

MgCl2 + Ca(OH)2 ====== Mg(OH)2 + CaCl2

MgSO4 + Ca(OH)2 ====== Mg(OH)2 + CaSO4

CO2 + Ca(OH)2 ====== CaCO3 + H2O

A little quantity of crystallised carbonate is added to the water, acting as an agglomerator, to enhance the sedimentation process. Therefore, The equipment used for the quicklime treatment process has to have a place where the entering raw water and crystallised agglomerates may combine. As that, The sedimentation process performs better and moves along faster when coagulants like ferric chloride or agglomerated polymeric materials, or both, are added.

The following relation gives the theoretical dosage of slaked lime at a concentration of 100% to achieve a satisfactory precipitation of calcium carbonate:

M= 7.4 (TAC + C)

"M" is the amount of added calcium hydroxide estimated in grams per one cubic meter of treated water. And "C" represents the concentration of free dissolved carbon dioxide. While "TAC" is the total alkalinity.