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.
References:
- [Introduction to Water Chemistry (Pollution- Treatment- Analysis). Dr. Nasser Al-Hayek. Publication of the Higher Institute for Applied Sciences and Technology (HIAST). Syrian Arab Republic, 2017.]
- Taparhudee, Wara (2002). "Applications of Paddle Wheel Aerators and Diffused-Air System in Closed Cycle Shrimp Farm System" (PDF). Witthayasan Kasetsart (Sakha Witthayasat). 36: 408–419. Retrieved 26 April 2020.
- Unsafe water kills more people than war, Ban says on World Day". UN News. 22 March 2010. Retrieved 10 May 2018
- Raymond Desjardins- Livre: Le traitement des eaux- 2éme edition- Ecole Polytechnique de Montréal- 1997- ISBN 2-553-00643-8
- Drinking Water Treatment- EDX- Delft University of Technology.
- Book- Drinking Water: Principles and Practices- by Hans J C Van Dijk (Author), Jasper Q J C Verberk (Author), Peter J De Moel.
