Water Treatment "Part 28": Water softening with sodium hydroxide.

While both methods aim to soften water, sodium hydroxide has a few benefits over quicklime in this regard:

• The reaction kinetics are extremely quick, resulting in a substantially higher yield.
• The precipitate is in the form of granules rather than a slurry.
• In comparison to employing quicklime, the amount of carbonate generated is cut in half.
• Avoid the problem of lime slurry preparation.
• Treating cold water is possible.

NaOH beads

The following reactions result from adding sodium hydroxide to water that contains carbonic acid:

Ca(HCO3)2 + 2NaOH ===== CaCO3 + Na2CO3 + 2H2O

The sodium carbonate formed interacts with the calcium bicarbonate:

Ca(HCO3)2 + Na2CO3 ===== CaCO3 + 2NaHCO3

Furthermore, in an alkaline media, sodium carbonate interacts with permanent hardness compounds:

Na2CO3 + CaSO4 ===== CaCO3 + Na2SO4

Na2CO3 + CaCl2 ===== CaCO3 + 2NaCl

To increase the amount of suspended particles and crystallise the sediment, crystalline calcium carbonate develops collecting sites.
According to the aforementioned chemical processes, one mole of sodium hydroxide (NaOH) eliminates one mole of calcium (Ca++) and two moles of bicarbonate, yielding one mole of carbonate.

Working method:

In a settling basin, where the pH of the medium rises and suspended particles are generated, the process of eliminating carbonates with sodium hydroxide is carried out. By using coagulants or tiny silica particles, the production speed and size of suspended particles can be improved. In order to remove suspended particles afterwards, the treated water is sent into a sand filter.

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.