Lime softening

Lime softening, also known as Clark's process, is a type of water treatment used for water softening which uses the addition of limewater (calcium hydroxide) to remove hardness (calcium and magnesium) ions by precipitation. The process is also effective at removing a variety of microorganisms and dissolved organic matter by flocculation.

Lime softening was first used in 1841 to treat Thames River water. The process expanded in use as the bactericidal effect of the process was discovered. Lime softening greatly expanded in use during the early 1900s as industrial water use expanded. Lime softening provides soft water that can, in some cases, be used more effectively for heat transfer and various other industrial uses. In the United States, it is used primarily in the Midwest, Florida and Texas.

As lime in the form of limewater is added to raw water, the pH is raised and the equilibrium of carbonate species in the water is shifted. Dissolved carbon dioxide (CO₂) is changed into bicarbonate (HCO−
3) and then carbonate (CO2-
3). This action causes calcium carbonate to precipitate due to exceeding the solubility product. Additionally, magnesium can be precipitated as magnesium hydroxide in a double displacement reaction.

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