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Precipitation in Water Treatment

A precipitate is a solid that separates from solution as the result of a chemical reaction. The chemical reaction that produces a precipitate is called a precipitation reaction. Precipitation reactions occur widely in nature. Many naturally occurring minerals are formed as the result of precipitation reactions.

Precipitation reactions are also widely used in industry. Chemical precipitation used in water treatment is a good example. Water contains many impurities, including tiny suspended particles called colloids. Colloids are removed from water by a series of processes called coagulation, flocculation, and sedimentation. Coagulation occurs when chemicals added to the water form one or more precipitates. Flocculation is the process by which the colloidal impurities adhere to the precipitate. The resulting "floc" is allowed to settle to the bottom by sedimentation before removal.

In a typical process, a coagulant such as alum (aluminum sulfate) is added to water undergoing treatment. Often alum alone will react with the water's natural alkalinity due to the presence of calcium hydrogen carbonate, or lime (calcium hydroxide) or soda ash (sodium carbonate) is added to boost the water's alkalinity.

The following chemical equations illustrate typical precipitation reactions, which occur in the water treatment process:

  1. Al2(SO4)3 + 3Ca(OH)2 2Al(OH)3 + 3CaSO4

  2. Al2(S04)3 + 3Na2C03 + 6H20 2Al(OH)3 + 3Na2SO4 + 3CO2

  3. Al2(SO4)3 + 3Ca(HCO3)2 2Al(OH)3 + 3CaSO4 + 6CO2


Go to Solubility Rules (click on Solubility Rules under Toolbox) and use the solubility rules to identify the precipitates in the above chemical equations.


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