Lesson - Chemicals in Softening

Chemical Reactions in Softening 

In chemical precipitation, the hardness causing ions are converted to insoluble forms. Calcium and magnesium become less soluble as the pH increases. Calcium and magnesium can be removed from water as insoluble precipitates at high pH levels.

Addition of lime to water increases the hydroxide concentrations so that the pH increases. Addition of lime to water also converts alkalinity from the bicarbonate form to the carbonate form which causes the calcium to be precipitated as calcium carbonate. As additional lime is added to the water, the phenolphthalein alkalinity increases to a level where hydroxide becomes present allowing magnesium to precipitate as magnesium hydroxide. 

Following the chemical softening process, the pH is high and the water is supersaturated with excess caustic alkalinity in the hydroxide or carbonate form. Carbon dioxide can be used to decrease causticity and scale forming tendencies of the water prior to filtration.

The chemical reactions that take place in water during the chemical precipitation process depend on whether the hardness to be removed is carbonate or non-carbonate hardness. Carbonate hardness, temporary hardness, can be removed with the use of lime only. Removal of non-carbonate hardness, permanent hardness, requires lime and soda ash.

Chemicals in Softening 

The lime used in the chemical precipitation softening process can be hydrated lime, (Ca(OH)2), calcium hydroxide, or calcium oxide, CaO. Hydrated lime can be used directly. The calcium oxide or quicklime must first be slaked. This process involves adding the calcium oxide to water and heating it to cause slaking, which is the formation of calcium hydroxide (Ca(OH)2) before use. Small facilities commonly use hydrated lime. Large facilities find it more economical to use quicklime (CaO) and slake it on site.

The application of lime for the removal of carbonate hardness also removes carbon dioxide. Carbon dioxide does not contribute to hardness and does not need to be removed. However, carbon dioxide will consume a portion of the lime that is used and must be considered in the dosing process.

When lime is added to water, carbon dioxide present in the water is converted to calcium carbonate if enough lime is added. Adding more lime, the calcium bicarbonate will be precipitated as calcium carbonate. To remove calcium and magnesium bicarbonate, an excess of lime must be used.

Magnesium carbonated hardness requires the addition of lime and soda ash, Na2CO3. 

The primary chemical reactions products from the lime-soda softening process are calcium carbonate and magnesium hydroxide. The water treated has been chemically changed and is no longer stable because of pH and alkalinity changes. Lime-soda ash softened water is usually supersaturated with calcium carbonate. The degree of instability and excess calcium carbonate depends on the degree to which the water is softened. Calcium carbonate hardness is removed at a lower pH than magnesium carbonate hardness. If maximum carbonate hardness removal is practiced so that a high pH is required to remove magnesium carbonate hardness, the water will be supersaturated with calcium carbonate and magnesium hydroxide. Under these conditions, deposition of precipitates will occur in filters and pipelines.

Excess lime addition to remove magnesium carbonate hardness results in supersaturated conditions and a residual of lime, which will produce a pH of about 10.9. The excess lime is called caustic alkalinity since it raises the pH. If the pH is then lowered, better precipitation of calcium carbonate and magnesium hydroxide will occur. Alkalinity will be lowered also. This process is usually accomplished by pumping carbon dioxide gas into the water. This addition to the treated water is called re-carbonation.

Re-carbonation can be carried out in two steps. The first addition of carbon dioxide would follow excess lime addition to lower the pH to about 10.4 and encourage the precipitation of calcium carbonate and magnesium hydroxide. A second addition of carbon dioxide after treatment removes non-carbonate hardness. The pH is lowered to about 9.8 and it encourages precipitation. By carrying out re-carbonation prior to filtration, the buildup of excess lime and calcium carbonate and magnesium hydroxide precipitates in the filters will be prevented or minimized.

Care must be exercised when using re-carbonation. Feeding excess carbon dioxide can result in no lowering of the hardness by causing calcium carbonate precipitates to go back into solution and cause carbonate hardness. 

An alternative method to the lime-soda ash softening process is the use of sodium hydroxide (NaOH), which is called caustic soda. The chemical reactions using caustic soda demonstrate that in removing carbon dioxide and carbonate hardness, sodium carbonate (soda ash) is formed, which will react with to remove non-carbonate hardness. Sodium hydroxide substitutes for soda ash and part of the lime to remove carbonate hardness. The use of caustic soda may have several advantages including stability in storage, less sludge formation, and ease of handling.