Lime softening water treatment process involves a relatively complicated series of chemical reactions. The goal of all of these reactions is to change the calcium and magnesium compounds in water into calcium carbonate and magnesium hydroxide, which are the least soluble compounds and thus will settle out of the water at the lowest concentrations.
In order to produce calcium carbonate and magnesium hydroxide, the pH of the water must be raised by the addition of lime. Calcium compounds in water will be removed at a pH of about 9.0 to 9.5 while magnesium compounds require a pH of10.0 to 10.5. When soda ash is used to remove non carbonate hardness, an even higher pH is required; 10.0 to 10.5 for calcium compounds and 11.0 to 11.5 for magnesium compounds.
The first step in lime softening is the addition of lime to react with substances like CO2 in the water before it can begin softening the water. The following reaction occurs:
Carbon dioxide + Lime = Calcium carbonate + Water
CO2 + Ca(OH)2 = CaC03 + HP
The resulting calcium carbonate precipitates out of solution.
When water, especially groundwater, has a high carbon dioxide concentration, the water is often pretreated with aeration before softening begins. Aeration removes the excess carbon dioxide and lowers the lime requirements.
Once the carbon dioxide demand has been met, the lime is free to react with and remove carbonate hardness from the water in the reaction shown below.
Calcium bicarbonate + Lime = Calcium carbonate + Water
Ca(HCO)2 + Ca(OH)2= 2CaC03 + 2HP
Calcium bicarbonate is the most common calcium compound in water, but other calcium-based hardness compounds have similar reactions. In any case, the calcium carbonate produced is able to precipitate out of solution.
Magnesium compounds have a slightly different reaction. First, magnesium bicarbonate reacts with lime and produces calcium carbonate (which precipitates out of solution).
Magnesium bicarbonate + Lime = Calcium carbonate + Magnesium carbonate + Water
Mg(HC03)2 + Ca(OH)2 = CaC03 + MgC03 + 2HP
Then the magnesium carbonate reacts with lime and creates more calcium carbonate and magnesium hydroxide. Both of these compounds are able to precipitate out of water.
Magnesium carbonate + Lime = Calcium carbonate + magnesium hydroxide I MgC03 +
Ca(OH)2 = CaC03 + Mg(OH)2
Removal of Non carbonate Hardness
In many cases, only the carbonate hardness needs to be removed, requiring only the addition of lime. However, if non carbonate hardness also needs to be removed from water, then soda ash must be added to the water along with lime.
Each non carbonate hardness compound will have a slightly different reaction. Here, we will consider the reactions of magnesium sulfate. The lime first reacts with the magnesium sulphate, as shown below:
Magnesium sulfate + Lime = Magnesium hydroxide + Calcium sulfate
MgS04 + Ca(OH)2 = Mg(OH)2 + CaS04
The resulting compounds are magnesium hydroxide, which will precipitate out of solution, and calcium sulfate. The calcium sulfate then reacts with soda ash:
Calcium sulfate + Soda Ash = Calcium carbonate + Sodium sulphate
CaS04 + Na2C03 = CaC03 + Na2S04
The calcium carbonate resulting from this reaction will settle out of the water. The sodium sulfate is not a hardness – causing compound, so it can remain in the water without causing problems.
The reactions which remove carbonate and non carbonate hardness from water re pH and produce water with a high concentration of dissolved lime and calcium carbonate, If the water is allowed to enter the distribution system in this state, the high pH would cause corrosion of pipes and the excess calcium carbonate would precipitate out, causing scale. So the water must be re carbonated, which is the process of stabilizing the water by lowering the pH and precipitating out excess lime and calcium carbonate.
The goal of re carbonation is to produce stable water, which is the water in chemical balance, containing the concentration of calcium carbonate in which it will neither tend to precipitate out of the water (causing scale) nor dissolve into the water (causing corrosion.) This goal is usually achieved by pumping carbon dioxide into the water. Excess lime reacts with carbon dioxide in the reaction shown below, producing calcium carbonate:
Lime + Carbon dioxide = Calcium carbonate + Water
Ca(OH)2 + CO2 = CaC03 + HP
Re carbonation process must be carefully controlled since carbon dioxide can ,ead with calcium carbonate and <haw it back into solution as calcium bicarbonate, reversing the softening process.
Alternatively, re carbonation can be achieved through the addition of acids such as sulfuric or hydrochloric acids or through poly phosphate addition. These types of re carbonation work is different from carbon dioxide addition.
Sludge disposal: Lime softening produces large quantities of sludge; for every kg of lime used, about two kg of sludge is formed. Landfill disposal is the most common method of sludge disposal.
Monitoring: After softening water treatment, the Langelier Index of the water should be tested to ensure that the water is not corrosive.