Processes of Water softening:

Softening processes are used to reduce water hardness. In this process, the hardness of water undergoes chemical reactions with the use of chemicals and precipitates, and in this way, the hardness of water is reduced.

The reason for the hardness of water is the presence of carbonates, sulfates, chlorides and nitrates of calcium, magnesium, iron and aluminum metals. Because the three metals iron, manganese and aluminum are present in small amounts in water, calcium and magnesium ions form the main part of water hardness. The conditions of the ground where the water flows largely determines the hardness of raw water, and this causes the amount of ions in surface and underground water to be different. Water hardness is expressed in two ways: permanent or non-carbonate hardness and temporary or carbonate hardness.

Increasing the softening process efficiency:

One of the influential factors in the softening process is the pH of the environment. According to the results, the best pH for liming is 10 to 10.5. At the same time, the increase of carbonate alkalinity decreases the solubility of calcium carbonate. If the pH in the liming pond is around 10.5, the magnesium hardness is significantly reduced and Mg(OH)2 acts as a coagulant. If the pH is less than 10, it is necessary to use a coagulant. The optimal amount of lime and soda water depends on the alkalinity (M) and (P) as well as the water hardness and can be calculated. In addition to pH, the presence of sludge in the environment increases the liming efficiency due to providing the primary crystal nucleus.

Removal of temporary difficulty:

One of the common ways to reduce the temporary hardness of water is to use lime water. The result of the reaction of lime water (calcium hydroxide) with calcium bicarbonate and magnesium bicarbonate is the precipitation of calcium carbonate and magnesium hydroxide. The main reactions take place according to the following:

Ca(HCO3)2 + Ca(OH)2 → 2CaCO3↓ + 2H2O                pH=10 _ 10.5

Mg(HCO3)2 + 2Ca(OH)2 → Mg(OH)2↓ + 2CaCO3↓ + 2H2O

In the above reactions, the amount of lime water needed to reduce magnesium hardness is twice the amount of lime water needed to reduce calcium hardness. In the first stage, magnesium carbonate is produced from the reaction of calcium hydroxide with magnesium bicarbonate, and in the next stage, the result of the reaction of magnesium carbonate with calcium hydroxide is the production of magnesium hydroxide precipitate.

Mg(HCO3)2 + Ca(OH)2 → MgCO3↓ + CACO3↓ +2H2O

Ksp[MgCO3]=2.6×10-5

MgCO3 + Ca(OH)2 → Mg(OH)2↓ + CaCO3↓

Ksp[Mg(OH)2]=4.0×10-14

Therefore, it is clear that the solubility of magnesium hydroxide is less. Carbon dioxide (CO2) in water reacts with lime and some of it is removed.

Water softening processes by sedimentation:

CO2 + Ca(OH)2 → Ca(HCO3)2

If the amount of CO2 is more than 20ppm, first it is reduced by blowing air and then the remaining amount is removed by using lime water.

CO2 + Ca(OH)2 → Ca(HCO3)2

Ca(HCO3)2 + Ca(OH)2 → 2CaCO3↓ + 2H2O

Removal of permanent hardness:

Lime/soda is used to reduce the non-carbonate hardness of water (Na2CO3). In temporary hardness, an alkaline material such as lime or soda is added, and the bicarbonate in the environment is converted into carbonate, and the resulting carbonate precipitates calcium and reduces calcium hardness. In this way, it is obvious that after adding sodium carbonate, carbonate ion enters the water and results in the production of calcium carbonate.

CaSO4 + Na2CO3 → CaCO3↓ + Na2SO4

MgCl2 + Ca(OH)2 + Na2CO3 → Mg(OH)2↓ + CaCO3↓ + 2NaCl

MgSO4 + Na2CO3 + Ca(OH)2 → Mg(OH)2 + CaCO3↓ + Na2SO4

Using caustic soda is another way that is used in softening water and is used in special cases. The superiority of using caustic soda is preferable because it produces sodium carbonate with calcium bicarbonate, which is effective in permanent hardness removal.

Ca(HCO3)2 + 2NaOH → CaCO3↓ + Na2CO3 + 2H2O

Since water is neutral in terms of electrical charge, the total gram equivalent of anions is equal to the gram equivalent of cations. The amount of anions and cations in water is separately determined by performing decomposition tests for each ion. The values ​​of three anions, carbonate, bicarbonate and hydroxyl, can be obtained using the results of the tests of alkalinity compared to phenolphthalein and alkalinity compared to methyl orange.