Source of Hardness
Hardness generally enters groundwater as the water percolates through minerals containing calcium or magnesium. The most common sources of hardness are limestone (which introduces calcium into the water) and dolomite (which introduces magnesium).
Types of Hardness
Hardness in water is caused by a variety of divalent cations, primarily calcium and magnesium. These cations have a tendency to combine with anions (negatively charged ions) in the water to form stable compounds known as salts. The type of anion found in these salts distinguishes between the two types of hardness – carbonate and non-carbonate hardness.
Carbonate hardness compounds
Calcium carbonate (CaCO3)
Magnesium carbonate (MgCO3)
Calcium bicarbonate (Ca (HCO3) 2)
Magnesium bicarbonate (Mg (HCO3) 2)
Calcium hydroxide (Ca (OH) 2)
Magnesium hydroxide (Mg (OH) 2)
Non-carbonate hardness compounds
Calcium sulfate (CaSO4)
Magnesium sulfate (MgSO4)
Calcium chloride (CaCl2)
Magnesium chloride (MgCl2)
The following equilibrium reaction describes the dissolving and formation of calcium carbonate:
CaCO3 + CO2 (aq) + H2O (l) => Ca2 + (aq) + 2HCO3- (aq)
Temporary hardness is a type of water hardness caused by the presence of dissolved bicarbonate minerals
(Calcium bicarbonate and magnesium bicarbonate anions).
Permanent hardness is hardness caused by the presence of calcium sulfate and / or magnesium sulfates in the water.
Total Permanent Hardness = Calcium Hardness + Magnesium Hardness
The calcium and magnesium hardness is the concentration of calcium and magnesium ions expressed as equivalent of calcium carbonate.
In addition to having different removal methods, carbonate and non-carbonate hardness can cause different problems. Carbonate hardness is the most common and is responsible for the deposition of calcium carbonate scale in pipes and equipment.
The equation below shows how this deposition is formed in the presence of heat:
Calcium bicarbonate = Calcium carbonate + Water + Carbon dioxide
Ca (HCO3) 2 = CaCO3 + H2O + CO2
In addition to the scale (calcium carbonate) produced, carbon dioxide resulting from this reaction can combine with water to give carbonic acid which causes corrosion of iron or steel equipment.
Non-carbonate hardness forms soap scum. Non-carbonate hardness reacts with the carbonate alkalinity found in soap and detergents in this reaction:
Calcium sulfate + Sodium carbonate = Calcium carbonate + Sodium sulfate
CaSO4 + Na2CO3 = CaCO3 + Na2SO4
Classification of Water Hardness
Soft Water 0 – 60 ppm
Moderately Soft Water 61 – 120 ppm
Hard Water 121 – 180 ppm
Very Hard Water Over 180 ppm
How to Remove Hardness from Water
As we all know that Water is very important factor of environment. So fresh water can be saved by recycling hard water as advised by environmental consultants to minimize consumption of water. There are a number of methods to remove the hardness present in water. Those methods are followed, the hard water gets converted to soft water. Some of the methods to remove hardness from water are
• Chemical Process of Boiling Hard Water
• Adding Slaked Lime (Clark's Process)
• Adding Washing Soda
• Ion Exchange Process
• Using Ion Exchange Resins
• Membrane Technology (Nano Filtration)
To reduce or eliminate hardness from the water, there are various processes that can be utilized.
The most common is the use of water softeners which exchange the calcium and magnesium for
Sodium. This technology is used in residential, commercial and industrial settings.
In Industrial and Municipal applications, lime softening can be used with clarifiers and the addition of lime
The calcium and magnesium precipitate out of the water and form sludge at the bottom of the clarifier.