Water Hardness Explained


Water Hardness

Water hardness is defined as the sum of the divalent metallic ions in the water. The main contributors to the hardness of the water are calcium and magnesium. Additional contributors to the hardness of the water include iron (Fe2+), strontium (Sr2+), zinc (Zn2+), manganese (Mn2+) and other ions. However, their concentrations are usually significantly lower than the concentration of calcium and magnesium.

In most cases, summing up the calcium and magnesium in the water gives an adequate hardness measure.

What are the effects of water hardness?

In addition, calcium and magnesium react with soap to form soap scum. Hard water will increase the consumption of soap, as soap will not create foam.


Does water hardness have any positive effects?

Yes, in adequate concentrations, calcium and magnesium have a positive effect on human health and also on plants. They both are essential nutrients and their deficiency might cause health problems, so in drinking water and irrigation water, a certain level of hardness is favorable.


Temporary hardness vs. permanent hardness

There are two types of hardness – temporary hardness and permanent hardness.

Temporary hardness – also called ‘Carbonate hardness’. This type of hardness refers to the calcium and magnesium carbonates and bicarbonates in the water. Heating the water or reacting it with lime removes this hardness. CO2 is released as gas and precipitates of insoluble calcium carbonate and/or magnesium hydroxide are formed.

Ca(HCO3)2 –> CaCO3↓ + CO2↑ + H2O.

Mg(HCO3)2 –> Mg(OH)2↓ + 2CO2

Permanent hardness, also referred to as ‘non-carbonate hardness;, is the hardness due to the presence of calcium or magnesium sulfates, chlorides and nitrates. For example, calcium sulfate, magnesium chloride etc.

Removal of permanent hardness is done by using lime or lime with soda ash, depending on the chemical composition of the water.


In what units is hardness expressed?

The most common unit for expressing the level of hardness in the water is ‘mg/L as CaCO3’.

The concentrations of calcium and magnesium are expressed as equivalent of CaCO3.

The following formula can be used to calculate hardness , when the concentrations of calcium and magnesium in the water are known:

Water hardness = 2.5Ca + 4.1Mg

Where calcium and magnesium are measured in ppm (1ppm=1 mg/L).

For example, water with 50 ppm Ca and 15 ppm magnesium will have hardness of:

2.5×50 + 4.1×15 = 186.5 mg/L CaCO3

Additional units include:

dGH – ‘Degrees of General Hardness’ or ‘German Degrees’.

1 dGH = 17.484 mg/L CaCO3

Grains per gallon: 1 gpg = 17.1 mg/L CaCO3

French degrees ⁰fH:

1 French degree = 10 mg/L CaCO3


Classification of water hardness levels:

Hardness in mg/l CaCO3

Hardness level

< 60 mg/l


60 – 120 mg/l


120 – 180 mg/l


> 180 mg/l

Very hard

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