Hard Water

Hard water
is usually defined as water which contains a high concentration of calcium and magnesium ions.  Measurements of hardness are given in terms of the calcium carbonate equivalent, which is an expression of the concentration of hardness ions in water in terms of their equivalent value of calcium carbonate.  Water is considered to be hard if it has a hardness of 100 mg/L or more as calcium carbonate. 

bathtub ring.
Hard water causes bathtub rings.

Softening is the removal of hardness from water.  This is not a required part of the water treatment process since hard water does not have any health consequences.  However, hard water is problematic for a variety of reasons.  Hard water makes soap precipitate out of water and form a scum, such as the ring which forms around bathtubs.  In addition to being unsightly, the reaction of hard water with soap results in excessive use of soaps and detergents.  Hard water may also cause taste problems in drinking water and may shorten the life of fabrics washed in hard water.  Finally, hard water harms many industrial processes, so industries often require much softer water than is usually required by the general public.

Calcium carbonate scale on a piece of pipe.

Excessively hard water will nearly always have to be softened in order to protect the water treatment plant equipment and piping systems.  At a hardness of greater than 300 mg/L as calcium carbonate, scale will form on pipes as calcium carbonate precipitates out of the water.  The scaling can damage equipment and should be avoided. 

Sources 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.)  Since hardness enters water in this manner, groundwater generally has a greater hardness than surface water.  There are also regional variations in hardness, shown by the map below.

Map of U.S. water hardness.

Since they are the two most widespread and troublesome ions in hard water, it is often said that hardness is caused by calcium (Ca2+) and magnesium (Mg2+) ions dissolved in water.  However, hardness can be caused by several other dissolved metals as well, including strontium (Sr2+), iron (Fe2+), and manganese (Mn2+).  You will notice that all of the hardness-causing ions are divalent cations, meaning that they have a charge of positive two.  Metals such as sodium (Na+) and potassium (K+) with a charge of positive one do not cause hardness. 

Types of Hardness

As mentioned above, 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 noncarbonate hardness. 

Carbonate hardness compounds
Noncarbonate 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)

Calcium sulfate (CaSO4)
Magnesium sulfate (MgSO4)
Calcium chloride (CaCl2)
Magnesium chloride (MgCl2

As you can see in the table above, carbonate hardness is caused by metals combined with a form of alkalinity.  Alkalinity is the capacity of water to neutralize acids and is caused by compounds such as carbonate, bicarbonate, hydroxide, and sometimes borate, silicate, and phosphate.  In contrast, noncarbonate hardness forms when metals combine with anything other than alkalinity.

Carbonate hardness is sometimes called temporary hardness because it can be removed by boiling water.  Noncarbonate hardness cannot be broken down by boiling the water, so it is also known as permanent hardness.  In general, it is important to distinguish between the two types of hardness because the removal method differs for the two. 

When measuring hardness, we typically consider total hardness which is the sum of all hardness compounds in water, expressed as a calcium carbonate equivalent.  Total hardness includes both temporary and permanent hardness caused by calcium and magnesium compounds. 

Hardness Problems

In addition to having different removal methods, carbonate and noncarbonate 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. 

In contrast, noncarbonate hardness is the culprit in forming soap scum.  Noncarbonate hardness reacts with the carbonate alkalinity found in soap and detergents in this reaction:

Calcium sulfate + Sodium carbonate Calcium carbonate + Sodium sulfate
CaSO4 + NaCO3 CaCO3 + Na2SO4