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.
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.
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:
