Water Hardness

Water Hardness

Almost everyone has heard of “hard” and “soft” waters. In hard water areas people notice how much more detergent or soap is needed in washing activities to generate a lather. Characteristic scum forms around the sides of the bowl and kettles become rapidly “furred up”.

The term “hard water” is used to describe water which has a high concentration of calcium and magnesium salts and it arises as a result of groundwater dissolving limestone rock

There are two types of hardness; temporary and permanent.

Temporary hardness is due to calcium and magnesium salts in the form of bicarbonates. These are alkaline salts and so make the water more alkaline. Bicarbonates are relatively unstable and are easily broken down by heating the water to form carbonates, carbon dioxide and more water as shown in the case of calcium below.

Calcium carbonate is a form of limestone rock and so it can be seen that just heating water will decompose the soluble bicarbonates to form carbonates or “rock hard” scale. Permanent hardness is due to other salts of calcium and magnesium, for example, sulphates and nitrates. Permanent hardness salts will not form scale unless the water is boiled down to a fraction of its volume. This will concentrate the salts to a level such that they no longer remain dissolved in the water. Both temporary and permanent hardness salts react with soap to form an insoluble compound which shows as scum. This explains why more soap is needed in hard water areas just to precipitate the hardness salts, the scum being the precipitate.

From the above, it may be expected that people in hard water areas are in a much worse position than those in soft water areas, when in fact, hard waters have beneficial health effects.

Firstly, permanent hardness salts are less soluble at low temperatures and so gradually build up a fine layer of scale in pipes. This can be beneficial in isolating the inside surface of the pipes from the water and preventing lead from dissolving into the water in properties fitted with lead piping. Additionally, soft waters tend to have a lower pH (more acidic due to the absence of temporary hardness) and so are more corrosive towards metals. This can significantly increase the uptake of lead from pipes into the water.

Secondly, a number of studies have shown that water hardness has a relationship with heart disease. The harder the water, the fewer the deaths from heart disease. It has been estimated that in England and Wales alone, there are as many as 10,000 extra deaths per annum due to heart disease occurring in men aged 45-64 years who live in soft water areas rather than those with hard water. However, it is suggested that it may be the lack of some trace elements rather than the presence of hardness salts which account for the differences.
Thirdly, calcium is need by the body to promote healthy bones and for the regulatory functions in blood serum. It is especially important for growing children and nursing and pregnant women. Hard water helps to supplement the supply of calcium. Water Authorities are aware of the health problems and are increasingly blending hard and soft waters to produce waters of intermediate hardness.

​The level of hardness in water can be reduced by using a water softener. The most common type works by using an ion-exchange resin to exchange calcium and magnesium for sodium. Sodium salts are very soluble and do not form scale. The only maintenance required is occasional regeneration of the resin by use of a strong salt solution.

One word of caution when using softeners; high levels of sodium cause dehydration in young babies and water from commercial softeners MUST NOT be used to prepare feeds for babies.

The following figures are given as a guide to hardness levels:

0 – 50 mg/l soft water,
50 – 150 mg/l medium,
150 – above mg/l hard water

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