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Water Hardness

Although there are slight differences between the Island's drinking water zones, Guernsey's water is classed as slightly to moderately hard. A number of household appliances require the input of water hardness in order for them to work to their full potential, this can be measured in a number of different units, a table of which can be found on the page below.

Water hardness

What is the difference between hard and soft water?

Certain materials in water react with soap, causing a precipitation which appears as a 'scum' or 'curd' on the water surface. Until enough soap has been dissolved to react with all the materials, no lather can be formed. Water which behaves like this is said to be 'hard'.

When synthetic detergents are used instead of soap, the inconvenience of hard water is not so strongly felt, because the detergents do not cause a precipitation of the hardness compounds. Nevertheless there are still many objections to hard water, the principle one being that when water is heated in boilers and heating systems, the hardness compounds precipitate to form a hard compound on the surface of the boiler and the interior of the pipes. The portion of the hardness which is deposited when water is boiled is called 'temporary hardness'; the remainder is called 'permanent hardness', or more precisely, 'non-carbonate hardness'.

Water hardness table

 Total Hardness (mg/L CA)Calcium Carbonate (mg/L CaCO3)Degrees Clarke (e, °e or °clark)French Degrees (°fH, °f or °HF)German Degrees (°dH)Millimoles per litre (mmol/l Ca2+ Mg2+)
Island Mean65.5163.611.416.49.31.6


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