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65
coal and acts as a very efficient filter that traps microscopic traces of pesticides
and herbicides. Water percolates down through the bed by gravity. Rapid sand
filters have the raw water pumped through a pressurized cylinder that contains
the filter medium. This filter material is either crushed silica, quartz or anthra-
cite coal. Filtering removes metallic salts, bacteria and turbidity (mud-diness).
It also removes colouring effects, odours and particles, which affect the taste of
the water. The naturally occurring pH value and the total dissolved salt concen-
tration are virtually unaltered by the water supply authority.
Water quality varies with the local geology and can be classified as hard,
soft, acidic or alkaline. Mineral salts of calcium and magnesium have soap-
destroying properties and are considered in the evaluation of water hardness.
Temporary hardness is due to the presence of calcium carbonate, calcium
bicarbonate and magnesium bicarbonate, which dissolve in water as it passes
through chalky soil. These salts are deposited as scale on heat transfer surfaces
during boiling, causing serious reduction in plant efficiency. They are known as
carbonate hardness.
Permanent hardness is due to the presence of the non-carbonate salts cal-
cium sulphate, calcium chloride, magnesium chloride and other sulphates and
chlorides. Neutralization of these is achieved by means of chemical reactions.
Soft water contains up to 100 mg/1 of hardness salts, as in Cornwall, and
hard water contains as much as 600 mg/1, as in parts of Leicestershire. Acidic
water is produced by contact with decomposing organic matter in peaty locali-
ties and normally occurs in soft-water regions. This water is very corrosive to
steel, is plumbo-solvent and can cause dezincification of gunmetal pipe fittings.
The pH value denotes acidity or alkalinity due to the presence of free hy-
drogen ions in the water: acidic water, pH < 7; neutral water, pH = 7; alka-
line water, pH > 7. Copper and plastic pipes and fittings can be used in acidic
water regions. Hard-water areas are generally alkaline. Water treatment for
large boiler plants includes chemical injection to reduce corrosion from dis-
solved oxygen, and the pH value is raised to 11. Galvanized metal can be used
where the pH value is 7.4 if the carbonate hardness is greater than 150 mg/1.
Users of large amounts of water may have a treatment plant that removes or
converts hardness salts to less harmful salts.
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