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The
presence of calcium and magnesium salts causes water to
be hard, with the degree of hardness being directly
proportional to the quantity of these heavy metals that
are present. Hardness of natural water will vary
considerably, depending upon the source from which it is
obtained. Sections that have limestone formations
generally have a high hardness content in the water.
Since surface waters are dilutee by rainfall, well water
in the same area will normally have a much higher
hardness than surface water since the flow is
underground over rock layers and through sand strate.
Hardness will cause numerous
detrimental effects domestically, such as excessive soap
consumption in the home and laundries, as well as scums
and curds formed on equipment; yellowing of fabrics;
toughening of vegetables; film formation in tea; and
scale formation in hot water heaters, pipes, and
utensils.
The classification of water supplies
as soft, moderately hard, and very hard is rather
unsatisfactory even where the domestic use of water is
concerned. Water with a hardness of 100ppm may be called
a soft water by one accustomed to using a water with 300
or 400 ppm hardness. Whereas. One accustomed to using a
water with less than 50ppm hardness may call waters with
a hardness of 100 ppm rather hard. A water with a
hardness of 100 ppm is not soft in terms of soap
consumption in cleaning, washing and laundering
operations. The accompanying table gives a general
classification of waters.
|
Hardness |
Classification |
| Less
than 15 ppm |
Very
soft water |
| 15
to 50 ppm |
Soft
water |
| 50
to 100 ppm |
Medium
hard water |
| 100
to 200 ppm |
Hard
water |
| Greater
than 200 ppm |
Very
hard water |
In industry, high hardness is
undesirable for laundries, metal finishing, dyeing and
textile plants, food processing, pulp and paper, bottle
washing, photography, leather good and many others.
Hardness is also the source of scale formation in boiler
feed water heaters, feed lines, and economizer. Boilers
also will be heavily scaled due to precipitation of
calcium and magnesium salts unless properly treated. In
cooling water systems, scale will develop in heat
exchange equipment, engine jackets, piping and in
general wherever water circulates and is exposed to a
temperature change.
Hardness can be removed by lime-soda
softening, ion exchange softening, hot lime-hot ion
exchange and various combinations of processes. It may
be removed in internal boiler water conditioning by
employing inorganic salts such as phosphate and
carbonate in conjunction with either protective or
reactive organic materials, which insure the
precipitation of hardness from solution as a fluid
nonadherent sludge.
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