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NATURALLY OCCURRING SOLUBLE SALTS 759

but forms scale in boilers and pipes, which reduces their efficiency.

All steam plants must therefore use soft water in their boilers.

For industrial purposes and even on a small scale in the home,

zeolites, sometimes called " permutite,'' are used to soften water.

They are complex silicates that remove the calcium, magnesium,

and iron ions and add sodium ions. The zeolites are renewed by

reversing the process, which consists of passing salt water through

the mineral.

Water Purification Is Also Characteristic of Modem Civilization.

Natural waters may contain objectionable suspended matter,

including bacteria, as well as soluble impurities. As a general rule,

the soluble materials in water,

with the exception of those that

cause hardness, are not objectionable.

The suspended materials

are usually removed by

settling,

or coagulation and settling,

followed by filtration

through sand. When coagulation

is

used, an iron or aluminum salt

is added to water whose hydronium

ion, (H3O)''', concentration

has been adjusted by adding

lime if necessary. These salts

hydrolyze to produce gelatinous

precipitates that carry down

most of the finely divided suspended

matter with them as they

settle out. They also form a very

efficient filtering layer over the sand in the sand filters which removes

a large portion of the bacteria. When necessary, the remaining bacteria

are then destroyed by adding very small amounts of chlorine or by

treating with ozone, ultraviolet rays, silver ions, or other materials.

Since 1900 the use of chlorinated water has reduced the death rate

due to typhoid fever and other water-borne diseases in most of our

large cities by from 80 to nearly 90 per cent.

Aeration by spraying the

water into the air helps to remove undesirable odors and tastes as well

as to kill bacteria.

Sewage is now treated so as to remove all objectionable dissolved

and suspended matter and is then usually disinfected with

chlorine.

Fig. 320. Scale produced in a pipe by

hard water. (Courtesy of the Permutit

Company.)

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