GROUND WATER IN NORTH-CENTRAL TENNESSEE

QUALITY OF GROUND WATER 105
nesium and sodium sulphate. All the products of this reaction are-
soluble. The total carbonate hardness is removed by adding enough
slaked lime (calcium hydroxide) to combine with the free carbon
dioxide and the bicarbonate to transform the calcium and magnesium
naturally present in the water as well as the calcium added in the-
form of lime into calcium carbonate and magnesium hydroxide.
These products, being essentially insoluble, form a solid precipitate,,
the flocculation and settling of which may be accelerated by adding a,
small quantity of alum or other coagulant with the lime and soda.
Small amounts of calcium carbonate and magnesium hydroxide re­
main in solution, however, so that the treated water has a small
residual hardness. Water softened by the lime and soda process con­
tains less dissolved mineral matter by the amount of calcium bicarbon­
ate and magnesium bicarbonate precipitated from the untreated
water less the excess of soda used. All the sodium of the soda used in
the process remains in the treated water, as all its products are soluble..
Hence, some natural waters that contain much noncarbonate hard­
ness can not be conditioned by this process for successful use in steam
boilers, because the treated water contains so much sodium that it.
would foam prohibitively. Besides lime and soda, many other sub­
stances have been proposed and used successfully as softening agents,,
but many of them are too costly if a large volume of water must be-
treated. Sodium aluminate has been used successfully with lime and.
soda to minimize the total dissolved solids in softened water used,
in locomotive boilers.68
The lime and soda process is generally less costly than the exchange-
silicate method of softening waters that contain only carbonate hard­
ness. On the other hand, it is more costly if much noncarbonate-
hardness must be removed. With the lime and soda process great
care must be exercised to avoid an excess of the chemicals added to the
water and to insure complete precipitation of sludge before the water
is put to use. Hence, close technical control is necessary. Moreover,
if a large volume of water must be treated, difficult problems of sludge-
disposal may arise.
In the exchange silicate process the active softening agent is the-
so-called "zeolite" or "permutite," an insoluble hydrous sodium-
aluminum silicate which has the property of exchanging its sodium
with the calcium, magnesium, iron, and manganese of the hard water.
The exchange silicate, which may be natural or artificial, is marketed
under several trade names. In practice, the hard water is filtered
through a bed of granular exchange silicate and in passing gives up to
the silicate its soap-consuming and scale-forming constituents and
** Qrime, E. M., Water treatment and railroad efficiency: Am. Water Works Assoc. Jour., vol. 18, No*.
4, pp. 440-441,1927.
100144r 32 8