278 Science and Human Welfare
A river is often the source of supply of raw water. At the
water-treatment plant coagulents are added and the water
passed into large settling basins. The water is then efficiently
aerated, oftentimes by sprays and waterfalls of architectural
splendor, in order to eliminate any hydrogen sulphide pres-
ent or to oxidize the iron and manganese. Filtration removes
most of the suspended materials present and, at the same
time, a large per cent of the bacteria. If odors and tastes per-
sist, activated carbon is added before the filters. If the water
is too hard it may be necessary to soften it to some desired
degree through the use of lime-soda chemical treatment. The
prevention of excessive scale formation from hard or un-
stabilized water has been one of the most stubborn problems
for the water plant operator to solve. By recarbonating the
water following the softeners, he could prevent after-precipi-
tation more or less completely; but if he were successful in
stopping all precipitation he would very likely be plagued by
red water. “Threshold Treatment” offers a new method of
control. This consists of adding a very small quantity of
hexametaphosphate. When two parts per million, or even
less in some cases, are added to the water, all precipitation
of calcium carbonate is prevented. The economic advantage
of the threshold treatment is the relatively high alkalinity
which may be maintained, thus inhibiting corrosion.
Although softening and filtering remove a very large per
cent of bacteria present in the water, yet the water is not
safe. The remaining bacteria must be killed. The practical
development of chemical disinfection has occurred during
the last twenty years and by far the most commonly used
disinfectant is chlorine. The chlorine attacks the bacteria,
causing their elimination. There are indications that there
are direct chemical combinations with some forms of bac-
teria. After such a chemical reaction there would be a com-