After the required quantity of lime and carbonate of soda which is
necessary for a total precipitation has been figured out from the
analysis of the water, respectively verified by practical experiments
in the laboratory, the heated water in the reservoir is mixed with the
lime, in form of thin milk of lime, and stirred up; we have to add so
much lime, that slightly reddened litmus paper gives, after ¼ minute's
contact with this mixture, an alkaline reaction, i.e., turns blue; now
the solution of carbonate of soda is added and again stirred well.
After twenty or thirty minutes (the hotter the water, the quicker the
precipitation) the precipitate has settled in large flocks at the
bottom, and the clear water is drawn off into the pure water reservoir.
The precipitating and settling of the impurities can also take place in
cold water; it will require, however, a pretty long time.
In order to avoid the weighing and slaking of the lime, which is
necessary for each precipitation, we use an open barrel, in which a
known quantity of slaked lime is mixed with three and a half or four
times its weight of water, and then diluted to a thin paste, so that one
kilogramme slaked lime is diluted to twenty-five liters milk of lime.
Example.--If we use for ten cubic meters water, one kilogramme lime,
or in one day (in twenty-four hours), 240 cubic meters 24 kg. lime, a
vessel four or five feet high and about 700 liters capacity, in which
daily 24 kg. lime with about 100 liters water are slaked and then
diluted to the mark 600, constantly stirring, 25 liters of this mixture
contain exactly 1 kg. slaked lime.
Before using, this milk of lime has to be stirred up and allowed to
settle for a few seconds; and then we draw off the required quantity of
milk of lime (in our case 25 liters) through a faucet about 8 inches
above the bottom, or we can dip it off with a pail. For the first
precipitate we always need the exact amount of milk of lime, which we
have figured out, or rather some more, but for the next precipitates we
do not want the whole quantity, but always less, as that part of the
lime, which does not settle with the precipitate, will be good for use
in further precipitations. It is therefore important to control the
addition of milk of lime by the use of litmus paper. If we do not add
enough lime, it prevents the formation of the flocky precipitate, and,
besides, more carbonate of soda is used. By adding too much lime, we
also use more carbonate of soda in order to precipitate the excess of
lime. We can therefore add so much lime, that there is only a very small
excess of hydrous lime in the water, and that after well stirring, a red
litmus paper being placed in the water for twenty seconds, appears only
slightly blue. After a short time of practice, an attentive person can
always get the exact amount of lime which ought to be added. On adding
the milk of lime, we have to dissolve the required amount of pure
carbonate of soda in an iron kettle, in about six or eight parts hot
water with the assistance of steam; add this to the other liquid in the
precipitating reservoirs and stir up well. The water will get clear
after twenty-five or thirty minutes, and is then drawn off into the pure
water reservoir.
EXAMINATION OF WATER WHICH HAS BEEN PURIFIED BY MEANS OF MILK OF LIME
AND CARBONATE OF SODA.
In order to be convinced that the purification of the water has been
properly conducted, we try the water in the following manner. Take a
sample of the purified water into a small tumbler, and add a few
drops of a solution of oxalate of ammonia; this addition must neither
immediately nor after some minutes cause a milky appearance of the
water, but remain bright and clear. A white precipitate would indicate
that not enough carbonate of soda had been added. A new sample is taken
of the purified water and a solution of chloride of calcium added; a
milky appearance, especially after heating, would show that too much
carbonate of soda had been added.