At station No. 2 the germicidal action was still proceeding but at No. 5, representing an outlying section of the city, the increase in the B. coli content is very apparent.

During 1915 and 1916 the author endeavoured to duplicate these results under laboratory conditions and entirely failed. These experiments, which were made with the same materials as were in use at the city chlorination plant, but in glass containers, were usually only carried to a forty-eight hours contact, as this was the extreme limit for the city mains; one, however, was prolonged to five days. Many experiments were made under varying conditions, with similar results. Typical examples are given in [Tables VI], [VIII] and [IX] on [pages 33] and [37].

TABLE XIX.—AFTERGROWTHS OF B. COLI

Percentage of Samples Showing B. Coli in 10 c.cms.
Sampling Point No.
12345
191315.214.416.316.826.8
19147.05.76.0....11.6

In every case there was persistent diminution in the number of B. coli with increase of contact period. Determination of the bacterial count on nutrient agar showed that, in several experiments, the aftergrowth had commenced, and in some instances there was evidence that the second cycle was partially complete i.e. the number had reached a maximum and then commenced to decline. The time required for the completion of the two cycles, comprising the first reduction caused by the chlorine, the increase or aftergrowth, and the final reduction due to lack of suitable food material, is dependent upon several factors of which the dosage and temperature are the most important. With a small dosage the germicidal period is short and the second phase is quickly reached; with large doses, the second phase is not reached in forty-eight hours; the higher the temperature the quicker is the action and the development of the aftergrowth. These statements refer only to the bacteria capable of development on nutrient agar. The B. coli group behaved differently and persistently diminished in every case. If B. typhosus acts in a similar manner to B. coli, the laboratory experiments show that aftergrowths are of no sanitary significance and can safely be ignored, but as the results obtained in practice are contradictory to the laboratory ones, the matter must be regarded as sub judice until more definite evidence is available.

It is common knowledge that samples of water from “dead ends” of distribution mains show high counts and much larger quantities of B. coli than the water delivered to the mains. This is another phase of aftergrowth problem that often causes complaints and can only be eliminated by “blowing off” the mains frequently or by providing circulation by connecting up the “dead ends.” One extreme case of this description might be cited. A small service was taken off the main at the extreme edge of the city to supply a Musketry School two miles away and was only in use for a few months in the summer season. This service pipe delivered water containing B. coli in a considerable percentage of the 10 c.cm. samples and in a few instances in 1 c.cm., although the water delivered to the city mains never exceeded 2 B. coli per 100 c.cms. and averaged about one-tenth that quantity. No epidemiological records of the effect of this water are available because it was put through a Forbes steriliser before consumption.

In some instances the rate of development of the organisms after chlorination is greater than in the same water stored under similar conditions. This is especially noticeable in the presence of organic matter and has been ascribed to the action of the chlorine on the organic matter with the production of other compounds that are available as food material for the organisms.

Houston, during the treatment of prefiltered water Lincoln in 1905, found that although the removal of B. coli and other organisms growing at 37° C. was satisfactory, there was almost invariably an increase in the bacteria growing on gelatine at 20° C. This was ascribed to the action mentioned above and the chemical results supported this view, more organic matter being found in the filter effluents than in the prefiltered water. Rideal’s experiments with sewage at Guildford indicate that a similar action may occur in contact beds. The addition of bleach to the prefiltered water at Yonkers also resulted in an increased count and in these instances the aftergrowths are due to a disturbance of the equilibrium by the action of the chlorine on the zooglea and other organic matter invariably found in ripe filters. Similar results can be produced by the addition of chlorinated water to small experimental sand filters. This is shown by the results in [Tables XX] and [XXI].

TABLE XX.—AFTERGROWTHS IN SAND

Available
Chlorine in
Water
p.p.m.		Bacteria Per
Gram of
Sand After		Typical B. coli After
24 Hours.		Free Chlorine
After 24 Hrs.
Without
Acidifi-
cation.		After
Acidifi
cation.
3 Hrs.24 Hrs.100 Gr.10 Gr.1 Gr.0.1 Gr.
Nil12,00021,000+++---
3.080114,000------
5.050150,000------
7.025214,000------
10.026500,000------