Available Chlorine 1.88 Parts Per Million
Bacteria Per c.cm. Agar.B. Coli. Index Per c.cm.
3 Days at 20 C.1 day at 37 C.
Raw water410 104 0.280
Treated water49 26 0.036
Percentage purification88.275.087.5

During August the point of application of the hypochlorite was changed from the inlet of the basin to the suctions of the pumps and the solution proportioned to the amount of water pumped by the starch and iodide test. The average of the daily tests for this month were:

Available Chlorine 1.55 Parts Per Million
Bacteria Per c.cm. Agar.B. Coli. Index Per c.cm.
3 Days at 20 C.1 day at 37 C.
Raw water448 100 0.600
Treated water26 12 0.005
Percentage purification91.988.099.2

Here again thorough admixture produced better results than inefficient admixture plus a longer contact period. Langer[10] has also noted the effect of local concentration and found that the disinfecting action is increased by adding the bleach solution in fractions, a cumulative effect replacing that of concentration.

The importance of the admixture factor was not thoroughly appreciated during the earlier periods of chlorination but later installations, and particularly the liquid chlorine ones, have been designed to take full advantage of it.

The point of application in American water-works practice varies considerably (Longley[11]). In 57 per cent of those cases in which it is employed as an adjunct to filtration, it is used in the final treatment; in 26 per cent it is used after coagulation or sedimentation and before filtration; in the remaining 17 per cent it is applied before coagulation and filtration. The report of the committee adds: “The data at hand do not give any reasons for the application before coagulation. In general, an effective disinfection may be secured with a smaller quantity of hypochlorite, if it is applied after rather than before filtration. It should be noted that the storage of chlorinated water in coagulating basins, and its passage through filters, tend to lessen tastes and odors contributed by the treatment and this fact may in some cases account for its use in this way.”

Contact Period. Other things being equal, the efficiency of the treatment will vary directly, within certain limits, with the contact period. When a chlorinated water has to be pumped to the distribution mains directly after treatment, the dosage must be high enough to secure the desired standard of purity within twenty to thirty minutes. The chlorine is sometimes not completely absorbed in this period and may cause complaints as to tastes and odours. The examples given above show that the lack of contact period can be largely compensated by ensuring proper admixture. Experience has amply demonstrated that there is no necessity to use heroic doses for water that is delivered for consumption almost immediately after treatment, and that, with proper supervision, complaints can be almost entirely prevented.

The general effect of the effect of contact period is shown in [Tables VIII] and [IX] on [page 37]. Another example of a coloured water is given in [Table XI], whilst [Table XII] shows the results obtained with a colourless water.

TABLE XI.[D]—EFFECT OF CONTACT PERIOD

Contact Period.Chlorine, Parts Per Million.
0.300.400.551.21
Nil 3,800.........
1minute1,40012000
10minutes720500
20minutes35000
[D] Resultsare B. coli per 10 c.cms.