Diagram II — Effect of Temperature on Absorption of Chlorine by Water
Absorption of Chlorine by water
containing 40 p.p.m. of colour
Value of K calculated from
absorption at 63° F.
K = Log (N1/N2)
t2 - t1
Time of
Contact
Minutes
Temperature of Water
t2
minutes
t1 = 0
t1 = 5
t1 = 10
32° F.
46° F.
63° F.
Nil
10.00
10.00
10.00
5
8.00
7.45
6.50
5
0.0374
10
7.23
7.09
5.91
10
0.0228
0.0082
20
7.00
6.60
5.18
20
0.0190
0.0066
0.0057
40
6.42
6.05
4.47
40
0.0087
0.0043
0.0040
60
6.22
5.60
3.90
60
0.0068
0.0040
0.0036
80
6.13
5.40
3.65
80
0.0056
0.0033
0.0029
Tables VIII and IX, however, show that the temperature coefficient of the germicidal action has a greater effect than the reduction in the amount of chlorine absorbed and removed from the reaction.
The results obtained on the works scale with these waters are very different to the laboratory ones and show that more chlorine is required during the summer season than in winter. The results with bleach and liquid chlorine are in the same direction (vide Diagrams III and IV). The bleach was regulated so as to maintain a constant purity, whilst in the other case the dosage was constant with a varying B. coli content. In Diagram IV the B. coli is plotted; this does not represent all the factors involved as the B. coli content of the treated water is also a function of that of the raw water, but in the example given this factor is of no moment because it was comparatively constant during the period plotted (extreme variation 80 per cent).
The discrepancies between the laboratory and works results cannot be easily explained. The only difference in the conditions is the nature of the containing vessel. Glass is practically inert at all temperatures but the iron pipes, through which the water passed before the samples were taken, may exert an absorptive influence on the chlorine at the higher temperatures experienced during the summer months.
Waters containing organic matter that differs much in quantity from the examples above may yield very different results and no generalisation can be made that will cover all cases. An increase of temperature increases the germicidal velocity and also the rate of absorption of chlorine by the organic matter; other factors determine which of these competitive actions predominates.
Method of Application (admixture). A thorough admixture of the water and chlorine is a sine qua non for successful operation. This should, if possible, be attained by natural means, but if there is any doubt as to the efficiency of the mixing process, mechanical appliances should be utilised. Pumps, especially centrifugal pumps, constitute a very convenient and efficacious method of mixing the germicide and the water, and the solutions should never be injected into the discharge pipes when it is possible to make connections with the suctions.
Inefficient admixture leads to local concentration of the chlorine, a condition which (vide p. 35), results in a wastage of the disinfectant. Two practical examples of this effect may be cited. In one case the water was free from colour and contained very little organic matter. This water was chlorinated at one plant by allowing the bleach solution to drop into one vertical limb of a syphon approximately 6,000 feet long, the other vertical limb being used as a suction well for the pumps which discharged into the distribution mains. At the other plant the bleach solution was injected into the discharge pipe of a reciprocating pump through a pipe perforated with a number of small holes. The results for two typical months are given in Table X.
Table X — Effect of Efficient Mixing
| Month. | Available Chlorine Parts Per Million. | Bacteria Per c.cm. | B. Coli Index Per 100 c.cms. | ||||
| Raw Water. | Treated Water. | ||||||
| A. | B. | A. | B. | A. | B. | ||
| July | 0.20 | 0.25 | 864 | 27 | 93 | < 0.2 | 8.5 |
| August | 0.20 | 0.27 | 1.108 | 12 | 120 | < 0.2 | 10.2 |
| A = efficient mixing. B = inefficient mixing. | |||||||
The results with the “B” plant were very irregular. The hypochlorite and water did not mix thoroughly and, as several suctions pipes were situated in the suction shaft, there was no subsequent admixture in the pumps; this also caused complaints regarding taste and odour but the complaints were localised, and not general as would result from an overdose of solution due to irregularities at the plant.
The second example deals with a water containing 40-45 p.p.m. of colour. This supply was taken from the river by low-lift pumps and discharged into a header which was connected with the high-lift pumps by two intake pipes about 5,000 feet in length. During 1914 a baffled storage basin of two hours capacity was constructed and in June the hypochlorite was added at the inlet to this basin by means of a perforated pipe. The object was to increase the contact period prior to the delivery of the water into the header. The results for this month were as follows:
| 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 water | 410 | 104 | 0 | .280 | ||
| Treated water | 49 | 26 | 0 | .036 | ||
| Percentage purification | 88 | .2 | 75 | .0 | 87 | .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 water | 448 | 100 | 0 | .600 | ||
| Treated water | 26 | 12 | 0 | .005 | ||
| Percentage purification | 91 | .9 | 88 | .0 | 99 | .2 |
Here again thorough admixture produced better results than inefficient admixture plus a longer contact period. Langer10 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 (Longley11). 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 — Effect of Contact PeriodD
| Contact Period. | Chlorine, Parts Per Million. | ||||
| 0.30 | 0.40 | 0.55 | 1.21 | ||
| Nil | 3,800 | ... | ... | ... | |
| 1 | minute | 1,400 | 120 | 0 | 0 |
| 10 | minutes | 720 | 5 | 0 | 0 |
| 20 | minutes | 35 | 0 | 0 | 0 |
| D Results are B. coli per 10 c.cms. | |||||
Table XII — Effect of Contact Period
| Available Chlorine 0.27 Part Per Million | |||||||
| Sampling Point. | Bacteria Per c.cm. | ||||||
| Average of series of samples | 5,000 | ft. | from | pumping | station | 300 | |
| 6,000 | „ | „ | „ | „ | 203 | ||
| 7,000 | „ | „ | „ | „ | 103 | ||
| 12,000 | „ | „ | „ | „ | 86 | ||
| 14,000 | „ | „ | „ | „ | 87 | ||
Table XIII is taken from the work of Wesbrook et al.4
Table XIII — Treatment of Mississippi River WaterE
| Aug. 8, 1910 | ||||||
| Available Cl. P.p.m. | Contact Period. (Temp. 22°‑26° C.). | |||||
| 30 Mins. | 1 Hr. 30 Mins. | 3 Hrs. | 6 Hrs. 30 Mins. | 24 Hrs. | ||
| 0 | 230,000 | 200,000 | 160,000 | 150,000 | 140,000 | |
| 0 | .5 | 14,000 | 7,400 | 2,000 | 6,000 | 11,000 |
| 1 | .0 | 20 | 14 | 170 | 450 | 60,000 |
| 1 | .5 | 10 | 6 | 16 | 45 | 70,000 |
| 2 | .0 | 7 | 8 | 10 | 97 | 70,000 |
| 2 | .5 | 7 | 14 | 30 | 116 | 65,000 |
| 3 | .0 | 6 | 12 | 5 | 12 | 16,500 |
| E Results are bacteria per c.cm. | ||||||
In Tables VIII, IX, XI, and XII, the bacteria decreased constantly with increase of contact period, but the results in Table XIII show that no advantage was to be gained by prolonging the contact beyond three hours; after this period the bacteria commenced to increase in number and when twenty-four hours had elapsed the number approached the original. This increase in the bacteria is technically known as “aftergrowth” and will be discussed more fully in Chapter IV.
The replies to queries sent out by the Committee on Water Supplies of the American Public Health Association11 indicate that the contact period after treatment varies considerably in American water-works practice. Forty per cent of the replies indicated no storage after treatment; 18 per cent less than one hour; 9 per cent from one to three hours; 5 per cent three to twelve hours; 11 per cent twelve to twenty-four hours, and 17 per cent a storage of more than twenty-four hours.
Turbidity is usually considered to exert an effect upon the dosage required but no definite evidence has been adduced in support of this hypothesis. Turbidity is generally caused by the presence of very finely divided suspended matter, usually silt or clay, which is inert to hypochlorites. The condition that produces turbidity, however, produces a concomitant increase in the pollution and some of the organisms are embedded in mineral or organic material that prevents access of the chlorine to the organisms which consequently survive treatment. A larger concentration is required to meet these conditions but it is not necessitated by the turbidity per se.
Effect of Light. Light exerts a marked photo-chemical effect on the germicidal velocity of chlorine and hypochlorites. When chlorinated water is passed through closed conduits and basins the effect of light is of course nil but in open conduits and reservoirs this factor is appreciable and reduces the necessary contact period. The effect of light on laboratory experiments made with colourless glass bottles is so marked as to make it impossible to compare the results obtained on different days under different actinic conditions. The following figures illustrate the effect of sunlight:
Effect of Sunlight
| Contact Period. | Available Chlorine 0.35 p.p.m. | |||
| Exposed to Bright Sunlight (April) | Stored in Dark Cupboard. | |||
| Nil | 215 | 215 | ||
| 30 | minutes | 130 | 145 | |
| 1 | hour | 122 | 136 | |
| 2 | 1⁄2 | hours | 61 | 130 |
| 3 | 1⁄2 | hours | 0 | 32 |
Determination of Dosage Required. The dosage required for the treatment of a water can only be accurately determined by treating samples with various amounts of chlorine and estimating the number of bacteria and B. coli after an interval of time equal to that available in practice. The temperature of the water during the experiment should be the same as that of the water at the time of sampling.
In order to limit the range covered by the experiments the approximate dosage can be ascertained from Diagram V if the amount of oxygen absorbed by the water is known. This diagram is calculated on the amount of available chlorine, present as chlorine or hypochlorite, that will reduce the B. coli content to the U. S. Treasury standard (2 B. coli per 100 c.cms.) in two hours. If the oxygen absorbed values are determined by the four-hour test at 27° C. they should be multiplied by two.
Another method which has been generally adopted for military work during the war, consists in the addition of definite volumes of a standard chlorine solution to several samples of the water and, after a definite interval, testing for the presence of free chlorine by the starch-iodide reaction. The details of the method of Gascard and Laroche, which is used by the French sanitary service, have been given by Comte.12 One hundred c.cms. of the water to be examined are placed in each of 5 vessels and 1, 2, 3, 4, and 5 drops of dilute Eau de Javelle (1: 100) are added and the contents stirred. After twenty minutes, 1 c.cm. of potassium iodide-starch reagent (1 gram each of starch, potassium, iodide and crystallized sodium carbonate to 100 c.cms.) is added and the samples again stirred. The lowest dilution showing a definite blue colour is regarded as the dose required, and the number of drops is identical with that required of the undiluted Eau de Javelle for 10 litres of water when the same dropping instrument is used. The actual concentration represented by these dilutions depends necessarily upon the size of the drops and the strength of the undiluted Eau de Javelle, but one drop per 100 c.cms. usually represents approximately 1 p.p.m.
In Horrocks’s method, as used in the British army, a standard bleach solution is added and is almost immediately followed by the zinc iodide-starch reagent. The two methods were compared by Massy,13 who found that the French method gave an average result of only 0.06 m.gr. per litre (0.06 p.p.m.) higher than the English method. Water in the Gallipoli campaign required from 0.21 to 1.06 p.p.m. as determined by both methods.
Diénert, Director of the Paris Service for investigating drinking water, adds 3 p.p.m. of available chlorine and allows the mixture to stand fifteen minutes after shaking; the residual chlorine is then titrated with thiosulphate. The amount absorbed is increased by 0.5 p.p.m. and in the opinion of Diénert this dosage is correct for a contact period of three hours.
For military camps where a standpipe usually provides a reasonable contact period, it has been found good practice to add sufficient chlorine to give a rich blue colour with the starch-iodide reagent and subsequently reduce the dosage gradually until the water, after standing one hour, gives but a faint reaction to the test reagent. This method should be checked up as soon as possible by bacteriological examinations. An example of this method is given in Table XIV.
Table XIV — Control of Dosage by Starch- Iodide Reaction
| Starch-iodide Reaction After One Hour. | Bacteria on Agar Per c.cm. | B. Coli Per 100 c.cms. | |
| 1 Day at 37 C. | 2 Days at 20 C. | ||
| 000⊕⊕ | 40 | 15 | 0 |
| 0000⊕ | 37 | 18 | 8 |
| 00000 | 68 | 268 | 34 |
| 00000 | 115 | 553 | 61 |
| Raw water | 114 | 685 | 89 |
| The number of ⊕ signs indicates the intensity of the reaction. | |||
Bibliography
Notes
1 Nissen. Zeit. f. Hyg., 1890, 8, 62.
2 Delépine, J. Soc. Chem. Ind., 1911, 29, 1350.
3 Phelps. Water Supply Paper No. 220, U. S. Geo. Survey.
4 Wesbrook, Whittaker, and Mohler, J. Amer. Public Health Assoc., 1911, 1, 123.
5 Lederer and Bachmann. Eng. Rec., 1912, 65, 360.
6 Harrington. J. Amer. Waterworks Assoc., 1914, 1, 438.
7 Ellms. Eng. Rec., 1911, 63, 472.
8 Heise. Philippine Jour. Sci., 1917, 12, A, 17-34.
9 Norton and Hsu, Jour. Inf. Dis., 1916, 18, 180.
10 Langer. Zeit. f. Hyg., 1916, 81, 296.
11 Longley. J. Amer. Public Health Assoc., 1915, 5, 920.
12 Comte. J. Pharm. Chim., 1916, 14, 261.
13 Massy. J. Pharm. Chim., 1917, 15, 209.