Chlorination of Water Part 6

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 c.u.mulative 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 quant.i.ty 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 pa.s.sage 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.[A]--EFFECT OF CONTACT PERIOD

-----------------+------------------------------- | CHLORINE, PARTS PER MILLION.

Contact Period. +-------+-------+-------+------- | 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 -----------------+-------+-------+-------+-------

[A] 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| 5,000 ft. from pumping station| 300 of samples | 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.[B]--TREATMENT OF MISSISSIPPI RIVER WATER AUG. 8, 1910

-------------+------------------------------------------------- | CONTACT PERIOD. (TEMP. 22-26 C.).

Available Cl.+---------+---------+---------+---------+--------- P.p.m. | | 1 Hr. | | 6 Hrs. | | 30 Mins.| 30 Mins.| 3 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 -------------+---------+---------+---------+---------+---------

[B] 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 a.s.sociation[11] 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 pa.s.sed 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 gla.s.s bottles is so marked as to make it impossible to compare the results obtained on different days under different actinic conditions. The following figures ill.u.s.trate the effect of sunlight:

EFFECT OF SUNLIGHT

-----------------+------------------------------------- | AVAILABLE CHLORINE 0.35 P.P.M.

Contact Period. +-------------------+----------------- | Exposed to Bright | Stored in Dark | Sunlight (April) | 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 pota.s.sium iodide-starch reagent (1 gram each of starch, pota.s.sium, 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.

[Ill.u.s.tration: DIAGRAM V

RELATION OF DOSAGE TO OXYGEN ABSORBED]

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 Ma.s.sy,[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.

Dienert, 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 t.i.trated with thiosulphate. The amount absorbed is increased by 0.5 p.p.m. and in the opinion of Dienert 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 | BACTERIA ON AGAR PER C.CM. | Reaction After One +-----------------+-----------------+ B. Coli Per Hour. | 1 Day at 37 C. | 2 Days at 20 C. | 100 c.cms.

--------------------+-----------------+-----------------+-------------- 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

[1] Nissen. Zeit. f. Hyg., 1890, =8=, 62.

[2] Delepine, 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 a.s.soc., 1911, =1=, 123.

[5] Lederer and Bachmann. Eng. Rec., 1912, =65=, 360.

[6] Harrington. J. Amer. Waterworks a.s.soc., 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 a.s.soc., 1915, =5=, 920.