Chlorination of Water Part 15

_Cost Per Million Imperial Gallons_[A]

-----------+--------------+-------------- | Hypochlorite | Hypochlorite | alone. | and ammonia.

-----------+--------------+-------------- Mar. 15-31 | $1.12 | $0.46 April | 1.26 | 0.54 -----------+--------------+--------------

[A] Calculated as Bleach at $3.80 per 100 pounds and aqua ammonia (26 Be.) at 5-1/2 cents per pound.

The results were so satisfactory that the author recommended the adoption of the process on the main chlorinating plant but owing to conditions imposed by the Provincial Board of Health the process was not operated until February, 1917.

In place of ammonia fort, aqua ammonia (26 Be.), containing approximately 29 per cent of anhydrous ammonia, was used. The material was first examined by the presence of such noxious substance as cyanides and found to be very satisfactory.

[Ill.u.s.tration: FIG. 12.--Sketch of Ottawa Chloramine Plant.]

The general design of the plant is shown in Fig. 12. The bleach is mixed in tank _A_ as a solution containing 0.3 to 0.6 per cent of available chlorine and delivered to tanks _B_ and _D_, each of which has a twenty-four-hour storage capacity. The ammonia solution is mixed and stored in tank _B_ and contains 0.3-0.5 per cent of anhydrous ammonia.

The two solutions are run off into boxes _E_ and _F_ which maintain a constant head on valves _V_ and _V'_ controlling the head on the orifices. Both orifices discharge into a common feed box _G_ from which the mixture is carried by the water injector _J_ through one of duplicate feed pipes and discharged into the suction well through a perforated pipe.

As tank _B_ was previously used as a bleach storage tank, the change from hypochlorite alone to chloramine necessitated very little expense.

The treatment was commenced by gradually increasing the quant.i.ty of ammonia, until a dosage of 0.12 p.p.m. was reached, and constantly increasing the dosage of bleach, which was formerly 0.93 p.p.m. of available chlorine. Owing to the restrictions imposed by the Provincial authorities it has not been possible to maintain a dosage as low as that indicated as sufficient by the experimental plants results, but some interesting data have been obtained. Table XXIX shows the results obtained from February to October, 1917, from the chloramine treatment at Ottawa and also those obtained with liquid chlorine at Hull where the same raw water is treated with 0.7-0.8 p.p.m. of chlorine.

TABLE XXIX.--CHLORAMINE RESULTS AT OTTAWA

---------+--------------+----------+--------+------------------+------- | _B. coli_ | | | | Hull |PER 100 C.CMS.| | | DOSAGE P.P.M. |_B. coli_ 1917 +------+-------+Turbidity.| Colour.+---------+--------+Per 100 | Raw | Tap | | | | |c.cms.

|Water.| Water.| | |Chlorine.|Ammonia.| ---------+------+-------+----------+--------+---------+--------+------- Feb. | 268 | 0.88 | 3 | 40 | 0.57 | 0.05 | ....

Mar. 1-18| 250 | 0.96 | 4 | 40 | 0.32 | 0.11 | ....

Mar. 1-31| 643 | 0.43 | 4 | 40 | 0.47 | 0.14 | ....

April | 5228 | 0.34 | 31 | 32 | 0.56 | 0.10 | ....

May | 162 | <0.08 |="" 3="" |="" 39="" |="" 0.52="" |="" 0.08="" |="">

June | 114 | <0.08 |="" 3="" |="" 41="" |="" 0.51="" |="" 0.08="" |="">

July | 237 | 0.08 | 5 | 41 | 0.51 | 0.08 | 44.4 Aug. | 165 | 0.08 | 4 | 42 | 0.51 | 0.10 | 28.0 Sept. | 55 | <0.08 |="" 6="" |="" 42="" |="" 0.50="" |="" 0.09="" |="" 15.2="" oct.="" |="" 31="" |="" 0.15="" |="" 5="" |="" 42="" |="" 0.42="" |="" 0.08="" |="" 1.1="" ---------+------+-------+----------+--------+---------+--------+="" average="" |="" 211="" |="" 0.22="" |="" 7="" |="" 40="" |="" 0.51="" |="" 0.09="" |="" ---------+------+-------+----------+--------+---------+--------+-------="">

At the height of the spring floods the raw water contained 80 p.p.m. of turbidity and over 500 _B. coli_ per c.cm. but 0.6 p.p.m. of chlorine and 0.13 p.p.m. of ammonia reduced the _B. coli_ index in the tap samples to 2.5 per 100 c.cms.; samples taken in Hull on the same day (treated with 0.7-0.8 p.p.m. of liquid chlorine) gave a _B. coli_ index of 26.7. Previous experiences in Ottawa has shown that a dosage of approximately 1.5 p.p.m. of available chlorine is required to reduce the _B. coli_ index to 2.0 per 100 c.cms. under similar physical and bacteriological conditions.

During the period of nine months covered by the results in Table XXIX, only five cases of typhoid fever were reported in which the evidence did not clearly indicate that the infection had occurred outside the city.

The reduction in the bleach consumed during the same period effected a saving of $3,200.

During one period of operation the hypochlorite dosage was gradually reduced to ascertain what factor of safety was maintained with a dosage of 0.5 p.p.m. of available chlorine and 0.06-0.08 p.p.m. of ammonia. The results are shown in Diagram VIII. The percentage of samples of treated water showing _B. coli_ in 50 c.cms. was calculated from the results of the examination of 4-7 samples daily.

The results showed that it was possible to reduce the chlorine dosage to 0.25 p.p.m. with 0.06 p.p.m. of ammonia without adversely affecting the bacteriological purity of the tap supply and fully confirmed the experimental results previously obtained.

The lowest ratio of available chlorine to ammonia used during this test was approximately 4:1. This is the ratio indicated by a consideration of the theory of the reaction, and not 2:1 as was formerly stated (Race[4]). If bleach is represented as Ca(OCl)_{2}, the equation

Ca(OCl)_{2} + 2NH_{3} = 2NH_{2}Cl + Ca(OH)_{2}

would indicate a ratio of 2:1; but only one molecule of Ca(OCl)_{2} is produced from two molecules of bleach and the theoretical ratio is therefore 4:1 (142:34),

2CaOCl_{2} = CaCl_{2} + Ca(OCl)_{2} and Ca(OCl)_{2} + 2NH_{3} = Cl = 142 34 = 2NH_{2}Cl + Ca(OH)_{2}.

The chlorine to ammonia ratio is very important because of its influence on the economics of the process (_vide_ p. 124).

[Ill.u.s.tration: DIAGRAM VIII

CHLORAMINE TREATMENT, OTTAWA]

All the laboratory and works results that have been obtained in Ottawa indicate the importance of an adequate contact period. The superiority of chloramine over other processes is due to the non-absorption of the germicidal agent and to obtain the same degree of efficiency the contact period must be increased as the concentration is decreased. For this reason the best results will be obtained by chlorinating at the entrance to reservoirs or under other conditions that will ensure several hours contact. At Ottawa the capacity of the pipes connecting the pumping station (point of chlorination) and the distribution mains provides a contact period of one and a quarter hours but even better results would be obtained if the contact period were increased.

The general results obtained during the use of chloramine at Ottawa in 1917 have shown that the aftergrowths noted during the use of hypochlorite (see p. 56) have been entirely eliminated and that the _B.

coli_ content of the tap samples from outlying districts has been invariably less than that of samples taken from taps near to the point of application of the chloramine. At Denver, Col., where the chloramine process has also been used, similar results were obtained[5]: four days after the initiation of the chloramine treatment the aftergrowth count on gelatine of the Capitol Hill reservoir dropped from 15,000 to 10 per c.cm. The hypochlorite dosage was cut from 0.26-0.13 p.p.m. of available chlorine and 0.065 p.p.m. of ammonia added.

_Economics of the Chloramine Process._ The chloramine process was introduced at Ottawa for the purpose of obtaining relief from the effect of the high price of bleach caused by the cessation of imports from Europe in 1915. The results obtained with the experimental plant indicated that, calculated on the prices current at the beginning of 1917, appreciable economies could be made. Although the reduction in the chlorine dosage has not been as great as was antic.i.p.ated, due to the restrictions previously mentioned, the cost of sterilising chemicals in 1917 was $3,200 less than the cost of straight hypochlorite treatment.

During the latter part of 1917 the relative cost of bleach and ammonia changed (see Diagram IX).

When calculated on the New York prices for January, 1918, the cost of chloramine treatment in the United States would be greater than hypochlorite alone unless a large reduction in the dosage could be secured by very long contact periods. This condition is only temporary, however, and the price of ammonia will probably gradually decline as the plants for fixation of atmospheric nitrogen commence operations and reduce the demand for the ammonia produced from ammoniacal gas liquor.

In Canada, the market conditions are still (1918) favourable to the chloramine process: bleach is 25 per cent higher than the U.S.A. product and ammonia can be obtained for one-half the New York prices.

[Ill.u.s.tration: DIAGRAM IX

BLEACH AND AMMONIA PRICES]

_Advantages of the Chloramine Process._ Although the market conditions may, in some instances, be unfavourable to the chloramine process, the method possesses certain advantages that more than offset a slight possible increase in the cost of materials. The taste and odour of chloramine is even more pungent than that of chlorine but since the introduction of the process in Ottawa no complaints have been received.

Owing to the reduced dosage, slight proportional fluctuations in the dosage do not produce the same variations in the amount of free chlorine which is the usual cause of complaints. A public announcement that the amount of hypochlorite has been reduced also has a psychological effect upon the consumers and tends to reduce complaints due to auto-suggestion.

The most important advantage of the process is the elimination of the aftergrowth problem. At Denver, where the aftergrowth trouble is possibly more acute than at any other city on the continent, it was effectively banished by the use of chloramine. At Ottawa, the sanitary significance of _B. coli_ aftergrowths is no longer of practical interest because such aftergrowths have ceased to occur. Whatever may be their opinion as to the sanitary significance of aftergrowths, all water sanitarians will agree that the better policy is to prevent their occurrence.

_Operation of Chloramine Process._ For the successful operation of the chloramine process, the essential factors are low concentrations of the hypochlorite and ammonia solutions. The author has found that hypochlorite containing 0.3-0.5 per cent of available chlorine and ammonia containing 0.3-0.5 per cent of anhydrous ammonia can be mixed in a 4:1 or 8:1 ratio without appreciable loss in t.i.tre. Solutions of these concentrations mixed in 4:1 ratio lost only 2-3 per cent of available chlorine in fifteen minutes and less than 10 per cent in five hours. The effect of mixing solutions containing 4.35 per cent of available chlorine and 2.2 per cent of ammonia is shown in Table x.x.x.

The stability of chloramine is a function of the concentration and the temperature and in practice it will be found advisable to determine in the laboratory the maximum concentrations that can be used at the maximum temperature attained by the water to be treated (cf. Muspratt and Smith[6]).

According to Raschig[1] two competing reactions occur when ammonia is in excess.

(1) NH_{2}Cl + NH_{3} = N_{2}H_{4}HCl hydrazine hydrochloride and (2) 3NH_{2}Cl + 2NH_{3} = N_{2} + 3NH_{4}Cl.

When the excess of ammonia is large, as on the addition of ammonia fort, the second reaction predominates and the yield of nitrogen gas is almost quant.i.tatively proportional to the quant.i.ty of available chlorine present. As ammonium chloride has no germicidal action, and hydrazine a carbolic coefficient of only 0.24 (Rideal), the formation of these compounds should be avoided.

TABLE x.x.x.--LOSS ON MIXING HYPOCHLORITE AND AMMONIA

Hypochlorite containing 4.35 per cent available chlorine. Ammonia contained 2.2 per cent NH_{3}

-----------------------------+------------------------------------- | LOSS OF AVAILABLE CHLORINE AFTER Ratio Chlorine to Ammonia by +--------------+----------+----------- Weight. | Few Minutes. | 1 Hour. | 24 Hours.

-----------------------------+--------------+----------+----------- | Per cent | Per cent | Per cent 6:1 | 19 | 19 | 19 4:1 | 24 | 25 | 25 2:1 | 45 | 47 | 47 1:1 | 91 | 91 | 92 1:2 | 20 | 28 | 65 -----------------------------+--------------+----------+-----------

The dosage of chloramine can be checked by t.i.tration of the available chlorine (see p. 82) immediately after treatment or by the estimation of the increment in the total ammonia (free and alb.u.minoid). Routine determinations of the latter made in Ottawa show that practically the whole (90-95 per cent) of the added ammonia can be recovered by distillation with alkaline permanganate and that 85-90 per cent is in the "free" condition.

In operating the chloramine process it is important that the pipes used for conveying the chloramine solution should be of ample dimensions and provided with facilities for blowing out the lime that deposits from the solution.