Chlorination of Water Part 14

An electrolyser of the non-submerged diaphragm type is the Allen-Moore cell which has been adopted by the Montreal Water and Power Co. This has been described by Pitcher and Meadows.[8] The general lay-out of the installation is shown in Fig. 10, and the essential features are: a salt storage bin having a capacity of 40 tons; the brine saturating and purifying apparatus; duplicate 15 horse-power motor-generator sets; four chlorine cells; and the silver ejectors and distributing lines for carrying the chlorine solution to the point of application.

[Ill.u.s.tration: FIG. 10--Brine Saturating and Purifying Equipment.]

The brine solution, which is prepared by pa.s.sing water through the saturators previously filled with salt, is delivered to the two concrete reaction tanks where an amount of soda ash and caustic liquor sufficient to combine with the calcium and magnesium salts is added, and the mixture filtered through sand and stored in the purified brine tanks. To prevent the formation of hypochlorites by the interaction of chlorine and alkali, the alkalinity of the liquor is determined and sufficient hydrochloric acid added to ensure an acidity of 0.01 per cent. The acid brine is delivered at one end of the four cells (Fig. 11) each of which is 7 feet long and 20-3/8 inches wide and consumes 600 amperes at 3.3 volts. The cell box is built of concrete and is provided with a perforated wrought iron cathode box and graphite anode plates which are separated by an unsubmerged asbestos paper diaphragm.

[Ill.u.s.tration: FIG. 11.--Sections of Allen-Moore Cell.]

Each cell has a capacity of 32 pounds of chlorine per day and the gas flow is determined by measuring the volume of caustic soda produced in a given period of time and calculating the weight from the volume and concentration as determined by t.i.tration with standard acid; each gram of NaOH is equal to 0.88 gram of chlorine. The efficiency of the cell is obtained by dividing the number of grams of chlorine produced per hour by the product of the current volume (in amperes) and the factor 1.33, the theoretical production of chlorine for one ampere hour. The average efficiency of the Montreal cells was found to be 93 per cent. The installation comprises four cells, one being held in reserve, and the annual cost of producing 90 pounds of chlorine per day is given as $2,500. The details are:

Salt at $8.00 per ton, delivered $500.00 Power, 15 H.P., at $30.00 flat rate 450.00 Labour and superintendence 500.00 Interest at 6 per cent on capital cost 300.00 Depreciation, 15 per cent 750.00 --------- $2,500.00

cost per pound of chlorine = 7.6 cents.

The diaphragm cells, like the non-diaphragm ones, operate most efficiently under a constant load; they are consequently suitable for treating the effluent of filter plants.

Where very cheap electrical power can be obtained, the cost per pound of available chlorine is less for the electrolytic method just described than for liquid chlorine or chlorine obtained from bleach; but this condition obtains in very few places. Mr. J. A. Meadows has suggested to the author that the cost could be reduced by converting the chlorine gas into hypochlorite and then adding dilute ammonia as in the chloramine process (_vide_ page 115). The caustic liquor, usually run to waste from the cathodic compartment, could be delivered into a feed box from which it would be drawn off by the water injector used for dissolving the chlorine gas.

BIBLIOGRAPHY

[1] Lunge and Landolt. Jour. Soc. Dyers and Colourists, Nov. 25, 1885.

[2] Kershaw. Jour. Soc. Chem. Ind., 1912, =31=, 54.

[3] Rickard. Quar. Bull. Ohio Board of Health, Oct.-Dec., 1904.

[4] Race. Jour. Amer. Waterworks a.s.soc., 1918, =5=, 63.

[5] Rabs. Hygienische Rundschau, 1901, 11.

[6] Winslow. Public Health Rpts. U. S. P. H. S., 1917, =32=, 2202.

[7] Tolman. Jour. Amer. Waterworks a.s.soc., 1917, =4=, 337.

[8] Pitcher and Meadows. Jour. Amer. Waterworks a.s.soc., 1917, =4=, 337.

CHAPTER IX

CHLORAMINE

Chloramine (NH_{2}Cl), a chemical compound in which one of the hydrogen atoms of ammonia has been replaced by chlorine, was discovered by Raschig[1] in 1907. Chloramine was prepared by cooling dilute solutions of bleach and ammonia and adding the latter to the former contained in a flask surrounded by a freezing mixture. The proportions were as the equivalent weights of anhydrous ammonia and available chlorine (approximately two parts by weight of chlorine to one part by weight of ammonia). After gas evolution had ceased the mixture was saturated with zinc chloride and the magma distilled under reduced pressure. The distillate was a dilute solution of comparatively pure chloramine.

The first to notice the effect of ammonia on the germicidal value of hypochlorites was S. Rideal[2] who noted that during the chlorination of sewage, the first rapid consumption of chlorine was succeeded by a slower action which continued for days in some instances, and was accompanied by a germicidal action after free chlorine or hypochlorite had disappeared. Rideal stated that: "It became evident that chlorine, in supplement to its oxidising action, which had been exhausted, was acting by subst.i.tution for hydrogen in ammonia and organic compounds, yielding products more or less germicidal." On investigating the effect of ammonia on hypochlorite it was found that the addition of an equivalent of ammonia to electrolytic hypochlorite increased the carbolic acid coefficient of 2.18, for one per cent available chlorine, to 6.36 (nearly three times the value). Further experimental work showed that the increase was due to the formation of chloramine.

The author, in 1915, during a series of experiments on the relative germicidal action of hypochlorites, attempted to prepare the ammonium salt by double decomposition of bleach and ammonium oxalate solutions.

Ca(OCl)_{2} + (NH_{4})_{2}C_{2}O_{4} = CaC_{2}O_{4} + 2NH_{4}OCl.

The velocity of the germicidal action of the solution was found to be about ten times greater than the germicidal velocities of other hypochlorites of equal concentrations, (Race[3]), and from a consideration of the chemical formula of ammonium hypochlorite it appeared probable that it would be very unstable and decompose into chloramine, which Rideal had previously shown to have an abnormal germicidal action, and water. NH_{4}OCl = NH_{2}CL + H_{2}O. After these results have been confirmed, the effect of adding ammonia to bleach solution was tried and it was found that 0.20 p.p.m. of available chlorine and 0.10 p.p.m. of ammonia produced equally good results as 0.60 p.p.m. of chlorine only. Similar results were obtained on the addition of ammonia to electrolytic hypochlorite.

Experiments made with a view to determining the most efficient ratios of ammonia gave very surprising results: chlorine to ammonia ratios (by weight) between 8:1 and 1:2 gave approximately the same germicidal velocity.[3] The action of the ammonia on the oxidising power of bleach, as measured by the indigo test, was also found to be disproportionate to the amount added.

The oxidising action of various mixtures of bleach and ammonia as measured by the rate of absorption of the available by the organic matter in the Ottawa River water is shown in Table XXV.

TABLE XXV.--RATE OF ABSORPTION OF AVAILABLE CHLORINE

--------------------------+----------------------------------- Ratio Chlorine / Ammonia | PERCENTAGE OF ORIGINAL FOUND AFTER by Weight. +-----------+-----------+----------- | 10 Mins. | 4 Hours. | 20 Hours.

--------------------------+-----------+-----------+----------- Infinity (ammonia absent) | 66.8 | 40.0 | 25.1 8:1 | 83.2 | 77.8 | 67.3 4:1 | 97.2 | 94.7 | 88.5 2.7:1 | 98.3 | 96.5 | 92.8 2:1 | 99.8 | 98.2 | 96.2 --------------------------+-----------+-----------+-----------

The 8:1 ratio caused a marked reduction in the rate of absorption of the chlorine whilst a 4:1 ratio was almost as active as the ratios containing more ammonia.

At the time when the abnormal results were obtained with ammonium hypochlorite and mixtures of bleach and ammonia, the phenomenon appeared to be of scientific interest only and especially so as Rideal had attributed the obnoxious tastes and odours, sometimes produced by chlorination, to the formation of chloramines. During the winter of 1915-1916 the price of bleach, however, advanced to extraordinary heights and the author then determined to try out the process on a practical scale for the purification of water. A subsidiary plant pumping about 200,000 Imperial gallons per day (240,000 U. S. A.

gallons) was found to be available for this purpose and the chloramine process was subst.i.tuted for the bleach method previously in operation.

The process was commenced by the addition of pure ammonia fort, in the amount required to give a chlorine to ammonia ratio of 2:1, to the bleach solutions in the barrels. The results were not in accordance with those obtained in the laboratory and it was found that the samples of bleach solutions received for a.n.a.lysis were far below the strength calculated from the amount of dry bleach used. This experience was repeated on subsequent days and the deficiency was found to increase on increasing the ammonia dosage. Solutions of similar concentration were then used in the laboratory with similar losses, and it was observed that on the addition of ammonia a copious evolution of gas occurred. An investigation showed that the ammonia and bleach must be mixed as dilute solutions and prolonged contact avoided (_vide_ p. 127). Alterations were accordingly made in the plant and the bleach and ammonia were prepared as dilute solutions in separate vessels and allowed to mix for only a few seconds before delivery to the suction of the pumps. This method of application was instantaneously successful and results equal to those obtained in the laboratory were at once secured. The dosage was reduced until the bacteriological results were adversely affected and continued at values slightly in excess of this figure (0.15 p.p.m.) for a short period to prove that the process was reliable.

From a consideration of the work of Raschig and Rideal, it appeared that the most efficient proportions of available chlorine and ammonia would be two parts by weight of the former to one part of the latter and this ratio was maintained during the run on the experimental plant. Lower ratios of chlorine to ammonia were contra-indicated by the laboratory experiments, which showed that the efficiency was not increased thereby whilst higher ratios were left for future consideration.

The results obtained on the experimental plant, together with those obtained on the main plant, where 24 million gallons per day were treated with bleach only, are given in Tables XXVI, XXVII and XXVIII.

The two periods given represent the spring flood condition and that immediately preceding it; these are respectively the worst and best water periods. The results in both cases are from samples examined approximately two hours after the application of the chemicals.

The cost data were calculated on the current New York prices of bleach and ammonia.

TABLE XXVI.--COMPARISON OF HYPOCHLORITE AND CHLORAMINE TREATMENT

BACTERIOLOGICAL RESULTS

-----+------------------+----------------------+---------------------------- | RAW WATER. | TREATED WITH HYPO- | TREATED WITH HYPOCHLORITE | | CHLORITE ALONE. | AND AMMONIA.

+------------+-----+-----------+-----+----+------------+-----+----+---- 1916 | Bacteria | | Bacteria | | | Bacteria | | | | per cubic | | per cubic | | | per cubic | | | |centimeter. | _B. |centimeter.| _B. | |centimeter. | _B. | | +-----+------+coli_+-----+-----+coli_| +------------+coli_| | |Agar | Agar |Index|Agar |Agar |Index| (1)|Agar |Agar |Index| (1)| (2) | 1 | 3 | per | 1 | 3 | per | | 1 | 3 | per | | | day | days | 100 | day |days | 100 | | day | days | 100 | | | at | at | cc. | at | at | cc. | | at | at | cc. | | | 37 | 20 | | 37 | 20 | | | 37 | 20 | | | | C. | C. | | C. | C. | | | C. | C. | | | -----+-----+------+-----+-----+-----+-----+----+-----+------+-----+----+---- Mar. | 44| 238| 35.7| 4 | 12 |<0.14|0.90| 4="" |="" 12="" |0.14="" |0.22|0.11="" 15-31|="" |="" |="" |="" |="" |="" |="" |="" |="" |="" |="" |="" april|3,099|14,408|195.5|="" 32="" |="" 56="" |="" 0.50|1.10|="" 33="" |246="" |0.74="" |0.25|0.13="" 1-19="" |="" |="" |="" |="" |="" |="" |="" |="" |="" |="" |="" |="" -----+-----+------+-----+-----+-----+-----+----+-----+------+-----+----+----="" legend:="" (1)="" available="" chlorine="" parts="" per="">

(2) Ammonia, parts per million.

TABLE XXVII

_Percentage Reduction_

-------+---------------------------------+-------------------------------- | HYPOCHLORITE ALONE. | HYPOCHLORITE AND AMMONIA.

+---------------+-------+---------+--------------+-------+--------- | Bacteria | |Available| Bacteria | |Available | per cubic | _B. |chlorine | per cubic | _B. |Chlorine | centimeter. | coli_ | parts | centimeter. | coli_ | Parts +------+--------+ Index | per +------+-------+ Index | per | Agar | Agar |per 100|million. | Agar | Agar |per 100|Million.

|1 Day | 3 Days | cubic | |1 Day |3 Days | cubic | | at | at |centi- | | at | at |centi- | |37 C.| 20 C. |meters.| |37 C.| 20 C.|meters.| -------+------+--------+-------+---------+------+-------+-------+--------- Mar. | 90.9 | 95.8 | 99.9+ | 0.90 | 90.0 | 95.0 | 99.7| 0.22 15-31| | | | | | | | April | 98.9 | 99.6 | 99.7 | 1.10 | 98.3 | 98.9 | 99.6| 0.25 1-19 | | | | | | | | -------+------+--------+-------+---------+------+-------+-------+---------

TABLE XXVIII