Chlorination of Water Part 11

(ii) Na_{2}S_{2}O_{3} + Cl_{2} = Na_{2}S_{4}O_{6} + 2NaCl.

Sodium bisulphite is a very efficient "antichlor," only 1.46 parts being required to remove 1 part of chlorine, but owing to its instability the action is uncertain. Sodium thiosulphate is a comparatively stable cheap salt, containing 5 molecules of water of crystallization, Na_{2}S_{2}O_{3} 5H_{2}O but 7 parts are necessary to remove 1 part by weight of chlorine.

"Antichlors" are used as aqueous solutions and the dosage controlled in the same manner as for bleach solutions. The action is an instantaneous one and it is consequently necessary that the germicidal action should be complete before the "antichlor" is added.

Filters, containing solid materials capable of absorbing free chlorine, have also been used for removing the excess of the germicidal reagent.

Iron borings and aluminium were used experimentally by Thresh[11] but the process was not commercially developed. The "De Chlor" filter, in which carbon is the active substance, has been installed at several water works in England (Reading, Exeter, Aldershot) with apparently successful results. The Reading experimental installation, described by Walker,[12] consisted of a steel drum, 8 feet 3 inches in width, the top and bottom being domed. In the upper portion, 10 feet 9 inches in depth, provision was made for thorough admixture of the bleach solution and water and a subsequent storage of thirty minutes. The lower section of the filter was divided into three compartments, the first and last of which contained graded silica; the middle compartment was filled with a layer (20 inches deep) of specially prepared granulated charcoal or carbon.

The filter was operated under pressure and pa.s.sed an average of 192,000 Imp. gallons per day, the rate being 32,000 Imp. gallons per square yard per day.

Water from the pre-filters (polarite and sand) was treated with bleach to give a concentration of 1 p.p.m. of available chlorine and pa.s.sed through the De Chlor filter. The average bacteriological results obtained during the first six months operation were as follows:

Bacteria Per c.cm. B. coli Index Gelatine 3 Days at 20 C. Per 100 c.cms.

Raw river water 6,775 600

Water from pre-filters 579 119

Water from De Chlor filter 33 Nil

Free chlorine could not be detected by chemical tests in the filtered water which was also free from abnormal tastes and odours. It is stated that the carbon has to be removed and revivified periodically. The filter was washed about once per week, the wash water being only one-tenth of one per cent.

The experimental filter was operated for nearly two years before being removed to permit the erection of larger units having a total capacity of one million Imp. gallons per day.

BIBLIOGRAPHY

[1] Hooker. Chloride of Lime in Sanitation, New York, 1913.

[2] Griffen and Hedallen. J. Soc. Chem. Ind., 1915, =34=, 530.

[3] Hale. Proc. N. J. San. a.s.soc., 1914.

[4] Adams. J. Amer. Pub. Health a.s.soc., 1916, =6=, 867.

[5] Ellms and Hauser. J. Ind. and Eng. Chem., 1913, =5=, 915 and 1030; _ibid._, 1914, =6=, 553.

[6] Wallis. Ind. Jour. Med. Res., 1917, =4=, 797.

[7] Le Roy. Comptes rend., 1916, =163=, 226.

[8] Winkler. Zeit. angew. Chem., 1915, =28=, 22.

[9]: Rideal, E. K. and Evans. a.n.a.lyst, 1913, =38=, 353.

[10] J. Amer. Pub. Health a.s.soc. 1915, =5=, 921.

[11] Thresh. Internat. Congress Appl. Chem., 1908.

[12] Walker. Jour. Roy. Inst. Pub. Health, Jan., 1911.

CHAPTER VII

LIQUID CHLORINE

The use of liquefied chlorine for the disinfection of water was first proposed by Lieutenant Nesfield[1] of the Indian Medical Service. He stated that: "It occurred to me that chlorine gas might be found satisfactory ... if suitable means could be found for using it.... The next important question was how to render the gas portable. This might be accomplished in two ways: By liquefying it, and storing it in lead-lined iron vessels, having a jet with a very fine capillary ca.n.a.l, and fitted with a tap or a screw cap. The tap is turned on, and the cylinder placed in the amount of water required. The chlorine bubbles out, and in ten to fifteen minutes the water is absolutely safe, and has only to be rendered tasteless by the addition of sodium sulphite made into a cake or tablet.... The cylinders could, of course, be refilled.

This method would be of use on a large scale, as for service water carts."

The first _practical_ demonstration of the possibilities of this method was made by Major Darnall[2] of the Medical Corps, United States Army, in 1910. Chlorine was taken from steel cylinders and pa.s.sed through automatic reducing valves which provided a uniform flow of gas for the water requiring treatment. A uniform flow of water was maintained through the mixing pipe and so secured a uniform dosage. This apparatus might be considered as the forerunner of the various commercial types of machines that were developed later and which are being so extensively used at the present time.

A working model, having a capacity of 500 gallons per hour, was erected at Fort Myer, Va., and was operated on water that had been treated with alum but had received no further purification. Despite the presence of the flocculated organic matter, satisfactory purification was obtained with 0.5 to 1.0 p.p.m. of available chlorine and no taste or odour was imparted to the supply.

From the results obtained at Fort Myer, and Was.h.i.+ngton, D.C., Darnall concluded that "In general, it may be said that with an average unfiltered river water such as that of the Potomac, about one-half of one part (by weight) of chlorine gas per million of water will be required. For clear lake waters three-tenths to four-tenths of a part per million will be sufficient."

A Board of Officers of the War Department examined the results and reported (June, 1911) "That the apparatus is as efficient as purification by ozone or hypochlorite and is more reliable in operation than either.... That it could be installed at a very low cost and that the cost of operation would be very slight."

In June, 1912, Ornstein experimented with chlorine gas, obtained from the liquefied gas in cylinders, for sewage and water disinfection but his method differed from Darnall's in first dissolving the gas in water and feeding the solution to the liquid to be treated.

Kienle[3] made experiments at Wilmington, Del., in November, 1912, and obtained a constant flow of gas by means of high- and low-pressure valves; the gas was dissolved in water in an absorption tower and afterwards fed to the water to be treated.

Van Loan and Thomas of Philadelphia experimented with liquid chlorine on a large scale at the Belmont Filter Plant in September, 1912. The chlorine was fed into the filtered water basin in the gaseous state and the quant.i.ty was regulated by the loss in weight of the containers. The dosage was approximately 0.14 p.p.m. (West[4]).

Jackson, of Brooklyn, made similar experiments about the same time at the Ridgewood Reservoir, Brooklyn, and his type of apparatus was shortly afterwards put on the market as the Leavitt-Jackson Liquid Chlorine Machine. The regulation of the flow in this machine was determined by the loss in weight of the gas cylinder which was suspended from a sensitive scale beam. By moving the counterbalancing weight on the beam at a constant rate, a uniform flow of gas was obtained, the area of the orifice being kept constant by the equilibrium in the balance operating controlling valves through a system of levers.

This type of apparatus was tried at several places but it was found that the adjustment of the regulating mechanism was too sensitive and produced considerable irregularities in the flow of gas.

The type used by Ornstein and Kienle were combined and commercially developed by the Electric Bleaching Gas Co. of New York.[A] In this combined type the gas was collected from one or more cylinders by means of a manifold which delivered it to the regulating mechanism at the pressure indicated by a gauge attached to the inlet pipe. Beyond this gauge were two pressure-regulating devices, the first being used primarily to reduce the initial pressure to about 15 pounds per square inch, and the second for controlling the pressure through a range sufficient to give the desired discharge of gas. The gas from the second regulator pa.s.sed through an orifice in a plate at a pressure indicated by a suitable gauge which was calibrated in terms of weight of chlorine per unit of time. The gas, on leaving the regulating apparatus, pa.s.sed up an absorption tower of hard rubber, where it met a descending stream of water. The solution was carried by suitable piping to the point of application. This type was modified in some cases by the subst.i.tution of a flow meter of the float type for the inferential pressure meter.

[A] This type has recently been withdrawn from the market.

[Ill.u.s.tration: FIG. 6.--Manual Control Chlorinator, Solution Feed, Type A.]

Another type of apparatus, developed by Wallace and Tiernan,[A] is shown in Figs. 6 and 7. The gas under the pressure indicated by the tank pressure gauge (Fig. 6) pa.s.ses into the pressure compensating chamber, which maintains a constant drop in pressure across the chlorine control valve, through the check valve, and into the solution jar after measurement in the pulsating meter. The water required for dissolving the chlorine enters the jar through the feed line and check valve and the solution pa.s.ses along the feed line after being water sealed in a special chamber. The meter is a volumetric displacement one and is regulated by observing the number of pulsations per minute. Each pulsation corresponds to 100 milligrams or 0.00022 pound of chlorine; diagrams for converting pulsations per minute into weight per twenty-four hours are usually provided with the apparatus. This type of meter is suitable for quant.i.ties between 0.1 and 12 pounds per day and possesses the distinct advantage of enabling the operator to see the actual delivery of the gas.

[A] Manufactured by Wallace and Tiernan Co. Inc. N. Y.

[Ill.u.s.tration: FIG. 7.--Manual Control Chlorinator, Solution Feed, Type B.]

The quant.i.ties of gas exceeding 12 pounds per day the type shown in Fig.

7 may be used. The gas from the control valve pa.s.ses through a visible gla.s.s orifice which is connected with the manometer. This manometer, or chlorine meter, contains carbon tetrachloride and is graduated empirically in terms of weight of chlorine per unit of time. A suitable gauge indicates the back pressure thrown by the check valve and registers the same pressure as the tank gauge when the flow of gas is stopped. The gas pa.s.ses into the gla.s.s cylinder where it is dissolved in water and pa.s.ses out by the feed pipe.

The most accurate range of the orifice type is from 1-6, i.e. if the minimum graduation on the scale is 10, the maximum is 60. If quant.i.ties less than the minimum graduation are desired, a smaller orifice with its corresponding scale can be subst.i.tuted in a few minutes.