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 canal, 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 passed 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 Washington, 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 quantity 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 passed 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, passed 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 substitution of a flow meter of the float type for the inferential pressure meter.

[A] This type has recently been withdrawn from the market.

Another type of apparatus, developed by Wallace and Tiernan,[B] is shown in Figs. 6 and 7. The gas under the pressure indicated by the tank pressure gauge (Fig. 6) passes 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 passes 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 quantities 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.

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

The quantities of gas exceeding 12 pounds per day the type shown in Fig. 7 may be used. The gas from the control valve passes through a visible glass 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 passes into the glass cylinder where it is dissolved in water and passes 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 quantities less than the minimum graduation are desired, a smaller orifice with its corresponding scale can be substituted in a few minutes.

These types are manually controlled, but automatic control types, to meet almost any condition, can be obtained and are in use in many cities.

In some instances (dry-feed types) the chlorine gas is not dissolved in water prior to addition to the water requiring treatment but is carried to the point of application as a dry gas and enters the water through a diffusion plate made of carborundum sponge. The sponge becomes saturated with water because of the capillary action of the carborundum upon the water. The pressure of the chlorine in the feed pipe forces the gas through the diffuser in the form of minute bubbles which become saturated with moisture. On meeting the water they immediately go into solution and no gas escapes.

The operation of liquid chlorine machines is exceedingly simple. After the cylinders have been connected, the cylinder valves are opened and the joints tested for leakage by holding a swab of absorbent cotton saturated with strong ammonia under them; a leakage is indicated by the appearance of white fumes of ammonium chloride. The control valve is then slightly opened and the auxiliary cylinder valves partially opened; whilst the pressure in the apparatus is slowly increasing the remainder of the joints are tested and if found to be tight, the cylinder valves are fully opened and the control valve opened to the desired amount. In the solution feed types the water required as solvent is turned on before the control valve is opened. Once the apparatus is working, no further attention is required, except for the regulation of the dosage in the manual control types, until the cylinders are replaced. When the stock of gas in the cylinders is almost depleted the pressure falls but it is always preferable to determine the stock by standing the cylinders on a platform scale and weighing at regular intervals. This also provides a check on the apparatus and can be utilised to check the operators.

The accumulation of substances that impede the flow of gas is usually slow and is indicated by a gradual increase in the back pressure. The orifice is calibrated at 25 pounds back pressure and any deviation from this figure will show a discrepancy between the actual weight of chlorine evaporated and the amount calculated from the scale reading.

Liquid chlorine is usually sent out by the manufacturers in steel cylinders which contain about 1.1 cubic feet of liquid or approximately 100 pounds (1 cu. ft. = 89.75 pounds).[C]

[C] An effort is now being made to standardise cylinders of 150 lbs. capacity.For small installations only one cylinder is necessary but it is always preferable to connect more than one. When the flow of gas is rapid the temperature of the liquid chlorine falls and reduces the pressure. The effect of the fall in temperature, due to the latent heat of evaporation, can be partially overcome by using a larger number of cylinders; in addition a source of external heat should be provided that will maintain the temperature of the cylinders at a minimum of 80° F. This is a “sine qua non” for successful operation. The effect of the temperature upon the pressure in the cylinders is shown in Diagram VII.

In practice it is found impossible to utilise all the gas contained in the containers; when the cylinders are almost empty the pressure necessary for the operation of the regulating device cannot be obtained and full cylinders must be attached. When sufficient heat is provided the weight of chlorine in the cylinder can be reduced to 1 - 11/2 pounds before the tank pressure becomes too low.Liquid chlorine machines will operate, with ordinary care, for long periods. The various parts are made of such metals as experience has demonstrated to be best able to resist the corrosive action of the dry gas and the apparatus is designed to prevent the access of moisture which would otherwise produce corrosion and impede the flow of gas. Stoppages are sometimes caused by brown deposits derived from impurities in the liquid chlorine. These are primarily due to variations in the graphite electrodes used in the electrolytic process for the manufacture of chlorine from salt.

To convey the dry gas from the apparatus to the point of application, copper or iron pipes may be used; for aqueous solutions, flexible rubber hose must be employed. Chlorine water is exceedingly active, chemically, and rapidly attacks all the common metals; ordinary galvanised iron pipe is eroded in a few days and should never be used.Liquid chlorine, for water disinfection, possesses several marked advantages over the ordinary bleach process.

(1) The sterilising agent is practically 100 per cent pure, the only impurities being traces of carbon dioxide and air, and does not deteriorate on storage; it will, in fact, keep almost indefinitely.

(2) Liquid chlorine practically eliminates all labour costs because of the simplicity of the apparatus and the concentrated form of the sterilising agent. The apparatus is so compact that all the cylinders and regulating apparatus required for delivering 200 pounds of gas per day can be placed in an area of about 50 square feet and it can consequently be almost invariably accommodated in locations where the trifling amount of attention required can be obtained without extra cost.

(3) The sludge problem, inseparable from bleach installations, is eliminated.

(4) Regulation of the dosage is simpler and consequently usually more accurate. The dosing apparatus in bleach plants invariably tends to choke and demands regular attention from intelligent operators; a similar tendency in liquid chlorine machines is easily detected and electrical devices can be installed to indicate automatically any changes in the flow.

(5) The first cost is smaller. The cost of liquid chlorine machines varies from $400, for the small manual control types, to $1,200, for the automatic control types. The capital outlay is mainly determined by the number of machines and accessories required and not, within certain limits, by the capacity. One machine will deliver up to 200 pounds of gas per day, an amount sufficient to treat 60,000,000 U. S. A. gallons (50,000,000 Imp. gals.) at 0.40 p.p.m. of available chlorine. Unless duplicate machines are installed for the higher rates, the first cost is inversely proportional, though not directly so, to the volume of water treated. It is in all cases less than the first cost of a bleach plant of equal capacity, accuracy, and durability.

(6) Liquid chlorine installations usually tend to produce less complaints as to tastes and odours. This is probably due, not to any merit of the chlorine per se, but to a more accurate regulation of the dosage and efficient distribution of the chlorine in the treated water. The advantages ensuing from thorough admixture had only become partially appreciated before liquid chlorine machines were fully developed and they have been more fully utilised in the design of these later installations.Claims have also been made that liquid chlorine prevents “aftergrowths” but no evidence can be adduced in support of this statement. Aftergrowths have occurred at many places where this process is employed and in this respect it possesses no advantage over hypochlorite installations.

It is also claimed that one pound of liquid chlorine is more efficient, as a germicide, than an equal weight of chlorine in the form of bleach. Jackson[5] has stated that 1 pound of chlorine is equal to 9 pounds of bleach; Kienle (loc. cit.) that it was equal to 8 pounds of bleach, whilst Huy claimed to have obtained an efficiency ratio of 1:10 at Niagara Falls, N. Y. The conditions of the experiment were not comparable however, in the last mentioned ratio. Catlett, at Wilmington, N. C. (West[4]) obtained a better bacterial reduction with 1 pound of liquid chlorine than with 6 pounds of bleach.

The efficiency ratio of chlorine to bleach has been reported upon by West.[4] From 1910-1913 the mixed filter effluents of the Torresdale plant at Philadelphia were treated with bleach but in November, 1913 the liquid chlorine process was substituted. On comparing the results obtained during the same months of the two periods it was found that, in general, 1 pound of liquid chlorine gave a slightly higher percentage purification than 6-7 pounds of bleach. Similar results were obtained at the other Philadelphia plants. The figures published by West show that the hypochlorite solutions used were abnormally strong (3.6-10.4 per cent of available chlorine), a condition that would increase the difficulty of extracting all the soluble hypochlorite. It was found indeed, that, under the most advantageous conditions, only 87 per cent of the available chlorine was extracted. The average chlorine content of the bleach used during 1912-1913 was 36.1 per cent but the figures given would indicate that at least 1.5 per cent, a reduction of 4.6 per cent of the total, was lost during storage. It would seem not improbable that the total loss under average conditions was not less than 20 per cent, which would reduce the efficiency ratio to 1:4.8-5.6.

Hale[6] also made a comparison of the relative efficiency of liquid chlorine and hypochlorite of lime at New York, and the earlier results agreed with West’s ratio of 1:6-7. An investigation showed that large quantities of chlorine were not extracted from the bleach and when this condition was rectified the total loss averaged only 4 per cent and the results obtained were equal to those given by the liquid chlorine machines. Hale’s comparative figures are given in Table XXIII.

TABLE XXIII.—COMPARISON OF LIQUID CHLORINE
WITH EFFICIENT USE OF BLEACH—(Hale)

Hale concluded that, when efficiently used, the ratio of chlorine to bleach required to produce equal bacterial purification, approached 1:3.

The results obtained by the author in Ottawa are similar to those of Hale. During the earlier period of the bleach treatment a dosage of 1.5 p.p.m. of available chlorine was required to obtain satisfactory purification but various improvements that were subsequently made enabled the quantity to be reduced to 0.8 p.p.m. The same raw water usually requires 0.75 to 0.80 p.p.m. of liquid chlorine to obtain the same purification. The total losses in the Ottawa bleach plant averaged 6-8 per cent and based on these figures the efficiency ratio is approximately 1:3.5.

Ratios as low as 1:3.5 can only be obtained by the supervision of a chemist and this analytical control involves additional expense that must be charged against the bleach process. No chemical analyses are necessary for the control of liquid chlorine plants.

Disadvantages of Liquid Chlorine Plants. The main objection to the use of liquid chlorine is that the slight leaks of gas occur occasionally and unless removed by forced ventilation may produce a concentration of chlorine that will injure the operators.

Pettenkofer and Lehmann[7] found that 0.001-0.005 per cent of chlorine in air affected the respiratory organs; 0.04-0.06 per cent produced dangerous symptoms, whilst concentrations exceeding 0.06 per cent rapidly proved fatal.

The danger of gas leakages can be eliminated by placing the apparatus in a small separate room provided with a fan and a ventilation duct. By the liberal use of glass in the construction of the room, the operation of the plant can be seen at all times without entering the chamber.

A portion of the liquid chlorine apparatus is made of glass and is consequently easily fractured. Duplicates of the glass parts should be kept in stock to prevent interrupting the supply of gas; a duplicate machine is also advisable in large installations.Cost of Treatment. Prior to the outbreak of war in 1914, liquid chlorine sold at 10-11 cents per pound in small quantities and for 8-9 cents per pound in large shipments. In 1917 the price was 18-20 cents per pound for small quantities and 15 cents upwards for large contracts. Canadian prices are 25 per cent higher.The amount of chlorine required for satisfactory disinfection (see Chapter III) depends upon the nature of the water and the cost of treatment varies accordingly. In the majority of plants the cost varies from 25-90 cents per million gallons.Popularity of Process. Since 1913, when the first commercial liquid chlorine machines were used, the popularity of this process has increased in a most remarkable manner. In 1913 over 1,700 million gallons per day were treated with hypochlorite; in 1915, 1,000 million gallons per day were treated with liquid chlorine and an approximately equal amount with hypochlorite; in January 1918, the amounts were 3,500 million gallons per day (liquid chlorine) and 500 million gallons per day (hypochlorite).

This wonderful development has been largely due to the intrinsic merits of the process and the reliability of the machines manufactured although it has been indirectly assisted by the excessive cost of hypochlorite during 1915-1916.

Liquid chlorine machines are being used for the purification of water on the Western Front of the European battlefield. The outfit is a mobile one and consists of a rapid sand filter, liquid chlorine apparatus, a small storage tank and solution tanks. Owing to the limited contact period available a large dosage of chlorine is employed and the excess afterwards removed by the addition of a solution of sodium thiosulphate.Chlorine Water. Marshall[8] has proposed the use of chlorine water for the sterilisation of water for troops. The solution is contained in ampoules which are of two sizes, one for water carts and the other for water bottles of one quart capacity.

The coefficient of solubility of chlorine, from 10°-41° C. is C = 3.0361 - 0.04196t + 0.0001107t2; when t = 10° C. 1 c.cm. of water absorbs 2.58 c.cms. of chlorine or 8.2 m.gr., a quantity sufficient to give a concentration of 1 p.p.m. in 8 litres of water. Marshall has stated that, when pure materials are used, chlorine water is stable but the author is unable to confirm this. A saturated solution of chlorine in distilled water lost over 50 per cent of its available chlorine content when stored for five days in the dark at 70° F. The chlorine present as hypochlorous acid increased slightly but the quantity never exceeded very small proportions. Chlorine solutions decompose in accordance with the equation, Cl2 + H2O = 2HCl + O.

Although chlorine water appears to be of little value because of its instability there appears to be no reason why chlorine hydrate should not be successfully employed. The hydrate was first prepared by Faraday[9] by passing chlorine into water surrounded by a freezing mixture. A thick yellow magma resulted from which the crystals of chlorine hydrate were separated by pressing between filter paper at 0° C. The hydrate prepared by Faraday was found to have the composition represented by the formula Cl·5H2O but later investigators have shown that more concentrated hydrates can be prepared. Roozeboom[10] prepared a hydrate represented by the formula Cl·4H2O and Forcrand[11] one containing only 31/2 molecules of water (Cl2·7H2O). Chlorine hydrate separates into chlorine gas and chlorine water at 9.6° C. in open vessels and at 28.7° C. in closed vessels. Pedler[12] has shown that when the ratio of Cl2:H2O is 1:64 or greater, the mixture of chlorine hydrate and water exhibits great stability and can be exposed to tropical sunlight for several months without decomposition.

Cl2·64H2O contains 5.8 per cent of chlorine and about 8. c.cms. would be required to give a concentration of 1 p.p.m. in 110 Imp. gallons of water, the usual capacity of a military water cart.