Chlorination of Water Part 12

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 acc.u.mulation 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).[A]

[A] 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.

[Ill.u.s.tration: DIAGRAM VII

CHLORINE GAS PRESSURES AT VARIOUS TEMPERATURES]

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 - 1-1/2 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.

[Ill.u.s.tration: FIG. 8.--Dunwoodie Chlorinating Plant Treating 400,000,000 Gallons Per Day for New York City.]

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 subst.i.tuted. 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 quant.i.ties 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)

----------------+----------+-----------+------------+--------------- Treatment. | Water | Number of | Chlorine | Reduction | Treated. | Samples. | p.p.m. | of B. coli.

----------------+----------+-----------+------------+--------------- Bleach | Croton | 84 | 0.27-0.36 | 93% Liquid chlorine | Bronx | 84 | 0.27-0.36 | 93% ----------------+----------+-----------+------------+---------------

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 quant.i.ty 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 a.n.a.lytical control involves additional expense that must be charged against the bleach process. No chemical a.n.a.lyses 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 gla.s.s 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 gla.s.s and is consequently easily fractured. Duplicates of the gla.s.s 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 quant.i.ties and for 8-9 cents per pound in large shipments. In 1917 the price was 18-20 cents per pound for small quant.i.ties 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 a.s.sisted 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 - 0.04196_t_ + 0.0001107_t_^{2}; when _t_ = 10 C. 1 c.cm. of water absorbs 2.58 c.cms. of chlorine or 8.2 m.gr., a quant.i.ty 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 quant.i.ty never exceeded very small proportions. Chlorine solutions decompose in accordance with the equation, Cl_{2} + H_{2}O = 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 pa.s.sing 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 Cl5H_{2}O but later investigators have shown that more concentrated hydrates can be prepared. Roozeboom[10] prepared a hydrate represented by the formula Cl4H_{2}O and Forcrand[11] one containing only 3-1/2 molecules of water (Cl_{2}7H_{2}O). 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 Cl_{2}:H_{2}O 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.

Cl_{2}64H_{2}O 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.

BIBLIOGRAPHY