(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].