The molecular weight of calcium hypochlorite is 127.0. This reacts to produce two atoms of available chlorine with a molecular weight of 70.9. If the bleaching powder were pure the available chlorine would therefore represent 70.9 ÷ 127, or 56 per cent of its weight. Then to obtain one pound of chlorine it would be necessary to have 1.79 pounds of pure bleaching powder. Since 1,000,000 gallons of water weigh approximately 8,300,000 pounds, in order to apply one part per million of chlorine to 1,000,000 gallons of sewage it is necessary to apply 1.79 × 8.3 or 14.9 pounds of pure bleaching powder. Commercial bleaching powder is only about 60 per cent calcium hypochlorite. It is therefore necessary to add 14.9 ÷ 0.60 or about 25 pounds of commercial bleach.
Since liquid chlorine is very nearly pure, approximately 8½ pounds of it applied to 1,000,000 gallons of sewage are equivalent to a dose of one part per million.
Commercial bleaching powder is a dry white powder which absorbs moisture slowly, and which loses its strength rapidly when exposed to the air. It is packed in air-tight sheet iron containers, which should be opened under water, or emptied into water immediately on being opened. The strength of the solution should be from ½ to 1 per cent. The rate of the application of the solution to the sewage may be controlled by automatic feed devices, or by hand-controlled devices.
Commercial liquid chlorine is sold in heavy cast steel containers, which hold 100 to 140 pounds of liquid chlorine under a pressure of 54 pounds per square inch at zero degrees C. or 121 pounds per square inch at 20 degrees.
The amount of chlorine used is dependent on the character of the sewage to be treated, the stage of decomposition of the organic matter, the desired degree of disinfection, the period of contact, and the temperature. The amount of chlorine is expressed in parts per million of available chlorine, regardless of the form in which the chlorine is applied. In general about 15 to 20 parts per million of available chlorine with 30 minutes’ contact at a temperature of about 15° C. will effect an apparent removal of 99 per cent of the bacteria from the raw sewage. The effect is only apparent because many of the bacteria encased in the solid matter of the sewage escape the effect of the chlorine, or detection in the bacterial analysis. Stronger and older sewages, higher temperatures, and shorter periods of contact will demand more chlorine to produce the same results. A septic effluent will require more chlorine than a raw sewage because of the greater oxygen demand by the septic sewage. The results of experiments on disinfection made at different testing stations have shown such wide variations in the amount of chlorine necessary, as to demonstrate the necessity for independent studies of any particular sewage which is to be chlorinated. For instance, at Milwaukee approximately 13 p.p.m. of available chlorine applied to an Imhoff tank effluent effected a 99 per cent removal of bacteria, whereas the same result was obtained at Lawrence, Mass., on crude sewage with only 6.6 p.p.m. and at Marion, Ohio, only 9 per cent removal of bacteria was obtained by the addition of 4,815 p.p.m. to crude sewage. The Ohio and Massachusetts reports show irrational variations among themselves. For instance, 6.2 p.p.m. applied to a septic effluent effected 88 per cent removal whereas in another case 7.6 p.p.m. effected only 36 per cent removal. At Lawrence in one case it took 8.6 p.p.m. to remove 99 per cent from a sand filter effluent, but only 6.3 p.p.m. to effect the same result in the effluent from a septic tank. The most consistent results are those found at Milwaukee which show a steadily increasing percentage removal with increasing amounts of chlorine.
Some time after sewage has received its dose of chlorine the number of bacteria may be greater than in the raw sewage. Such bacteria are called aftergrowths. Certain forms of bacteria, particularly the pathogenic or body temperature types, are most susceptible to disinfecting agents. These are killed off and leave the sewage in a condition more favorable to the growth of more resistant forms of bacteria. As the latter are non-pathogenic and are generally aërobic their presence is usually more beneficial than detrimental, as they hasten the action of self-purification.
REFERENCES
The following abbreviations will be used: E.C. for Engineering and Contracting, E.N. for Engineering News, E.R. for Engineering Record, E.N.R. for Engineering News-Record, M.J. for Municipal Journal, p. for page, and V. for volume.
No. 1. Grease and Fertilizer Base for Boston Sewage, by Weston, E.N. V. 75, 1916, p. 913 and Journal American Public Health Association, April, 1916. 2. Getting Grease and Fertilizer from City Sewage, by Allen. E.N. V. 75, 1916, p. 1005. 3. New Haven Tests Five Processes of Sewage Treatment. E.N.R. V. 79, 1917, p. 829. 4. Recovery of Grease and Fertilizer from Sewage Comes to the Front. E.N.R. V. 80, 1916, p. 319. 5. Miles Acid Process may Require Aëration of Effluent, by Mohlman. E.N.R. V. 81, 1918, p. 235. 6. Promising Results with Miles Acid Process in New Haven Tests. E.N.R. V. 81, 1918, p. 1034. 7. Baltimore Experiments on Grease from Sewage. E.N. V. 75, 1916, p. 1155. 8. Report on Industrial Wastes from the Stock Yards and Packingtown in Chicago to the Trustees of the Sanitary District of Chicago, 1914, pp. 187–195. 9. The Separation of Grease from Sewage, by Daniels and Rosenfeld. Cornell Civil Engineer. V. 24, p. 13. 10. The Separation of Grease from Sewage Sludge with Special Reference to Plants and Methods Employed at Bradford and Oldham, England, by Allen. E.C. V. 40, 1913, p. 611. 11. Acid Treatment of Sewage, by Dorr and Weston. Journal Boston Society of Civil Engineers, April, 1919. E.C. V. 51, 1919, p. 510. M.J. V. 46, 1919, p. 365. 12. The Miles Acid Process for Sewage Disposal. Metallurgical and Chemical Engineering, V. 18, p. 591. 13. Miles Acid Treatment of Sewage, by Winslow and Mohlman. Journal American Society Municipal Improvements, Oct., 1918. M.J. V. 45, 1918, pp. 280, 297, and 321. 14. New Electrolytic Sewage Treatment. M.J. V. 37, 1914, p. 556.