Disinfection
277. Disinfection of Sewage.—Sewage is disinfected in order to protect public water supplies, shell fish, and bathing beaches; to prevent the spread of disease; to keep down odors, and to delay putrefaction. Disinfection is the treatment of sewage by which the number of bacteria is greatly reduced. Sterilization is the destruction of all bacterial life, including spores. Ordinarily even the most destructive agents do not accomplish complete sterilization. Chlorine and its compounds are practically the only substances used for the disinfection of sewage. The lime used in chemical precipitation, the acid used in the Miles Acid Process, the aëration in the activated sludge process, all serve to disinfect sewage, but are not used primarily for that purpose. Copper sulphate has been used as an algaecide but never on a large scale as a bactericide.[[199]] Heat has been suggested, but its high cost has prevented its practical application to the disinfection of sewage.
| TABLE 102 | |||
|---|---|---|---|
| Cost of Electrolytic Treatment, Elmhurst, Long Island, and Easton, Pennsylvania | |||
| Item | One Million Gallon | Three Million Gallon | |
| unit at Easton, Dollars | unit at Elmhurst, Dollars | unit at Elmhurst, Dollars | |
| Hydrated lime: Elmhurst, 1300 pounds at $7.90 ton. Easton, 3720 pounds at $6.75 ton. | 12.56 | 5.14 | 15.42 |
| Electric power electrolysis: Elmhurst, 85 kw-h. at 4 cents Easton, 6.25 kw-h. at 8.05 cents | 4.19 | 3.40 | 9.60 |
| Electric power, light and agitation: Elmhurst, 60 kw-h. at 4 cents Easton, 6.25 kw-h at 8.05 cents | 0.50 | 2.40 | 7.20 |
| Heating | 1.25 | ||
| Labor and supervision | 15.00 | 12.50 | 15.00 |
| Maintenance, repairs and supplies | 1.50 | 1.00 | 3.00 |
| Sludge pressing and removal | 5.11 | 15.33 | |
| Total | 35.00 | 29.55 | 65.55 |
| Cost per million gallons | 35.00 | 29.55 | 21.85 |
The action which takes place on the addition to sewage of chlorine or its compounds is not well understood. The idea that the bacteria are burned up with “nascent” or freshly born oxygen, has been exploded.[[200]] Likewise the idea that the toxic properties of chlorine have no effect has not been borne out by experiments. It has been demonstrated, particularly by tests on strong tannery wastes, that the action of chlorine gas is more effective than the application of the same amount of chlorine in the form of hypochlorite. All that we are certain of at present is that the greater the amount of chlorine added under the same conditions, the greater the bactericidal effect.
Chlorine is applied either in the form of a bleaching powder or a gas. In ordinary commercial bleach (calcium hypochlorite) the available chlorine is about 35 to 40 per cent by weight. In order to add one part per million of available chlorine to sewage it is necessary to add about 25 pounds of bleaching powder or 8½ pounds of liquid chlorine per million gallons of sewage. This can be computed as follows:
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.