In this process, sometimes called the direct-oxidation process, all grit is removed and the sewage is passed through fine screens before entering the electrolytic tank. In the electrolytic tank the sewage passes in thin sheets between electrodes and an electric current is discharged through it. A recent development has been the addition of lime to the sewage at some point in its passage through the electrolytic tank. From the electrolytic tank the sewage flows to a sedimentation tank, where sludge is accumulated, and from which the liquid effluent is finally disposed of.
It is claimed that the action of the electricity electrolyzes the sewage, releasing chlorine, which acts as a powerful disinfectant. The constituents of the sewage are oxidized so that the dissolved oxygen, nitrates, and relative stability are increased and the sludge is rendered non-putrescible. It is said that the addition of lime increases the efficiency of sedimentation and enhances the effect of the electric current. The results obtained by tests at Easton, Pa., are shown in Table 101. It will be observed from this table that the combination of lime and electricity does not have a more beneficial effect than either one of them alone. The amount of sludge produced by the combination is about the same as by chemical precipitation alone, but the character of the sludge produced with electricity is less putrescible. The cost of the treatment as estimated at Elmhurst is shown in Table 102.
As a result of the tests at Decatur, comparing lime alone with lime and electricity together, Dr. Ed. Bartow stated:
The purification by treatment with lime alone was greater than that obtained in several of the individual samples treated with lime and electricity.
| TABLE 101 | ||||||
|---|---|---|---|---|---|---|
| Comparative Results Obtained from the Treatment of Sewage by Lime Alone, Electricity Alone, and Lime and Electricity Combined | ||||||
| (Creighton and Franklin, Journal of the Franklin Institute, August, 1919) | ||||||
| Lime and Electricity | Lime Alone | Electricity Alone | ||||
| Change, Parts per Million | Change, Per Cent | Change, Parts per Million | Change, Per Cent | Change, Parts per Million | Change, Per Cent | |
| Chlorine | +1.2 | +1.9 | +12.3 | +18.2 | +1.6 | +2.2 |
| Nitrites | +0.014 | +58.3 | -.005 | –10.0 | –0.01 | –20.0 |
| Nitrates | +0.13 | +23.6 | +.005 | +0.8 | –0.15 | –20.0 |
| Ammonia | –3.3 | –18.3 | +0.2 | +1.3 | +0.9 | +6.6 |
| Albuminoid ammonia | –3.6 | –12.1 | –0.4 | –1.7 | –0.5 | –2.3 |
| Oxygen demand | –13.0 | –20.5 | –7.7 | –8.9 | –6.5 | –10.0 |
| Dissolved oxygen | +1.78 | +40.9 | –0.93 | –19.1 | +1.61 | +40.1 |
| Total bacteria at 37° (Thousands) | –343 | –92.7 | –373 | –82.4 | –165 | –37.8 |
| Total bacteria at 20° (Thousands) | –688 | –92.7 | –1074 | –90.1 | –635 | –70.0 |
| B. Coli (Thousands) | –77.9 | –99.85 | –96.3 | –92.3 | –45 | –81.8 |
| Oxygen absorbed in 5 days | –3.40 | –81.6 | –1.03 | –21. | +1.24 | +31 |
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: