| TABLE 84 | ||||
|---|---|---|---|---|
| Capacities of Septic Tanks for Small Installations | ||||
| Rule Recommended by State Board of Health | Number, Persons | Capacity, Gallons per Person | Period of Retention | Remarks |
| Wisconsin | 30 | 24 hours | ||
| Ohio | 4 to 10 | 50 | Not less than 560 gallons | |
| Kentucky | 24 to 48 hours | Not more than 5 feet deep | ||
| Texas | 24 hours | |||
| Illinois | 45 | 24 hours | ||
| U.S. Dept. Agriculture. | 40 | 24 hours | 25 per cent additional | |
| capacity for sludge | ||||
| North Carolina | Large Schools | 15 | Not less than 500 gallons | |
| North Carolina | 20 pupils | 25 | ||
| North Carolina | Medium School | 20 | ||
| North Carolina | Homes | 25 to 30 | ||
249. Imhoff Tanks.—In the discussion of septic tanks it has been brought out that one of the objections to their use is the unloading of sludge into the effluent which occasionally causes a greater amount of suspended matter in the effluent than in the influent. The Imhoff tank is a form of septic tank so arranged that this difficulty is overcome. It combines the advantages of the septic and sedimentation tanks and overcomes some of their disadvantages. An Imhoff tank is a device for the treatment of sewage, consisting of a tank divided into 3 compartments. The upper compartment is called the sedimentation chamber. In it the sedimentation of suspended solids causes them to drop through a slot in the bottom of the chamber to the lower compartment called the digestion chamber. In this chamber the solid matter is humified by an action similar to that in a plain septic tank. The generated gases escape from the digestion chamber to the surface through the third compartment called the transition or scum chamber. Sections of Imhoff tanks are shown in Fig. 161. It is essential to the construction of an Imhoff tank that the slot in the bottom of the sedimentation chamber does not permit the return of gases through the sedimentation chamber, and that there be no flow in the digestion chamber.
Fig. 161.—Typical Sections through Imhoff Tanks.
Eng. News, Vol. 75, p. 15.
The Imhoff tank was invented by Dr. Karl Imhoff, director of the Emscher Sewerage District in Germany. Its design is patented in the United States, the control of the patent being in the hands of the Pacific Flush Tank Co. of Chicago, which collects the royalties which are payable when construction work begins. The fee for a tank serving 100 persons is $10, for 1,000 persons is $80 and for 100,000 persons is $2550. The rate of the royalty reduces in proportion as the number of persons served increases.[[154]] As designed by Imhoff and used in Germany the tanks were of the radial flow type and quite deep. The depth, as explained by Imhoff, is one of the chief requirements for the successful operation of the tank. As adapted to American practice the tanks are generally of the longitudinal flow type and are not made so deep. An isometric view of a radial flow Imhoff tank is shown in Fig. 162. The sewage enters at the center of the tank near the surface and flows radially outward under the scum ring and over a weir placed near the circumference of the tank. One type of longitudinal flow tank is shown in isometric view in Fig. 163.
Fig. 162.—Sketch of Radial Flow Imhoff Tank at Baltimore, Maryland.
Eng. Record, Vol. 70, p. 5.
Fig. 163.—Isometric View of Longitudinal Flow Imhoff Tank at Cleburne, Texas.
Eng. News, Vol. 76, p. 1029.
250. Design of Imhoff Tanks.—The velocity of flow, period of retention, and the quantity of sewage to be treated determine the dimensions of the sedimentation chamber as in other forms of tanks. The velocity of flow should not exceed one foot per minute, with a period of retention of 2 to 3 hours. A greater velocity than one foot per minute results in less efficient sedimentation. A longer period of retention than the approximate limit set may result in a septic or stale effluent, and a shorter period may result in loss of efficiency of sedimentation. The bottom of the sedimentation chamber should slope not less than 1½ vertical to 1 horizontal, in order that deposited material will descend into the sludge digestion chamber. Provision should be made for cleaning these sloping surfaces by placing a walk on the top of the tank from which a squeegee can be handled to push down accumulated deposits. It is desirable to make the material of the sides and bottom of the sedimentation chamber as smooth as possible to assist in preventing the retention of sludge in the sedimentation chamber. Wood, glass, and concrete have been used. The latter is the more common and has been found to be satisfactory. The length of the sedimentation chamber is fixed by the velocity of flow and the period of retention. Tanks are seldom built over 100 feet in length, however, because of the resulting unevenness in the accumulation of sludge. Where longer flows are desired two or more tanks may be operated in series. The width of the chamber is fixed by considerations of economy and convenience. It should not be made so great as to permit cross currents. In general a narrow chamber is desirable. Satisfactory chambers have been constructed at depths between 5 and 15 feet. The depth of the sedimentation chamber and the depth of the digestion chamber each equal about one-half of the total depth of the tank. This should be made as deep as possible up to a limit of 30 to 35 feet, with due consideration of the difficulties of excavation. C. F. Mebus states:[[155]]