| TABLE 83 | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Analysis of Tank Sludges | ||||||||||
| Place | Specific Gravity | Per Cent Moisture | Per Cent in Terms of Dry Matter | Cubic Yard per Million Gallons, Wet | Pounds per Million Gallons, Dry | Kind of Sludge | Reference | |||
| Volatile | Fixed | Nitrogen | Fat | |||||||
| Mansfield, O. | 1.11 | 80.8 | Septic | 1908 Report, State Board of Health | ||||||
| Chicago, Ill. | 1.03 | 90 | 40 | 60 | 1.9 | 7.0 | 1.0 | 200 | Septic | |
| 1.5 | 300 | |||||||||
| Columbus, O. | 1.09 | 83.3 | 4.4 | 16.7 | 0.25 | 0.94 | Septic | G. A. Johnson 1905 Report | ||
| Atlanta, Ga. | 1.02 | 87.1 | 39.1 | 60.9 | 1.25 | 6.11 | Imhoff | Eng. Rec., V. 72, 1915, p. 4 | ||
| Baltimore, Md. | 1.02 | 91.9 | 66.2 | 2.45 | 4.02 | Digestion Tank | Eng. News-Rec., V. 87, 1921, p. 98 | |||
| Baltimore, Md. | 1.02 | 92.4 | 62.7 | 2.75 | Imhoff | do. | ||||
| Baltimore, Md. | 79.2 | 73.8 | 2.64 | 9.00 | Raw Sludge | do. | ||||
| Baltimore, Md. | 92.4 | 58.0 | 3.19 | Settling Basin | do. | |||||
Two or more tanks should be constructed to allow for the shut down of one for cleaning and to increase the elasticity of the plant. The number of tanks to be used is dependent on the total quantity of sewage and the fluctuations in rate of flow. An average period of retention of about 9 to 10 hours with a minimum period of 6 hours during maximum flow is a fair average to be assumed for design. The period of retention should not exceed about 24 hours, as the sewage may become over-septicized. The sludge storage period should be from 6 to 12 months.
A cover is not necessary to the successful operation of a septic tank. Covers are sometimes used with success, however, in reducing the dissemination of odors from the tank. They are also useful in retaining the heat of the sewage in cold weather and thus aid in promoting bacterial activity. Types of covers vary from a building erected over the tank to a flat slab set close to the surface of the sewage. In the design of a cover, good ventilation should be provided to permit the escape of the gases, and easy access should be provided for cleaning. Tightly covered tanks or tanks with too little ventilation have resulted in serious explosions, as at Saratoga Springs in 1906 and at Florenceville, N. C., in 1915.[[152]]
The sludge may be removed through drains in the bottom of the tank as described for sedimentation basins, or where such drains are not feasible the sludge and sewage are pumped out. For this purpose a pump may be installed permanently at the tank, or for small tanks portable pumps are sometimes used. Septic tanks should be cleaned as infrequently as possible without permitting the overflow of sludge into the effluent. The less frequent the cleaning the less the amount of sludge removed since digestion is continuous throughout the sludge. It is necessary to clean when the tank becomes so filled with sludge, that the period of retention is materially reduced, or sludge is being carried over into the effluent.
The details of the septic tank at Champaign, Illinois, are shown in Fig. 159. This tank was designed by Prof. A. N. Talbot, and was put in service on Nov. 1, 1897. It was among the first of such tanks to be installed in the United States. The tank shown in Fig. 159 is an example of present day practice in single-story septic tank design.
Fig. 159.—Septic Tank at Champaign, Illinois.
Fig. 160.—Design for a Residential Septic Tank for a Family of Ten. Illinois State Board of Health.
Small septic tanks for rural homes of 5 to 15 persons, or on a slightly larger scale for country schools and small institutions, are little more than glorified cesspools. Nevertheless much attention has been given to the construction of such tanks by the National Government and by state boards of health.[[153]] The recommendations of some of these boards have been compiled in Table 84. A typical method for the construction of such tanks, as recommended by the Illinois State Board of Health, is shown in Fig. 160. A subsurface filter, into which the effluent is discharged, is an important adjunct where no adequate stream is available to receive the discharge from the tank.