Septic Tank Design

In septic tank design, environmental engineers must consider the treatment following the septic tank as a part of the septic tank system (U.S. Department of Health, Education, and Welfare 1967). A two-compartment design arranged in a series is preferred (see Figure 7.20.1). The first chamber should contain two-thirds and the second chamber should contain one-third of the total volume. The liquid depth should be between 4 and 7 ft.

FIG. 7.20.1 Septic tank configurations. A. Typical household septic tank; B. Typical large institutional septic tank with dosing siphon. For large fields, uniform distribution is obtained by periodic flooding of the field followed by periodic drying. Dosing tanks are used to flood these fields; they collect the sewage, and automatic bell siphons or pumps transport the waste to the field.

FIG. 7.20.1 Septic tank configurations. A. Typical household septic tank; B. Typical large institutional septic tank with dosing siphon. For large fields, uniform distribution is obtained by periodic flooding of the field followed by periodic drying. Dosing tanks are used to flood these fields; they collect the sewage, and automatic bell siphons or pumps transport the waste to the field.

FIG. 7.20.2 Relationship between allowable sewage application rate and soil percolation rates for soil absorption trenches or seepage pits. (Reprinted from U.S. Department of Health, Education, and Welfare, 1967, Manual of septic tank practice, Public Health Service Publication no. 526, Washington, D.C.)

FIG. 7.20.2 Relationship between allowable sewage application rate and soil percolation rates for soil absorption trenches or seepage pits. (Reprinted from U.S. Department of Health, Education, and Welfare, 1967, Manual of septic tank practice, Public Health Service Publication no. 526, Washington, D.C.)

FIG. 7.20.3 Septic tank absorption field. Trench surface area required: If the water in the test hole takes 1 min to recede 1 in, 70 ft2 per bedroom is needed. If the water in the test hole takes 30 or 60 min, 250 or 330 ft2 per bedroom is required.

The minimum effective tank capacity should be as follows: for flows up to 1500 gpd, 1As times the daily sewage flow; for flows in excess of 1500 gpd, the volume V in gallons can be calculated from the following equation:

where:

Q = The daily sewage flow

Environmental engineers must check soil porosity and base the design of the soil absorption field on the rate of percolation. This rate is the number of minutes required for the effluent to recede 1 in in a test hole that has been

FIG. 7.20.4 Imhoff tank configuration.

bored, filled with water, and allowed to swell the day previous to the test (see Figure 7.20.2). Figure 7.20.3 provides information on absorption area requirements.

If the septic tank must handle wastewater from garbage grinders and automatic washing machines, the soil absorption area (see Figure 7.20.3) should be increased by 60%, and the tank volume should be increased (see Part A in Figure 7.20.1) by 25%. If the soil absorption trench required exceeds 500 ft2 or the septic tank is larger than 1500 gal, a dosing tank is needed.

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