Skeptic Tank Systems: ABSORPTION SYSTEMS

Subsoil Absorption System

Sizing the System

The basic question to be answered before a subsoil absorption system can be designed is: What is the permeability of the soil? This question is answered by performing the percolation test described earlier.

The next question to be answered is: For the given rate of seepage, how many square feet of dispersal area are required? Table 2 gives the answer to this question in terms of the number of bedrooms in the dwelling. If, for example, the percolation rate is a 1-in, drop in the water level in 10 minutes, and if the dwelling will have four bedrooms, then Table 2 shows that the number of square feet of dispersal area required is either

100 sq ft X 4 = 400 sq ft, or

120X4=480 or

135X4=540 or

165X4 = 660

depending on whether or not a garbage disposal unit or an automatic clothes washer, or both, are to be installed in the dwelling. Most health codes assume that if a house doesn’t have these appliances when the septic tank system is being built, it will later. They thus throw out the first three columns of Table 2 altogether and make the figures in column 4 the basis for sizing the absorption system.

Table 2. Determination of Size of Subsoil Absorption Systems under Various Conditions

Table 3. Determination of Size and Spacing of Subsoil Absorption Trenches


A trench 18 to 24 in. wide at the bottom is dug to the required depth (which should, of course, be as shallow as practicable), and a 6-in.-deep layer of gravel, slag, clinkers, crushed stone, or a similar material, from 1/2 to 2 1/2 in. in size, is laid down at the bottom of this trench (see Fig. 7). The drain tiles are then laid on top of this bed. The drain tiles can also be laid directly in the soil, but only when the soil has an exceptionally granular, porous structure through which the effluent can drain freely. Most soils do not have this extreme permeability and require a coarse gravel bed that will enable the effluent to make its way into the soil with greater efficiency.

Fig. 7. Construction of a subsoil absorption system in either permeable or semi-permeable soil.

In the example in which we determined the size of the dispersal area required for a 4-bedroom house, we found that 660 sq ft were required. If the width of the trenches is 18 in., then only 440 linear ft of drain tiles need be laid down since 660 sq ft — 18 in. = 440 ft. If the width of the trenches is 24 in., then only 330 linear ft of drain tiles are required since 660 sq ft 24 in. = 330 ft.

No single line of tiles should, however, be longer than 100 ft for the simple reason that it is very unlikely that the effluent will ever travel farther than 100 ft in any absorption system, no matter how level it is. In our example, therefore, if we assume the width of the trenches is 24 in., at least four separate lines of tiles, each at least 821k ft long, will be necessary in order to dispose of the effluent.

If these lines of tiles are laid out parallel to each other, they must be spaced a certain distance apart to ensure that each will remain capable of draining freely and without interference from the other lines of tiles. Table 3 shows, for example, that for a 24-in.-wide trench, adjacent trenches must be at least 6 ft apart.

The actual layout of the system will depend on how much land the homeowner has available and on whether the land is flat or sloped. On flat land, the subsoil absorption system can be laid out in a variety of ways—in parallel lines, in a closed circle or rectangle, in a V shape, or whatever is most convenient or suitable.

When a subsoil absorption system is being installed on flat land, perhaps the single most important point to watch is that the drain tiles are laid as level as possible. If the lines should be laid at a slope, this slope should be very slight, not exceeding (depending on the local code) 1 in. in 24 ft, 1 in. in 36 ft, or 1 in. in 50 ft.

When the land slopes, the levelness of the installation is attained by having the lines of drain tiles follow the contours of the land. Or a cascading system of drainage tile can be installed in which the effluent is led from one level to another through sewer lines. The same requirements regarding the width and depth of the trenches must be met when this type of system is installed.

Distribution Box

All absorption systems having two or more branch lines must have a distribution box installed in the sewer line, this box being located at the point where the individual branch lines originate (see Fig. 1). The distribution box is usually made of concrete.

The sewer line carrying the effluent from the septic tank discharges the effluent into the box, about 6 in. from its bottom. The outlet lines are usually about 2 in. lower than the sewer-line inlet. The outlet lines must be exactly level with each other to make sure that each line carries its fair share of effluent to the drainage tiles.

Each branch line can be blocked off by a stop board or gate, the position of which is changed manually. In a 2-outlet box, the homeowner can block off the flow from any given line completely, or in a 4-outlet box, he can control to some degree the proportion of effluent that enters each line. He can let an overworked section of the absorption system ‘ for a while, or he can balance the flow of effluent between the sections of the absorption system. In addition, if any section of the system should ever need to be repaired, the flow to that part of the system can be blocked off until the repairs have been completed.

Sizing a Seepage Pit

Seepage Pit System

Table 4 shows the total surface area, in square feet, required for a seepage pit, given different percolation rates and the installation of different household appliances. If, for example, the percolation rate for a particular soil is a 1-in, drop in the water level in 10 mm, and the dwelling will have a garbage disposal unit and a clothes washer, then, according to Table 4, the required surface area of the seepage pit will be 95 sq ft.

Table 4. Surface Area Required for Seepage Pits for Different Percolation Rates and Wastewater Flows

Local health codes usually require that a seepage pit be at least 4 to 6 ft in diam. For a seepage pit 6 ft in diam., the surface area per foot of depth is 18.85 sq ft. Therefore, 95 sq ft ÷ 18.85 sq ft = 5.04 ft will be the required depth of the seepage pit.

If the percolation rate should be 1 in. in 30 mm, a seepage pit 10.08 ft deep would be required. This, of course, is impossible.

What is done in a case like this is to divide the total required depth into two or three interconnected seepage pits. If, for example, two seepage pits are decided upon, each pit need be only about 5 ft deep (that is, about 5 ft below the level of the incoming sewage line).

These calculations assume that the seepage rate will be the same throughout the entire depth of the soil, that is, that the soil will have a homogenous structure. This may not always be the case. Very often one finds that the deeper one digs the less permeable the soil becomes, or layers of soil having different permeabilities may be present. When this is the case, then an average percolation rate must be calculated for the soil.

Sizing a Sand Filter

Sand Filter System

A sand filter system (Fig. 4) requires the same size dispersal area as a subsoil absorption system, if the percolation rate and dwelling size are the same. The only difference between the two is that a sand filter can occupy a much more compact area. If we assume, for example, that 660 sq ft of dispersal area is required for a dwelling (as calculated in our example above), a sand filter can be installed in an area approximately 24 by 28 ft in size, if the distribution tiles are laid parallel to each other and 6 ft apart. In comparison, the 82.5-ft-long drain lines of the subsoil absorption system that has been described above require about 1485 sq ft of area, or an area about 82.5 by 18 ft in size.


There are two basic kinds of sand filter system—an open system and a closed system (see Fig. 8). A closed system is constructed entirely underground.

In an open system, the sand filter is not covered with topsoil nor are the distribution tiles buried in a layer of gravel. Instead, the distribution drain tiles rest on planks laid on top of the sand, which is in turn exposed to the air and the weather. An open system can become rather smelly and should only be considered if it can be installed somewhere by itself, far from any houses or public thoroughfares. An open system has the advantage, of course, that is somewhat cheaper to construct than a closed system.


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