Part 4

A convenient method of assuring correct location of the tank is to build a frame as shown in figure 6. Care is necessary to aline it with the center line of the inlet and outlet and to level it so that the distance from the bottom of the 2 by 4's on the form to the lower edge of the inlet hole in the form will permit it to be set at the grade of the house sewer. This frame is used to support the form for the tank. To avoid caving the edges, drive the stakes supporting the frame before beginning the excavation. The lumber in the frame can be used later to make part of the tank baffles.

Figure 7 shows how an inside form can be built and hung in place. The inlet and outlet tees should be carefully set and tied in place before the concrete is poured. A single length of pipe should be joined to the tee, so that the two can be set in the form as one unit. In most cases the earth walls of the excavations will serve as the outside forms unless the soil is sandy or gravelly and the excavation is deeper than 5 feet. If outside forms are used, space must also be provided for them. Forms should be constructed before the excavation is made and the tank built as soon as practical, to avoid warping of forms and caving of earth walls.

County agricultural agents, local health departments, building-material dealers, and other agencies often have forms that may be borrowed or rented.

THE EFFLUENT SEWER

The effluent sewer should be constructed in similar manner and of the same materials as the house sewer and on a slope of 1/8 inch to 1 foot. This line, however, may be laid of terra-cotta pipe, as cast-iron is not considered necessary except in unusual cases. This line should always terminate in a distribution box from which the tile lines of the disposal field lead away. For steep slopes the arrangement shown in figure 9 (p. 15) is practical. Joints must be of root-tight construction if the sewer is in the vicinity of trees or shrubs. The length of the sewer depends upon the distance from the tank to a safe site for the disposal field.

THE DISPOSAL FIELD

Correct installation of the disposal field is of great importance for proper functioning of the septic tank. Therefore, the width, depth, and spacing of the tile trenches must be carefully selected. Line of 4-inch, open- jointed, agricultural drain tile laid in shallow trenches are ordinarily used. Perforated fiber drain pipes also may be used and are obtainable in 4-foot lengths.

A distribution box with an inlet for the effluent sewer and an outlet for each individual run of disposal tile is the best means of dividing the flow. The outlet serving a large or double disposal field may be alternately opened and closed by means of a sewage switch that permits half the disposal field to work and rest alternately several weeks. A switch is especially helpful in tight soils but should not be provided unless proper maintenance is assured, so that a portion of the disposal field will not be left to handle the entire load of the system for an indefinite period. There are many variations of boxes, but figure 8 shows a practical type.

Shallow Tile Lines

The disposal tile should not be more than 18 to 24 inches below the surface, and where the ground-water level rises to the bottom of the trench special underdrains, described on page 16, are necessary. Special provisions must also be made where tight soils are encountered. These methods are described in the section entitled "Disposal methods in tight or wet soils."

The table in figure 9, together with the information given in table 1, below, may be used for estimating the number of tiles needed in any particular soil type. If there is any doubt about this requirement, a percolation test should be made in the disposal field, as follows:

Dig a hole 1-foot square and to the depth at which the tile is to be laid. This depth in most instances will be about 24 inches and should not exceed 36 inches. Fill the hole with water to a depth of 6 inches and observe the time required for the water to seep away; divide by 6 to get the average time for the water to fall 1 inch. The test should be repeated at three or four different points in the disposal field and the average time noted for all tests used. The data in table 1 can then be used to determine the number of tiles needed. Where 1 hour is required for the water to fall 1 inch the soil is totally unsuitable, and another site should be selected. Soil conditions at the time of the test may vary from year-round average conditions, and this factor must be taken into account. If the soil appears exceptionally dry, greater depths of water may be used or the test repeated in the same hole. In no case should tests be made in filled or frozen ground. Where fissured rock formations are encountered, advice should be sought from sanitation specialists.

Table 1.--Determining tile-disposal field requirements from percolation tests

------------+----------------------++------------+--------------------- Minutes | Effective absorption|| Minutes | Effective absorption required | area required, per || required | area required, per for water to| person, in bottom ||for water to| person, in bottom fall 1 inch | of disposal trenches ||fall 1 inch | of disposal trenches ------------+----------------------++------------+--------------------- | Square feet || | Square feet | || | 2 or less | 26 || 10 | 52 | 30 || 15 | 63 | 36 || 30 | 90 | 40 || 60 | 120 ------------+----------------------++------------+---------------------

A minimum of 150 square feet should be provided, equal to 100 feet of 18-inch trench.

If more than 60 minutes, use special design with seepage pits or sand-filter trenches.

Figure 9 suggests methods of arranging the tiles in disposal fields under varying conditions and the length of tiles needed.

SIZE AND MINIMUM SPACING REQUIREMENTS FOR DISPOSAL TRENCHES +---------+----------+----------------------+-------------+ | TRENCH | TRENCH | EFFECTIVE ABSORPTION | TILE LINES | | WIDTH-W | DEPTH-D | AREA IN SQUARE FEET | SPACING-S | |IN INCHES| IN INCHES| PER LINEAL FOOT | IN FEET | +---------+----------+----------------------+-------------+ | 18 | 18 to 30 | 1.5 | 6.0 | | 24 | 18 to 30 | 2.0 | 6.0 | | 30 | 18 to 36 | 2.5 | 7.5 | | 36 | 24 to 36 | 3.0 | 9.0 | +---------+----------+----------------------+-------------+ Wider spacing of the lines desirable where available area permits

DISPOSAL-TILE TRENCH

Disposal-tile lines--Maximum length for each line 100 feet. All lines to be equal in length.

Disposal-tile lines to slope 2" to 4" per 100 feet, not over 6". Sewer-tile lines to slope 1/8" to 1/4" per foot.

DISPOSAL METHODS IN TIGHT OR WET SOILS

If the soil is heavy clay or has tight formation, yet shows some porosity from percolation tests, the efficiency of the field may be increased by placing below the tile lines 12 to 15 inches of additional filter material (washed gravel, crushed stone, slag, clean cinders, or clean bank-run gravel 3/4 to 2-1/2 inches in size). When the surface soil is tight and is underlain by porous soil, sufficient drainage is sometimes obtained for the smaller installations by omitting the tile field and providing a dry well at the end of the effluent sewer, provided the water table will not be contaminated. Larger systems under such soil conditions should have a tile field, and absorption can be increased by boring 6- or 8-inch holes down to the porous stratum and filling them with gravel or sand; the holes should be 4 to 6 feet apart. Another and perhaps the best practice is to excavate the tile trenches 4 to 6 feet and install a lower tile line, as shown in figure 10. This latter method is especially desirable if the upper tight stratum is especially thick, or if there is no porous lower stratum, or if in irrigated regions and where the disposal field is limited in area.

Where the underdrain tile is not used, the absorption capacity of the field can be increased by providing a rock-filled trench across the lower end of the tiles for the full width of the field. The depth should be not less than 5 feet and the width not less than 3 feet.

On account of the beneficial action of bacteria in the upper soil layers it is highly desirable to confine the effluent near the surface rather than to use underdrains. Purification becomes slower and less effective, the deeper the drains.

In situations where the soil contains considerable moisture or is even saturated, the field may be improved by partially encircling it with a tile line laid to serve as a drain. Such a line should be on the high side and have surface outlets for removing the water from the soil. It should not be laid so close to a disposal tile line that it will drain the sewage effluent from the disposal field onto the surface of the ground.

Plug upper end of underdrain tile lines, lower end to discharge into rock-filled seepage pit or into other approved outlet.

Figure 10.--Filter trench with underdrains.]