Part 4

The following table gives the principal dimensions with quantities of materials for four sizes of tank as illustrated in figure 25:

_Dimensions and quantities for septic tanks._

+--------+--------+-------------------------------------------- |Quantity| | Settling chamber. | of |Capacity| Number | sewage | below +-------+-------+-------+----+-------+---+--- of | in 24 | flow |Length.| Depth.|Width. | W. | X. | Y.| Z. persons.| hours. | line. | | | | | | | --------+--------+--------+-------+-------+-------+----+-------+---+--- | Galls. | Galls. |Ft. In.|Ft. In.|Ft. In.| In.|Ft. In.|In.|In. 5 | 180-280| 240 | 4 0 | 5 0 | 2 0 | 6 | 2 0 | 4 | 6 10 | 320-480| 420 | 5 0 | 5 6 | 2 6 | 6 | 2 3 | 4 | 6 15 | 520-680| 620 | 5 6 | 6 0 | 3 0 | 8 | 2 6 | 5 | 8 20 | 720-960| 860 | 6 0 | 6 6 | 3 6 | 8 | 2 9 | 5 | 8 --------+--------+--------+-------+-------+-------+----+-------+---+---

+----------+------------------------------------------------ | Quantity | Siphon chamber. Number |of sewage +-------+--------+--------+----+----+----+------- of | in 24 |Length.| Depth. | Width. | A. | B. | C. | D. persons.| hours. | | | | | | | --------+----------+-------+--------+--------+----+----+----+------- | Galls. |Ft. In.| Ft. In.| Ft. In.| In.| In.| In.| In. 5 | 180-280 | 5 0 | 2 8 | 2 0 | 3 | 4 | 15 | 18-1/4 10 | 320-480 | 8 0 | 2 8 | 2 6 | 3 | 4 | 15 | 20-1/4 15 | 520-680 | 8 8 | 2 10 | 3 0 | 4 | 4 | 17 | 20-1/4 20 | 720-960 |10 0 | 2 10 | 3 6 | 4 | 4 | 17 | 20-1/4 --------+----------+-------+--------+--------+----+----+----+-------

+---------+---------+--------+--------+---------+------------- |Quantity | | | | |Reinforcement | of | | | | | in top slab Number | sewage | | | | | (strip of of | in 24 |Concrete.| Cement.| Sand. | Stone. | heavy stock persons.| hours. | | | | | fencing). | | | | | +-------+------ | | | | | |Length.|Width. --------+---------+---------+--------+--------+---------+-------+------ | Galls. | Cu. yds.| Bbls. |Cu. yds.| Cu. yds.| Ft. | In. 5 | 180-280 | 3 | 4-1/2 | 1-1/3 | 2-2/3 | 10 | 3 10 | 320-480 | 4-1/4 | 6-1/4 | 2 | 3-3/4 | 14 | 3 15 | 520-680 | 6-2/3 | 9-3/4 | 3 | 6 | 15-2/3| 4 20 | 720-960 | 8 | 12 | 3-1/2 | 7 | 17-1/2| 5 --------+---------+---------+--------+--------+---------+-------+------

=Siphons.=--Reference has already been made to the vital importance of air in sewage filtration. If the spaces within a filter or soil are constantly filled with water, air is excluded, and the action of the filtering material is merely that of a mechanical strainer with its clogging tendency. The purpose of a siphon is twofold: (1) To secure intermittent discharge, thus allowing a considerable period of time for one dose to work off in the soil and for air to enter the soil spaces before another flush is received; (2) to secure distribution over a larger area and in a more even manner than where the sewage is allowed to dribble and produce the conditions of the old-fashioned sink drain--namely, a small area of water-logged ground.

Three types of sewage siphon are shown in figure 26. In all, the essential principle is the same: A column of air is entrapped between two columns of water; when the water in the chamber rises to a predetermined height, called the discharge line, the pressure forces out the confined air, destroying the balance and causing a rush of water through the siphon to the sewer. The entire operation is automatic and very simple. The siphons shown are commercial products made of cast iron; they have few parts and none that move, and the whole construction is simple and durable. The table (fig. 26) lists stock sizes adapted to farm use. Manufacturers furnish full information for setting their siphons and putting them in operation. For example, take type 2, figure 26: (1) Set siphon trap (=U=-shaped pipe) plumb, making E (height from floor to top of long leg) as specified; (2) fill siphon trap with water till it begins to run out at B; (3) place bell in position on top of long leg, and the siphon is ready for service. Do not fill vent pipe on side of bell.

[Transcriber Note: The data for Type 1 Siphon has been added to the table.

Type 2 INCHES +---+-------+-------+-------+-------+ Diameter of siphon | A | 3 | 3 | 4 | 4 | Diameter of outlet | B | 4 | 4 | 4 | 4 | Drawing depth | C | 13 | 15 | 14 | 17 | Depth to floor | D | 16¼ | 18¼ | 17¼ | 20¼ | Height above floor | E | 7¼ | 9¼ | 8¾ | 11¾ | Clearance under bell | F | 2 | 2 | 2 | 2 | Inside bottom of outlet, | | | | | | to discharge line | G | 20½ | 22½ | 22¾ | 25¾ | Discharge line, to top of wall | H | | | | | Depth of outlet sump | I | | | | | Length and width of outlet sump | J | | | | | +---+-------+-------+-------+-------+ | R | 4 | 4 | 4 | 4 | | S | 2 ft. | 2 ft. | 7 ft. | 8 ft. | Diameter of carrier (R), and +---+-------+-------+-------+-------+ minimum fall (S) in feet | R | 5 | 5 | 5 | 5 | per 100 feet | S | 1½ ft.| 1½ ft.| 2 ft.| 2½ ft.| +---+-------+-------+-------+-------+ | R | 6 | 6 | 6 | 6 | | S | 1 ft. | 1 ft. | 1 ft. | 1 ft. | +---+-------+-------+-------+-------+

Type 3 Type 1 INCHES INCHES +---+----+----+----+----+ +----+ Diameter of siphon | A | 3 | 3 | 4 | 4 | | 4 | Diameter of outlet. | B | | | | | | | Drawing depth | C | 13 | 15 | 14 | 17 | | 5 | Depth to floor. | D | 17 | 17 | 19 | 22 | | 15 | Height above floor | E | 13 | 13 | 15 | 18 | | 19 | Clearance under bell. | F | | | | | | | Inside bottom of outlet, | | | | | | | | to discharge line. | G | 19 | 21 | 20 | 25 | | | Discharge line, to top of wall. | H | 6 | 6 | 8 | 8 | | | Depth of outlet sump | I | 13 | 13 | 11 | 12 | | | Length and width of outlet sump | J | 18 | 19 | 19 | 18 | | | +---+----+----+----+----+ +----+ ]

The overhead siphon, type 3, figure 26, may be installed readily in a tank already built by addition of an outlet sump. If properly set and handled, sewage siphons require very little attention and flush with certainty. Like all plumbing fixtures they are liable to stoppage if rags, newspaper, and similar solids get into the sewage. If fouling of the sniffing hole or vent prevents the entrance of sufficient air into the bell to lock the siphon properly, allowing sewage to dribble through, the remedy is to clean the siphon. Siphons are for handling liquid; sludge if allowed to accumulate will choke them.

=Submerged outlet.=--The purpose of a submerged outlet is to take the outflow from a point between the sludge at the bottom and the floating solids or scum. The outlet in figure 25 may be readily made of sheet metal by a tinsmith. Wrought iron or steel pipe with elbows or light lead pipe may be used, the pipe being set in the concrete and left in place. Sometimes a galvanized wire screen (1/4-inch mesh) is fitted over the inner end to prevent large solids leaving the settling chamber and possibly clogging the siphon or distribution tile. If a screen is used it should be easily removable for cleaning.

=Manhole frame and cover.=--The frame and cover shown in figure 25 are stock patterns made of cast iron and weighing about 250 pounds per set. The cover is 21 inches in diameter; it is tight and, on account of its weight, is unlikely to be disturbed by small children. The frame or rim is about 7 inches high and 31 inches in longest diameter. If desired, light cast-iron cistern or cesspool covers obtainable from plumbing supply houses, home-made slabs of reinforced concrete (see fig. 27), or wooden covers (see fig. 23) may be used.

=Overflow.=--The purpose of an overflow is to pass sewage to the distribution field should the siphon stop working. The overflow (fig. 25) is a 3-inch riser pipe with top 3 inches above the discharge line and the bottom calked or cemented into the side outlet of a =T=-branch. The run of the =T=-branch should correspond with the size of the sewer from the tank to the distribution field. If this sewer is 4-inch pipe, a 4 by 3 inch =T=-branch is used, the 4-inch spigot end of the siphon being calked or cemented into the branch, as shown in figure 25; if the sewer is 5-inch, a 5 by 3 inch =T=-branch is used and connected to the siphon with a 5-inch to 4-inch reducer (in vitrified specials the equivalent is a 4-inch to 5-inch increaser); if the sewer is 6-inch, a 6 by 3 inch =T=-branch is used and connected to the siphon with a 6-inch to 4-inch reducer.

=Concrete work.=--Before excavation for the tank is begun, two wooden forms should be built for shaping the inside of the settling and siphon chambers. In most instances the ground is fairly firm, so that the lines of excavation may conform to the outside dimensions of the tank, the back of the walls being built against the earth. The forms may be made of square-edged boards, braced and lightly nailed, as shown in figure 28. The forms should have no bottom. If it is desired to lay the sides and covering slab in one operation, the top of the forms must be boarded over. All pipe and manhole openings should be accurately placed and cut. The faces of the forms may be covered with paper or smeared with soap or grease to facilitate removal later.

The ground should next be excavated to the proper depth for placing the floors in both chambers. The settling chamber floor, being the lower, should be placed first. Effort should be made to secure water-tight work, a feature of especial importance where leakage might endanger a well or spring. A concrete mixture of 1:2:4 is generally preferred (1 volume cement, 2 volumes sand, 4 volumes stone). The ingredients should be of best quality and thoroughly mixed. The concrete should be poured promptly and worked with a spade or flat shovel to make the face smooth and eliminate pockets or voids within the mass.[10] Before the settling chamber floor has hardened the form should be set upon the floor and the concrete work continued up the sides. The pipe form for the submerged outlet should be set. When the side walls of the settling chamber have reached the bottom of the excavation for the siphon chamber, the siphon trap with its connecting branch and short piece of pipe should be set to proper line and grade and blocked in position. The floor of the siphon chamber should now be poured and the form for that chamber placed thereon, leaving a 6-inch or 8-inch space (according to the thickness of the division wall) between the ends of the two forms. Pouring of all side walls and the top slab should continue without stop, making the entire structure a monolith.

[10] See footnote, p. 17. For more detailed information on form and concrete work the reader is referred to U. S. Department of Agriculture Farmers' Bulletin No. 481, "Concrete Construction on the Live-Stock Farm."

=Steel reinforcement.=--To stiffen the cover slab and guard against cracking, a little steel should be embedded in the concrete about 1 inch above the inside top. For this purpose a strip of heavy stock fencing is convenient and inexpensive. The line wires should be not less than No. 10 gauge (about 1/8 inch) and the stay wires not less than No. 11 gauge. The reinforcement should be cut at manholes and fastened around manhole openings. If desired a standard wire-mesh reinforcement weighing about one-third of a pound per square foot may be used. Another alternative is to use 1/4-inch round rods, spacing the crosswise rods 6 inches apart and the lengthwise rods 12 inches apart. Poultry netting should not be used, because of its lightness.

=Sewer from tank to distribution field.=--The length of this sewer depends on the situation of the field and the fall to it. The size of the sewer depends on the fall that can be obtained and the size of siphon. The table in figure 26 shows the minimum fall at which 4-inch, 5-inch, and 6-inch sewers should be laid to take the discharge of the 3-inch and 4-inch siphons specified. The line and grade should be set in the same manner as for the house sewer (see fig. 20), and the construction should be as specified under that caption.

=Distribution field.=--The distribution field or area is a sewage filter, and its selection and the manner of preparing it largely determine the success of subsoil disposal of sewage. As a rule farm land is not the best filtering material. It is too fine grained and fertile. Its tendency is to hold water too long, to admit insufficient air, to clog when even small quantities of sewage are applied. Hence the distribution area should be of liberal size--on the average 500 square feet for each person served. It should be dry, porous, and well drained--qualities that characterize sandy, gravelly, and light loam soils. It should be devoid of trees and shrubbery, thus giving sunlight and air free access. It should be located at least 300 feet downhill from a well or spring used for domestic water supply. Preferably it should slope gently, but sharp slopes are not prohibitive. Subsoiling the area is always desirable.

Clay and other compact, impervious soils require special treatment. Less sewage can be applied to them, and hence it is well to have the area larger than 500 square feet per person. Clay should be subsoiled as deep as possible with a subsoil plow. In some instances dynamite has been of service in opening up the ground to still greater depth. Drainage and aeration should be further promoted by laying tile underdrains, as outlined in figure 19 and shown in more detail in figure 31.

After the construction work the distribution area should be raked and seeded with thick-growing grass. Grass is a safe crop; its water requirement is high, and it affords considerable protection from frost. Suitable grasses are redtop, white clover, blue grass, and Bermuda grass. The area may be pastured or kept as grass land.

=Distribution system.=--Poor distribution of the sewage and failure to protect the joints of the distribution tile account for most of the failures. Each flush of the siphon should be so controlled that every part of the field will receive its due proportion. The distribution tile must be so laid that loose dirt will not fall or wash into the open joints.

Different methods of dividing the flush and laying out the distribution tile are shown in figures 29 and 32. Layouts 1, 2, and 3, figure 29, are suitable for flat or gently sloping areas and are planned for the shallow siphon chambers tabulated on page 37. Layout 4, figure 29, is suitable for steep slopes. In all four layouts use is made of one or more =V=-branches (not =Y=-branches) to divide the flow equally among the several lines. =V=-branches, sometimes called breeches, should be leveled with a carpenter's level crosswise the ends of the legs, thus insuring equal division of the flow.

The size and length of distribution tile and the spacing of the lines or runs admit of considerable variation in different soils. Water sinks rapidly in gravels and sands, and hence larger tile and shorter length are permissible than in close soils. Lateral movement is slow in all soils, but extends farther in gravels and sands than in close soils. In average soils the effect on vegetation 5 feet away from the line is practically nil.

From these considerations, with the siphon dose 20 gallons per person, it is usually a safe rule to provide 50 feet of 3-inch tile for each person served and to lay the lines 10 feet apart. Such provision gives a capacity within the bore of the tile lines about equal to the siphon dose, and as some sewage is wasted at each joint a reasonable factor of safety is provided. A spacing of 10 feet will, it is believed, permanently prevent the extension of lateral absorption from line to line, provided the area is fairly well drained. As between 3-inch and 4-inch tile the smaller size costs less and is better calculated to taper the dose to small proportions. Four-inch tile is less likely to get out of alignment or to become clogged; a length of 28 feet has the same capacity in the bore as 50 feet of 3-inch.

Good-quality drain tile in 1-foot lengths or second-quality sewer pipe in 2-foot lengths may be used. The lines are generally laid in parallel runs, but may be varied according to the topography. Layouts 1, 2, and 3, figure 29, for flat or gently sloping land, run with the slope; layout 4, for steep slopes, runs back and forth along the contour in a series of long flat sweeps and short steep curves. The grade of the runs and sweeps should be gentle, rarely more than 10 or 12 inches in 100 feet. In layouts 1, 2, and 3, figure 29 especially, it is desirable that the last 20 feet of each run should be laid level or given a slight upward slope, thus guarding against undue flow of sewage to the lowest ends of the system.

Fig. 29.--Methods of laying distribution system: Methods 1, 2, and 3 for flat or gently sloping land; method 4 for steep slopes (see also fig. 32); _A_, direction of slope; _B_, contour of field; _C_, sewer from tank, preferably size 5 inch, though 4 or 6 inch may be used, depending on the fall and the size of the siphon (see table, fig. 26); _D_, =V=-branch set to divide the flow exactly; _E_, reducer, to 4 inches; F, 1/8 bend, 4-inch; _G_, increaser, from 4 inches; _H_, increaser, 3 to 4 inches; _I_, reducer, 4 to 3 inches; _J_, distribution tile, 3-inch; _K_, distribution tile, 4-inch.

The runs should be laid no deeper than necessary to give clearance when plowing and prevent injury from frost. Ten inches of earth above the top of the tile is sufficient generally throughout the southern half of the United States and 18 inches generally in the North, but if the field is exposed or lacks a thick heavy growth of grass the cover should be increased to 2-1/2 or 3 feet near the Canadian line. What is better, the tile in all instances may be laid with a 10-inch cover and in cold weather the runs may be covered with hay, straw, or leaves weighted down, which may be removed in the spring.