Part 7

=The tank.=--The septic tank should be in an isolated location at least 50 to 100 feet from any dwelling. This is not always possible, because of flat ground, but in many such instances reasonable distance and fall may be secured by raising both the house sewer and tank and embanking them with earth. Cases are known where tanks adjoin cellar or basement walls and the top of the tank is used as a doorstep; in other cases tanks have been constructed within buildings. Such practices are bad. It is difficulty to construct an absolutely water-tight masonry tank, and still more difficult to make it proof against the passage of sewage odors.

In Northern States, particularly in exposed situations, it is desirable to have the top of the tank 1 to 2 feet underground, thus promoting warmth and uniformity of temperature in the sewage. In Southern States this feature is less important, and the top of the tank may be flush with the ground. Every tank should be tightly covered, for the reason above stated and to guard against the spread of odors, the transmission of disease germs by flies, and accidents to children.

Considerable latitude is allowable in the design and construction of septic tanks. No particular shape or exact dimensions can be presented for a given number of people. One family of 5 persons may use as much water as another family of 10 persons; hence the quantity of sewage rather than the number of persons is the better basis of design. Exact dimensions are not requisite, for settlement and septicization proceed whether the sewage is held a few hours more or a few hours less. As to materials of construction some form of masonry, either brick, building tile, rubble, concrete, or cement block, is employed generally. Vitrified pipe, steel, and wood have been used occasionally.

A plant for use all year round should have two chambers, one to secure settlement and septicization of the solids and the other to secure periodic discharge of the effluent by the use of an automatic sewage siphon. The first chamber is known as the settling chamber, the second as the siphon or dosing chamber. The siphon chamber is often omitted and the effluent is allowed to dribble away through subsurface tile, as illustrated in figures 17 and 18. The latter procedure is not generally advised, but may be permissible where the land slopes sharply or has long periods of rest, as at summer houses and camps.

The septic tanks shown in this bulletin are designed to satisfy the following conditions:

1. Water consumption of 40 gallons per person per day of 24 hours.

2. A detention period of about 24 hours; that is, the capacity of the settling chamber below the flow line is approximately equal to the quantity of sewage discharged from the house in 24 hours.

3. Where a siphon chamber is provided, its size is such that the dose of sewage shall be approximately equal to 20 gallons per person; that is, the capacity of the siphon chamber between the discharge and low-water lines is roughly equal to the quantity of sewage discharged in 12 hours.

A simple one-chamber brick tank suitable for a household discharging 180 to 280 gallons of sewage daily is shown in figure 23. A small two-chamber tank constructed of 24-inch vitrified pipe, suitable for a household discharging about 125 gallons of sewage daily, is shown in figure 24. A typical two-chamber concrete tank is shown in figure 25. Excepting the submerged outlet, all pipes within the tank and built into the masonry are cast-iron soil pipe with cast-iron fittings. Vitrified or concrete sewer pipe and specials are generally used as they are frequently more readily obtainable and a slight saving in first cost may be effected. Cast iron is less liable to be broken in handling or after being set rigidly in masonry, and the joints are more easily made water-tight. The submerged outlet is midway of the depth of liquid in the settling chamber. The inside depth of the siphon chamber is the drawing depth of the siphon plus 1 foot 5 inches.

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.

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. 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.

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."

1. Make the forms as shown and to the dimensions required by fig. 25 and the table on p. 37; nails to be driven from the inside and left projecting for drawing with a claw hammer.

2. Excavate to lines 6 or 8 inches, as may be required, outside of the forms and to the depths required for both chambers.

3. Pour settling chamber floor and place form thereon.