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, homemade slabs of reinforced concrete (see fig. 25), or wooden
covers (see fig. 21) may be used.
[Illustration: Fig. 25.--Homemade reinforced concrete covers. (1) Slabs
placed crosswise permit uncovering the whole tank for cleaning, but as
inspection is somewhat difficult, cleaning is the more likely to be
neglected; (2) manhole, 18 inches square; cover, 22 by 22 by 3 inches
thick, easy to make and to slide or lift from the opening]
=Overflow.=--The purpose of an overflow is to pass sewage to the
distribution field should the siphon stop working. The overflow (fig. 23)
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 23; 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
26. 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.
[Illustration: Fig. 26.--Forms for concrete work--how to use them]
1 Make the forms as shown and to the dimensions required by Figure 23
and the table on p. 29; 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.
4 Pour settling chamber walls to level of siphon chamber excavation,
inserting submerged outlet pipe at the proper height. 5. Block siphon and
short pipes to correct line and grade, and fill with concrete around the
trap.
6. Pour siphon chamber floor, and place the form thereon.
7 Continue pouring all walls to their full height, inserting the inlet
pipe when the concrete reaches that elevation.
8. Do not remove forms till the concrete is hard; with favorable weather,
forms for walls only may be removed in 1 to 2 days; forms supporting a
cover slab should remain 1 to 2 weeks.
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. 12. For more detailed information on form and
concrete work the reader is referred to U. S. Department of Agriculture
Farmers' Bulletin 1480-F, "Small Concrete Construction on the 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 14-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 24 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. 18) 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 17 and shown in more detail in Figure 29.
After the construction work the distribution areas 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 27 and 30. Layouts 1, 2, and 3, Figure 27, are
suitable for flat or gently sloping areas and are planned for the shallow
siphon chambers tabulated on page 29. Layout 4, Figure 27, 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.
[Illustration: Fig. 27.--Methods of laying distribution system: Methods
1, 2, and 3 for flat or gently sloping land; method 4 for steep slopes
(see also fig. 30); _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. 24);
_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]
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 27, 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 27 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.
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 3 to 6 feet near the Canadian line. Where frost goes down 5
to 7 feet, it is better to lay the tile at moderate depth and cover the
runs with hay, straw, or leaves weighted down, removing the covering in
the spring.
Making the joints of the distribution tile demands especial attention.
For a short distance on the upper end of each run the tile should be laid
with ends abutting; the joint opening should be increased gradually to
one-eighth inch and this increased to one-fourth in the last 20 feet of
the run. All joints should be protected against the entrance of loose
dirt. Four methods are shown in Figure 28. The lower end of each run
should be closed with a brick or flat stone; or, what is better, an elbow
or =T= branch may be placed on the end and vented above the surface of the
ground, improving the flow of sewage, the ventilation of pipes, and the
aeration of the soil.
If the distribution tile must be laid in clay or other close, poorly
drained soil, special treatment is necessary. A common method is to
subsoil and underdrain the area thoroughly, as shown in Figure 29.
It is not always possible to run the underdrain in lines between the
distribution lines as shown in Figures 17 and 29, but it is a desirable
thing to do, as the sewage must then receive some filtration through
natural soil.
In some instances it is sufficient to lay the distribution tile on a
continuous bed, 8 to 12 inches thick, of coarse gravel, broken stone, or
brick, slag, coke, or cinders and complete the refill as shown in Figure
16 or 29.
Figure 30 shows two other methods of controlling the flow on steep slopes
and diverting proper proportions to the several lateral distributors laid
along the contour of the field. This work can not be effected properly
with =T= or =Y= branches; the flow tends to shoot straight ahead,
comparatively little escaping laterally. To overcome this difficulty
recourse is had to diverting boxes, of which two types are shown in Figure
30. These boxes involve expense, but permit inspection and division of the
flow according to the needs. They may be built of brick, stone, concrete,
or even wood.
Type 1 consists of a single box, into which all the lateral distributors
head. It will be noted that the laterals enter at slightly different
elevations, the two opposite the inlet sewer being the highest, the next
two slightly lower, and the next two the lowest. This staggering of the
outlets, in a measure, offsets the tendency of the flow to shoot across
and escape by the most direct route.
[Illustration: Fig. 28.--Four methods of protecting open joints in
distribution lines--an all-important work. Sketches show cross-section and
longitudinal views; the depth from the surface of the ground to the top of
the tile is about 10 inches]
1. _A_, Subsoiled ground; _B_, 3 or 4 inch drain tile; _C_, strip of
tarred paper about 6 inches wide and extending three-fourths the distance
around the tile, allowing sewage to escape at the bottom; _D_, coarse
sand, gravel, broken stone or brick, slag, cinders, or coke, the coarsest
material placed around the tile (where the ground is naturally very
porous and well drained, special filling in the trench may be omitted);
_E_, natural soil.
2. Drain tile covered with a board laid flat, leaving the entire joint
open.
3. Drain tile laid in stoneware gutter pieces and the joint covered
with stoneware caps; gutter and cap pieces are inexpensive commercial
products; their radius is longer than that of the outside of the tile,
thus leaving open most of the joint space; the gutter aids in keeping the
tile in line.
4. Vitrified sewer pipe with hubs facing downhill; the spigot end should
be centered in the hub with a few small chinks or wedges.
[Illustration: Fig. 29.---Close soils should be deeply subsoiled and
underdrained. Porous, well-drained, air-filled soil is absolutely
necessary. _A_, Subsoiled ground; _B_, 3 or 4 inch distribution tile; _C_,
depth variable with the climate, 1¼ to 3½ feet; _D_, 4-inch underdrain;
_E_, depth such as would prepare land for good crop production, generally
3½ to 4 feet; _F_, stone or other coarse material; _G_, gravel grading
upward to coarse sand; _H_, loose soil]
Type 2 calls for one or more diverting boxes, according to the number of
lateral distributors, and readily permits of wasting sewage at widely
separated elevations and distances. The outlet pipes enter the box at
slightly different elevations, for the reason already stated. With either
type, should the outlets not be set at the right elevations, partial
plugging of the holes and a little experimenting will enable one to
equalize or proportion the discharges.
[Illustration: Fig. 30.--Two systems of distribution on steep slopes--use
of diverting box. _A_, Direction of slope; _B_, contour of field; _C_, 4,
5, or 6 inch sewer from tank; _D_, diverting box; _E_, 3-inch or 4-inch
distribution tile]
=Sewage switch.=--The clogging of filters and soils after long-continued
application of sewage has been previously referred to. It is, therefore,
desirable to arrange the distribution system in two units with a switch
between them, so that one area may drain and become aerated while the
other is in use. This procedure is especially desirable where the soil is
close and the installation of considerable size. It adds to the life and
effectiveness of the distribution area and permits use of a plant in case
it is necessary to repair, extend, or relay the tile in either unit.
Arrangement in two units does not necessarily mean doubling the amount
of tile and the area required in a single field. However desirable that
may be, expense or lack of suitable ground will often prevent. With open
sands and gravels and the assumed siphon dose of 20 gallons per person,
15 to 20 feet of 4-inch tile in each unit for each person will usually
suffice. With more compact soil it is advisable to more nearly double the
requirements previously described. Two simple types of switch are shown in
Figure 31. The switch should be turned frequently, certainly as often as
is necessary to prevent saturation or bogginess of either area.
[Illustration: Fig. 31.--Two simple types of sewage switch. _A_, Sewer
from tank; _B_, switch box; _C_, cover; _D_, blade or stop board (in the
left-hand box the direction of flow is controlled by placing the blade in
alternate diagonal position; in the right-hand box the stop works in iron
guides cast integral with a short piece of light-weight pipe set in the
masonry; if desired the guides may be wood, fastened to the masonry with
expansion bolts); _E_, sewer to distribution area; _F_ (right-hand box),
alternate position of outlets or additional outlets if required]
=A complete installation.=--The general layout and working plans of
a complete installation built in 1915-16 are shown in Figure 32. The
plant is larger than those heretofore considered, and involves several
additional features. The settling chamber below the flow line has a
capacity of 1,000 gallons, and on a basis of 40 gallons per person per day
would serve 25 people.
For many years sewage had been discharged through two 4-inch sewers to
a cesspool in the rear of the house. The proximity of the well made it
unsafe, and the overflow of the cesspool dribbled over the low portion of
the garden and barnyard, cheating nuisance.
The first step was to make borings with a soil auger in the pasture 400
or 500 feet from the house. The borings showed a heavy clay soil to a
depth of about 4 feet, underlaid with a sandy stratum only a few inches in
thickness. It was decided to locate the distribution area in the pasture
and to aid the seepage of sewage by digging numerous filter wells through
the clay to the sandy stratum. Levels were taken and a contour plan
prepared to serve for laying out the plant and establishing the grades.
[Illustration: Fig. 32.--A complete installation for a large rural
home. General layout on a contour plan and construction drawings. Note
abandonment of old cesspool near the well and garden and removal of sewage
to a lower and safer location in the pasture, where the treatment is
subsurface distribution, aided by numerous filter wells about 4 feet deep
filled with coarse gravel. Note that sludge is removed from the bottom of
the settling chamber by opening the gate on the sludge drain]
The septic tank is built in one corner of the barnyard, and a 5-inch sewer
connects it with the old 4-inch sewers to the cesspool. All sewer-pipe
joints were poured with a flexible jointing compound. The settling chamber
is of hopper shape at the bottom, and a 4-inch sludge drain with gate
provides for the gravity removal of sludge. The lower end of the sludge
drain is above the surface of the ground and 9 feet below the flow line.
The end is protected by a small retaining wall, and the sludge is readily
caught in barrels and hauled out on the land for burial. The outlet is low
enough to drain the settling chamber completely. If it is desired merely
to force out the sludge, the drain may be brought to the surface under a
head of 3 to 5 feet, discharging the sludge into a trench or drying bed,
to be applied later to the land. A 2-inch waste pipe about mid-depth of
the settling chamber permits drawing off the cleared portion of the sewage
to the siphon chamber and from thence through another 2-inch waste pipe
into the 6-inch sewer leading to the distribution field.
The 4-inch siphon has a drawing depth of 33 inches, and as the siphon
chamber is 4 feet wide by 6 feet long the dose is about 500 gallons. The
siphon cost $35. The 6-inch sewer to the switch box falls about 6 inches
in 50 feet. The distribution field was thoroughly subsoiled, and about 800
feet of 3-inch tile was laid in each unit. At intervals of 25 feet along
the distribution trenches 6-inch holes were dug through the clay stratum
with a posthole digger. These holes were filled with stone and constitute
the filter wells previously mentioned. All tile lines are surrounded with
stone and coarse gravel, and the ground has been trimmed to give a uniform
cover of 12 inches. All work was done by day labor in a thorough manner.
As the men were doing other work at the same time the actual cost is not
known, but it is believed the installation cost about $700.
=Cost data.=--Reliable cost figures are difficult to estimate. Labor,
materials, freight, haulage, and other items vary greatly in different
localities. The septic tank shown in Figure 21 contains about 1,000 bricks
and is estimated to cost $60 complete. The septic tank shown in Figure
23 for 5 persons is estimated to cost $135; for 10 persons, $170; for
15 persons, $240; for 20 persons, $280. In Maryland, in 1916, the cost
of installing a septic tank similar to that shown in Figure 23 (for 5
people), including 86 feet of 5-inch house sewer (55 feet of cast-iron
pipe passing a well, and 31 feet of vitrified pipe) and 214 feet of
second-quality 4-inch sewer pipe in the distribution area, was as follows:
Excavation, labor $7.50
Materials delivered 46.60
Three-inch siphon, including freight 15.75
Construction, labor 28.00
Supervision 5.00
------
Total 102.85
The quotations in the following table will be found useful in making
estimates of cost:
_Cost per foot of pipe and drain tile_
(Approximate retail prices, Washington, D. C., February, 1928)
------------------------------------+-----------------------------------
| Size, in inches.
Kind of pipe. +--------+--------+--------+--------
| 3 | 4 | 5 | 6
------------------------------------+--------+--------+--------+--------
| | | |
Extra heavy cast-iron soil pipe | $0.23 | $0.31 | $0.40 | $0.48
Vitrified salt-glazed sewer pipe | .15 | .15 | .22½ | .22½
Clay or shale drain tile | .06 | .07 | .10 | .13
| | | |
------------------------------------+--------+--------+--------+--------
The cost of cast-iron fittings may be roughly estimated as follows; Bends,
one and one-half times the price of straight pipe; =T=-branches, two times
the price of straight pipe; reducers, average of the prices of straight
pipe at each end. The cost of clay bends, =T=-branches, reducers, and
increasers may be roughly estimated at four times the price of straight
pipe.