other form of sewage tank is foul and dangerous. Whether or not the
solids are removed by screening, by short periods of rest, as in plain or
modified forms of settling tanks, or by longer quiescence, as in septic
tanks, the effluent generally requires further treatment to reduce the
number of harmful organisms and the liability of nuisance. This further
treatment usually consists of some mode of filtration. In the earliest
example of such treatment the sewage was used to irrigate land by either
broad flooding or furrow irrigation. By another method the sewage is
distributed underground by means of drain tile laid with open joints, as
illustrated in Figures 27 and 30.
Artificial sewage filters are composed of coarse sand, screened gravel,
broken stone, coke, or other material, and the sewage is applied in
numerous ways. Since, filtration is essentially an oxidizing process
requiring air, the sewage is applied intermittently in doses.[9]
[9] Artificial filters of various types are well described and illustrated
in Public Health Bulletin No. 101, "Studies of Methods for the Treatment
and Disposal of Sewage--The Treatment of Sewage from Single Houses and
Small Communities." U. S. Public Health Service, December, 1919.
If properly designed and operated, filters of sand, coke, or stone are
capable of excellent results. Under the most favorable conditions it is
unwise to discharge the effluent of a sewage filter in the near vicinity
of a source of water supply. Under farm conditions filters are usually
neglected or the sewage is improperly applied, resulting in the clogging
and befouling of sand filters and the discharge from stone filters of an
effluent which is practically as dangerous and even more offensive than
raw sewage. Moreover unless the filters are covered there are likely to be
annoying odors, and there is always the possibility of disease germs being
carried by flies where sewage is exposed in the vicinity of dwellings.
Hence it seems more practical for the farmer, avoiding the expense of
earth embankments or masonry sides and bottom for a filter bed, to waste
the tank effluent beneath the surface of such area of land as is most
suitable and available. This method of applying sewage to the soil or
subsoil is often spoken of as subirrigation, but subsoil distribution of
sewage is different in principle and practice from subirrigation for the
increase of crop yields. Subirrigation is rarely successful unless the
land is nearly level, the topsoil porous and underlaid with an impervious
stratum to hold the water within reach of plant roots, and unless a
relatively large quantity of water is used and the work is skillfully
done. On the other hand, the quantity of sewage on farms being small, it
may be wasted in hilly ground, which should be as porous, deeply drained,
and dry as possible.
=Parts of a system.=--The four parts of a septic-tank installation with
subsurface distribution of the effluent are outlined in Figure 17: (1)
The house sewer from house to tank; (2) the sewage tank consisting of one
or more chambers; (3) the sewer from tank to distribution field; (4) the
distribution field, where the sewage is distributed and wasted, sometimes
called the absorption field. These parts will be discussed in the order
named, although the last should have the first consideration.
[Illustration: Fig. 17.--Parts of a septic-tank installation]
=House sewer.=--The length will vary with the slope of the ground and
position of buildings, well, and distribution field. Fifty to 100 feet is
a fair length; a greater is still more sanitary. Wherever possible the
house sewer should be laid straight in line and grade. Figure 18 shows
how this work may be done. Suppose the distance from A to E be 100 feet;
that grade boards be set 25 feet apart crosswise of the trench at A, B,
C, D, and E; that the ground at A be 4 feet lower than at E; that the top
of the sewer be 2½ feet below the surface of the ground at A and 4½ feet
below the surface of the ground at E; the fall of the sewer between A
and E is 2 feet (4 + 2½ - 4½ = 2). If the fall in 100 feet be 2 feet, in
25 feet it is one-fourth as much, or 6 inches. Hence, grade board B is 6
inches higher than grade board A, C is 6 inches higher than B, and so on
to E. The top edges when all the boards are set with a carpenter's level
and fastened in position should be in line. The grade thus established
may be any convenient height above the top of the proposed sewer, and the
measuring stick used to grade the pipe is cut accordingly. This height is
usually a certain number of whole feet. Fixing the line of the sewer is a
mere matter of setting nails in the top edges of boards A and E directly
over the center of the proposed sewer and tightly stretching a fish line
or grade cord; nails should be set where the cord crosses boards B, C, and
D.
[Illustration: Fig. 18.--Setting line and grade for house sewer. To
the observer at A the top edges of the grade boards appear as one; the
half-driven nails are set to line]
If the cellar or basement contains plumbing fixtures, the house sewer
should enter 1 to 2 feet below the cellar floor. If all plumbing fixtures
are on the floors above, the sewer may enter at no greater depth than
necessary to insure protection from frost outside the cellar wall. Digging
the trench and laying the pipe should begin at the tank or lower end. The
large end of the pipes, called the hub, should face uphill, and the barrel
of each pipe should have even bearing throughout its length. Sufficient
earth should be removed from beneath the hubs to permit the joints to be
made in a workmanlike manner.
The house sewer may be vitrified salt-glazed sewer pipe, concrete pipe, or
cast-iron soil pipe. The latter, with poured and calked lead joints makes
a permanently water-tight and root-proof sewer, which always should be
used where the vicinity of a well must be passed; 4, 5, or 6-inch pipe may
be used, depending mainly on the fall and in less degree on the quantity
of sewage discharge. As a measure of economy the 4-inch size is favored
for iron pipe. If vitrified pipe is used, either the 5 or 6-inch size is
preferable, as these sizes are made straighter than the 4-inch size and
are less liable to obstruction. Of the two the 5-inch size is preferable.
The fall in 100 feet should never be less than 2 feet for 4-inch size, 1½
feet for 5-inch size, 1 foot for 6-inch size.
Figure 19 shows methods of making good joints. _A_, _B_, _C_, _D_, _E_,
_F_, and _G_ are ordinary sewer pipe joints; _H_, cast-iron soil pipe.
[Illustration: Fig. 19.--How to make good joints. See text for directions
and specifications]
_A_ shows the use of a yarning iron to pack a small strand of jute into
the joint space, thus centering the pipes and preventing the joint filler
running inside. The joint surfaces should be free of dirt and oil. The
jute is cut in lengths to go around the pipe; a small strand is soaked
in neat Portland cement grout, then twisted and wrapped around the small
end of the pipe to be pushed into the hub of the last pipe laid. After
the pipe is pushed home the jute is packed evenly to a depth of not over
½ inch, leaving about 1½ inches for the joint filler. Old hemp rope or
oakum dipped in liquid cement or paper may be used, in place of jute, and
the packing may be done with a thin file or piece of wood.
_B_ shows the use of a rubber mitten or glove to force Portland cement
mortar into the joint space. The mortar should be thoroughly and freshly
mixed in the proportion of one volume of cement to one volume of clean
sand and should be pressed and tamped to fill the joint completely.
_C_ shows a section of finished joint. The fresh mortar should not be
loosened or disturbed when laying the next pipe.
_D_ shows method of pouring a joint with grout, which is quicker,
cheaper, and better than using a rubber mitten. A flexible sheet-metal
form or mold, oiled to prevent the grout sticking, is clamped tightly
around the joint and is completely filled with grout consisting of equal
parts of Portland cement and clean sand mixed dry, to which water is
added to produce a creamy consistency. The pipes should not be disturbed
and the form should not be removed for 24 hours.
_E_ shows a section of grouted joint, well rounded out, strong, and tight.
_F_ shows the use of a pipe jointer for pouring a hot filler. The pipe
jointer may be an asbestos or rubber runner or collar or a piece of
garden hose clamped around the pipe leaving a small triangular opening
at the top. The jointer is pressed firmly against the hub, and any
small openings between the jointer and pipe are smeared with plastic
clay to prevent leakage of the filler. A clay dike or funnel about 3
inches high built around the triangular opening greatly aids rapid and
complete filling of the joint space. The filler may be a commercially
prepared bituminous compound or molten sulphur and fine sand. The former
makes a slightly elastic joint; the latter a hard unyielding joint. With
good workmanship both kinds of joint are practically water-tight and
root-proof, and cost about the same as cement mortar joints. The filler
is heated in an iron kettle over a wood, coke, or coal fire. It should
be well stirred, and when at a free running consistency should be poured
with a ladle large enough to fill the joint completely at one operation.
As soon as the compound cools the jointer is removed. Sulphur-sand
filler is made by mixing together dry and melting equal volumes of
ordinary powdered sulphur and very fine clean sand, preferably the finest
quicksand. A 5-inch sewer pipe joint requires from three-tenths to
nine-tenths of a pound (according to the kind of pipe) of sulphur, worth
3 to 5 cents per pound, and a like quantity of sand. From ½ to 1½ pounds
of bituminous filler are required for a 5-inch pipe joint.
_G_ shows section of finished joint.
_H_ shows the use of a pouring ladle in making lead joints in cast-iron
soil pipe. This pipe is in lengths to lay 5 feet, and the metal of the
barrel is ¼ inch thick. The joint is yarned with dry jute or oakum, as
described above, and is poured full with molten, soft, pig lead to be
afterwards driven tightly with hammer and calking tools. About ¾ pound
of lead for each inch in diameter of pipe is required. Prepared cements
of varying composition have proved effective and, as they require no
calking, are economical. Among the best is a finely ground, thoroughly
mixed compound of iron, sulphur, slag, and salt.
_I_ is a homemade pipe jointer or clay roll for use in pouring molten
lead. A strand of jute long enough to encircle the pipe and the ends to
fold back, leaving an opening at the top, is covered with clay moistened,
rolled and worked to form a plastic rope about 1 inch in diameter. The
jointer gives the very best results but must be frequently moistened and
worked to keep the clay soft and pliable. The jointer shown in _F_ is
frequently used for pouring lead joints.
Obstructions in house sewers are frequent. Among the causes are broken
pipes, grade insufficient to give cleansing velocities, newspaper, rags,
garbage, or other solids in the sewage, congealing of grease in pipes
and main running traps (house sewer traps), and poor joint construction
whereby rootlets grow into the sewer and choke it. Good grade and good
construction with particular care given to the joints, will avert or
lessen these troubles. The sewer should be perfectly straight, with the
interior of the joints scraped or swabbed smooth. When the joint-filling
material has set, the hollows beneath the hubs should be filled with good
earth free of stones, well tamped or puddled in place. It is important
that like material be used at the sides of the pipe and above it for at
least 1 foot. The back filling may be completed with scraper or plow. No
running trap should be placed on the house sewer, because it is liable
to become obstructed and it prevents free movement of air through the
sewer and soil stack. Conductors or drains for rain or other clean water
should never connect with the house sewer, but should discharge into a
watercourse or other outlet.
Where obstruction of a house sewer occurs, use of some of the simple tools
shown in Figure 20 may remedy the trouble. It is not likely that farmers
will have these appliances, except possibly some of the augers; but some
of them can be made at home or by a blacksmith, and most of them should
be obtainable for temporary use from a well-organized town or city sewer
department. The purpose of the several tools shown is indicated in the
notation.
=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 difficult
to construct an absolutely water-tight masonry tank, and still more
difficult to make it proof against the passage of sewer odors.
[Illustration: Fig. 20.--Sewer-cleaning tools--how to use them. _A_,
Ordinary 1½ or 2 inch auger welded to a piece 1¾-inch extra strong wrought
pipe about 5 feet long: the stem is lengthened by adding other pieces of
pipe with screw couplings, and is fitted with a pipe handle; all cleaning
work should proceed upstream; _B_, twist or open earth auger; _C_, ribbon
or closed earth auger; _D_, spiral or coal auger; _E_, ship auger; _F_,
root cutter; _G_, sewer rods, with hook coupling, usually of hickory
or ash 1 or 1¼ inches in diameter and 3 or 4 feet long; _H_, gouge for
cutting obstructions; _I_, scoop for removing sand or similar material;
_J_, claw, and _K_, screw, for removing paper and rags; _L_, scraper; _M_,
wire brush for removing grease, drawn back and forth with a wire or rope;
_N_, homemade wire brush (for a 5-inch sewer use a 1½-inch wooden pole
to which is securely tacked a piece of heavy rubber, canvas, or leather
belting or harness leather 5½ by 8 inches, spirally studded, as shown,
with ordinary wire nails 1½ inches in length)]
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 reasons 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.
[Illustration: Fig. 21.--One-chamber septic tank--does nothing more than
a tight cesspool. Brick construction, heavily plastered inside; size
suitable for 180 to 280 gallons of sewage daily (nominally 4 to 7 persons)]
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 Figure 16. 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 21. 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 22. A
typical two-chamber concrete tank is shown in Figure 23. 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 23:
_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 | 32
10 | 320-480 | 4-1/4 | 6-1/4 | 2 | 3-3/4 | 14 | 39
15 | 520-680 | 6-2/3 | 9-3/4 | 3 | 6 | 15-2/3| 47
20 | 720-960 | 8 | 12 | 3-1/2 | 7 | 17-1/2| 56
--------+---------+---------+--------+--------+---------+-------+------
=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.
[Illustration: Fig. 22.--Two-chamber septic tank, simple and inexpensive,
constructed of 24-inch vitrified sewer pipe, size suitable for 125 gallons
of sewage daily (nominally 3 persons). _A_, House sewer; _B_, settling
chamber, made of double =T= branch and one length of straight pipe, each 3
feet long and 2 feet in diameter, supported by 4 inches of concrete, all
joints made water-tight; _C_, submerged outlet, consisting of a metal =T=
slipped into the sewer-pipe branch; _D_, wire screen ¼-inch mesh; _E_,
siphon chamber made of one =T= branch 3 feet long and 2 feet in diameter;
_F_, siphon; _G_, 3-inch overflow; _H_, sewer to distribution field; _I_,
tight cover with lifting ring; _J_, concrete protection around sewer-pipe
hubs]
[Illustration: Fig. 23.--Typical two-chamber concrete septic tank. (See
table for dimensions and quantities for different sizes)]
Three types of sewage siphon are shown in Figure 24. 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. 24) 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 24: (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.
[Illustration: Fig. 24.--Three types of sewage siphon. The table gives
dimensions for setting standard 3 and 4 inch siphons; also the appropriate
size and grade of the sewer to carry the siphon discharge]
The overhead siphon, type 3, Figure 24, may be installed readily in a tank
already built by addition of an outlet sump. If properly set are 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 23 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 (¼-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.