A tight, underground septic tank with shallow distribution of the effluent
in porous soil generally is the safest and least troublesome method of
treating sewage upon the farm, while at the same time more or less of the
irrigating and manurial value of the sewage may be realized.
The late Prof. Kinnicutt used to say that a septic tank is "simply a
cesspool, regulated and controlled." The reactions described under the
captions "How sewage decomposes," "Liquefying closet," and "Cesspools"
take place in septic tanks.
In all sewage tanks, whatever their size and shape, a portion of the solid
matter, especially if the sewage contains much grease, floats as scum on
the liquid, the heavier solids settle to form sludge, while finely divided
solids and matter in a state of emulsion are held in suspension. If the
sludge is retained in the bottom of the tank and converted or partly
converted into liquids and gases the tank is called a septic tank and the
process is known as septicization. The process is sometimes spoken of as
one of digestion or rotting.
=History.=--Prototypes of the septic tank were known in Europe nearly 50
years ago. Between 1876 and 1893 a number of closed tanks with submerged
inlets and outlets embodying the principle of storage of sewage and
liquefaction of the solids were built in the United States and Canada. It
was later seen that many of the early claims for the septic process were
extravagant. In recent years septic tanks have been used mainly in small
installations, or, where employed in large installations, the form has
been modified to secure digestion of the sludge in a separate compartment,
thus in a measure obviating disadvantages that exist where septicization
takes place in the presence of the entering fresh sewage.
=Purposes.=--The purposes of a septic tank are to receive all the farm
sewage, as defined on page 4, hold it in a quiet state for a time, thus
causing partial settlement of the solids, and by nature's processes of
decomposition insure, as fully as may be, the destruction of the organic
matter.
=Limitations.=--That a septic tank is a complete method of sewage
treatment is a widespread but wrong impression. A septic tank does not
eliminate odor and does not destroy all organic solids. On the contrary,
foul odors develop, and of all the suspended matter in the sewage about
one-third escapes with the effluent, about one-third remains in the tank,
and about one-third only is destroyed or reduced to liquids and gases.
The effluent is foul and dangerous. It may contain even more bacteria
than the raw sewage, since the process involves intensive growths. As
to the effects upon the growth and virulence of disease germs little is
known definitely. It is not believed that such germs multiply under the
conditions prevailing in a septic tank. If disease germs are present many
of their number along with other bacteria may pass through with the flow
or may be enmeshed in the settling solids and there survive a long time.
Hence the farmer should safeguard wells and springs from the seepage or
discharges from a septic tank as carefully as from those of liquefying
closets and cesspools.
=Further treatment of effluents.=--The effluent of a septic tank or any
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 17, 29, and 32.
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 top soil 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 19: (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. 19.--Parts of a septic-tank installation.]
[Illustration: Fig. 20.--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.]
=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 20 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 2-1/2 feet below the surface of the ground at A and 4-1/2 feet
below the surface of the ground at E; the fall of the sewer between A and
E is 2 feet (4 + 2-1/2 - 4-1/2 = 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 settling 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.
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 discharged. 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,
1-1/2 feet for 5-inch size, 1 foot for 6-inch size.
Figure 21 shows methods of making good joints. _A_, _B_, _C_, _D_, _E_,
_F_, and _G_ are ordinary sewer pipe joints; _H_, is cast-iron soil pipe.
[Illustration: Fig. 21.--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 1/2
inch, leaving about 1-1/2 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 1/2 to 1-1/2
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 1/4 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 1 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 home-made 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 22 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
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.
[Illustration: Fig. 22.--Sewer-cleaning tools--how to use them. _A_,
Ordinary 1-1/2 or 2 inch auger welded to a piece of ¾-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 11 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 or
rags; _L_, scraper; _M_, wire brush for removing grease, drawn back and
forth with a wire or rope; _N_, home-made wire brush (for a 5-inch sewer
use a 1-1/2-inch wooden pole to which is securely tacked a piece of heavy
rubber, canvas or leather belting or harness leather 5-1/2 by 8 inches,
spirally studded, as shown, with ordinary wire nails 1-1/2 inches in
length).]
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: