SEWAGE DISPOSAL
The disposal of sewage, in a convenient and sanitary manner is a
problem of serious importance in the equipment of isolated dwellings
with modern household conveniences. The manner of heating, lighting
and of water supply are questions of selection among a number of
established systems, but the problem of sewage disposal must in a
great measure be determined by local conditions. Unless the natural
surroundings are such as will permit sewage to be emptied directly
into a stream of considerable volume, the problem of its safe disposal
becomes one of serious importance.
Sewage is understood to mean the fluid waste from the kitchen, toilet
and laundry and has nothing whatever to do with garbage. Sewage
disposal has to do with conducting away the house waste and disposing
of it in a sanitary manner. Sewage disposal does not necessarily have
anything to do with sewage purification; although a sewage disposal
plant may be so constructed as to discharge a purified effluent, it
usually is understood to have to do alone with its disposal in a manner
that does not offend the aesthetic sense. A simple sewage plant is
anything that will take the sewage away from the house in such a way
as to produce no unsightly accumulations that will decay and produce
offensive odors.
A sewage purification plant is one in which the raw sewage from the
house drain is first liquefied, after which the liquid is passed into
a filter where it undergoes a process of bacterial disintegration and
the organic matter reduced to the inorganic state, where no further
change is possible. The water which flows from such a filter is clear
and sparkling, and is often taken for spring water. The degree of
purification given to the sewage will depend on the style of filter and
the length of time necessary for the water to pass through it.
Sewage is composed of organic matter in a fluid or part fluid
condition, contained in a large volume of water. It is not usually
the dark, heavy, foul-smelling fluid that is imagined by many, but a
turbid liquid possessing only a few of the qualities usually ascribed
to sewage. Under favorable conditions practically all of the organic
matter will be readily dissolved and the sewage will become entirely
liquid.
As a liquid, the raw sewage is in the most favorable condition for
rapid decay and if left standing in the air it soon develops properties
that render it highly objectionable.
The decay of all organic matter is a process of disintegration that
ultimately ends in the elements from which it came. In the disposal of
sewage, the aim is to permit this disintegration to take place under
conditions that will be least offensive to the aesthetic sensibilities,
and in some cases to render it free from harmful properties should
there be present the bacteria of communicable disease.
The successful disposal of sewage from cities is accomplished under a
great variety of conditions. It is much easier to arrange for sewage
purification on a large scale than in a small way. The reason for this
is that in the care of a city the sewage-disposal plant is under the
supervision of a competent person, whose business it is to see that
the conditions are kept at the highest efficiency. Private plants are
left almost entirely without care, until they fail from causes that
are usually preventable. Sewage may be successfully purified under a
great many conditions, but no type of plant has as yet proven itself
successful that does not receive intelligent attention.
The most successful of small sewage disposal plant is the septic tank
system alone or in connection with an adequate form of bacterial
filter. Cesspools are not to be countenanced by people of intelligence.
The cesspool has been so universally condemned by authorities on
sanitation, that all intelligent people look upon it as a thing filthy
beyond description. Although the septic tank is little more than an
improved cesspool, the condition under which it acts is entirely
different from that which takes place in the latter and with care
and watchfulness, it may be made to work to a degree of perfection
that is surprising. The one great cause of the failure of small
sewage-disposal plants is the lack of proper care.
The process of sewage purification as now practised in the most
successful plants is largely mechanical, but bacterial action plays a
part of great importance in the completion of the process. It consists
of two stages: the tank treatment, in which the sewage is liquefied;
and the process of filtration where the liquefied sewage--commonly
called the effluent--from the septic tank undergoes a process of
filtration and bacterial purification.
=The Septic Tank.=--The septic tank alone, as used for sewage disposal,
is often termed a sewage purifying plant, when in reality it is only
intended to change the sewage into a form in which it can be readily
carried away. The word septic means putrifying, and when applied to
sewage disposal it furnishes a convenient term but has nothing to do
with purification. The septic tank furnishes only the first stage of
the purifying process, and although its effluent may be clear and
possess little odor, it is nevertheless unpurified. The septic tank
discharges an effluent of more or less completely digested sewage,
instead, as in the cesspool, of permitting it to remain a constantly
festering mass, to be slowly absorbed by the earth.
The sewage is first collected in a tank of sufficient size to contain
the discharge from the house for 24 hours. In the process of digestion
which the sewage undergoes while in the tank, it is rendered almost
entirely liquid; at the same time it is acted upon by the bacteria that
are developed, and that tend to reduce the sewage to its elemental
form. The effluent liquid which passes out of the tank is almost
colorless and possesses relatively little odor.
The tendency of the change which takes place in the tank is to nitrify
the organic matter but under ordinary conditions the action is not
fully complete. The effluent sometimes undergoes but little change
except to be reduced to a liquid. If the effluent is now allowed to
flow into a ditch where it will stand in pools, further putrification
will take place with its resulting annoyance. In case the septic tank
is to be used alone, the effluent should be conducted to a stream for
final disposal. A septic tank must be built to accommodate a certain
number of people and of sufficient size to take care of the entering
sewage. The action which goes on in the tank will render the contents
almost entirely fluid, and under good conditions the sewage will be
completely digested. When working properly, a thick scum will form on
the surface, through which filters the gases that are liberated in the
process of disintegration. The formation of the scum is an indication
that the filter is doing its best work. Should the tank be required
to take care of more sewage than it can conveniently handle, the scum
will not form and the effluent will be turbid because of the undigested
matter.
The change that takes place in the sewage while it remains in the tank
is first that of being liquefied and then disintegrated by bacterial
action. That such a change does take place is evidenced by the residue
that is found in the tank in the process of cleaning. This is a black
granular substance, composed mostly of humus and commonly known as
sludge. The amount of accumulated sludge is relatively small, and the
operation of cleaning is not necessary more than twice a year and is
not the disagreeable task one might suppose.
=The Septic Tank With a Sand-bed Filter.=--In places where the use
of the septic tank alone is not possible, it sometimes happens that
the natural conditions are such as will permit the effluent to be
drained directly into the soil. With such a condition, the effluent
goes into a filter bed composed of gravel or other loose material,
where it undergoes still further bacterial action and if the process
is complete, the water which comes from the filter bed is clear and
odorless. Under good conditions it is clear sparkling water and
contains but a small amount of impurities.
Septic tanks are made in many forms but that illustrated in Figs. 151
and 152 is commonly used. In Fig. 151 the tank is shown in position to
receive the sewage from the house drain, where it is to undergo the
first treatment and then to be conducted to a filter bed made of porous
tile, set in loose soil. The tank is shown in detail in Fig. 152. It is
a cemented brick cistern with an opening to the surface that contains
a double cover as a protection during cold weather. A brick partition
divides the tank into spaces _G_ and _H_, that contain volumes that
are to each other as 1 to 2. The tank is of such size as will hold
a volume of sewage equal to 24 hours’ use; that is, it is expected
that any portion of sewage will remain in the tank for that length of
time. The sewage enters at _A_, in such a way as will give the least
disturbance of the liquid of the tank. An opening _C_ allows the liquid
to pass from _H_ into _G_, where any additional sewage entering _H_
will displace an equal amount in _G_, which will pass out at _B_ to
the filter bed. The partition _D_ is high enough so that the scum
that forms on the surface will not pass directly into the space _G_.
The entrance and exit pipes are made of vitrified sewer tile with the
openings below the surface.
[Illustration: FIG. 151.--Sectional view of a septic tank, connected
with a sand-bed filter; for the disposal of sewage from a residence.]
As the sewage enters the tank _A_, a considerable portion will sink to
the bottom, while some will float to the top where a thick scum will
gather. By far the greatest portion of solids will be readily dissolved
in the water and the remainder will be still further reduced to liquid
form by bacterial solution. The process of disintegration that goes
on evolves a considerable amount of carbon dioxide and ammonia which
filters through the scum. The process that now goes on in the tank is
that of liquefying the organic matter and changing it from organic to
the inorganic state.
The bacteriologist recognizes in the process of sewage disintegration
the work of two classes of bacteria, the aerobic or those bacteria
that work by reason of air and do their work only in its presence and
the anaerobic or those that work in the absence of air. In the action
of the sewage-disposal plant both kinds of bacteria are at work. If,
in the final stage where the sewage passes into the filter, air can be
carried into the earth the action will be hastened.
[Illustration: FIG. 152.--Section of the septic tank in Fig. 151
showing details of construction.]
It is evident that, since the sewage entering the tank is almost
entirely dissolved, under ideal action this system would give very
little trouble, but actually as the sewage enters the tank the
disturbance caused by the incoming water forces some of the undigested
matter into the outlet and being carried into the filter bed it will be
deposited at the first opportunity. This will cause the filter bed to
fill up with undigested sewage at the point nearest the entrance, and
in course of time it will stop the pipe because of this accumulation.
To avoid such an occurrence, tanks have been built in which an
automatic siphon discharges the effluent whenever a certain quantity
has collected. Such a tank is shown in Fig. 153. With this arrangement,
the sewage enters the first tank at _A_, and passes into the second
tank at _B_. At _S_ is shown an automatic siphon, so made that when
the effluent has collected to the height of the water line, the siphon
automatically discharges the contents of the tank. This is known as a
dosage tank because periodically a dose of the effluent is discharged
into the filter bed. The volume discharged is sufficient to fill the
greater portion of the bed, and force out the air in the loose soil. As
the water filters from the bed the air is drawn in to take its place
and gives the bacteria which work in the presence of air an opportunity
to do their work. The work done by this filter bed is first to filter
out any suspended matter carried in the effluent which will lodge on
the surface of the filter material and then to undergo the slow process
of integration, and to permit the oxidation of the dissolved sewage. If
this matter is deposited faster than it disintegrates then the filter
will fill up and finally refuse to work.
[Illustration: FIG. 153.--Sectional view of a two-chamber septic tank
with a dosage siphon.]
=The Septic Tank and Anaerobic Filter.=--In places where the use of the
simple septic tank is not possible and where the character of the soil
will not permit of a natural sand-bed filter, an anaerobic filter may
be constructed through which to pass the effluent from the septic tank.
The anaerobic filter is one in which anaerobic bacterial action is
given opportunity to reduce the organic matter in the sewage to its
elemental condition. The filter may be constructed in any form that
will permit the process of filtration to be carried out in a way
that will afford good anaerobic action. The extent to which the
purification is to be carried will determine the form and size of the
filter.
In Fig. 154 is shown such a plant, where a combined septic tank and
anaerobic filter discharges its effluent into a filter ditch in which
the purifying process is continued through a bed of gravel of any
desired length. The figure illustrates a plant that was designed for
a country residence. The septic tank and anaerobic filter are located
relatively as shown in the drawing, the filter ditch following the
course of a roadway. The water is finally discharged into a little
stream, where it mingles with the water from a spring, and flows
through a meadow.
[Illustration: FIG. 154.--Sectional view of a septic tank combined with
an anærobic filter; together with the details of construction and plan
of arrangement.]
The septic tank in Fig. 154 is quite similar in construction to the
others described except that a section of sewer tile takes the place
of the brick wall between the two parts of the tank. The opening _O_,
through which the effluent is discharged, is located a little above the
center of the tank.
The anaerobic filter is a tank, rectangular in cross-section, made
with brick walls and cemented on the inside. The effluent from the
septic tank enters the anaerobic filter in a chamber, that is separated
from the main tank by a wooden grating against which rests the filter
material. As indicated in the drawing, the bottom is filled with coarse
material; stones or broken tiles about 4 inches in diameter. Above
this is a layer of material about 2 inches in diameter and above that
another layer of 1-inch material; the top is made of gravel. This forms
the anaerobic filter, in which takes place the bacterial action away
from the presence of air. The interspaces in the filter material allows
the effluent from the septic tank to seep through and deposit the
particles of matter held in suspension. The arrangement is such as is
best suited to the anaerobic action. Here, shut away from the light and
air, the organic matter in the effluent undergoes disintegration just
as would happen in the earth.
It is evident that some of the matter that should remain in the septic
tank and be removed as sludge will be carried into the anaerobic
filter. This will, of course, form an insoluble deposit that will
accumulate and in the course of time the filter will become clogged. It
should be expected that such a filter will ultimately need renewing,
for this reason the top is made of a slab of reinforced concrete that
may be raised to allow the removal and refilling of the filter material.
The automatic siphon discharges the water from the chamber _S_,
whenever it fills. The discharged water from the siphon is conducted
into a drain tile, placed in a ditch filled with gravel or other
loose material, which serves as an additional filter and in which the
water undergoes a still further purification. This filter ditch is
constructed as indicated in longitudinal section. The water from the
siphon enters the tile _C_ and seeping through the filling is drained
away in the tile shown at _D_.
The tiles are not set close together, but the joints are left open and
covered by pieces of broken tile as shown in _H_. This is to prevent
the filter material from entering the tile and thus stopping the ready
flow of the water.
The filter ditch of the plant will be constructed according to the
contour of the ground and will follow the natural drainage. The course
of the ditch--if it is desired to use one--will accommodate itself to
the character of the ground. The final discharge of the water will be
determined by the natural drainage.
That a plant of this kind will work perfectly when new is is beyond
a doubt but that it will continue indefinitely to give perfect
satisfaction is not reasonable to expect. The septic tank will require
cleaning, probably once a year. The anaerobic filter will require
renewing at intervals, depending on the amount of sewage the filter is
required to take care of and the rate at which the plant is worked,
probably once in 4 or 5 years. If the septic tank is of insufficient
size to readily digest the sewage, the accumulation of sludge in the
anaerobic filter will be greater than should occur.
It would be only reasonable to suppose that the siphon will sometimes
refuse to discharge. Even though it is an automatic siphon,
circumstances may cause it at times to fail to act. For this reason the
manhole is placed so that the siphon may be inspected and repaired,
should it be necessary. It must not be supposed that once such a
plant is in place that all of the work is over. The success of a good
sewage-disposal plant of this kind demands eternal vigilance.
[Illustration: FIG. 155.--Septic tank with a settling basin and
windmill pump.]
In the level areas where the possibilities of natural drainage is
not good, it sometimes occurs that plants such as those described
are not permissible. To overcome such conditions the plant in Fig.
155 represents an installation where the effluent is carried several
hundred feet through a drain tile before it is finally discharged into
an outlet. This plant is made up of two separate tanks, the first
acting as a septic tank, while the second tank is a settling chamber.
The water from the second chamber is pumped by windmill power and
discharged into a drain tile at the required height through which it is
carried to the place of final deposit.
=Limit of Efficiency.=--Much that has been written on the subject
conveys the impression that the septic tank alone, used under various
conditions, will eliminate disease germs and all offending features
of sewage and render it a pure water with a small amount of residue
remaining in the tank. That such is not the case is all too evident to
many who have constructed plants expecting perfect results and have
attained only partial success.
It is not reasonable that a plant giving satisfaction under the usual
conditions could accomplish its purpose under stress of work. It is
quite evident that the amount of sewage from any source cannot be
constant. It is equally evident that the effluent from the plant cannot
always be the same; but with reasonable limits of variation, a suitably
designed tank ought to take care of the sewage from a house at all
times and discharge an effluent that is reasonably clear and without
offending odor.
It should be kept in mind that, as commonly used, the chief office of
the septic tank is to do away with the things that offend the senses,
and not to make an effluent that might serve as drinking water. It must
also be kept in mind that if the disease germs enter the plant because
of sickness in the house that there is every possibility that the germs
will be in the discharged water.
The plant must be located as is directed by the natural surroundings
but the drainage must be away from buildings and particularly from
wells.
Small sewage plants are reasonably efficient and add immensely to the
comfort and healthful conditions of the home. They are not perfect in
their action but there is excellent reason to believe that the plant of
ideal construction will yet be attained.
In a flat country where drainage is difficult, the form of plant must
be modified to suit the prevailing conditions but some form of working
plant can always be devised. Small plants do not give so efficient
results as those of large size but they do very acceptable work. To do
good service they must receive attention but the actual amount of labor
they demand is small. Small sewage-disposal plants are not expensive
nor difficult to construct, and for the amount of labor and money
expended they give returns that cannot be estimated.
In determining the character of plant to be constructed, in any
particular place, local conditions will in a great measure decide the
type to be used. The degree of purity to which it will be necessary
to reduce the effluent will depend on the location of the plant and
the means of final disposal. If the effluent can be run into a stream
of sufficient volume, the septic tank alone will probably answer the
purpose.
The septic tank reduces sewage to a liquid form which has some odor.
It may be carried away in an open ditch which has good flow, but if
allowed to collect in pools it will undergo further putrescence and be
objectionable.
It may be possible to use a small creek for final disposal but one in
which the effluent from a septic tank alone would be objectionable.
In such a case the use of the septic tank combined with an anaerobic
filter would probably give a permissible degree of purity.
With a plant composed of a septic tank and anaerobic filter, sewage
is rendered almost free from odor and the effluent will not undergo
further putrescence when collected in pools.
In many cases it is desired to purify the effluent still further,
either because of lack of means for final disposal or because the
effluent would contaminate the water into which it is discharged. In
such cases the plant will consist of the septic tank, an anaerobic
filter and a filter ditch or sand-bed filter. The effluent from such a
plant will be clear sparkling water that might be mistaken for spring
water.
The design and construction of sewage-disposal plants has been
made a subject of investigation in a number of State engineering
experiment stations. In addition, manufacturers of cement have prepared
descriptive literature that is sent gratis on application. These
bulletins contain detailed information as to the working properties
and construction of private plants to suit the various conditions
of disposal. The following is taken by permission of the Universal
Portland Cement Co. from their bulletin on “Concrete Septic Tanks.”
[Illustration: FIG. 156.--Septic tank. This shows the construction as
if cut away along a center line following its length, also a section of
the siphon chamber and a plan of the whole construction.]
[Illustration: FIG. 156_a_.--Photographic reproduction of a concrete
septic tank, similar to that of Fig. 153. The tank requires only the
cover to make it complete.]
“The design in Fig. 156 shows a septic tank as it would appear
if partly cut away to expose the interior to view, and as if cut
in half along a center line following its length. This type
will be found to operate effectively where final disposal is
accomplished by sub-surface irrigation. This system once started
is self-operating due to the siphon shown in the second, or
right-hand compartment, which at regular intervals empties the
contents and discharges them into the line of tile from which
the liquids leach out through joints into the soil. In a tank
constructed as shown in the design mentioned, it is very important
to use a siphon to empty the second compartment at intervals
instead of allowing a continuous outward flow of contents, because
of the tendency for drains to become clogged when liquids are
constantly trickling through.
“The size of tank required for residence use depends upon the
quantity of sewage to be handled in the first chamber during a day
of 24 hours; therefore, this compartment should be large enough
to contain an entire day’s flow. This frequently amounts to from
30 to 50 gallons per person per day, so the required capacity
can readily be computed from these figures, although it must be
remembered that the required depth for the tank should be figured
from the top of the concrete baffle wall or partition which
separates the first and second compartments. Another point to bear
in mind is that the width of the first compartment should be about
one-half its length.”