_SEWAGE DISPOSAL_
The subject of sewage disposal for a single house in the country does
not at all present the elaborate problem that is suggested when the
disposal of sewage of a city is under discussion. In the first place,
the amount of sewage to be dealt with is moderate in quantity; and in
the second place the area available on which the sewage may be treated
is in almost all cases more than ample for the purpose. Nor is there the
complication that arises with city sewage, due to the admixture of
manufacturing wastes. The material to be handled is entirely domestic
sewage and varies only according to the amount of water used in the
house, making the sewage of greater or less strength according as less
or more water is used. Sewage from a single house differs only in one
respect disadvantageously from city sewage, namely, in the fact that the
sewage, not having to pass through a long length of pipe, comes to the
place of disposal in what is known as a fresh condition; that is, no
organic changes have taken place in the material of which the sewage is
composed.
_Definition of sewage._
The great bulk of sewage is water, and, in quantity, the amount of
sewage to be cared for is about equal to the amount of water consumed
in the household, although this will depend somewhat on the habits of
the family. If, for example, part of the water-supply is used for an
ornamental fountain in the front yard, or if in the summer time a large
amount of water is used for sprinkling the lawns, that water is not
converted into sewage, and the amount of the latter is thereby
diminished; but, ordinarily, it is safe to say that the quantity of
water supplied to the house and the quantity of sewage taken away from
the house is identical, and since it is much easier to measure the
water-supply than the sewage flow, the former is taken as the quantity
of sewage to be treated.
In the course of its passage through the house, however, the water has
added to it a certain amount of polluting substances, largely derived
from the kitchen sink, where dirt from vegetables and particles of
vegetable material, together with more or less soap, are carried by the
waste water from the sink into the drain. In the bath-room, also, some
small amount of organic matter is added to the water, but the proportion
of such matter to the total volume of water used is very small, probably
not exceeding one tenth of one per cent. This small proportion is
nevertheless sufficient to become very objectionable if allowed to
decompose, and the problem of sewage disposal for a single house is to
drain away the water, leaving behind the solids so disposed that they
shall not subsequently cause offense by their putrefaction.
The process of decay is normal for all organic matter and is due to the
agency of certain bacteria whose duty it is, providentially, to
eliminate from the surface of the earth organic matter which otherwise
would remain useless, if not destructive, to man. It is impossible to
leave any vegetable or animal matter exposed to the air without this
process of decay at once setting in. Apples left in the orchard at the
end of the season inevitably are reduced and disappear in a short time.
Dead animals, whether large or small, in the same way succumb to the
same process of nature, and it has been pointed out that, unless this
provision did exist, the accumulation of such organic wastes since the
settlement of this country would be so great as to make the country
uninhabitable. Fortunately, however, this inevitable process breaks down
the structure of all organic material, partly converting fiber and pulp
into gas, partly liquefying the material and converting the remainder
into inorganic matter which is of vast importance as food for plant
life. A cycle is thus formed which may be best illustrated in the case
of cows which feed on the herbage of a meadow, the manure from the cows
furnishing food for the grass which otherwise would soon exhaust the
nutriment of the soil.
_Stream pollution._
The first fundamental principle of sewage disposal, therefore, is to
distribute the organic matter in the sewage so that these beneficent
bacteria may most rapidly and thoroughly accomplish their purpose.
During the last fifty years, a great deal of study has been expended on
this problem, and while it has not as yet been entirely solved, certain
essential features have been well established.
The most important factor promoting the activity of these agents of
decay is the presence of air, since in many ways it has been proved that
without air their action is impossible. Thus it has been shown that
discharging sewage into a stream, whether the stream be a slow and
sluggish one or whether it be a mountain stream churned into foam by
repeated waterfalls, has little other power to act on organic matter
than to hold it for transportation down stream, or to allow it to settle
in slower reaches until mud banks have been accumulated which will be
washed out again at the first freshet. Experiments have shown that the
agencies to which certain diseases are attributed, commonly known as
pathogenic bacteria, are frequently, if not always, found in sewage, and
that when these bacteria are discharged into streams they may be carried
with the stream hundreds of miles and retain all their power for evil,
in case the water is used for drinking purposes. No right-minded person
to-day will so abuse the rights of his fellow-citizens as deliberately
to pour into a stream such unmistakable poison as sewage has proved
itself to be. The fact is so well known that it is not worth while
pointing out examples. It is enough to say that some of the worst
epidemics of typhoid fever which this country has known have been traced
to the agency of drinking water, polluted miles away by a relatively
small amount of sewage.
In a number of states, laws have been passed which expressly prohibit
the discharge of sewage, even from a single house, into a stream of any
sort, even though the stream is on the land of the man thus discharging
sewage and where it would appear as if he alone might control the uses
of that stream. Unfortunately, the machinery of the law does not always
operate to detect and punish the breakers of the law, but any law which,
as in this case, has so positive a reason for its existence, and
violation of which is so certain to bring disaster on persons drinking
the water of the stream below the point where the sewage is discharged,
any law which appeals for its enforcement so directly to the common
sense and right feeling of all intelligent people, seems hardly to need
legal machinery for its enforcement. It must depend, as indeed all laws
must depend, upon the intelligent support of the community, and surely
no law would commend itself more urgently than this one forbidding the
pollution of drinking water.
In spite of the fact that the lack of air in the water will prevent
bacterial action, there are, nevertheless, many cases where the
discharge of sewage into a stream may be permitted as being the best
solution of the disposal problem, provided always that the stream is not
used and is not likely to be used for drinking water. Such cases occur
where the stream is relatively large and where the level of the stream
is fairly regular, so that there is no likelihood of the deposit of
organic matter on the banks during the falling of the stream level.
Examples of this sort might be cited in the vicinity of the Mohawk or
Hudson River, or in the vicinity of any of the larger rivers of any
populous state, since although the water of the Mohawk is used by the
city of Albany for drinking purposes, yet the amount of organic matter
which inevitably finds its way into such rivers precludes its use for
drinking without filtration. Into the Hudson below Albany there can
hardly be any question of the propriety of discharging sewage from a
single house.
Again, houses in the vicinity of large bodies of still water may without
question be allowed to discharge into those lakes. For example, houses
in the vicinity of Lake Ontario or Lake Michigan, or even of much
smaller lakes, should not contribute any offensive pollution to the
waters of the lake. In New York State, some of the smaller lakes are
used as water-supplies for cities, as, for example, Owasco Lake for the
city of Auburn and Skaneateles Lake for the city of Syracuse, and,
acting under the statutes, special laws have been passed by the State
Department of Health, forbidding any discharge of any kind of household
wastes into these lakes. The same is done in other states. Here, again,
it is a question of the drinking supply which is being considered, and
not a question of the possibility of any nuisance being committed.
_Treatment of sewage on land._
If no stream suitable for the reception of sewage is available, then the
sewage must in some way be treated on land before it passes into the
nearest watercourse. For the second fundamental principle about the
treatment of sewage is that of all places the action of putrefactive
bacteria is most energetic in the surface soil and that it is there that
the organic matter of sewage can be most rapidly accomplished.
Experiments already referred to have shown not only this, but also that
their activity is most noticeable in the surface layers of the soil and
that their action continues for scarcely two feet downward, and it is
customary to assume that the largest amount of work done is accomplished
in the top twelve inches. Further than this, it has been established
that in order to persuade the bacteria involved to do their work as
promptly as possible, the application of sewage to any particular
locality should be made intermittent; that is, that a resting period
should be given to the bacteria between successive applications of
sewage.
For example, one can recall without difficulty the conditions on the
ground at the back of the house where the kitchen sink-drain commonly
discharges. At the beginning of summer perhaps a rank growth of grass
starts up vigorously in the vicinity, and the path of the surface drain
can be traced by the heavy vegetation along the line of the drain. If
the slope of the surface away from the house is considerable, no other
effect may be noticed through the season, since the surface slope
carries away the sewage, spreading it out over the ground so that the
soil really has a chance to breathe between successive doses. But if the
ground is flat, it will be remembered that before many weeks the sewage
ceases to sink into it; the ground becomes "sewage-sick," as they say in
England, and a thick, dark-colored pool of sewage gradually forms, which
smells abominably. If a piece of hose a dozen feet long had been
attached to the end of the drain and each day shifted in position so
that no particular spot received the infiltration two days in
succession, it is probable that no such pondage of sewage would occur,
but that the mere intermittency of the application thereby secured would
permit the successful disposal of this sink waste throughout the season.
The same effect is to be noted in some cesspools where, because of the
great depth to which they are dug and because no overflow into the
surface layers of the soil is provided, the pores of the ground around
the cesspool become clogged and choked, and the cesspool becomes filled
with a thick, viscous, dark-colored, objectionable-looking, and
evil-smelling liquid.
The three principles which will avoid these conditions are, as already
stated, plenty of air, presence of bacteria normally found in the
surface layers of the soil, and intermittency of application.
In order to secure the operation of these three principles in the
application of sewage onto land, the sewage must be made to pass either
over the surface of the land in its natural condition in such a way that
the sewage may sink into the soil and be absorbed and at the same time
give up its manurial elements to whatever vegetation the soil produces;
or, as a modification of this principle, the sewage may be required to
pass through an artificial bed of coarse material by which the rate of
treatment may be considerably increased. In the latter case, although
probably the greater part of the action of the bacteria takes place in
the top twelve inches, it is customary to make the beds about three feet
thick, chiefly in order to prevent uneven discharge of the sewage
through the bed. Finally, wherever, for aesthetic reasons, it is
desirable that the sewage should not be in evidence, either before
passing through the natural soil or exposed in an artificial bed, the
practice may be resorted to of distributing the sewage through
agricultural tile drain laid about twelve inches below the surface. In
this way, the sewage is scattered through the top soil, where bacteria
are most active, without being apparent, and a front lawn thus treated
would not give any indication of its use.
Taking up now in order these three methods of treatment, we may consider
some of the details of construction. In spreading the sewage over the
lawn or in distributing it on the surface, due regard must be paid to
the kind of soil. Clay soils and peaty soils are useless for the
purpose of sewage disposal unless as the result of continuous
cultivation a few inches of top soil may have accumulated on the clay.
This top soil is adapted to sewage purification, provided the quantity
applied is not excessive.
_Surface application on land._
Two methods of operation may be pointed out. The sewage (and this is the
simplest method of disposal possible) may be brought to the upper edge
of a small piece of ground, usually sowed to grass, and allowed merely
to run out over the surface of the ground. There should be, however,
some method of alternating plots of ground, one with another, so that
the sewage is turned from one to the other every day. Each plot will
then have one day's application of sewage and one day's rest, and this
would complete the disposal, were it not for the interference of rain
and cold. The winter season practically puts a stop to this method of
treatment, and rainy weather reduces the power of the soil to absorb
sewage. For these two reasons, it is desirable to have one plot in
reserve, or three in all, and the area of each plot should be based on
the amount of sewage contributed. For a family of ten persons using
twenty-five gallons of water per day the total area provided should be
one tenth of one acre, or an area seventy feet square divided into three
plots. Figure 67 shows six beds arranged to care for the sewage of a
public institution in Massachusetts. As a guide to the amount of land
needed, it will be safe to provide at the rate of one acre for each
forty persons where the soil is a well-worked loam but underlaid with
clay. The effect of this irrigation on the grass will be to induce a
heavy, rank growth which must be kept down by repeated cutting or by
constant grazing. Both methods are practiced in England, and it may be
said in passing that no injury to stock from the feeding of such
sewage-grown grass has been recorded. The grass cut from such areas (and
the cutting is done every two weeks through the whole summer) is packed
into silos and fed to cattle through the winter with advantage. Or, if
grazing is resorted to to keep the grass down, the herd is alternated
with the sewage from one field to the other, so that the bed which has
received sewage one week is used for pasture the next week, and the
number of head which can thus be fed is astonishing. In order to secure
an even flow of sewage over such grass land as is here contemplated,
there must be a gentle slope to the field, and the ditch or drain
bringing the sewage to the field should run along its upper side.
Openings from the drain, controlled by simple stop planks, are provided
at intervals of about ten feet, and no attention is needed further than
the opening and closing of these admission gates.
[Illustration: FIG. 67.--Sewage beds.]
Another method of applying sewage to the surface of the ground is to
lead it in channels between narrow beds on which vegetables have grown.
These beds are made about eight feet wide with two rows of root crops,
such as turnips or beets, set back about two feet from the edge. The
beds are made by properly plowing, the channels between the beds being
back-furrowed. Here, again, the principle of intermittent application is
essential, and the area to be provided is the same as already given for
the surface irrigation. Three beds should be provided, as before; but,
in general, no provision need be made for carrying off the sewage at the
lower end of the beds, since it may be safely assumed that all of the
sewage will be absorbed by the soil. Of course, a sandy soil will
absorb more water than a clay soil, and if the soil is entirely clay, it
is not suitable for such treatment. Sewage passed over the surface of
clay soil, however, will, in the course of a few months, so modify the
clay as to convert it into a loam, and in this way increase its
absorptive power.
When possible, it is desirable to have a plot of plowed ground over
which the sewage may pass before reaching the beds, so that the grosser
impurities may be left behind and harrowed in or plowed under. If proper
regard is paid to intermittent application, no danger from odors need be
feared, and the repeated plowing in will increase immensely the
fertility of the soil. Nor need one be afraid that all of the manurial
elements will be left behind on this plowed ground. About two thirds of
the organic matter in sewage is in solution, and this will be carried
onto the beds just as if passage over the plowed ground had not
occurred.
_Artificial sewage beds._
In order to secure a higher rate of discharge of sewage through the soil
it is best to arrange an artificial bed which shall be made of coarse,
sandy material which will allow a rate of at least 10 times that already
given. The best material out of which to make such an artificial bed is
a coarse sand; that is, a sand whose particles will not pass through a
sieve which has 60 meshes to the inch and which would pass through a
sieve of 10 meshes to the inch. Such an ideal sand will purify sewage at
the rate of 50,000 gallons per acre per day, or an acre will take care
of the sewage of at least 1000 persons. This means that it is necessary
to provide about 50 square feet for each person in the family, or a
family of 10 persons could have all the sewage taken care of on an area
25 feet square. The same principle of intermittency of application,
however, must be observed by dividing the bed into three parts, so that
the sewage may be alternated from one bed to another. Practice has
indicated that it is better to shift from bed to bed about once a week
and to deliver the sewage onto each bed intermittently; that is, to
discharge a bucketful at a time with short intervals between, rather
than to allow a small stream to flow continuously onto a bed. Such a bed
should be about 3 feet deep, as already stated, and preferably should
have light concrete side walls and bottom, as shown in the sketch (Fig.
68). Ordinarily, the surface of the sand will be level, and the dose of
sewage applied to the bed will cover it a fraction of an inch deep, and
in the course of an hour or so will disappear into the sand and reappear
in the underdrains as clear water.
[Illustration: FIG. 68.--Sewage beds.]
In cold weather a thin sheet of sewage spread out over the surface of
the sand would freeze before penetrating the bed; therefore, in the
winter time, it is usual to furrow the beds; that is, dig furrows across
the beds 2 or 3 inches wide at the bottom and about 10 inches deep, so
that in the bottom of these furrows the sewage may be, partly at least,
protected against frost. It has been found that, if sewage is discharged
intermittently,--that is, in bucketfuls into such furrows,--the beds
open and allow the filtration of the sewage. To be sure, the
purification effected in cold weather is not quite that accomplished in
warm weather, but the results are sufficiently satisfactory, and no
nuisance ensues.
_Subsurface tile disposal._
The other method of distributing sewage over land is by means of
draintile placed in shallow trenches, so that the sewage may leach out
into the soil through the open joints of the pipe. These draintiles
receive the sewage intermittently, and by the constant rush of water are
presumably filled throughout their length. The sewage then gradually
works out of the joints into the surrounding soil, and the pipes are
empty and ready to receive another dose when next delivered.
Two essential points must be considered in the successful operation of
such a plant: the grade of the tile and the length of the tile.
The grade of the tile must be properly adjusted to the porosity of the
soil; that is, in open, porous, and gravelly soils a grade must be
steeper than in loamy and dense soils. The reason is manifest. In a
gravel soil, the sewage is at first rapidly absorbed, so that as the
sewage goes down the pipe line the first joints take up the water and
deliver it to the soil, where it disappears, and probably no flow
reaches the end of the line at all. This means that the soil surrounding
the first joints does the work which the entire pipe line was intended
to do and thus becomes overworked. When overworked, the soil always
refuses to do anything, so that when the succeeding joints take up the
sewage and in their turn become overworked, the line is useless. If, on
the other hand, the grade had been steep enough to carry the sewage down
the pipe line gradually so as to secure a uniform distribution, then the
same or approximately the same amount of sewage would be taken out of
the pipe at each joint, securing a long life for the system. In loamy
soil, on the contrary, there is not the same absorption at the joints,
and so on a steep grade there is the tendency for all the sewage to
follow down the pipe line to the lower end and there escape to clog the
soil and thus spoil the system. As a general average, it may be said
that the proper grade for such a subsurface distribution pipe line in a
fairly good sandy loam should be 5 inches in 100 feet; less than this as
the loam becomes clay and more as the loam becomes gravel.
The other essential point for the successful operation of this method
of distribution is to provide a proper length of pipe for the number of
persons contributing sewage. The soil itself will absorb about the same
amount as when the sewage is spread over the surface, so that a family
of ten persons would require, as before, an area about 70 feet square.
The pipe lines may be laid in different sections, provided the different
lines of pipe are not nearer together than 10 feet. On an area 70 feet
square there would be, therefore, 7 lines of pipe each 70 feet long, or
490 lineal feet of pipe in all, or 49 feet per person. The writer
generally allows 40 feet in well-cultivated soil as a reasonable length
of pipe for each person in the family. If the soil is sandy, this may be
reduced one half, but need not be increased under any conditions, since
a soil requiring a greater length of pipe than 40 feet per person would
be so dense as to be unfit for use. To properly arrange the lines of
pipe on a sloping ground requires careful study of the inclination of
the ground and of the relation of direction of lines of pipe to slope.
Usually the slope of the ground is greater than the 5 inches per 100
feet just referred to, but by laying out the lines of pipe across the
slope instead of with it any grade desired may be obtained. Nor is it
necessary that these lines of draintile be run in straight lines; they
may very properly follow the curving slope, the proper grade being
always carefully maintained.
[Illustration: FIG. 69.--Plan of subsurface irrigation field.]
Common agricultural tiles three inches in diameter and costing about two
cents per running foot are suitable material for these distribution
lines. The sewage enters these distribution lines from a larger pipe,
usually six inches in diameter, and a difficult adjustment is presented
that each branch tile line shall receive its own proportionate share of
the sewage. If only one line of tile is provided, say 200 feet long for
5 members in the family, then all the sewage goes into that line with no
question of distribution arising, but if a number of short parallel
lines must be used, as shown in the sketch (Fig. 69), the difficulty of
subdividing the sewage properly among the different branch lines becomes
very great. For that reason the writer prefers to use not more than two
lines, with the possibility of delivering the sewage alternately in the
one and the other. In this way, the bed not receiving sewage is resting,
while the other bed is acting, and also the outlet for the sewage is
always definitely known. And particularly in the case of these
subsurface tile, the necessity for the intermittent dosing is apparent,
since with small, constant trickling discharges the difficulty of
distribution through the long length of tile is gradually increased, and
usually saturation of the soil occurs from joint to joint, as already
described. Therefore it becomes most necessary, in this case, for the
best results on the soil not merely to alternate the beds receiving
sewage, but also to effect the intermittent discharge onto the beds or
through the pipes although the sewage itself may flow very uniformly in
volume.
_Automatic syphon._
This intermittent discharge is accomplished by constructing on the pipe
line from the house and before it reaches the beds an "automatic
syphon," as it is called, the operation of which may be described as
follows: As the sewage enters the tank containing the syphon and rises
outside the syphon-bell, air is compressed between the water surface
inside the bell and the water left inside the syphon-leg. With greater
and greater height of water outside, this compression inside becomes
greater and forces the water in the syphon-leg lower and lower. Finally,
the water sinks so low as to allow the compressed air to escape suddenly
around this bend, instantly relieving the compression, and the water
outside rushing in to fill up the space occupied by the air starts the
syphon (see Fig. 70).
[Illustration: FIG. 70.--Section of "Miller" syphon.]
This syphon, in size suitable for a single house, costs about $12
delivered, and will always be available to secure an intermittent dosing
of the bed or pipe line. Usually the chamber in which this syphon is
placed holds about one hour's flow, so that it may be estimated that
this syphon will discharge on the bed every sixty minutes. The exact
interval of time is not essential nor, perhaps, important, although it
may be noted that the coarser the material,--that is, the nearer uniform
all the sand particles are to the largest size passing the ten-mesh
size,--the smaller must be the dose applied, but the more frequently
must the application be made. This has been very thoroughly studied in
Massachusetts, and the views of experts on this subject may be found in
the report of that Board.
Such an intermittent discharge may be made and often is made by a hand
valve leading out from this chamber in institutions or in private houses
where some one constantly is available for the purpose. Thus it becomes
the duty of the man in charge every hour or perhaps three times a day to
pull the valve and allow the sewage to discharge (see Fig. 71). An
overflow pipe should always be provided, so that if he forgets to pull
the valve, the sewage will still find its way into the system rather
than out on the ground.
[Illustration: FIG. 71.--Plan and section of a septic tank with valve.]
_Sedimentation._
As a matter of economy of operation, it has been found desirable to take
out from the sewage before the treatment already described as much of
the solid matter as may be reasonably done, and for this purpose
sedimentation is made use of. Most of the solids in sewage are slightly
heavier than water, so that if they be allowed to stand in the water for
a short length of time, they will settle to the bottom of the tank and
allow the liquid above to pass on, considerably clarified. It has been
found worth while to do this, since all three processes described are
interfered with if the solids taken out by sedimentation are allowed to
be deposited either upon the surface of the ground, giving rise to odors
as well as to objectionable appearances, or onto the surface of the sand
beds, which they clog up, or in the three-inch tile drain, which may be
filled in a short time.
It has been further found by experience that if these sedimentation
tanks are made large, really larger than necessary for sedimentation, in
some way a large proportion of the matter accumulating in the tank will
disappear, so that the amount of sediment to be taken out of the tank is
not as large as might be expected. In fact it is usual for such tanks to
run one or two years without cleaning, although the amount of solids
shown by chemical analysis to have been removed from the sewage would
fill the tank twice over.
It has been found that a tank, in order to do successful work in
separating solids and in eliminating as much as possible of the
sediment, needs to be of a capacity to equal about one day's flow of the
sewage, and this is a good basis for computation. Here, again, the fact
that the sewage from a single house is considerably fresher than the
sewage from a city must be remembered, since, while many cities build
tanks holding only one third or one fourth of their daily flow with good
results, in the case of a single house this is not possible, and the
tanks, if built at all, ought to hold at least the full day's flow. Ten
persons, at 25 gallons each, furnish 250 gallons per day or 33 cubic
feet. The tank, then, must be large enough to hold this volume, and
suitable proportions generally require that the tank be at least 5 times
as long as wide. A certain allowance must always be made for deposit in
the bottom and for the accumulation of scum on the top, so that an extra
foot or more of depth is desirable. The tank, then, to furnish the
required 33 feet, might be made 3 feet wide, 3 feet deep, and 5 feet
long, and probably in no case would a tank much smaller than this be
used.
[Illustration: FIG 72.--Section of a septic tank with syphon chamber.]
There are two or three details of tank construction which may be
suggested, although almost any kind of tank will answer the purpose. It
is desirable in order that the surface scum may not be disturbed, and in
order that the inflowing sewage may distribute itself as uniformly as
possible across the tank, to attach an elbow to the entering pipe so
that the sewage enters about halfway between the top and bottom of the
tank (see Fig. 72). Similarly, at the outlet or weir an elbow should be
provided because it is not desirable to allow the floating matter of the
surface to be carried onto the bed, and a pipe taking off liquid, open
halfway between top and bottom, will carry away but little of either the
surface scum or bottom sediment. Such a tank must be built of concrete
or masonry or timber, although the latter is not to be recommended
because of its short life. The walls of an ordinary tank may be built 6
inches thick at the top and 12 inches to 18 inches thick at the bottom,
the latter being necessary if the depth is over 8 feet. The tank should
have 6 inches of concrete on the bottom, and the roof may be made of
flagstone or of concrete slabs in which some wire mesh has been buried.
It is not necessary to ventilate this tank, although it is desirable to
have perhaps a foot of air-space between the water level and the roof of
the tank. During the first few months of its operation such a tank is
very likely to smell badly, and, if ventilators are provided, the
presence of the tank will be well known by the odors sent off. After the
tank has been in operation two or three months these odors gradually
disappear, due presumably to the fact that the surface of the water in
the tank has become coated with a thick blanket through which odors
cannot penetrate. On the other hand, there have been a few cases
recorded where the production of gas in a septic tank was so great that
an explosion occurred, tearing off the roof and otherwise doing
considerable damage.
The full plant, therefore, will consist of the settling tank, receiving
the raw sewage from the house and discharging it into a small tank
holding about one hour's flow and containing the automatic syphon
apparatus for intermittent discharge. This dosing tank must provide for
one hour's flow at the maximum rate of flow, and should hold about one
fourth of the total daily flow. Then the ground area, either natural or
artificial, which receives the intermittent discharge from the dosing
tank, completes the installation (see Fig. 73).
[Illustration: FIG. 73.--Plan of sewage disposal for single house with
details of receiving tank.]
_Underdrains._
The question of installing underdrains will arise only in cases where
the ground water, always to be found below the surface somewhere, comes
up so high as to affect the disposal of sewage. Usually no underdrains
will be needed unless the ground water gets up to within three feet of
the surface, and, in a number of cases, underdrains have been laid under
a sewage filter at considerable expense, only to find when the filter
was in operation that they were never in use. In clay soils the
underdrain is not necessary. In fact, it may be noticed that the
underdrain is not for the purpose of taking care of the sewage, but
rather of draining off the soil-water and preventing its interference
with the action of soil on sewage. This principle will indicate where
underdrains are necessary and where not.
When used, underdrains should be laid from three to four feet below the
surface in parallel lines about fifteen feet apart and on grades of not
less than one foot in one hundred. It is always better to have the
underdrains too large than too small, and drains less than three inches
in diameter should not be used, and they should increase in size to four
inches and then to six inches as the separate drains are brought
together. The writer has seen a six-inch underdrain running full of
ground water collected within a distance of a hundred feet, but this was
in gravel soil through which the water passed very freely. No exact
rules can be given for the size of the underdrains, but it will be
noticed that, since water passes through clay soil slowly and through
gravel soil rapidly, larger pipes must be used where the soil is
coarse.