SUB-SURFACE IRRIGATION
The disposal of sewage by the method of sub-surface irrigation,
sometimes known as the Waring system, consists in its distribution by
means of open jointed tiling over a comparatively large area of soil and
at a depth of a few inches beneath the ground surface. The sewage should
first be passed through settling tanks to remove as much as practicable
of the suspended matters contained in it, as explained in the chapter on
settling tanks. The partially clarified effluent from the settling tank
should then be collected in a dosing chamber, or separate compartment of
the settling tank, and discharged intermittently, preferably by means of
a siphon, into the sub-surface irrigation system. This intermittency of
discharge of accumulated quantities of effluent is necessary for an even
distribution of the effluent throughout the entire system of sub-surface
tiling, and for a continuous and successful operation of the system as a
whole. It has been found necessary, also, to alternate the discharge of
effluent from the dosing or siphon chamber over different portions of
the irrigation area. One siphon is all that is necessary to install in
the siphon chamber for sub-surface irrigation systems, and if the
settling tank has two compartments, as shown in Fig. 3, Chapter II, the
single siphon would be placed in the centre of the chamber.
The principle involved in this method of sewage purification is that of
any general method of sewage reduction in whatever form carried on,
namely, its oxidation or nitrification. This oxidation, or breaking down
of the organic matter in the sewage, is accomplished in this case, as in
the case of intermittent sand filters, contact beds, and sprinkling
filters, through the agency of bacterial action.
Householders have long been familiar with the fact that although solids
contained in sewage may have been discharged for long periods of time
into a cesspool, the latter, if located in dry, porous soil, did not
seem to become filled with the solid residue. This is due to the
liquefaction of the solid matter in the sewage after its discharge into
the cesspool, and to the seepage and bacterial reduction of the liquid
matter in the surrounding soil. To replace the cesspool and eliminate
the insanitary conditions which, in most instances, result from its use,
other methods have been devised which utilize the agencies of nature to
the best advantage. Thus the sedimentation and, in some cases, the
liquefaction of the solid matters in sewage are carried on in specially
designed settling tanks which are easily cleaned and which provide for
greater efficiency in settling out suspended matters than the cesspool.
Similarly, the filtration of liquids from cesspools through the soil is
replaced by the scientific method of sub-surface irrigation, which is
much more efficient in three distinct ways: (1) the limited seepage area
represented by the walls of the cesspool is increased many times by
distributing the effluent from the settling tank over a large area of
soil in a system of sub-surface tiling; (2) the bacterial reduction is
more effective, since it has been found that the bacterial action
necessary to purify sewage takes place in the upper layers of the soil
and is almost absent at depths of five feet or more; (3) the soil is
given an opportunity to rest and to dry out by alternately using
different portions of the sub-surface irrigation system. In the cesspool
the seepage of the effluent and whatever bacterial action takes place in
the surrounding soil must go on continuously, which often results in
clogging of the soil and overflowing of the cesspool.
The purification and final disposition of sewage by means of sub-surface
irrigation is the method best adapted to the single residence, and
oftentimes to the hotel or institution, if soil conditions are favorable
and proper area is available. This system requires less oversight in its
operation than the various forms of artificial filters. Furthermore, the
sewage is entirely hidden from sight after it leaves the settling tank,
and this is usually desirable near private residences and on the grounds
of country homes, country clubs, and summer hotels. Also, where the
sewage must be treated in close proximity to a residence or hotel or at
a point on the windward side of a residence, this method, more
effectually than any other, precludes the possibility of a nuisance
resulting from the operation carried on, since the settling-tank
effluent is at no point exposed directly to the air. Furthermore, its
cost is less than that of other works for final treatment of sewage,
and, finally, the system is more easily installed.
The method is in reality modified broad irrigation, but the sub-surface
irrigation field can be utilized much more effectively and with
considerably less attention than a broad irrigation area, and, as noted
above, is less liable to be the cause of a nuisance or to be the means
of spreading infectious disease through the agencies of flies and other
insects.
If an area of sandy soil is available on which to locate the sub-surface
irrigation field, if the settling tank and siphon chamber have been
correctly built, and if the sub-surface tiling system has been properly
laid, the success of the system is well assured. On the other hand,
failure is certain if either broad irrigation or sub-surface irrigation
methods of sewage disposal are attempted on stiff, impervious clay
soils. Between the ranges of porosity of soil represented by these
limits there are many soils in which sewage may be successfully disposed
of by sub-surface irrigation. A sandy or gravelly loam will, without
question, successfully care for sewage effluent when such effluent is
properly distributed by sub-surface tiling, and even in a rather heavy
soil the effluent from a settling tank may often be disposed of
satisfactorily by providing for a greater length of sub-surface tiling
per person served by the settling tank than that which would suffice in
the more porous soils. However, if the soil is very heavy so that
surface water does not readily seep away, or if the ground-water level
is within two or three feet of the surface, this method is not suitable
and some form of filter, described in the succeeding chapter, should be
used for final treatment of sewage.
When soil conditions and the area available are favorable to this method
and such a system is to be installed, the irrigation area selected
should be at the point where the ground-water level is lowest, and this
will generally be on a plateau or bench at the head of a slope of
ground. The relative elevation of the ground surface should, of course,
be low enough to insure operating head or fall to operate the siphon in
the chamber adjoining the settling tank and to distribute the effluent
by gravity to the sub-surface tiling. If the soil is composed of loose
gravel, or lies over a limestone or shale formation, the location of the
irrigation area should be selected with a view to preventing the
contamination of any wells or springs which may exist on the
premises,—that is, the area should be on lower ground, and as far
removed from wells as is convenient.
As will be explained later, the length of sub-surface tiling necessary
to receive a given quantity of sewage effluent should vary, within
certain limits, with the character and porosity of the soil, thus
requiring larger quantities of effluent to be delivered from the siphon
or dosing chamber in the case of the more compact soils. Also the size
of this chamber should be determined with reference to the number of
sections into which the sub-surface tiling system is divided.
TABLE II
FOR USE IN THE CONSTRUCTION OF SUB-SURFACE IRRIGATION SYSTEMS IN SANDY OR
VERY POROUS SOILS
Dimensions given are for inside measurement.
══════╤═══════╤═══════╤═══════╤═══════╤═══════╤════════╤════════╤════════
Number│ Total │No. of │ Mean │ Mean │ Depth │Diameter│ Dis- │Distance
of │Length │ Sect- │ Width │Length │ of │ of │charging│ from
Per- │ of │ions in│ of │ of │Siphon │ Siphon │Depth or│Roof to
sons │3–inch │ Sub- │Siphon │Siphon │Chamber│ (Inch- │Maximum │ Top of
Served│ Sub- │surface│Chamber│Chamber│ from │ es). │Depth of│ Wall
by │surface│System.│(Feet).│(Feet).│Roof of│ │Effluent│between
Sewer.│Tiling.│ │ │ │ Tank │ │ above │Settling
│ │ │ │ │(Feet).│ │ Lower │Tank and
│ │ │ │ │ │ │Edge of │ Siphon
│ │ │ │ │ │ │ Siphon │Chamber
│ │ │ │ │ │ │ Bell │ (Inch-
│ │ │ │ │ │ │ (Inch- │ es).
│ │ │ │ │ │ │ es). │
──────┼───────┼───────┼───────┼───────┼───────┼────────┼────────┼────────
4│ 140│ 2│2´ │2´ │ 2´ 4˝│ 3˝│ 13˝│ 12˝
8│ 280│ 2│2´ 6˝ │2´ 6˝ │ 2´ 4˝│ 3˝│ 13˝│ 12˝
12│ 420│ 2│2´ 6˝ │4´ │ 2´ 4˝│ 3˝│ 13˝│ 12˝
15│ 525│ 2│3´ 6˝ │4´ │ 2´ 4˝│ 3˝│ 13˝│ 12˝
25│ 875│ 2│3´ │4´ │ 3´ 2˝│ 5˝│ 23˝│ 12˝
35│ 1225│ 2│3´ 8˝ │4´ 6˝ │ 3´ 2˝│ 5˝│ 23˝│ 12˝
50│ 1750│ 2│4´ │6´ │ 3´ 2˝│ 5˝│ 23˝│ 12˝
75│ 2625│ 2│3´ │6´ │ 3´ 2˝│ 5˝│ 23˝│ 16˝
100│ 3500│ 2│4´ │7´ │ 3´ 2˝│ 5˝│ 23˝│ 16˝
125│ 4375│ 3│5´ 6˝ │8´ │ 3´ 2˝│ 5˝│ 23˝│ 16˝
150│ 5250│ 3│7´ │8´ │ 3´ 2˝│ 5˝│ 23˝│ 16˝
175│ 6125│ 3│7´ 6˝ │8´ │ 3´ 9˝│ 6˝│ 30˝│ 16˝
200│ 7000│ 3│8´ │8´ │ 3´ 9˝│ 6˝│ 30˝│ 18˝
──────┴───────┴───────┴───────┴───────┴───────┴────────┴────────┴────────
TABLE III
FOR USE IN THE CONSTRUCTION OF SUB-SURFACE IRRIGATION SYSTEMS IN THE
HEAVIER LOAMS (NOT CLAY OR HARDPAN)
Dimensions given are for inside measurements.
══════╤═══════╤═══════╤═══════╤═══════╤═══════╤════════╤════════╤════════
Number│ Total │No. of │ Mean │ Mean │ Depth │Diameter│ Dis- │Distance
of │Length │ Sect- │ Width │Length │ of │ of │charging│ from
Per- │ of │ions in│ of │ of │Siphon │ Siphon │Depth or│Roof to
sons │3–inch │ Sub- │Siphon │Siphon │Chamber│ (Inch- │Maximum │ Top of
Served│ Sub- │surface│Chamber│Chamber│ from │ es). │Depth of│ Wall
by │surface│System.│(Feet).│(Feet).│Roof of│ │Effluent│between
Sewer.│Tiling.│ │ │ │ Tank │ │ above │Settling
│ │ │ │ │(Feet).│ │ Lower │Tank and
│ │ │ │ │ │ │Edge of │ Siphon
│ │ │ │ │ │ │ Siphon │Chamber
│ │ │ │ │ │ │ Bell │ (Inch-
│ │ │ │ │ │ │ (Inch- │ es).
│ │ │ │ │ │ │ es). │
──────┼───────┼───────┼───────┼───────┼───────┼────────┼────────┼────────
4│ 300│ 2│ 2´ 5˝ │ 3´ │ 2´ 4˝│ 3˝│ 13˝│ 12˝
8│ 600│ 2│ 3´ │ 5´ │ 2´ 4˝│ 3˝│ 13˝│ 12˝
12│ 900│ 2│ 4´ │ 5´ │ 2´ 4˝│ 3˝│ 13˝│ 12˝
15│ 1125│ 2│ 4´ │ 6´ 6˝ │ 2´ 4˝│ 3˝│ 13˝│ 12˝
25│ 1875│ 2│ 4´ │ 6´ │ 3´ 2˝│ 5˝│ 23˝│ 12˝
35│ 2625│ 2│ 4´ │ 4´ 6˝ │ 3´ 2˝│ 5˝│ 23˝│ 16˝
50│ 3750│ 2│ 4´ 6˝ │ 6´ │ 3´ 2˝│ 5˝│ 23˝│ 16˝
75│ 5625│ 2│ 6´ │ 7´ │ 3´ 2˝│ 5˝│ 23˝│ 16˝
100│ 7500│ 2│ 7´ │ 8´ │ 3´ 2˝│ 5˝│ 23˝│ 16˝
125│ 9375│ 3│ 8´ │10´ │ 3´ 9˝│ 6˝│ 30˝│ 20˝
150│ 11250│ 3│ 9´ │12´ │ 3´ 9˝│ 6˝│ 30˝│ 20˝
175│ 13125│ 3│10´ │12´ │ 4´ 2˝│ 8˝│ 35˝│ 20˝
200│ 15000│ 3│11´ │12´ │ 4´ 2˝│ 8˝│ 35˝│ 20˝
──────┴───────┴───────┴───────┴───────┴───────┴────────┴────────┴────────
The dimensions of siphon chambers to effectively deliver the effluent in
proper volumes to the sub-surface irrigation system are given in the
following tables, which indicate widths of siphon chambers to agree in
general with the widths of the settling tanks to serve a given number of
persons, as shown in Chapter II. These tables of dimensions for siphon
chambers provide for two different capacities where the same number of
persons are served by the sewer, depending on the total lengths of
sub-surface tiling required, which in turn depend on the character of
the soil in which the sub-surface system is laid. The tables provide for
a division of the sub-surface tiling system into two parts up to a
system for 100 persons, and into three parts for a greater number of
persons. These tables also show the total length of lateral distributing
tiling in the sub-surface irrigation system necessary to distribute over
a sufficient area at the irrigation field, in both sandy soils and in
the heavier loams, the various quantities of sewage to be treated in the
different-sized tanks and discharged from the siphon chambers. The
tables also indicate the diameter of the siphon and the discharging
depth of each siphon.
As discussed in Chapter III, the siphon, in discharging, may draw upon
the upper 4 to 8 inches of sewage in the settling tank without
interfering with the efficiency of the tank. The dimensions of siphon
chambers for 75 or more persons in Table II, and for 35 or more persons
in Table III (see page 59), provide for such a draught upon the
settling-tank contents of from 4 to 8 inches when the siphon discharges.
This will decrease the cost of the plant somewhat and provide for a more
efficient form of siphon chamber. The last column in each table provides
for the proper height of dividing-wall between the settling tank and
siphon chamber to allow the drawing down of the settling-tank contents
as noted above.
[Illustration: FIG. 26.—Plan and Section of Sub-surface Irrigation
System.]
[Illustration: FIG. 27.—Plan and Section of a Portion of a Sub-surface
Irrigation System.]
The sub-surface irrigation or distributing system consists of a main
carrier or effluent sewer leading away from the siphon chamber to the
irrigation field, of two or more branches of this main carrier, and of
parallel lines of lateral distributing tiling extending at intervals of
4 to 6 feet from the branch carriers, or, in some locations, from each
side of the branch carriers.
The frontispiece shows the relation between the several portions of a
sub-surface irrigation system. The house sewer is shown leading to the
settling tank, and from the siphon chamber adjoining the settling tank
the main carrier or effluent sewer is shown leading to a diverting
manhole from which the effluent is carried at each discharge of the
siphon to the lateral lines of sub-surface tiling by the two branch
carriers.
Fig. 26 shows in plan and section a sub-surface irrigation system. The
section, which is drawn to a larger scale than the plan, shows the
settling tank and the adjoining siphon chamber. From this siphon chamber
the effluent sewer carries the discharge from the siphon to the
diverting manhole, at which point the effluent is diverted to the
different portions of the sub-surface tiling.
In Fig. 27 is shown in plan the diverting manhole and a small portion of
the sub-surface tiling system together with a section through the
diverting manhole and one of the lines of distributing tiling.
The main carrier should be of vitrified tile sewer pipe with cemented
joints, and should always have two or more branches at the irrigation
field in order to allow the use of different portions of the field in
turn for three days or a week at a time, thus allowing one of the
portions of the field to be resting for corresponding periods. The
branch carriers should be of vitrified tile also, and should have
cemented joints. If the diameter of the siphon is 5 inches, the main
carrier should be of 8–inch vitrified tile with a fall of at least 6
inches per 100 feet in order to quickly carry the dose from the siphon
chamber to the several lines of sub-surface tiling forming the
distributing system. With 3–inch siphons, a 6–inch main carrier may be
used, but the gradient or fall of the main carrier should then be at
least 12 inches per 100 feet, owing to the smaller capacity of the
6–inch pipe. In placing the siphon in position, when the siphon chamber
is being built, care should be taken to see that the trap or U-shaped
pipe is set plumb or in a vertical position. Concrete should then be
placed around the siphon to hold it in proper position and at the proper
height, and the trap should be filled with water before the bell is
placed in position. The bell should then be placed in position over the
long leg of the trap to prevent the materials used in construction from
being dropped into the siphon. The siphon should be set so that the
lower edge of the bell, or of that portion of the bell under which the
effluent is to flow, is three inches above the floor of the siphon
chamber.
[Illustration: FIG. 28.—Y-Branch of Vitrified Tile Pipe.]
In laying the distributing system, every second or third length of the
branch carriers, according to the porosity of the soil and the spacing
of the lines of distributing tiling, should consist of a Y-branch (see
Fig. 28), to which a one-eighth bend (see Fig. 29) should be fitted if
the lines of lateral tiling are to be laid at right angles to the main
carriers, as shown in Fig. 27; or the lateral tiling may be fitted
directly to the Y-branch if the lateral lines are to be led away from
the carrier at an angle of 45°, as shown in Fig. 30. The Y should branch
from the lower portion of the pipe, as shown in Fig. 28.
[Illustration: FIG. 29—Eighth Bend of Vitrified Tile Pipe.]
[Illustration: FIG. 30.—Sub-surface Tiling.]
The lateral tiling should be of three-inch agricultural tile (see Fig.
31), laid with a space of one-quarter inch between each length and with
a piece of tar paper or a half-collar of larger diameter pipe, as shown
in Fig. 32, placed over the joints to prevent clogging of the pipe with
earth. In the heavier soils the lateral lines of sub-surface tiling are
sometimes set in trenches eight to fourteen inches deep and about twelve
inches wide, filled with broken stone or gravel placed around the tiling
to within two or three inches of the ground surface, as shown in Fig.
32. This allows the effluent to seep away more readily, but while of
advantage in those soils the provision is not necessary in the more
porous soils.
[Illustration: FIG. 31.—Photograph of Tile laid as if for Sewage
Disposal.]
It is generally found that a sufficient length of sub-surface tiling
should be laid to provide for not more than one to three gallons of
effluent per day for each linear foot of tiling. In sandy soils there
should be at least thirty to forty feet of tiling for each person served
by the sewer, with six feet of space between the lines of tiling. This
length per person should be increased up to seventy or eighty feet for
the more compact sandy or gravelly loams, or the lighter clay loams,
with the lateral tiling spaced four feet apart. It is not considered
feasible to attempt to dispose of sewage by sub-surface irrigation in
soils which will not care for effluent when the greater lengths of
tiling per person, as stated above, will not prevent the appearance of
effluent on the surface. If, however, after the installation of a
sub-surface system in a rather heavy soil, it is found that proper
seepage of the effluent does not occur, the lateral branches may
sometimes be lengthened and the system then found to operate
satisfactorily.
[Illustration: FIG. 32.—Sub-surface Tiling with Broken Stone or Gravel
Surrounding Pipe.]
The lines of lateral tiling should be laid with the invert, or bottom of
the pipe, inside, from six inches to one foot below the surface, as
shown in Fig. 27. They should be parallel with the contours or at right
angles with the slope of the field, and should have a gradient or fall
of one-sixteenth of an inch to the foot when laid in sandy soil or sandy
loam, and of not more than one thirty-second of an inch to the foot when
laid in the heavier loams. To obtain such gradients for the sub-surface
tiling it is sometimes necessary to lay out the trenches along irregular
or curved lines, as shown in Fig. 33. The tiling should be laid near the
surface, as stated, and never deeper than twelve inches. The temperature
of the sewage will prevent its freezing even in very severe winter
weather, especially when the ground is covered with snow.
[Illustration: FIG. 33.—Sub-surface System on Irregular Ground.]
To provide for diverting the flow from the siphon chamber first into one
of the two portions into which the sub-surface system is divided, and
then, after an interval of three days or a week, into the other portion
of the system, at the point where the main carrier is to branch, a
ten-inch iron pipe casting (see Fig. 34), with its lower portion forming
the body of a double Y-branch of six-inch or eight-inch pipe, may be
placed, having a swinging blade or gate attached inside in a vertical
position. When, for example, the effluent has been passed for a week
into section B of the sub-surface system, the gate _C_, shown in Fig.
34, may be swung to the dotted position and the effluent, at each
discharge of the siphon chamber, will then pass through the branch
carrier _A_ to section A of the sub-surface system; or a double Y-branch
of iron pipe (see Fig. 35) or a cross may be placed at this point on the
main carrier when there are to be three sections of the sub-surface
system, and valves may be placed on the three branches of the main
carrier thus formed to permit of alternately shutting off the flow to
the various sections of the sub-surface tiling system (see also Fig.
30). Perhaps the simplest and most serviceable device, however, for
alternately resting different portions of the irrigation field is a
diverting manhole with stop planks or wooden sluices sliding in grooves
in the concrete walls or in a wooden frame, as shown in Fig. 27. (See
also Fig. 43, Chapter V.)
Where the ground-water level is not very deep below the surface, or a
clay or hardpan stratum occurs at a depth of a few feet, it is advisable
to underdrain the irrigation field by lines of open-jointed tiling laid
at right angles to the lateral distributing tiling and spaced about
fifteen feet apart. (See Fig. 36.)
[Illustration: FIG. 34.—Special Casting of Double Y-Branch with Swinging
Gate.]
[Illustration: FIG. 35.—Double Y-Branch with Valves on Branches of Main
Carrier.]
These underdrains should be placed at least four feet below the surface,
and inspection pipes should be placed over the outlets of the
underdrains or at the points where they discharge into a main
underdrain, in order to afford opportunity to determine if all portions
of the irrigation field are properly caring for the effluent. To provide
for the placing of the inspection pipes, a length of vitrified tile with
a Tee may be placed on each line of underdrain tiling near its junction
with the main underdrain. On this Tee, two or three lengths of vitrified
tile may be set, reaching to the ground surface and provided with a
removable wooden cover or a vitrified tile cap. This provision for
inspection is necessary where underdrains must be laid and where the
pollution of a stream is to be prevented, since it is often found that
through the activities of burrowing animals direct outlets from the
distributing tiling to the underdrains are formed and the final effluent
is therefore not sufficiently purified by seepage through the soil. It
is desirable for this reason to omit the underdrains when possible, and
in some instances a blind ditch may be constructed around two or three
sides of the field in order to intercept the ground-water flow and to
lower the ground-water level at the field, thus better insuring proper
seepage of the effluent distributed by the sub-surface tiling.
[Illustration: FIG. 36.—Sub-surface Tiling System with Underdrains.]
The essential features of the sub-surface irrigation system of sewage
disposal have been outlined above, and it may be said that this method
is especially adapted to the residence or single house. The method may
be employed with success to dispose of sewage from country clubs and
summer hotels, provided the soil conditions are favorable and proper
areas may be utilized. In these cases the comparatively short period
during each year in which the system is in use and the resulting long
periods of rest give opportunity for a recuperation of the soil and
permit the use of this system in comparatively large installations
where, under continuous operation, a different method of disposal would
be indicated. It should be borne in mind, however, that when any doubt
arises as to the suitability of the soil to care for sewage by this
method, and especially where considerable expense would be involved in
the installation of the system, competent engineering advice should be
sought by property owners before the installation is undertaken. In
fact, it is advisable in the case of all large plants of this type to
employ the services of a sanitary engineer to lay out the system, since
the matter of accurate gradients and proper operating arrangements then
becomes very essential to the success of the undertaking.
While it is not generally advisable to arrange for the disposal of
sewage by sub-surface irrigation when the number of persons served by
the sewer exceeds two hundred, this method will be found a most
satisfactory one if the general conditions at any point are favorable to
its use as heretofore described, and in such cases the adoption of this
system is strongly recommended to the owners of residences, summer
camps, summer hotels and boarding-houses, and to the managers of
moderate-sized institutions and of country clubs who must meet the
problem of properly disposing of sewage on their own premises.