SEWAGE FILTERS
It has been shown that the selection of the type of plant best suited to
solve the sewage-disposal problem at any given place depends on several
factors and can be safely made only after a consideration and study of
such local conditions as the character of the soil, the area available,
the presence and nearness to the surface of ground water, and the local
topographical conditions. If sub-surface irrigation is not feasible,
when, for instance, the soil is nearly impervious to water or when, in
the case of a wet soil, adequate underdrainage is not possible, some
form of artificial filter must be constructed to complete the reduction
of the sewage where the effluent from the settling tank may not properly
be discharged directly into a stream.
If such a filter is to be constructed, the kind most suitable depends,
in turn, upon several factors, such as the degree of purification to be
attained, the suitability of the available areas or locations for the
different types of filters, the operating head or fall available, and
the relative cost of the sand, gravel, broken stone, or furnace slag
which may be used as material for the filter bed.
With respect to the degree of purification of sewage that is desired it
may be said that, of the three general methods of sewage purification,
namely, intermittent sand filtration, treatment in contact beds, and
filtration through sprinkling or trickling filters, the first method
produces the most highly purified effluent. Such an effluent, if from a
properly constructed and operated sand filter, may generally be
considered sufficiently purified to allow its discharge into a stream,
even if the stream is subsequently used as a source of potable water
supply. In some instances, however, subsequent sterilization or
disinfection of the effluent may be required, particularly if the
waterworks intake is relatively near the point of discharge from the
sewage filter, or if the flow of the stream is small in comparison with
the sewage flow. However, if a stream used as a source of potable water
supply receives the effluent from a properly operated sand filter, the
further safeguarding of the quality of the water should generally be
accomplished by filtration or sterilization of the water supply, or
both.
In many cases the local conditions are such that contact beds or
sprinkling filters may be constructed more easily or more economically
than sand filters and, at the same time, the lesser efficiency of the
contact bed or the sprinkling filter, owing to the fact that the stream
is not used for water supply, may not preclude the adoption of these
latter types of plants.
However, where natural deposits of sand of not too finely divided
particles occur or where such sand may be readily procured, intermittent
sand filters are most satisfactory for the final treatment of sewage.
INTERMITTENT SAND FILTERS
The agencies employed in purifying sewage by intermittent sand
filtration involve its oxidation, or nitrification by bacterial action,
while the mechanical straining effected by its passage through the sand
plays a very small part in its reduction.
Where natural deposits of sand of suitable quality occur, sand filters
are constructed by levelling off definite areas of sand and making
embankments eighteen inches high to enclose these areas, the embankments
being generally formed of the surface loam and subsoil which must
usually be removed in order to expose the sand layer. There should be
from three to five beds prepared in order to provide for alternating the
discharge of the effluent from the settling tank over different portions
of the filtration area and thus to provide resting periods for each bed
while in operation. Also, the preparation of several equal areas permits
discontinuing the use of any single area for several days or a week at a
time in order to allow it to dry out and permanently retain its
filtering capacity. In Fig. 37 is shown a view of a set of sand-filter
beds arranged in terraces on sloping ground, the embankments being
formed by the material excavated to uncover the natural sand layer.
[Illustration: FIG. 37.—View of Sand-filter Beds for Village in
Massachusetts.]
The proper number of beds and the area of each bed corresponding to the
number of persons to be served by the sewer are given in Table IV. This
table also gives the required dimensions of siphon chambers (assuming
that this chamber forms a separate compartment of the settling tank) for
the capacities necessary in order that the effluent may be distributed
in proper quantities over each bed or over each pair of beds, as in the
case of plants serving two hundred or more persons. The widths of siphon
chambers given correspond in general with the widths of settling tanks
given in Table I. As in Table III, the dimensions of siphon chambers
given are based on a drawing down of the effluent in the tank when the
siphons discharge, amounting to from four to eight inches. The last
column in the table gives the space which should be left between the
roof of the tank and the top of the dividing wall between the settling
tank and the siphon chamber to provide for this draught upon the
settling-tank contents. It will be seen that no draught upon the
contents of the settling tank when the siphons discharge is arranged for
in the case of tanks serving from four to twenty-five persons.
TABLE IV
FOR USE IN CONSTRUCTING INTERMITTENT SAND FILTERS
═════════╤═════════╤═════════╤════════════╤═════════╤═════════
Persons │ No. of │ Area of │ Mean Width │Diameter │Distance
Served by│ Beds. │Each Bed │ and Length │of Siphon│from Roof
Sewer. │ │ (Square │ of Siphon │(Inches).│ of
│ │ Feet). │ Chamber │ │Settling
│ │ │ (Feet). │ │ Tank to
│ │ │ │ │ Top of
│ │ │ │ │ Wall
│ │ │ │ │ between
│ │ │ │ │Settling
│ │ │ │ │Tank and
│ │ │ │ │ Siphon
│ │ │ │ │ Chamber
│ │ │ │ │(Inches).
─────────┼─────────┼─────────┼────────────┼─────────┼─────────
4│ 3│ 60│ 3 × 3 │ 3│ 12
8│ 3│ 120│ 3 × 5 │ 3│ 12
12│ 3│ 180│ 4 × 5 │ 3│ 12
15│ 3│ 224│ 4 × 6.5│ 3│ 12
25│ 3│ 350│ 4 × 6 │ 5│ 12
35│ 3│ 480│ 4.5 × 5 │ 5│ 16
50│ 3│ 660│ 5 × 6 │ 5│ 16
75│ 3│ 1000│ 6 × 7 │ 5│ 18
100│ 3│ 1320│ 7 × 8 │ 5│ 18
125│ 3│ 1660│ 5.5 × 8 │ 6│ 20
150│ 3│ 2000│ 8 × 8 │ 6│ 20
175│ 3│ 2330│ 8 × 9 │ 6│ 20
200│ 5│ 1600│ 8 × 12 │ 8│ 20
250│ 5│ 2000│ 10.5 × 12 │ 8│ 20
300│ 5│ 2400│ 12 × 13 │ 8│ 20
350│ 5│ 2800│ 13 × 14 │ 8│ 20
400│ 5│ 3200│ 13 × 17 │ 8│ 20
450│ 5│ 3600│ 13 × 19 │ 8│ 20
500│ 5│ 4000│ 13 × 21 │ 8│ 20
─────────┴─────────┴─────────┴────────────┴─────────┴─────────
The siphons in each instance should be so placed that the lower edge of
the bell of the siphon will be at a distance below the roof of the tank
equal to twelve inches plus the drawing depth or discharging depth of a
siphon of the diameter indicated. There should be three inches of space
between the siphon bell and the floor of the chamber. The discharging
depths of siphons as used in forming Tables 2, 3, 4, and 5 are as
follows:
Diameter of Siphon. │ Discharging Depth.
│
3 inches│ 13 inches
5 inches│ 23 inches
6 inches│ 30 inches
8 inches│ 35 inches
10 inches│ 60 inches
12 inches│ 72 inches
If the siphons installed are larger or smaller than those shown in these
tables, or if the particular make of siphon purchased has the same
diameter but a different discharging depth, proper allowance must be
made in proportioning the size of the dosing chamber.
In order to quickly convey the dose from the siphon chamber to the
filter beds at the rate at which the siphon discharges, the sewer from
the siphon chamber should be of proper size and should have a sufficient
gradient. For instance, with a 3–inch siphon the sewer should be 6
inches in diameter, with a gradient or fall of at least 12 inches per
100 feet; with a 5–inch siphon, the sewer should be 8 inches in
diameter, with a gradient of at least 6 inches in 100 feet; with a
6–inch siphon, the diameter of the sewer should be 8 inches, and should
have a gradient of at least 12 inches per 100 feet, or 10 inches with a
gradient of at least 3 inches per 100 feet; with an 8–inch siphon, 12
inches, with a gradient of at least 12 inches per 100 feet.
Sewage is sometimes applied directly to the beds without treatment in
settling tanks, generally, in such cases, after having been screened to
remove the larger suspended matters, but it is decidedly preferable in
the case of the smaller plants under discussion to pass the sewage first
through settling tanks, as in the method of sub-surface irrigation.
Therefore, the areas of beds given in the table are for sewage which has
been passed through settling tanks. It is even necessary, in the case of
sand filters for institutions where considerable grease and soaps are
contained in the sewage, to provide grease traps through which the
sewage must pass before it reaches the settling tank. The effluent from
the tank should be discharged intermittently by means of a dosing
chamber and siphon and should be distributed quickly over the surface of
the bed as uniformly as possible. This is generally accomplished in the
case of the larger beds by laying on the surface of the bed, wooden
troughs, with short branches, as shown in Figs. 38 and 39. A detail of a
portion of these distributing troughs is given in Fig. 40. This view
shows the hinged gates which are used to effect a proportionate division
of the flow of the various branches of the main trough. The view also
shows the slots in the sides of the troughs which allow the sewage to
flow out onto the bed.
[Illustration: FIG. 38.—Layout for Intermittent Sand Filtration.]
If the ground-water level is within three feet of the surface at any
time, or if the sand is very fine and contains a slight proportion of
clay, underdrains should be laid at depths of four feet to prevent the
beds from becoming waterlogged.
[Illustration: FIG. 39.—Intermittent Sand-filtration Beds.]
Where sand deposits do not occur at a point suitable for the location of
the disposal plant, but where sand may be procured at a reasonable cost,
the beds may be formed artificially similar to the natural sand beds
heretofore described, but should not be less than three feet deep. It is
generally necessary in the case of artificially constructed sand filters
to provide underdrains as described below.
Two views of such an artificial sand filter are shown by Figs. 41 and
42. In Fig. 41 the settling tank and siphon chamber may be seen,
situated between two of the four beds composing the filter. In Fig. 42
is shown a nearer view of one of the beds with the distributing trough
and its branches on the surface of the bed. This bed, of the four
composing the filter, was not in operation at the time the photograph
was taken.
[Illustration: FIG. 40.—Portion of Distributing Troughs for Sand
Filters.]
In Fig. 38 is shown a sand filter layout with three beds. In this
drawing are shown the sewer leading from the house, the settling tank,
the siphon chamber, in which are placed two siphons, the effluent
sewers, and the diverting manhole, from which three pipe lines convey
the sewage to the filter beds. In Fig. 39 is shown also a view of the
three filter beds, one of the beds being shown in section. Figs. 43 and
44 show a plan and view of the diverting manhole.
Where sand must be carted in to form the filters, the embankments to
retain the sand should generally be formed by excavating for a depth of
two feet the whole area upon which the beds are to be placed. The
material thus excavated will usually be sufficient to form the
embankments. The embankments should usually be at least two feet wide on
top and should have side slopes of one and a half to one; that is, the
bottom width of the embankment should be two feet plus three times the
height. In clay soils the pits for the filter beds may be excavated with
the sides vertical, or nearly so. The bottom of each bed, as it is
prepared for the placing of the sand which is to compose the filter,
should slope slightly from the sides toward the centre line of the bed.
[Illustration: FIG. 41.]
Where the character of the underlying strata of soil or the presence of
ground water requires that sand filters, whether natural or artificial,
should be underdrained, this may be accomplished by laying a
longitudinal main drain through the centre of the bed at a depth of at
least three or four feet below the surface, with branches each way at
intervals of about fifteen feet. The main underdrain should be six
inches in diameter, of agricultural tile or of vitrified sewer pipe,
laid with open joints, and should have a fall of at least six inches per
hundred feet. The branches may be of three-inch agricultural tile.
[Illustration: FIG. 42.]
[Illustration: FIG. 43.—View of Diverting Manhole.]
In large installations for cities and villages it is usual to install
either plural alternating siphons or apparatus known as sewage feeds, by
means of which the contents of the dosing chamber are discharged upon
the different beds in rotation, there generally being four or five beds
constructed in each unit. This requires a separate siphon or sewage feed
for each bed, and entails considerable expense. However, for smaller
plants such as are now being considered, two ordinary siphons may be
placed in the same dosing chamber as described in Chapter III, and so
primed as to discharge alternately. Then, by means of a diverting
manhole or chamber through which the dose must pass, the effluent may be
diverted onto two beds in rotation, allowing a third bed to rest, or, if
there are five beds, it may be diverted onto two pairs of beds in
rotation, allowing a fifth bed to rest. For instance, in the case of
five beds, a diverting manhole may be constructed as shown in Fig. 45,
and arrangements may be made to couple bed No. 3 with No. 2 or No. 4,
allowing bed No. 1 or No. 5 to rest by means of the stop-plank to cut
off the flow to either of these beds, as shown in the illustration.
Then, when bed No. 3 is to be rested, stop-planks _A_ and _B_ are both
closed, and the stop-planks against all pipe outlets are raised. If it
is desired to throw bed No. 1 out of use, the stop-plank is placed
against the end of the pipe leading to this bed, stop-plank A is raised,
and stop-plank _B_ is lowered. One siphon will then discharge onto beds
Nos. 4 and 5, and with the next filling of the siphon chamber the second
siphon will discharge onto beds Nos. 2 and 3. By a proper combination of
the stop-plank positions, any two sets of two beds each may receive
alternately the discharge from the siphon chamber while the remaining
single bed may be left resting. The method for operating the beds in
rotation described above may, of course, be easily applied when only
three beds are constructed. A provision for allowing one bed to be
thrown out of use for a week or so at a time is very necessary for the
reasons stated above.
[Illustration: FIG. 44.—Plan of Diverting Manhole.]
At intervals of several weeks it will be found necessary to break up the
surface of each bed by raking or else to remove a thin coating of
clogging material. This should be done after the bed has been rested and
dried out, when the surface matting may be taken off without removing
much sand. To provide for operating the beds in winter, in the late
fall, before the ground has frozen, ridges and furrows should be formed
on the surface of the beds, similar to those shown in Fig. 51. The
furrows should be two or three feet apart and eight to twelve inches
deep. Then when effluent is discharged onto the beds in freezing
weather, as it fills the furrows, an ice roof will gradually form,
spanning the furrows and protecting the sides and bottoms of the furrows
from freezing, especially if a snowfall occurs before severe weather
sets in. It will sometimes be found necessary, especially with small
beds that are well underdrained, to provide board coverings for the
furrows to take the place of the natural ice roofs.
[Illustration: FIG. 45.—Five-way Diverting Manhole.]
The effluent from the tank should be discharged in such quantities as to
flood the entire bed to a depth of from one to two inches, except that
some of this effluent will immediately begin to seep into the bed.
Respecting the quality and relative fineness of sand suitable for sewage
filters, it should be noted that certain empirical methods of
measurement have been developed for use in comparing the size and
uniformity of particles of various sands. These measures are (1) the
“effective size,” and (2) the “uniformity coefficient.” The “effective
size” is the size of sand particle expressed in millimetres compared to
which ten per cent by weight of the particles in the sample is finer.
The “uniformity coefficient” is the ratio of the size of grain which has
sixty per cent of the sample finer than itself to the size which has ten
per cent finer than itself.
Concerning the grades of sand through which sewage may be successfully
and properly treated by intermittent filtration, it has been found that
the “effective size” should not be less than .20 of a millimetre, nor
greater than .50 of a millimetre, and the “uniformity coefficient”
should generally be from 1.5 to 3.0, when sewage is applied at the usual
rate. If, however, the sand is clean and sharp, but has an “effective
size” somewhat smaller than the limit above stated, it may sometimes be
found suitable.
In the case of any sewage-disposal project of considerable magnitude,
where any doubt exists as to the suitability of the sand available for
use in sand filters, analyses of representative samples of the sand
should be arranged for, and competent engineering advice should be
sought before any large outlay is incurred. In general, however, it may
be said that any clean, sharp sand suitable for building use is suitable
for sand-filter beds in any situation. Obviously, the coarseness of the
sand plays no part in its suitability as a filtering medium if the sand
occurs in a natural bed and underdrains are not necessary, since no
question of the discharging of an unpurified effluent would ordinarily
arise in such cases.
CONTACT BEDS
The treatment of sewage in contact beds consists in distributing the
effluent from settling tanks over beds of broken stone, furnace slag, or
other similar material contained in water-tight compartments and
allowing the beds to fill so that the spaces between the filtering
material will be filled with the sewage effluent. These beds are so
arranged that the effluent is held in contact with the filtering
material for a fixed interval of time and then, usually by means of
special siphons called “timed siphons,” or other automatic devices, it
is discharged from the beds onto sand filters for further treatment, or
into streams, as the case may be.
The process involves, as in intermittent sand filtration, the nitrifying
or oxidizing agencies of bacterial action, and differs from intermittent
filtration and from treatment of sewage on sprinkling filters
principally in the fact that the flow of effluent through the beds is
arrested and the liquid sewage held in contact with the filtering
material, as noted above.
Much smaller areas of filter beds are required than in the case of sand
filters, and for this reason this form of filter will often be found
preferable. The conditions which result in its selection are usually
either the unsuitable character of the soil or the presence of ground
water, making the installation of sub-surface irrigation systems
impracticable; or the absence of sand deposits or the high cost in any
locality of sand suitable for sand-filtration beds, making their
construction difficult or expensive.
The walls and floor of a contact bed are generally constructed of
concrete, and the filter should be rectangular in form, as it is easier
to distribute the effluent uniformly over a bed of this shape. The
details given in Chapter II for constructing the walls and floors of
settling tanks will serve as a general guide in the construction of
contact beds.
The real work of the contact filter is carried on during the period of
“resting empty,” that is, after the effluent has been withdrawn from the
bed. While the effluent fills the beds, much of the suspended solid
matter, together with a large proportion of the bacteria contained in
the sewage, adheres to a gelatinous film which has formed on the
surfaces of the stones or other materials forming the beds. This
interval of “resting full” should usually be about two hours. Then, when
the liquid portion is withdrawn from the bed, air is drawn in between
the stones, enabling the nitrifying or aërobic bacteria to do their work
of breaking down both the suspended and the partially dissolved organic
matters which have been contained in the sewage and which have adhered
to the filter material. It is believed, that some oxidation of that
portion of the organic matter which is in true solution is also
accomplished when the effluent passes over the gelatinous covering of
the stones by reason of the oxygen which has been absorbed by this
covering.
The interval when the bed is “resting empty” should be considerably
longer than the combined intervals when the bed is filling, “resting
full,” and emptying. For this reason there should be a series of from
three to five beds in order that it will not be necessary to turn the
effluent from the settling tank continuously onto one bed, which would
result in the clogging of this bed with suspended matters. The
additional third (or fifth) bed also gives opportunity for allowing each
bed in turn to be thrown out of use for intervals of a week or so at a
time, which is also necessary to keep the beds up to their proper
efficiency and obviate the necessity of cleaning or renewing the filter
material oftener than once in seven or eight years.
[Illustration:
PLAN OF SEWAGE DISPOSAL WORKS
FOR
MR. CHARLES L. A. WHITNEY
ALBANY, N.Y.
]
In Fig. 46 is shown in plan and section a sewage-disposal plant for the
residence of Mr. Charles L. A. Whitney, of Albany, N. Y., consisting of
a settling tank, dosing chamber, and contact beds. This plant is
designed to serve twenty-five persons, although the settling tanks have
a capacity for double the amount of sewage on the usual basis of design.
The depth of filtering material in the beds should preferably be four or
five feet, although, where operating head or fall is limited, this depth
may be decreased to three feet. The floor of the bed should slope toward
the outlet end at a rate of about one-eighth of an inch per foot.
Various materials are used to form the body of the filter, such as
broken stone, coke, broken brick, and furnace slag, but the material
used should not be such as will disintegrate readily, and for this
reason broken limestone, from one-half inch to one and one-half inches
in size, with perhaps two-inch stones for the bottom six inches of the
bed surrounding the underdrains, is most suitable for small plants.
These underdrains should be constructed of horse-shoe tiling, and in the
case of beds more than eight or ten feet wide should preferably be laid
with short branches reaching from a main drain laid along the centre of
the floor of the bed; or these drains may be laid in parallel lines, as
shown in Fig. 46.
In order to alternate the discharge of effluent from the settling tank
onto different beds in turn and to provide for more uniformly
distributing the effluent over all portions of the bed, the
settling-tank effluent should be collected as in the other methods of
disposal described, in a siphon or dosing chamber, from which, by means
of alternating siphons, it may be delivered to the proper bed.
In the case of a group of three beds or five beds, diverting chambers
with stop-planks, similar to those described in connection with
intermittent sand filters, may be provided to allow the throwing out of
use of each of the beds in turn for a week or so at a time. In the
smaller plants accommodating up to one hundred and fifty persons, it is
hardly necessary to provide for more than three beds, thus allowing
opportunity for each one to rest for one week in every three to six
weeks, which will result in a temporary increase of fifty per cent in
the rate of application of effluent to the remaining beds. In the case
of the larger plants, especially if they are to be operated
continuously, it is better to construct five beds so that two pairs of
two beds each may be used alternately, leaving one bed, or twenty per
cent of the total area, out of use. This will result in an increase of
but twenty-five per cent in the rate of application of effluent to the
four beds in use.
With the usual rates of operation for contact beds, one filling per day
of the beds will result, and, if the dosing of the beds is carried on as
above and as described in the portion of this chapter dealing with the
dosing of intermittent sand filters, but two siphons in the dosing tank,
constituting double alternating siphons, will be necessary. Such an
arrangement will eliminate the necessity of installing plural
alternating siphons consisting of three or more siphons, the cost of
which is not warranted in connection with small plants, since the double
alternating siphons will insure proper operation of the beds at much
less cost. Of course, in the larger plants where two beds are dosed at
each discharge of a siphon, a larger siphon chamber is necessary with
the two siphons, but the extra cost of a larger siphon chamber would in
most cases be more than offset by the increased cost of plural
alternating siphons.
The main effluent carrier from the siphon chamber to each contact bed
should discharge into a half-tile carrier, with branches, laid on the
surface of the contact bed, as shown in Fig. 46.
Each contact bed should be provided at the outlet end with a “timed”
siphon set in a separate chamber of two compartments, as shown in the
drawing. The diameter of the timed siphons should generally be that of
the next larger size than that indicated for the dosing-chamber siphons.
As shown in the illustration, where only three beds are necessary, the
third timed siphon may be dispensed with if arrangements are made to
permit the use of one siphon for discharging either the middle or the
outside bed on that side, and to permit the use of the other siphon for
discharging either the middle bed or the bed on the other side. In such
installations gates or valves must be placed on the outlets of the
contact beds to prevent the filling of the bed that is out of use by
back flow from the timed siphon chamber used to discharge the adjacent
bed.
The cost of contact beds is considerably greater than the cost of
intermittent sand filters, especially when sand of proper quality is
available, but their construction is advised in many cases where
sub-surface irrigation is not feasible, where the premises are subject
to overflow or the ground-water level is high, and where it is not
practicable to construct sand filters.
In the following table are given the proper number of units or beds for
contact filters of different-sized installations, together with the
required area of each filter, the depth of the filter medium in all beds
being four feet. The table also shows the dimensions of the siphon
chamber adjacent to the settling tank and the diameter of the siphons
necessary to discharge the effluent in proper volumes onto each contact
bed, or each pair of contact beds. Where it is necessary to decrease the
depth of the contact beds to three and one-half or three feet, owing to
lack of operating head or fall, a proportionate increase should be made
in the area of each bed.
TABLE V
FOR USE IN CONSTRUCTING CONTACT BEDS
══════════╤══════════╤══════════╤═══════════╤══════════╤═══════════════
Persons │ No. of │ Area of │Mean Width │ Diameter │ Distance from
Served by │ Beds. │ Each Bed │and Length │of Siphons│ Roof of
Sewer. │ │ (Square │ of Siphon │(Inches). │ Settling Tank
│ │ Feet). │ Chamber │ │to Top of Wall
│ │ │ (Feet). │ │ between
│ │ │ │ │ Settling Tank
│ │ │ │ │ and Siphon
│ │ │ │ │ Chamber
│ │ │ │ │ (Inches).
──────────┼──────────┼──────────┼───────────┼──────────┼───────────────
4│ 3│ 20│3 × 4.5 │ 5│ 12
8│ 3│ 40│4 × 6.5 │ 5│ 12
12│ 3│ 60│ 6 × 7 │ 5│ 12
15│ 3│ 70│ 6 × 8 │ 5│ 12
25│ 3│ 100│ 6 × 8 │ 6│ 16
35│ 3│ 130│ 7 × 9 │ 6│ 16
50│ 3│ 180│ 8.5 × 10 │ 6│ 16
75│ 3│ 280│ 10 × 11 │ 8│ 18
100│ 3│ 370│ 12 × 12 │ 8│ 18
125│ 3│ 460│ 12 × 14 │ 8│ 20
150│ 3│ 550│12 × 16.5│ 8│ 20
175│ 5│ 390│ 13 × 14 │ 10│ 20
200│ 5│ 440│ 14 × 15 │ 10│ 20
250│ 5│ 550│ 15 × 18 │ 10│ 20
300│ 5│ 660│ 17 × 19 │ 10│ 20
350│ 5│ 770│ 18 × 21 │ 10│ 20
400│ 5│ 880│ 18 × 24 │ 10│ 20
450│ 5│ 990│ 20 × 21 │ 12│ 20
500│ 5│ 1110│ 20 × 23 │ 12│ 20
──────────┴──────────┴──────────┴───────────┴──────────┴───────────────
In the above table, as in the previous tables, in indicating the height
to which the dividing wall between the settling tank and siphon chamber
should be carried, allowance is made for a draught upon the contents of
the settling tank at each discharge of a siphon of from four to eight
inches. In the discussion relating to siphon chambers in connection with
the description of intermittent sand filters will be found the necessary
details as to the discharging depths of siphons of different diameters
and the necessary depths of the siphon chambers in which such siphons
are to be placed. The construction of contact beds will naturally be
approached with hesitancy by property owners and others not familiar
with such work, and it is strongly recommended that where it is possible
the services of a sanitary engineer be engaged to design and supervise
the construction of a plant involving any considerable outlay, unless it
is felt that the descriptions and directions given above have afforded a
clear understanding of the design and construction of this type of
sewage-disposal works.
SPRINKLING FILTERS
One of the more recently developed methods of sewage disposal,—the
sprinkling- or trickling-filter system,—has for its principal feature
the thorough aëration of the settling-tank effluent before its passage
through the filter. This filter, like the contact filter, is of the
rapid, coarse-grained type, but in its operation resembles the process
of intermittent sand filtration in that the sewage effluent passes
through the filter continuously without being held in contact with the
filtering material as in the contact bed. The aëration of the sewage
effluent, which very greatly aids the final process of nitrification or
oxidation in the filter, is accomplished by spraying the sewage effluent
over the surface of the beds through a series of riser pipes with
nozzles, or allowing it to fall in fine streams on dash plates which
cause it to sprinkle over the beds and thus to absorb oxygen from the
air.
Sprinkling filters produce an effluent with a considerably less degree
of purification than sand filters, but may in general be said to produce
a more stable effluent, that is, one less liable to subsequent
putrefaction than the effluent from contact beds. Furthermore, with the
usual depths of contact beds and sprinkling filters, an area
approximately four times greater is required to treat the same amount of
sewage on contact beds than is necessary if sprinkling filters are
constructed. However, since the effluent from sprinkling filters is much
more turbid, necessitating, in most cases, subsequent sedimentation
before discharge; since considerably greater operating head or fall is
necessary; and since their operation requires much more supervision, the
construction of sprinkling filters is not generally as advisable as
contact beds for small installations, especially in cold climates.
The construction of sprinkling filters as compared to contact filters
differs principally in the size and depth of filtering material, in the
means provided for distributing the effluent over the beds, and in the
arrangements for draining the filter.
The depth of filtering material in a sprinkling filter is usually from
five to ten feet, preferably not less than eight feet. The material for
the filter is the same as that used for contact filters but the
fragments should be from one to three inches in diameter. Instead of a
system of tile underdrains on the floor of the filter, a false floor of
perforated tile, rectangular in section, or of half tile, circular in
section, with drainage holes cut out along the sides, should be laid
over the entire floor of the filter. As the settling-tank effluent is
sprayed on the filter it passes downward between the spaces of the
filtering material, and reaching the floor of the filter is collected in
a main drainage channel, through which it passes to the outfall sewer,
and thence to the final settling tank or into the stream.
As with contact beds and sand filters, intermittency of application of
the effluent to the filter is essential for proper action of the filter,
and this is accomplished as in the other types of filters by means of
automatic siphons placed in dosing tanks. These tanks, however, are of
special design in the case of sprinkling filters. Each siphon discharges
into a main carrier of iron pipe which extends sometimes over the
surface of the filter, but generally along the floor of the filter. The
main carrier has branch pipes of smaller diameter extending at right
angles nearly to the sides of the filter. On these branches vertical
riser pipes spaced about ten feet apart are connected, and these riser
pipes extend a few inches above the surface of the filter. Nozzles are
fitted to the ends of the riser pipes by means of which the sewage
effluent, under pressure, is sprinkled, at short intervals, in the form
of a fine, umbrella-shaped spray, over the surface of the filter. This
results in a thorough aëration of the effluent before it reaches the
filtering material, and makes this form of filter very effective.
[Illustration: FIG. 47.—View of Sprinkling Filter at Danville, Pa., in
Winter.]
In Fig. 47 is shown a view of a sprinkling filter at Danville, Pa.,
operating when the temperature was 14° below zero. It is generally
believed, however, that in cold climates it is advisable to house small
sprinkling filters.
Owing to the rather complicated hydraulic features and the somewhat
difficult engineering principles involved in the construction and
operation of sprinkling or trickling filters, it is not deemed advisable
to attempt to describe them in sufficient detail to furnish directions
for their construction. The design of a sprinkling-filter system for
small as well as for large installations should always be entrusted to
an engineer conversant with this line of sanitary engineering. It is
believed, however, that the description of such filters given above will
aid those who are about to install sewage-disposal plants in the
selection of the type of plant best suited to their particular needs and
conforming to the conditions peculiar to their situation.
Summarizing the foregoing descriptions and directions with reference to
sewage filters, it may be stated that where natural facilities for
disposing of sewage by simpler methods do not exist, the construction of
a sewage filter of some one of the above described types offers a
solution of every problem thus encountered. It is well to repeat that
sub-surface irrigation, where feasible, should be adopted, and this will
be the method indicated in a large majority of cases where small
disposal plants are to be constructed.
The principal point to be remembered in connection with sewage filters
is that their construction is but a good beginning, and that their
proper operation is very necessary to the success of the undertaking.
They constitute, with the developed bacteria in the filter, a rather
sensitive mechanism capable of efficient work if properly handled, but
each filter unit must be carefully operated and must be regularly given
extended periods of rest for the restoration of the void or open-space
capacity of the filter, and to provide for the necessary aëration of the
filtering material.
The peculiar action which takes place in a filter in the reduction of
sewage, and which is not even yet fully known, is best evidenced by the
fact that sewage filters, especially of the coarse-grained type, do not
attain their highest efficiency until after several weeks or months of
operation.
With a knowledge of these points, it will be seen that there should be
little divergence from accepted standards in the construction and
operation of sewage filters if they are to prove satisfactory when
installed.