In approaching a subject upon which so much has already been written,
it may be desirable to point out that the improvements which have
taken place in recent years in connection with sewage disposal are
so extensive and varied, and have developed at such a comparatively
rapid rate, that most treatises now in existence are in many respects
more or less out of date. It is true that further developments may
be anticipated in the future, but these will probably be concerned
more with additions and improvements in matters of detail than of
principles, which are now to a great extent agreed upon. The time
would thus appear to have arrived when it is desirable to describe in
detail the various methods of construction now generally adopted in the
design, of present-day sewage disposal works.
In order to avoid a repetition of the usual preliminary details to be
found in all the existing literature devoted to this subject, it is
assumed that readers are acquainted with the nature of the problem to
be solved in the design of sewage disposal works, with the varying
characteristics of the different liquids included under the term
“sewage,” and with the engineering formulæ and requirements involved
in the design of tanks, filters, and similar constructional works.
The present volume will thus consist exclusively of descriptions,
illustrated with drawings and photographs, of the various tanks,
chambers, filters, beds, and other details of sewage works, including
the numerous types of appliances required in connection therewith.
In other works dealing with this subject it has been customary to use
as illustrations, drawings of works actually carried out by their
authors or other engineers. While these are interesting and valuable to
a certain extent, their application under other conditions is limited,
and their usefulness is thus much reduced. In the present volume the
illustrations of the various details of construction do not, as a
rule, represent actual working drawings adapted to any particular set
of conditions, but are shown in diagrammatic form for the purpose of
serving as suggestions to engineers in search of ideas which they can
adapt to meet the requirements of any particular scheme upon which they
may be engaged. It will follow that the engineer must in all cases
rely upon his own practical experience and judgment in deciding to
adopt any of the various methods of design and construction illustrated
and described in the following pages; and it may be found that a
combination of several types, or even a combination of several details
of different types, combined with practical experience and mature
judgment, will frequently produce the most suitable and efficient
scheme.
_SCREENS._
On arrival at the disposal works, the first stage of the process
through which the sewage passes is generally that of screening for the
purpose of arresting the grosser solids in suspension. In a number
of cases where the sewage is delivered by gravitation, there are no
screens of any kind in use, reliance being placed upon the detritus
chambers to perform the duty of arresting the floating solids, as well
as the grit and other mineral matters, of such a specific gravity
that they are readily deposited by simply reducing the velocity of
the flow. Where the levels involve the use of pumping plant, screens
are a necessity, and, as the Royal Commission on Sewage Disposal have
expressed the opinion that _all_ sewage should be screened, it will
apparently be necessary to provide screens in all future schemes.
[Illustration: FIG. 1.—SIMPLE SCREEN.]
_Simple Screens._—The simplest type of screen is in the form
of a grating, consisting of vertical iron bars in a stout iron
frame, arranged to fit into grooves cut in the side walls of the
screen-chamber, or in channel-iron guides attached to the sides of the
chamber. As a general rule the vertical bars are round in section, but
some engineers prefer to use flat bars with their longer side parallel
to the line of flow, while others even go so far as to use wedge-shaped
bars with the thick end facing the flow of sewage. In the latter case,
the idea is to facilitate the passage through the screen of those
matters which are too small to be arrested on the _front_ of the bars,
but large enough to be caught _between_ the bars, and thus possibly
choke the intervening spaces. As all simple fixed screens must of
necessity be cleaned by hand, they are usually arranged at an angle of
about 60 degrees to the floor of the chamber, in order that the matters
arrested may be more easily drawn up by a hand rake to the top of the
screen. Fig. 1 shows a screen of this type in plan and section, with a
large scale detail of the round, flat and wedge-shaped bars previously
described. It will be noticed that a narrow platform of boards is
shown across the chamber, at the top of the screen, to receive the
screenings, which are then thrown into a barrow for removal to their
final destination. One important point to be remembered in the design
of the chamber for screens of this type is, that the bottom of the
screen should be placed in a sump some 12 inches or so below the invert
of the incoming sewer, so as to provide space for the accumulation of
a certain amount of sludge and screenings without choking the screen.
This sump should be provided with a washout valve. It is advisable
to have all screen-chambers in duplicate, so that one of them may be
in use while the other is being cleaned. The spaces between the bars
vary in width with the character of the sewage, but the distance most
generally adopted is half an inch. The important point to be considered
is, that while the screen should arrest all the larger suspended
matters it is intended to intercept, it should allow a free passage to
all others without becoming rapidly choked. Another important factor in
the efficiency of a fixed screen is its width. The greater the width,
the less will be the liability to choke, and consequently it will not
require raking so frequently to keep it in proper order.
_Rotary Screens._—Where the flow of sewage is sufficient for the
purpose, and it is desired to reduce the necessary attention to the
minimum, the self-cleansing rotary screen, manufactured by Messrs. John
Smith and Co., may be adopted. This is illustrated in Fig. 2, from
which it will be seen that it consists of a revolving wire screen,
extended between two rollers, one below and the other above the sewage
level. The upper roller is rotated by means of a water wheel driven by
the sewage. A rotary brush is fitted to the shaft and driven in the
opposite direction to the screen roller, so that it brushes off the
screenings into a trough, from which they are removed by hand.
[Illustration: FIG. 2.—ROTARY SCREEN.]
_Screens for Deep Sewers._—In cases where the depth of the sewer makes
it inconvenient to adopt a fixed screen, the double lifting screen,
manufactured by Messrs. Adams Hydraulics Ltd., may be used, as shown
in Fig. 3. This consists of duplicate screens, arranged to slide up
and down in cast-iron guides attached to the walls of the chamber.
These screens are raised and lowered by a chain, which passes over a
drum revolved by hand. The main screen is in the form of a basket,
with a hinged front, which falls to the floor of the chamber when this
screen is lowered into position. When it is desired to clean out this
basket screen, the other plain guard screen is lowered into position
in front of the basket-screen, and the latter is then raised. As the
chain by which the basket-screen is raised is attached to the top of
the hinged front, the action of raising this screen first draws up
the hinged front and this prevents the screenings falling out. After
this screen has been emptied, it is again lowered into position, and
the guard-screen raised to permit the sewage to flow direct into the
basket-screen.
[Illustration: FIG. 3.—DOUBLE LIFTING SCREEN.]
[Illustration: FIG. 4.—MECHANICAL SCREEN.]
[Illustration: FIG. 5.—MECHANICAL SCREEN.]
[Illustration: FIG. 6.—MECHANICAL SCREEN.]
_Mechanical Screens._—In larger schemes, where power is available for
the purpose, mechanically operated screens are frequently adopted,
as they are not only self-cleansing but the screenings are delivered
automatically at or above the ground level, and thus very little labour
is involved in removing these matters. Figures 4, 5, 6, 7 illustrate
four examples of this type of screen, manufactured respectively by
Messrs. Ham, Baker and Co., Ltd., Messrs. J. Blakeborough and Son,
Ltd., Messrs. S. S. Stott and Co., and Messrs. J. Wolstenholme and
Co. The general features of these screens are an inclined screen or
strainer, fixed in the channel or catchpit through which the sewage
flows to the tanks or to the pumps, and a raking apparatus with special
shaped prongs, which travel in the spaces between the bars forming the
screen and remove the refuse. The Stott screen includes a rake cleaning
gear, consisting of a revolving steel comb, by means of which the
screenings are removed from the prongs of the rake while they are in
motion.
[Illustration: FIG. 6A.—RAKE CLEANING GEAR FOR FIG. 6.]
[Illustration: FIG. 7.—MECHANICAL SCREEN.]
In the case of the screen, manufactured by Messrs. Whitehead and
Poole, illustrated in Fig. 8, the bars are of tapered steel, and are
so arranged that they can be removed and replaced if necessary. The
special friction drive with which this machine is fitted, prevents
the breaking of the chain should the rake prongs become caught in the
screen. The rake-cleaning gear consists of two swing levers, which
carry a cleaning comb and a balance weight to hold it in position
over the dirt tray. As the rakes bring up the screenings and reach
the delivery position, they pass through the cleaning comb, which is,
at the same time, forced down by a catch on the chain engaging with
flanged rollers on the end of the swing levers. In this way the rakes
are effectively cleaned, and it is impossible for the rake prongs and
the comb to foul each other.
In addition to the screens already described, mention may be made of
the special drum-shaped screen invented by Mr. Baldwin Latham and the
numerous types of mechanically-operated screens in use in Germany,
all more or less elaborate in character. Further details of these are
probably unnecessary, as the aim of the engineer engaged in the design
of sewage disposal works should be to adopt those appliances which are
of the simplest possible form consistent with the requirements of the
case with which he is called upon to deal. Some engineers prefer to use
screens specially designed by themselves to meet the requirements of
each particular scheme, and while this method provides scope for the
exercise of a considerable amount of ingenuity, it is liable to involve
greater expense than would be incurred by the adoption and possible
adaptation of one of the various types already on the market.
[Illustration: FIG. 8.—MECHANICAL SCREEN WITH RAKE CLEANING GEAR.]
_STORM-WATER OVERFLOW WEIRS._
The proper design of weirs for diverting the excess volume of sewage
in times of storm has not in the past always received sufficient
consideration. Too frequently it has been dealt with by rule of thumb.
In the first place the position for the weir has not always been well
chosen; but, as a result of the recommendations of the Royal Commission
on Sewage Disposal, it will be necessary in the future to construct
these weirs, in all cases which require the approval of the Local
Government Board, _after_ the screen. This is a wise precaution, as it
prevents the possibility of a storm-water overflow coming into action
as a result of want of attention to the screen. In this position the
factor which has the greatest influence upon the proper working of such
weirs is the rate of flow into the detritus tanks, i.e. the area of
the inlets to these tanks. It is true that these may be regulated by
the use of valves, but unless these valves, when once adjusted to the
correct height, can be permanently locked in that position, it leaves
them at the mercy of an unscrupulous workman, who may, if he wishes,
close them entirely, and thus cause the entire flow of sewage to pass
over the storm overflow weir in order to save himself the trouble of
attending to the tanks and filters. It is probably with the intention
of preventing the possibility of such mismanagement that the Local
Government Board object to valves on the inlets to the detritus tanks.
One method of preventing trouble is to use simple hand-stops, and
provide the frames in both inlets but only _one_ door, so that it is
impossible for the man to close both inlets at the same time. The Local
Government Board are also usually averse to the use of any type of
movable weir, and prefer the simple fixed weir.
_Diverting Plate._—Many ingenious devices have been adopted in the
past for the purpose of ensuring the diversion of all the excess volume
above a certain fixed quantity. One of these is shown in Fig. 9, where
it is assumed that all in excess of the volume which is taken by the
sewer flowing four-fifths full is to be discharged over the overflow.
In order to facilitate this result, an iron plate is fixed at the level
of the weir (say four-fifths of the diameter of the sewer), over the
whole of the outlet end of the chamber or man-hole, with a sharp edge
on the side facing the flow, so that when the sewage in the chamber
rises above this level, the excess volume above that flowing at a depth
of four-fifths of the diameter of the sewer, is automatically diverted
by the plate and caused to pass away over the weir. The invert of the
chamber must naturally correspond with the diameter of the sewer.
[Illustration: FIG. 9.]
_Fixed Weirs._—Even this ingenious method of diversion is, however,
not accurate, as no provision is made to counteract the effect of the
increased head on the outlet from the chamber, due to the backing up
of the sewage in passing over the weir. Where a fixed weir is alone
permissible, the only really satisfactory method of securing the
desired result, is to increase the width of the overflow weir to such
an extent that the maximum depth of storm-water, which may possibly
flow over the weir, is reduced to the minimum, say one inch, and thus
the effect of this head on the normal outlet from this chamber (i.e. on
the inlet to the detritus or sedimentation tanks) is also reduced to
the minimum. This will necessitate careful consideration, and a special
set of calculations in each case. Where it is found that the execution
of the above suggestion involves the construction of a weir of abnormal
and unpractical width, it will be found convenient to arrange the
normal dry-weather outlet from this chamber in the form of a narrow
vertical slot, which can be most easily provided in a simple door or
stop in a grooved frame, fixed in the outlet from this chamber. Fig.
10 shows an example of this slotted door, and when the correct width
of the slot has been ascertained by actual experiment, the door should
be bolted to the frame, so that it cannot be removed or altered by any
unauthorised person. From the drawing it will be seen that it is not
difficult to calculate the dimensions of the slot orifice, so that with
the head due to the height of the storm overflow weir it shall discharge
the desired volume (say three times the dry-weather flow), and if the
width of the overflow weir is then calculated to take the excess volume
with a depth of one inch of water over the weir, this extra one inch
of head will have very little effect on the discharge through the slot
outlet.
[Illustration: FIG. 10.]
[Illustration: FIG. 11.—FLOATING WEIR.]
[Illustration: FIG. 12.—SWINGING SIPHON.]
_Movable Weirs._—If, however, it is desired to provide for an
absolutely correct diversion of the storm-water, this can only be
done by the use of a movable weir. There are two types of this form
of weir on the market at present, both manufactured by Messrs. Adams
Hydraulics, Ltd. Fig. 11 shows a floating weir, circular in form,
arranged by means of floats to rise and fall freely with the level
of the sewage in the chamber. The joint between the fixed and moving
portions of the apparatus consists of an air-lock, and is thus
frictionless. The floats are adjusted to bring the lip of the weir
at such a depth below the top water level, that the volume which can
pass over the weir without raising it is the maximum volume which it
is desired to pass to the tanks and filters. As soon as the flow of
sewage exceeds this volume, it naturally causes the floats to raise
the lip of the weir, and in this way the volume passing to the tanks
and filters can never exceed the predetermined fixed volume, and all
in excess must pass over the overflow weir. Fig. 12 shows a swinging
syphon, which has the same effect as the floating weir. In this case
the syphon has both legs trapped, so that it acts as a continuous
syphon, and it is pivoted on the top of the division wall to swing
freely. To the inlet leg, on the sewer side of the division wall, is
attached an adjustable float, of sufficient buoyancy to raise this
leg of the syphon (and with it the outlet leg as well) as the sewage
rises in the chamber. It will be seen that the difference in level
between the lip of the inlet leg and the buoyancy point of the float,
represents the head which controls the maximum rate of flow through the
syphon, and that immediately this is exceeded the float rises, and with
it the syphon leg, so that all the excess volume of sewage, above the
fixed maximum rate of flow through the syphon, must of necessity pass
over the storm-water overflow weir.
_DETRITUS TANKS OR GRIT CHAMBERS._
The function of these tanks is to arrest all mineral matter, such
as stones, sand, road-grit, and similar substances which cannot be
decomposed in the subsequent stages of treatment, and would thus choke
the tanks and filters. The essential factor in their operation is a
reduction of the rate of flow of the sewage, so that all matters of
a greater specific gravity than the water and the organic matters
in suspension may be deposited by subsidence. At the same time the
velocity should not be reduced to such an extent as to allow the
organic matters in suspension to settle out, as these can be more
suitably dealt with in the subsequent tanks provided for that purpose.
From this it will be seen that considerable care is needed in designing
these tanks if they are to have the desired effect. Further, it is
very essential that every facility shall be provided for removing the
matters which are deposited with as little trouble as possible.
_Capacity of Detritus Tanks._—Too frequently there is very little
evidence of design in these tanks, especially in the provision of
suitable sludge outlets. Before all, there should be at least two
detritus tanks in every case, so that one may remain in work while the
other is being cleaned out, and, if the recommendations of the Royal
Commission on Sewage Disposal are followed, each should have a capacity
of not less than one-hundredth of the daily dry-weather flow. A simple
form of detritus tank is shown in Fig. 13. The essential features
are, a floor with a sharp fall towards the inlet end of the tank and
a sludge outlet at its lowest point. In this case a sludge plug valve
is shown. This is suitable for all cases where the sludge can be
discharged to the sludge bed by gravitation. Where the levels do
not permit of this, and it becomes necessary to raise the sludge, a
chain-pump may be fixed in the detritus tank itself. As, however, this
would involve a separate chain-pump for each detritus tank, as well
as for each of the other subsequent tanks, it is usually found more
convenient in such cases to construct a separate sludge-well provided
with a chain-pump, and arranged at such a depth that the sludge from
all the tanks will reach it by gravitation. This arrangement will be
shown later in connection with the sedimentation tanks. The inlets to
detritus tanks must be provided with valves, so that the flow of sewage
may be shut off when it becomes necessary to empty the tanks. In order
to prevent any misuse of these valves either in error, or wilfully, by
closing both simultaneously and thus causing the whole of the sewage
to pass over the storm-water overflow weir, the inlet valves should
consist of grooved frames with one interchangeable door. By this means
it is impossible for anyone to close both inlets at the same time.
[Illustration: FIG. 13.—DETRITUS TANKS.]
[Illustration: FIG. 14.]
_Dortmund Type of Tank._—Where the volume of sewage is fairly large,
and it would be convenient to have the sludge outlet at 2 feet to 3
feet below the level of the invert of the outfall sewer, the advantages
of designing the detritus tanks on the lines of the Dortmund tank
may be considered. An example of this kind of tank is shown in Fig.
14. Tanks of this type have the following special features:—Great
depth—from 16 feet to 20 feet below water level—and the bottom in the
form of an inverted cone, with an outlet at its apex connected to a
cast-iron sludge delivery-pipe, which may be carried up either outside
the tank, as shown in solid lines, or on the inside of the tank, as
shown in dotted lines. In either case this pipe should be continued
vertically up to, and finish with, an open end at the level of the
top of the wall of the tank, so as to form a means for inspection and
rodding in case the pipe should become choked. From this vertical pipe
a right-angled branch is arranged at about 2 feet _below_ the top-water
level in the tank, and provided with a sluice-valve. Ordinarily this
valve is closed. When it is desired to remove the sludge deposited in
the cone-shaped bottom of the tank, the sluice-valve on the sludge
outlet is opened and the sludge is forced up by the head of water, due
to the difference in level between the top-water level in the tank and
the invert of the sludge-outlet. It has been stated that this method of
sludge removal is subject to difficulties, due to the consolidation of
the sludge in the cone, to such an extent that it becomes of too thick
a consistence to flow up the vertical pipe. In some cases a special
mechanical contrivance is adopted, by means of which the sludge may be
stirred up at the apex of the cone-shaped bottom while the sludge-valve
is open. Again, in a special form of tank which has been brought into
use in Germany, the sludge is stirred up by means of jets of water,
under pressure from the main, forced through a ring-shaped perforated
pipe laid near the apex of the cone. In both cases it is evidently
assumed that the sludge will only be removed at long intervals, and in
the author’s opinion the difficulties referred to above may be avoided
by the application of the motto “Little and often,” as described in his
book on the management of Sewage Disposal Works.
[Illustration: FIG. 15.—SLUDGE SCRAPER IN DETRITUS TANK.]
_Special Apparatus for Sludge Removal._—A further type of detritus
tank is illustrated in Fig. 15, in which the tank is circular in form
but has a flat bottom. The sludge is discharged by the same means as
that shown in Fig. 14, but a special scraping machine operated by hand
is used to facilitate the removal of the sludge by drawing it towards
the inlet to the sludge pipe, which is situated at the centre of the
floor. The scraper, which is manufactured by Messrs. Ham, Baker and
Co., is helical in form, and is attached to and rigidly supported by a
framework mounted on the central shaft, which is rotated by suitable
gearing fixed at the side of the tank over the sludge discharge
inspection chamber, so that the operator may be able to regulate the
rate of the sludge delivery. It will be noticed that the outlet from
this tank is by means of cross-channels, described in detail later in
connection with sedimentation tanks.
_TANKS._
Under this heading are included a large number of tanks of various
types and systems, for each of which some particular advantage is
claimed in ordinary circumstances, or some peculiar suitability for
special conditions. All are, however, ostensibly designed for the
purpose of arresting the organic matters in suspension, in order to
prepare the sewage for the subsequent stage of oxidation in contact
beds, on percolating filters or on land.
_Types and Capacities of Ordinary Tanks._—In addition to detritus
tanks described in the preceding chapter, the Royal Commission on
Sewage Disposal, in its fifth Report, has dealt with five different
methods of tank treatment in detail. These are:—
1. Septic tanks, having a total capacity of about 24 hours’
dry-weather flow.
2. Continuous-flow settlement tanks without chemicals, having
a total capacity of about 15 hours’ dry-weather flow.
3. Continuous-flow settlement tanks with chemicals, having a
total capacity of about 8 hours’ dry-weather flow.