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                         SEWAGE DISPOSAL WORKS

                              _THEIR_
                     _DESIGN AND CONSTRUCTION_

                  BY W. C. EASDALE, M.S.E. M.R.SAN.I.

                 AUTHOR OF “THE PRACTICAL MANAGEMENT OF
                         SEWAGE DISPOSAL WORKS”

                           155 ILLUSTRATIONS

                             [Illustration]

                                London:
                 E. & F. N. SPON, LIMITED, 57 HAYMARKET

                               New York:
                 SPON & CHAMBERLAIN, 123 LIBERTY STREET

                                  1910




PREFACE


In the course of the preparation of a series of articles for “Surveying
and the Civil Engineer,” dealing with the numerous and varied types of
appliances used in connection with Sewage Disposal Works, it occurred
to the Author that it might be useful to many Engineers, and especially
to Students, to have the whole series published in a permanent form
for reference. At the same time, it appeared to afford an excellent
opportunity to include full details of all the various methods of
design and construction in general use, and thus provide a complete
work dealing with the whole subject. The result is the present volume,
which, it is hoped, will prove of value to those engaged in this
branch of engineering. In any future editions that may be required,
it will be the endeavour of the Author to omit any details which may
have become obsolete, and to include particulars of any new methods of
construction, systems or appliances, which may be brought into use from
time to time, and he will therefore be glad to receive particulars of
new appliances and systems as they are introduced.

                                                 W. C. EASDALE.

   28 VICTORIA STREET,
      WESTMINSTER, S.W.
         1910.




                          CONTENTS


                                                                  PAGE
  INTRODUCTION                                                       1

  SCREENS: Simple—Rotary—For deep
       sewers—Mechanical—Other types 3

  STORM-WATER OVERFLOWS: Diverting plate—Fixed
       weirs—Movable weirs                                          16

  DETRITUS TANKS: Capacity—Dortmund type—Apparatus
       for sludge removal                                           23

  TANKS: Various types—Capacities—General
       construction details—Flow through tanks—Sludge
       well—Roofs—Inlets and outlets—Precipitation
       tanks—Dortmund types—Hydrolytic tank—Imhof
       tank—Skegness tank—Candy-Whittaker tank—Non-septic
       cylinder—“Kessel”—Separator—Fieldhouse tank—Dibdin
       slate beds                                                   29

  SLUDGE DISPOSAL: Sludge removal—Apparatus—Chemical
       mixers—Sewage mixers—Sludge presses—Hydro-extractor
       for sludge—Sludge draining beds                              71

  PERCOLATING FILTERS: General design—Various
       types of construction—Floors of filters—
       Sub-drainage—Floor-tiles—Walls of filters—Planning
       of filters—Filtering material—Grading of material
       —Methods of distribution—Appliances for distribution
       —Automatic revolving distributors—Power-driven
       revolving distributors—Automatic travelling
       distributors—Power-driven travelling distributors—Fixed
       distributors—Troughs—Spray-nozzles—Methods of feeding
       percolating filters—Continuous versus intermittent
       supply—Supply channels and pipes-Dosing apparatus            85

  EFFLUENT SETTLING TANKS OR HUMUS PITS                            182

  SAND FILTERS                                                     185

  CONTACT BEDS: Causes of failure—General principles
       of design—General construction—Methods of distribution
       —Sub-drainage—Material for filling contact beds
       —Automatic apparatus                                        189

  CAPACITY OF PERCOLATING FILTERS AND CONTACT BEDS:
       Relative capacities—Table of requirements                   222

  STORM-WATER TREATMENT: Stand-by tanks—Methods of
       construction and operation                                  227

  MEASURING APPARATUS                                              232

  STERILISATION OF SEWAGE EFFLUENTS: Purpose and
       practicability—Apparatus for manufacture of hypochlorite
       solutions—Appliance for injection of chemicals              240

  INDEX                                                            251




                         SEWAGE DISPOSAL WORKS

                    _THEIR DESIGN AND CONSTRUCTION_




INTRODUCTION.


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.

    4. Quiescent settlement tanks without chemicals.

    5. Quiescent settlement tanks with chemicals.

The two last-mentioned have each a total capacity of about 24 hours’
dry-weather flow.

[Illustration: FIG. 16.]

[Illustration: FIG. 16.]

In all these five types of tanks the method of construction is very
similar, generally rectangular in plan and of a moderate depth. As a
rule they are connected by means of a supply channel to the preceding
detritus tanks, and the total capacity is divided up into a number of
units varying with the size of the scheme. The Royal Commission suggest
the following divisions:—

    1. Septic tanks: 5 tanks, with an additional spare tank.

    2. Continuous-flow settlement without chemicals: 6 tanks,
       with 2 additional spare tanks.

    3. Continuous-flow settlement with chemicals: 6 tanks, with
       2 additional spare tanks.

    4. Quiescent-settlement without chemicals: 8 tanks, with 2
       additional spare tanks.

    5. Quiescent settlement with chemicals: 8 tanks, with 2
       additional spare tanks.

The general features of construction are:—

Substantial walls in brickwork, concrete, plain or reinforced, and
of a suitable thickness to withstand with safety the pressures they
are required to resist; sloping floors provided with suitable outlets
for both liquid and solid contents at the bottom, and specially
arranged inlets and outlets at the top. In connection with the
floors, sufficient care has not always been devoted in the past to
the consideration of the most convenient method to adopt, in view
of the necessity of removing the sludge. In some cases, the floors
have been laid with a slope towards the outlet end, and, as the
greatest accumulation of deposit takes place at the inlet end, great
difficulties have been experienced in removing the sludge. There is
very little doubt that if suitable arrangements are made, by means of
which the accumulation of solids deposited at the inlet ends of tanks
can be removed without drawing off the total contents of the tank, much
labour will be saved. With this end in view, the design illustrated in
Fig. 16 is suggested as a model which may be adopted exactly as shown,
or, with some modifications, adapted to meet the special requirements
of particular cases. It will be noticed that a submerged weir wall is
introduced at some distance (which will vary with the method upon which
the tank is operated and with the character of the sewage) from the
inlet end of the tank, so as to retain the larger portion of the solids
in this separate compartment. The floor of this section is laid with a
comparatively steep gradient leading to the sludge outlet. A separate
outlet, fitted with a floating arm, may be provided for drawing off the
top water down to the level of the top of the weir wall. Below this
level, in ordinary circumstances, only the contents of the separate
compartment at the inlet end of the tank will be drawn off in removing
the sludge. A valve is provided at the bottom of the weir wall, so that
the entire contents of the tank may be drawn off should it be found
necessary at long intervals. An alternative to the submerged weir wall
is shown in Fig. 17, in the form of a division wall carried up to the
top of the tank, with orifices below the top water level through
which the sewage passes when the tank is in use. These apertures are
provided with valves, so that they may be closed when the solids in
the compartment at the inlet end of the tank are drawn off, and thus
obviate the necessity for emptying the whole of the tank.

[Illustration: FIG. 17.]

From observations which have been made in various places, it has been
found that although the actual capacity of the tanks corresponded to
anything from 12 up to 24 hours of the daily dry weather flow, the
period during which the sewage remained in the tank, or rather the
time taken for the sewage to pass through the tank, was much less than
it was anticipated would be the case. In one instance, it was noticed
that the sewage passed through a tank of a capacity equal to 15 hours’
dry weather flow in 4 hours, and, although it is obvious that the
same efficiency of sedimentation could not be secured by passing the
sewage at the same rate through a tank of a capacity of 4 hours’ flow,
it would seem that the full effect of the larger tank was not brought
into play. A possible explanation is that the form of the tank and the
arrangement of the inlet and outlet were such that the flow of sewage
through the tank was more or less in a direct line from the inlet to
the outlet, and this, if correct, would lead to the conclusion that
there is room for improvement in the design of the tank, in order to
cause the sewage in its passage to be spread out over the whole area of
the tank. With this end in view the author has specially designed the
arrangement illustrated, Fig. 18, as a suitable method of preventing
the sewage passing direct from the inlet to the outlet. It will be
noticed that the sewage enters the first compartment about 3 feet below
the top water level, and by means of three cross walls is made to flow
down to within a short distance of the floor in one compartment, and up
to within a short distance of the top water level in the next, and
that this occurs twice in the total length of the tank. By sloping the
floor from the centre both ways, i.e. to the inlet and outlet ends,
and providing sludge outlets at the lowest points in each case, every
facility is made for removing the deposit and for emptying each half
of the tank whenever it may be found necessary. Further, by arranging
the sludge outlets in pockets or sumps, situated below the level of
the lowest point of the floor itself, it is possible to draw off the
sludge in small quantities at frequent intervals without emptying the
tank itself. The chief factors in causing the sewage to be uniformly
spread out over the whole area of the tank are, however, the valves
or penstocks on the inlet and outlet pipes, and on the pipes in
the central cross wall. By suitable adjustment of these penstocks,
partially closing those through which the sewage has a tendency to flow
most freely and opening the others, there should be no difficulty in
securing a uniform distribution of the sewage. In any case the actual
direction of the flow of the sewage is, by means of these penstocks,
entirely under control. The inlets to the tank being submerged below
the water level in the supply channel, will secure a more uniform rate
of flow through all the inlet pipes than if they were placed at the top
water level, and the valves on these pipes provide facilities for any
further regulation that may be required. The most important point to
be observed, however, is that the rate of flow from the outlets of the
tank should be uniform. In order that this may be secured, these pipes
are submerged on the inside of the tank, but have their outlets set at
the top water level, so that the actual discharge may be visible, and
thus render it possible to regulate the rate of flow from each pipe by
means of the penstocks provided for the purpose. Further, the openings
in the middle cross wall may be adjusted to control the direction of
the flow through the tank by means of the penstocks, which also serve
to shut off either half of the tank when the other is emptied.

[Illustration: FIG. 18.]

[Illustration: FIG. 18.]

[Illustration: FIG. 19.]

Another method of ensuring uniformity of flow over the whole area of a
tank, is to arrange it in the form of a wedge, with the inlet at the
narrow end and the outlet in the form of a weir at the wide end. This
form of tank is shown, Fig. 134, page 183, for settling out the humus
in filter effluents. The same tank, with a greater depth, would be
equally suitable as an ordinary sedimentation tank for sewage, and
several could be arranged in such a way that three or four would form
a half-circle, i.e. the angle between the two side walls of each tank
would be 60 degrees or 45 degrees.

The principles embodied in the preceding suggestions can be applied to
most types of rectangular tanks.

_Sludge Well._—In connection with the actual method of conducting
the sludge from these tanks to the sludge disposal area, the remarks
made under the heading of detritus tanks will apply. A convenient
arrangement for a sludge well, where a number of tanks are involved,
is shown in Fig. 19, which is self explanatory. For small schemes a
chain-pump operated by hand may be used to raise the sludge from the
well. In larger schemes where power is available, sludge elevators of
the bucket type, as shown in Figs. 20, 21 and 21A, are very
convenient.

_Roofs over Tanks._—With regard to the question of roofs over
tanks, it is now generally admitted that these have very little, if
any, effect upon the working of the tank, and they may therefore
be dismissed in a few words. Under certain circumstances it may be
desirable for sentimental reasons to cover sewage tanks, and in such
cases the general practice is to form concrete arches covered with
earth and sown with grass. Reinforced concrete construction may
sometimes be found very suitable, while, in other cases, galvanized
corrugated-iron roofs, supported on an iron framework carried on
the walls of the tanks, are preferred. In very small installations,
1½-inch or 2-inch creosoted deal boards, laid loose, but fitting close
together with their ends supported in a rebate in the top of the wall,
make a very good cover, as they are easily removed whenever it becomes
necessary to inspect or gain access to the tank.

[Illustration: FIG. 20.—SLUDGE ELEVATOR.]

_Details of Inlets and Outlets._—Among the most important points to
be considered in designing sewage tanks is the arrangement of the
inlets and outlets, as upon these depends to a very great extent the
efficiency of the process. In order to afford a means of selecting the
most suitable arrangement for any particular case a number of different
methods are illustrated.

[Illustration: FIG. 21.—SLUDGE ELEVATOR.]

[Illustration: FIG. 21A.—SLUDGE ELEVATOR.]

Fig. 22 shows the simplest form of trapped inlet and outlet, consisting
of cast-iron Tee junction pipes, the junction being built into the
wall of the tank and fitted with a valve or penstock. The lower end of
the trapped pipe is generally about 3 feet below the top water level,
but in special cases may be much deeper. The upper end of this pipe
terminates at some distance (e.g. about twice the diameter of the
inlet junction) above the top water level, and the top is left open or
fitted with a blank flange for purposes of inspection. Where a roof is
provided over the tank, it is desirable to continue this pipe up and
through the roof, so that it may still be available for inspection. In
large tanks, or any tanks having a width of more than 6 feet, several
of these inlet and outlet pipes should be provided, one for about every
6 feet of width, in order to spread the sewage as much as possible over
the whole area of the tank. A valve should be provided on the inlet
pipe. This is essential in order that the flow of sewage to the tank may
be shut off whenever it needs attention or has to be emptied. Where
there are several tanks with their outlets discharging into a common
channel, it will be found desirable to have valves on the outlets as
well as on the inlets. A slight fall should always be allowed from the
invert of the inlet to the invert of the outlet pipe, and again from
the latter to the tank effluent channel or pipe leading to the filters.

[Illustration: FIG. 22.]

[Illustration: FIG. 23.]

[Illustration: FIG. 24.]

In Fig. 23 a somewhat similar arrangement is shown, but instead of Tee
junctions the inlets and outlets are formed of easy bends, which may be
in cast-iron or glazed stone ware as indicated. The observations made
above in connection with Fig. 22 apply generally to Fig. 23.

Fig. 24 is a plan of Fig. 23, to show a number of inlets and outlets to
one tank.

In Fig. 25 the trapped inlet and outlet is formed by means of a cross
wall carried up to the top of the tank with openings at the floor level
in the form of arches. It is considered by some engineers that this
method is a more substantial form of construction, and that it assists
to a great extent in spreading the flow of the sewage over the whole
area of the tank.

In Fig. 26 both the inlet and outlet is in the form of a weir, running
the full width of the tank, and it is probable that this is the most
efficient means of ensuring that the flow of sewage shall spread over
the whole area of the tank. The trapping of the inlet and outlet in
this case is obtained by the use of scum boards or plates, as shown.
When more than one tank of this type is required, it becomes necessary
to provide a separate feed channel or carrier in addition to the
channel immediately in front of the inlet weir, in order to arrange
means for shutting one or more tanks out of work when required.

[Illustration: FIG. 25.]

The method of arranging the inlets and outlets shown in Fig. 27,
consists of constructing extra deep sewage carriers and tank effluent
channels, and making the connections from these to the tank at the
desired depth below the top water level in the tank. It is true that
these deep channels always stand full of sewage or tank effluent while
the tank is in operation, but it is assumed that the passage into the
tank of all solid matters in suspension is facilitated, especially
during the minimum flow of sewage. It is essential that both channels
should be well dished towards the tank on either side, so as to avoid
all corners where solids may lodge, and render it easy to clean out the
channels when the tank is emptied.

[Illustration: FIG. 26.]

[Illustration: FIG. 27.]

[Illustration: FIG. 28.]

[Illustration: FIG. 29.—TYPE OF FLOATING ARM.]

The various types of inlets and outlets described above are more
particularly suitable for tanks which come under the terms “septic” and
“continuous-flow sedimentation without chemicals.” It is not necessary
that the inlets and outlets should both be of the same type. Various
combinations may be adopted, according to the requirements of each case
and the judgment of the engineer. Similar methods may be utilised for
“continuous-flow sedimentation tanks with chemicals,” but they need
the addition of floating arms for the purpose of drawing off the top
water before the sludge is removed. The type of inlet and outlet more
generally in use for chemical precipitation processes is shown in Fig.
28, as in these cases there is no need to preserve a scum on the
surface. The connection between the sewage carrier and the tank is
usually in the form of a sluice gate, and simple wooden boxes are
provided round the inlet and outlet in order to divert the flow towards
the bottom of the tank. It is also found desirable in some cases to
provide scum-boards for the purpose of arresting the grease, which
naturally rises to the surface, and must not be allowed to pass away
with the effluent. The floating arm outlet is essential, particularly
for tanks which are designed for “quiescent sedimentation with or
without chemicals,” and the usual form of outlet into a channel a few
inches only below the inlet level is not needed, as tanks of this type
are filled and allowed to stand full for a certain period, and the
contents are then drawn off through the floating arm. The function of
this appliance is to draw off the whole of the clear liquid contents,
from a point a few inches below the surface, at a slow rate, and
without disturbing the sludge at the bottom.

[Illustration: FIG. 30.—DECANTING VALVE.]

A type of floating arm is shown in detail in Fig. 29. In order to
prevent any possibility of these arms drawing off sludge by an
oversight, when approaching the floor of the tank, the chain attached
to the float should be arranged to check the fall of the arm at a
point which will be above the level of the sludge, or, if there is
any possibility of the chain being tampered with by unauthorised
persons, the fall of the arm may be arrested with certainty by means
of a bracket, built into and projecting from the wall of the tank, or
by means of a short pier of brickwork and concrete, built up on the
floor of the tank under the arm to the required level. Another method
of drawing off the top water from tanks has been introduced by Messrs.
Willcox and Raikes, Civil Engineers, and is manufactured by Messrs.
Adams Hydraulics, Ltd. As will be seen from the illustration, Fig.
30, it consists of a cast-iron stand-pipe, in sections, each of which
makes a tight joint with the one below it. A spindle, working in a
screwed nut in a bracket or pillar at the top, passes through crossbar
guides inside the stand-pipe sections. This spindle has projections at
irregular intervals, arranged in such a manner that as the spindle is
screwed up it lifts the top section first, then the second, and lastly
the third, and thus makes it possible to draw off the supernatant
water in three layers, each of which may if desired be discharged in
different directions. Finally, the sludge may be drawn off through the
same outlet to the sludge-disposal area.

[Illustration: FIG. 31.]

As the distance which the sewage travels in “continuous flow settlement
tanks with chemicals” is frequently an important factor in securing the
maximum efficiency, it may be found economical to arrange the tanks in
the form shown in Fig. 31, where each tank has a division wall, carried
through from the inlet end to within a few feet of the opposite end,
so that the sewage travels a distance equal to twice the length of the
tank before passing to the outlet. This arrangement requires only one
carrier, but this must be provided with suitable sluice-gates opposite
to each tank, in addition to similar gates on the inlet and outlet from
each tank.

The Dortmund type of tank, described under the heading of detritus
tanks, may also be adapted for sedimentation tanks, but the outlet
should be arranged in such a manner as to reduce the velocity of the
flow at this point to the minimum. This is usually secured in by
causing the liquid to flow over a weir formed by the circular wall of
the tank, or by a number of weirs consisting of cast-iron channels laid
transversely across the top of the tank. In either case it becomes
necessary to form a circular effluent channel round the top of the
tank, to receive the effluent after it has passed over the weirs.
These two arrangements are illustrated in Figs. 32 and 33, the former
showing the circular weir wall, and the latter the transverse cast-iron
channels. Both edges of each of these channels act as weirs, so that
the total effective length of weir is thus greatly increased. The
inlets, conical bottoms, and sludge outlets for these two tanks, would
be similar to those shown in connection with this form of detritus tank
(Fig. 14). Mr. S. R. Lowcock, M.Inst. C.E., has stated that in his
experience an excellent effluent can be obtained by drawing off the
liquid at one point, and at about two feet below the top water level. A
method of accomplishing this is shown in dotted lines on Fig. 32.

[Illustration: FIG. 32.]


_Special Types of Tanks._—One of the troubles which frequently arises
in the operation of all types of natural sedimentation or septic tanks
is a nuisance from smell, due to offensive gases given off by the
effluent. These are the result of the decomposition under anaerobic
conditions of the organic matter deposited in the tanks. It is possible
to arrange them in such a way, that the conditions which cause the
trouble may to a great extent be avoided even in the ordinary types of
tanks.

[Illustration: FIG. 33.]


_Hydrolytic Tank._—There are, however, several types specially
designed to eliminate these troubles altogether, by separating the flow
of sewage through the tank from that section in which the composition
of the sludge takes place. Among these is the hydrolytic tank. This
tank is already well known to most engineers, in the original form
designed by Dr. W. Owen Travis, and adopted at Hampton-on-Thames, but
a new and improved method of construction has recently been brought
out. The principle of this tank may be described as the deposition and
collection of the impurities in sewage by a process of physical
de-solution, the matters being separated in the order of their
grossness and specific gravity, namely (_a_) the removal of the grosser
solids by means of screens; (_b_) the settling of the heavy inorganic
solids in a detritus chamber; and (_c_) the separation of the lighter
solids in suspension and in a colloidal state. Finally, means are
provided whereby the deposit in the various chambers may be collected
and removed with facility. It is impossible in the space available to
describe in full the reasons for the various details of construction
which have been adopted, but the accompanying illustrations, Figs. 34
to 42, which have been kindly furnished by Messrs. Shone and Ault,
Civil Engineers, illustrate an example of the latest type of the tank.
Fig. 34 is a plan section of the tank, and Figs. 35 to 42 are vertical
sections on the lines indicated. The tank is by preference circular,
as shown. The sewage is delivered from the pipe S through a screening
chamber, in which the gross matters, such as rags and vegetable debris
are retained on the screen A, and are from time to time removed by hand
or mechanically. The sewage passes over the weir _a_ into the first
section B, which occupies about one-eighth of the circumference and is
divided into two parts by the diaphragm _b_, Fig. 35. The flow of the
sewage through this first section B may, by the weirs _b_^1 and _b_^2,
be so appointed that two-thirds of it flows from the outer compartment
over _b_^1 and one-third over _b_^2 from the inner compartment, the
only entrance to which is by the opening _b_^3, Fig. 35, in the bottom
of the diaphragm _b_; so that the deposition of the solids by gravity
is accelerated by the flow of the one-third of the sewage into the
inner part of the compartment B. The solids collect in the conical
bottom part _b_^4, Fig. 35. The overflows from the weirs _b_^1 and
_b_^2, are, by the channels _b_^5 and _b_^6, directed to the downtake
_c_, Figs. 34, 35, 38 and 39, which delivers the sewage near the bottom
of the outer compartment C, which latter, with the inner compartment
D, forms the second section of the tank. These two compartments are
divided by the diaphragm _c_^1, Fig. 39, having openings _c_^2 in the
lower edge. In the drawing the second section of the tank is shown
divided in two parts by the wall and weirs _c_^3, Figs. 34 and 40,
and they occupy together about seven-eighths of the circumference of
the tanks. The weirs _c_^3 are so proportioned that 85 per cent. of
the liquid passes directly through the outer compartment, and 15 per
cent. indirectly through the inner compartment of the first portion of
the second section of the tank, into the respective compartments of
the second portion of that section. It should be noted that the only
passages for the flow of liquids into the inner compartment are the
openings _c_^2, Figs. 34 and 39; and consequently the deposition of
solids is accelerated by this flow, so that they collect in the lower
part, _c_^4, Fig. 35, of the inner compartment B. The flow through the
second portion of this second section of the tank is governed by the
weirs _e_^5 and _e_, Figs. 34 and 36, which weirs are shown of such
proportion as to cause 70 per cent. of the liquid to flow directly
through the outer compartment, and 30 per cent. indirectly through
the inner compartment. The colloiders _c_^6, Figs. 34 and 35, are
fixed vertically in the outer compartments to attract and absorb the
solids in pseudo solution. It will thus be clear that 70 per cent. of
the sewage flows in a direct manner through the outer compartment,
and in doing so deposits practically the whole of its permanent and
a considerable portion of its convertible solids. The effluent from
the inner compartment D of the second section of the tank is, by the
submerged channel _e_^3, Fig. 36, passed into the supplementary section
E, which is fitted with colloiders, _e_^1, Figs. 34, 35, and 41. This
effluent, which has become fouled by the disturbance caused by the
evolution of gases in the inner compartment of the second section, is
thus submitted to a further de-solution action by absorption and other
processes. Finally the outflow from the outer compartment C, of the
second section over the weir _e_^5, Figs. 34 and 36, and the outflow
from the supplementary section E, over the weir _e_, are passed away
from the tank by a common channel, _e_^4, Figs. 34 and 37, whence
the effluent may, for further treatment, be led to filters or on the
land. The overflows from the two weirs may, however, be led away from
the tank by independent channels for separate treatment. The solids,
collected in the form of sludge in the lower parts of the sections,
can be drawn off periodically through the pipes _c_^7, Figs. 34, 35,
41, and 42, governed by valves into the central chamber F, Figs. 34
and 35, from which it may be led by the pipe _f_ to adjoining land,
or elsewhere for further treatment. The lighter solids, that collect
in the form of scum on the surface of the liquid in the tank, may be
skimmed off or drawn into the channels _g_, Figs. 34, 38, and 40, and
conducted to the central chamber F, and disposed of similarly to the
sludge. The tank is, or may be, constructed of concrete, which may be
reinforced as required according as it is wholly or partly above the
ground. Its shape may be greatly varied according to local requirements
and other considerations.

[Illustration: FIG. 34.]

[Illustration: FIG. 35.]

[Illustration: FIG. 36.]

[Illustration: FIG. 37.]

[Illustration: FIG. 38.]

[Illustration: FIG. 39.]

[Illustration: FIG. 40.]

[Illustration: FIG. 41.]

[Illustration: FIG. 42. HYDROLYTIC TANK.]


_Imhof Tank._—A somewhat similar tank has been introduced in Germany,
and is known as the Imhof tank; but in this case the whole of the
sewage is passed through direct to the outlet, and none is allowed to
flow through the portion in which the decomposition of the sludge takes
place. These tanks are known in Germany as “Emscherbrunnen,” from the
district in which they were first introduced. The present type has been
designed by Dr. Imhof, the engineer to the Emschergenossenschaft at
Essen, in Germany, and is shown, Fig. 43. It should be noted that the
arrangements may be varied in special cases. Where the daily flow is
considerable, at least two such tanks are recommended, and the inlets
and outlets are so arranged that the direction of the flow may be
reversed at regular intervals, in order that both tanks may receive
an equal proportion of the solid matters. The method of removal of
the sludge is usually arranged on the same lines as that previously
described in connection with the Dortmund type of detritus tank. It
is, however, evident that difficulties are occasionally experienced in
drawing off the sludge when it has been allowed to remain in the tanks
for long periods untouched, as it is suggested that a connection from
the water supply service may be carried down to the bottom of these
tanks, to permit of a jet of water under pressure being directed upon
the sludge in order to stir it up and thus facilitate its withdrawal.

[Illustration: FIG. 43.—IMHOF TANK.]


_Skegness Tank._—With the same end in view—the separation of
the process of sludge liquefaction from the bulk of the sewage
flow—Messrs. Elliott and Brown, Civil Engineers, devised an ingenious
arrangement of tanks for the scheme of sewage disposal which they
carried out at Skegness. In this installation the sewage first enters
a settling tank on the Dortmund principle, from which it overflows at
the top into a dosing tank which gives intermittent discharges to the
filters. The usual sludge delivering pipe from the settling tank is
connected into the _bottom_ of a separate sludge liquefying tank, the
floor of which is some four or five feet below the top water-level
of the settling tank. The upper part of the sludge liquefying tank
is also connected to the dosing tank in such a way, that when the
latter discharges it draws off several inches depth of the supernatant
water from the top of the sludge liquefying tank at each discharge.
The result of this operation is, that each time the dosing tank is
discharged an artificial difference in level is created between the top
water levels in the settling tank and the sludge liquefying tank—the
latter being the lower of the two—and as they are in direct
communication through the sludge pipe, the extra head in the settling
tank causes a movement to take place through the sludge pipe, and thus
forces some sludge up into the sludge liquefying tank, where it remains
for any desired period for liquefaction without unduly fouling the tank
liquor delivered to the filters.

[Illustration:

  =_Candy-Whittaker Patent Bacterial Sewage Purification Tank_=
  _Sectional Elevation_
  FIG. 44.]

_Candy-Whittaker Bacterial Tank._—Somewhat similar in form to some of
the previously described tanks, the Candy-Whittaker bacterial tank is
circular in plan and provided with a deep inner cone, which divides it
into two compartments as shown, Fig. 44. The sewage enters the outer
compartment through a pipe, by means of which it is evenly distributed.
The outlet is through submerged effluent troughs situated inside the
cone, so that the sewage must flow down to within a short distance
of the bottom of the tank in order to pass under the bottom of the
cone and reach the outlet troughs. In consequence of this method of
construction, the bulk of the solids in suspension are deposited in
a circular =V=-shaped gutter or sump, from which the sludge is
removed by the pressure due to the head of water forcing it up a sludge
pipe similar to that previously described in connection with the
Dortmund type of tank. In the Candy tank, however, the inlet end of the
sludge pipe has a returned end with a swivel joint, which is rotated by
means of a vertical spindle operated by a crank handle at the side of
the tank, working through suitable gearing. It is claimed that any scum
which may be formed on the surface by floating solids, or by sludge
freed from the bottom of the tank by gases produced by fermentation, is
retained in the outer compartment, and thus prevented from passing away
at the outlet with the clarified sewage.


_Non-septic Cylinder._—The troubles due to foul-smelling gases arising
from the over-septicisation of sewage in tanks, are very liable to
occur in small installations for country houses, where the daily volume
varies periodically, and may drop to a mere dribble when the family
is away and only one or two servants are left in the house. To meet
the requirements of these cases, an arrangement has been designed
by Messrs. Adamsez, Ltd., which consists of a deep glazed fir-eclay
cylinder, provided with special inlet and outlet pipes. In consequence
of the small diameter of the cylinder, the sewage passes direct through
to the outlet in a very short space of time, but leaves the solids in
suspension in the cylinder, where they undergo decomposition without
affecting to any great extent the character of the fresh sewage on its
way to the filter. This tank is shown in connection with a small filter
and special distributing apparatus in Fig. 45, and is known as the
“Non-septic” cylinder. The sewage, as it leaves this cylinder, is well
suited for further oxidation in properly constructed filters, or on
suitable land without any possibility of causing a nuisance from smell.

[Illustration: FIG. 45.—“NON-SEPTIC” CYLINDER WITH SMALL FILTER.]


“_Kessel._”—In addition to those already described, other ingenious
devices have been designed with the same end in view, viz. the
prevention of nuisance from smell. Two of these, introduced by the
Septic Tank Co., are based upon the theory that it is desirable to
separate the solids in sewage from the liquid at the earliest possible
moment after they enter the sewer. These are illustrated in Figs. 46
and 47. The former shows what is known as the “Kessel,” its name in
Germany, where it was first used. Briefly described, it consists of a
vacuum chamber, in which the sewage rises, by reason of the pressure
of the atmosphere, to a height of about 25 feet, and then flows down
again through a vertical tube, emerging from the apparatus at a level a
few inches below the level of the invert of the incoming sewer. It is
claimed that the deposition of the solids in suspension, due to their
specific gravity being slightly greater than that of the liquid
sewage, is greatly assisted by taking place in vacuum, and that a
high percentage of the suspended solids is removed. The bottom of the
“Kessel” is in the form of an inverted cone, to the apex of which
a sludge pipe is connected, with its outlet end delivering into a
separate sludge well. The deposit which takes place in the “Kessel”
is drawn off at frequent intervals, before it has had time to become
foul, and the capacity of the “Kessel” is so small by comparison with
the daily flow of sewage, that the latter passes out very slightly
altered in character from the state in which it entered. The apparatus
is provided with various arrangements, for ensuring its continuity of
action, for producing the necessary vacuum, and for facilitating the
removal of the sludge. Other advantages claimed for the system are
that it is constructed _above_ the level of the sewer, so that costly
construction below ground is avoided, and that only a few inches of
fall are lost between the inlet and the outlet.

[Illustration: FIG. 46.—“KESSEL” TANK.]


“_Separator._”—The second apparatus shown in Fig. 47 is of an entirely
different character, and is aptly designated by the term “Separator.”
It consists of a number of comparatively shallow settling tanks, each
provided at the top with a metal grating, the separate bars of which
are in the form of narrow channels, with open ends discharging into a
common effluent carrier. The edges of these channels are accurately
planed to form weirs, over which the liquid portion of the sewage flows
in an extremely thin film. These channels are provided with adjusting
set-screws, so that they may all be set at exactly the same level, and
thus ensure a uniform depth of flow over the edges of the whole of the
channels in each tank. The combined length of the channels in each tank
form a weir of comparatively enormous width, so that the velocity with
which the sewage approaches the edges of the channels is extremely low,
with the result that a high percentage of the matters in suspension
are arrested in the tank and are slowly deposited to form sludge. The
bottom of each separate compartment of these tanks is in the form of a
sump provided with a sludge valve connected to a common sludge delivery
pipe, leading to the sludge disposal area by gravity if the latter is
at a lower level or to a sludge well if the tanks are below ground. In
order to prevent the decomposition of the sludge from proceeding so far
as to cause a nuisance from smell, the deposit in the tanks is drawn
off at frequent intervals.

[Illustration: FIG. 47.—“SEPARATOR” TANK.]

[Illustration: FIG. 48.—THE “FIELDHOUSE” TANK.]


_The “Fieldhouse” Tank._—This is illustrated in Fig. 48 (from a
drawing supplied by the patentee, Mr. J. Fieldhouse) from which it
will be seen that the sewage enters the central chamber A^1 by the
inlet pipe M, the end of which is turned down to deliver the sewage
immediately over the inverted cone C. Between the inverted cone C and
the side of this chamber an annular space E is provided, so that the
solids which are deposited may find their way into the cone-shaped
sludge chamber below, from which they are drawn off by means of valve
D and sludge pipe F. The liquid passes from the central chamber A^1
through the walls on all sides into the outer tank B^1, by way of the
oblique passages H, by which the liquid is deflected in a downward
direction, and eventually flows over the outer circular weir K into
the effluent channel L. The outer tank B^1 is divided into sections,
each of which is provided with a sludge sump and sludge valve N.
Scum-boards are provided both radially T, and in front of the weir J,
and the latter may be lowered when it is desired to draw off the scum.
This operation is performed by closing slides S_{1}, so as to cause the
sewage to head up in the tank, and the scum of any section may then
be drawn off by lowering the particular end board J next to the weir
K, and allowing the scum to overflow into the effluent channel L and
thence to the sludge bed. The special features of this tank are:—(_a_)
the cone-shaped bottom of each section, to facilitate the withdrawal of
the sludge without discharging the liquid contents; (_b_) the oblique
passages H in the wall between the inner and outer tanks, for the
purpose of deflecting the flow of the sewage in a downward direction,
and thus assisting the deposition of the matters in suspension; (_c_)
the removable scum boards in the outer tank, to allow of the removal of
the scum; (_d_) the general design by which the sewage enters at the
centre, and thence spreads in all directions until it flows in a thin
film over a weir of comparatively enormous length, thereby causing a
gradually increasing reduction in the velocity of the flow, and thus
providing every facility for the deposition of a very large percentage
of the matters in suspension.

[Illustration: FIG. 49.—SLATE BEDS IN COURSE OF CONSTRUCTION.]


_Slate Beds._—From the foregoing it will be gathered that there is
a growing tendency to reduce the process of putrefaction in tanks
under anaerobic conditions to the minimum, consistent with the removal
of solids. If this theory is carried to its logical conclusion, it
would appear to point to the elimination of all anaerobic conditions.
That this is not generally done is probably due to the fact that a
preliminary process of putrefaction to some extent, is, by many,
considered essential in the removal of solids in sewage. On the other
hand, there are some who are not of this opinion. Mr. W. J. Dibdin has
always contended that putrefaction is not necessary, and his system of
slate beds is designed as a preliminary process in which the conditions
are purely aerobic. Fig. 49 shows details of this system, from which it
will be seen that it consists essentially of a watertight tank filled
with superimposed layers of plates, usually about 2 inches apart. In
order to prevent any misunderstandings, it should be noted that the
description “slate beds” has arisen through the adoption of thin slate
slabs, with distance pieces of slate blocks, as the most economical
method of construction. No special value is ascribed to the slate
itself, beyond its cheapness in the particular form required and its
durability, it being practically everlasting. The essence of the system
is the use of horizontal plates to receive and retain the deposit of
solid matters in suspension in the sewage, so that they are decomposed
or digested, after the settled liquid has been drawn off, by aerobic
bacteria and other higher forms of life, including worms, all of which
thrive only in the presence of air. The beds are filled with the raw
sewage, which is then allowed to remain for a period of about two hours
for quiescent settlement, after which the liquid is slowly drawn off.
It is true that during the period of standing full the solids in the
sewage are not actually in the presence of air, but it is claimed that
a certain amount of air is retained on the under side of the plates,
and the oxygen thus available, in addition to the oxygen present in the
raw sewage, is sufficient to prevent the setting up of putrefaction
during the comparatively short period of standing full. As the liquid
is drawn off, air enters freely between all the layers, so that the
deposited solids are then immediately brought into close contact with
air, from which the aerobic bacteria and other organisms can draw the
oxygen they need for their life functions. The result is that the
ultimate residue of solids is of quite a different character from
sludge of the ordinary type. It is of a granular nature, which rapidly
dries on a properly constructed draining bed, and, when dry, resembles
ordinary peaty mould. Independent information as to the actual amount
of ultimate solid residue resulting from this system is not yet
available, but it is generally admitted that, when properly operated,
putrefaction does not occur at any stage of the process, and that there
is an entire absence of nuisance from smell throughout the works. When
new, these slate beds have a liquid capacity of over 80 per cent. of
the gross capacity of the beds, but it is usual, in calculating the
size of the beds required for a particular volume of sewage, to allow
for a normal working capacity of 66 per cent. of the gross capacity,
and to provide for one filling per day in dry weather. These beds are
generally constructed with a working depth of 3—4 feet, but they may be
as little as 1 foot in depth where it is necessary to reduce the total
fall required for the works to the minimum. The residue of the solids
after treatment in these beds passes out in the effluent, and it is
understood that it has not been found necessary to wash out the beds or
remove the deposit on the slates themselves, even after several years
of operation with strong sewage. In designing beds for this system, the
chief points to be borne in mind are that the constructional work shall
be absolutely watertight, and that the fall on the floor shall be
sufficient to allow the solid residue to pass freely to the outlet with
the effluent. The beds may be operated by hand by means of penstocks
on the inlets and outlets, or automatically by means of special
apparatus of the type which will be described later in connection with
contact-beds. It is, however, important that the liquid shall not be
discharged from the beds at too rapid a rate.




_SLUDGE DISPOSAL._


_Sludge Removal._—In connection with the discharge of sludge from
tanks of any kind, there are several appliances adapted to meet the
requirements of particular cases. Where the sludge-disposal area is
at a lower level than the bottom of the tank, a simple sludge-plug or
penstock on the inlet to the sludge-pipe may be used, or a sluice valve
may be inserted on the sludge-pipe after it leaves the tanks. Where the
sludge-disposal area is 2 feet or more below the level of the surface
of the sewage in the tank, and the floor of the latter is provided
with a suitable sump in which the sludge may accumulate, the method of
withdrawing the sludge by utilising the pressure of the head of liquid
in the tank, as described in connection with the Dortmund type of
detritus tank, may be adopted with advantage.

In cases where it is necessary to raise the sludge to the disposal
area, a hand-operated chain-pump may be used for small schemes, or for
large volumes, and where power is available, sludge elevators of the
bucket type, as shown on pages 40 to 42, and manufactured by Messrs.
S. S. Stott and Co., Messrs. Ham, Baker and Co., Ltd., and Messrs.
Adams Hydraulics, Ltd., will be found convenient. These appliances
are usually erected in special sludge wells, to which the sludge is
delivered by gravity. In the case of long tanks, in which the floors
are comparatively flat, and especially where the sludge is allowed
to accumulate until it has become consolidated to a great extent,
difficulties are experienced in causing the sludge to flow to the
outlet by gravity. This usually involves the employment of men to
descend into the tank and force the sludge towards the outlet by means
of squeegees, a slow and laborious process.

[Illustration: FIG. 50.—CHEMICAL MIXER.]


_Chemical Mixers._—The methods adopted for adding the necessary
chemicals to sewage for chemical precipitation are various. Where
alumina-ferric is used, the simplest method is to place blocks of the
precipitant in wire cages placed in the inlet channel so that the flow
of the sewage itself dissolves the block as required. It has been found
that this method is not economical in some cases, and the precipitant
is dissolved beforehand in a suitable mixer in order that it may be
added to the sewage in the form of a solution. This applies specially
to the lime process, and several forms of these mixing machines are
shown in Figs. 50, 51 and 52, made by Messrs. Goddard, Massey and
Warner, Messrs. Manlove, Alliott and Co., Ltd., and Messrs. S. H.
Johnson and Co., Ltd. These may be driven by power or by the flow
of the sewage itself, but the most important point which requires
attention is that the strength of the solution shall vary with the
strength of the sewage, either by varying the rate of flow of a
solution of uniform strength, or by varying the strength of a solution
flowing at a uniform rate.

[Illustration: FIG. 51.—CHEMICAL MIXER.]


_Sewage Mixers._—Even after the chemical solution has been added to
the sewage, it is necessary to make sure that it is thoroughly mixed
with the sewage. The simplest method of doing this is by means of
baffle-plates fixed in the channel leading to the tanks. Other methods
are by paddle-wheels driven by the sewage itself; by allowing the
sewage to drop in a chamber on to a projecting pier or stone; by using
power to drive (_a_) a plunger moving up and down in a sump, (_b_) a
vertical shaft to which horizontal paddles are attached to rotate in
the sewage channel, (_c_) to operate a device similar to the well-known
mechanical egg-whisk, (_d_) or to force compressed air through a
perforated pipe laid in the sewage channel. Indeed, there is no end to
the various mechanical devices which are used for this purpose.

[Illustration: FIG. 52.—PNEUMATIC CHEMICAL MIXER.]

[Illustration: FIG. 53.—SLUDGE PRESS.]

[Illustration: FIG. 54.—SLUDGE PRESS.]


_Sludge Presses._—When it is desired to reduce the liquid content of
the sludge as far as possible, the general practice is to make use of
sludge presses for this purpose. Several types are illustrated in Figs.
53, 54 and 55, manufactured by Messrs. Manlove, Alliott and Co., Ltd.,
Messrs. Goddard, Massey and Warner, and Messrs. S. H. Johnson and Co.,
Ltd. All are based upon the principle of compressing the liquid sludge
under high-pressure between iron plates which support cloth or other
filtering material, through which the liquid passes into grooves on
the faces of the plates, and thence by way of conduits in the plates
themselves to the floor below. The several makes have different methods
of opening and closing the plates, and the presses are made of various
sizes for operation by hand or by power. Fig. 56 shows a complete
sludge-pressing plant as designed by Messrs. S. H. Johnson and Co.,
Ltd. The description of the details of this plant is as follows.

[Illustration: FIG. 55.—SLUDGE PRESS.]

The sewage enters the works by the channel A, and passes first through
the bar screen B. The screening is necessary to remove anything that
would tend to produce obstruction in the inlets to the press chambers
and be liable to cause breakage of the press plates. The sewage next
meets with the milk of lime from the lime mixer C, with which it is
mixed by flowing along the gravitation mixer D. The pneumatic lime
mixer produces lime milk of a constant strength, and the flow is
adjusted in proportion to the requirements of the sewage. Should it be
necessary to add sulphate of alumina to the sewage, this is produced
by the pneumatic alumina mixer E, and is added to the sewage after
the latter has been thoroughly mixed with lime. Air for working the
pneumatic lime and alumina mixers is provided by the blowing engine
R. The treated sewage then passes further along the zigzag channel
into the precipitating tanks F, the ends of two of which are shown
in the drawing. It is advisable to have two or more tanks, so as to
allow sufficient time for precipitation. The usual capacity of the
precipitation tanks is equal to 6 hours’ flow of the sewage, and they
may continue running, overflowing continuously, for a considerable
time, but not so long as will produce putrefactive decomposition and
thereby cause a nuisance. The precipitation tanks, which are cleared
out alternately, are provided with hinged flap valves G and underground
stoneware pipes to convey the sludge into the liming sump H, the top
water being first decanted off. In the liming sump the sludge is limed
with milk of lime from the lime mixer I, which is also worked by the
blowing engine R above referred to. From the liming sump the sludge
passes into the sludge tank J, by means of the pair of automatic
rams K. The automatic rams work alternately, one filling by means of
vacuum, whilst the other is being discharged by means of compressed
air. As soon as the one is emptied and the other filled, the action is
reversed, and so on, each filling and emptying alternately, thereby
keeping up a continuous discharge. By being drawn into the rams, and
thence forced into the sludge tank, the sludge becomes thoroughly mixed
with the lime. This liming of sludge causes a considerable further
deposition and concentration of the sludge, and after standing all
night the supernatant water is decanted off by the skimmer L. The
sludge, now ready for pressing, is allowed to run by gravitation into
the automatic rams K previously referred to, and thence discharged into
the sludge presses M by means of compressed air, the compressed air
being supplied by the air-compressor N, which also acts as a vacuum
pump for drawing the sludge from the sludge pump into the automatic
rams. The solid portion of the sludge is retained in the chambers of
the sludge presses by the filter cloths, the effluent being discharged
into the trough at the side of each press, and thence by down pipes
and gullies into the effluent channel O, being treated again in the
gravitation mixer, and finally flowing away with the effluent from the
precipitating tanks. The press chambers are known to be filled with
solid sludge cakes, when effluent ceases to flow from the outlets of
the chambers. The presses are then opened and the cakes discharged into
a tipping truck Q, by which they are removed to the final disposal site.

[Illustration: FIG. 56.—COMPLETE SLUDGE-PRESSING PLANT.]

[Illustration: FIG. 57.—SLUDGE-DRYING APPARATUS.]

[Illustration: FIG. 57.—SLUDGE-DRYING APPARATUS.]


_Hydro-extractor for Sludge._—An entirely different method has been
adopted in the special apparatus in use at Hanover and other towns in
Germany, the Schaefer-ter-Mer centrifugal sludge de-hydrating apparatus
manufactured by the Hanoversche-Maschinenbau A.-G., vormals Georg
Egestorff, and illustrated in Fig. 57. In this apparatus the
centrifugal force resulting from the rapid rotation of the drum into
which the liquid sludge is fed, is utilised to throw out the solid
matters from the centre towards the circumference, where they are
caught in the outer part of the drum of the machine. The drum revolves
continuously, but at regular intervals it is opened automatically in
sections for a brief period, so that the dry sludge is thrown outwards
against the fixed casing and thus becomes broken up and falls to the
bottom, and thence to an endless-band transporter by which it is
discharged outside the building. At the moment when the sections of
the outer casing of the drum of the machine are opened to allow the
dry sludge to be thrown out, the wet sludge is prevented from passing
into these sections by the automatic closing of the inner slide door,
which is opened as soon as the outer slide is closed. The water
extracted falls into an annular channel below, from which it flows,
by way of a pipe, back to the settling tanks to be treated again. The
result of a series of special tests of this apparatus showed that the
liquid contents of the sludge was reduced from 92 per cent. to 50 per
cent. The installation at Hanover has now been in operation since June
1908, dealing with a daily volume of 6·6 million gallons of sewage
from a population of 280,000. From particulars supplied by the town
authorities, it appears that the total cost of operating the complete
plant, including the settling tanks and the sludge treatment apparatus,
amounts to about 8_s._ per million gallons of sewage treated, or about
0·8_d._ per head of population per annum.

Messrs. Manlove, Alliott and Co., Ltd., have now entered into an
arrangement with the above-mentioned firm to take up the control of
the patents and the sole manufacture and sale of the Schaefer-ter-Mer
Sludge-Drying Apparatus in Great Britain and the Colonies.

[Illustration: FIG. 58.—SLUDGE-DRAINING BED.]


_Sludge Draining Beds._—Although the methods of disposal of sludge
must vary in different localities according to the means available
for the purpose, and most of them involve very little, if any,
constructional work, it may be desirable to describe the various
points which should be taken into consideration in the construction of
suitable draining beds, as these should be included in the original
design of any scheme in which they are to be used. Their chief function
is to provide means for removing the maximum amount of the liquid
contents of the sludge in the minimum of time, and it is obvious that
this desideratum can only be secured by spreading out the liquid
sludge in thin layers upon material through which the liquid may
readily pass without carrying with it any of the sludge. The first
of these requirements necessitates the provision of an ample area of
draining surface, and the second involves the use of a suitably graded
material provided with ample means of drainage. The beds themselves
may be simple excavations in the ground, as shown in Fig. 58, or may
be constructed of brickwork or concrete, but in either case it is
absolutely essential that the floor should be covered with tiles, or
other means of sub-drainage, leading to a free outlet, which should be
connected to the screen chamber, detritus tanks, pump well, or some
other point at the inlet to the works, so that it may be treated over
again with the crude sewage. Whatever material is used for filling
the bed, the lower portion which is placed on the floor and over the
drainage tiles should be of large size, 2 inches to 3 inches in
diameter. The next layer should be 1½ inches to ½ inch in diameter, and
the top layer 6 inches to 9 inches in depth, should be fine material ¼
inch to ⅛ inch in diameter. In the author’s opinion, coke-breeze will
probably be found to be the best material for the top layer, and it
would be a good precaution to provide beforehand a quantity of this
material in reserve to replace what is lost in removing the dried
sludge from the surface of the beds.

It will be found advisable in operating these beds to discharge the
sludge from the tanks in small quantities at frequent intervals, rather
than in large quantities at long intervals, and it is very important
that each layer of dried sludge should be removed before the next layer
is delivered to the bed. It cannot be too strongly urged that sludge
disposal needs as much care and attention as any other stage of the
process of sewage disposal, and if this is available, and ample area
of draining beds is provided, there should be no difficulty in solving
this usually troublesome problem.




_PERCOLATING FILTERS._


In approaching the subject of the design of filters for the purpose of
oxidising the organic matters—in solution and in suspension—contained
in the liquid which leaves the preliminary process in tanks, the first
consideration is the question of site. Where the slope of the ground
permits of the construction of the filters on or above the ground
level, much expense for excavation may be avoided, so long as the base
of the filter can be laid on solid ground. In cases where the site of
the works is comparatively flat, it is impossible to avoid excavation,
and other means must be adopted to keep the actual cost as low as
possible consistent with efficiency.


_General Design._—Taking the latter case first, it should be observed
that some engineers consider it desirable to construct retaining walls
under all circumstances, but the author does not agree with this idea.
In the first place the walls do not, of themselves, have any influence
on the efficiency of the filters in producing a satisfactory effluent,
and if a filter can be constructed without them there is no reason why
they should not be omitted. This applies especially where the filters
have to be constructed entirely below the surface of the ground. The
chief point to be considered is that the effluent shall have a free
outlet, with facilities for inspection. In the case of all filters, the
best method of securing a free outlet for the effluent is to provide
the floor with a suitable slope from the centre to all sides. When the
floor is at some depth below ground this requirement necessitates an
effluent channel on all sides of the filter. Two methods of carrying
this into effect are illustrated, Figs. 59 and 60. Fig. 59 shows one
retaining wall for the filter and an additional retaining wall for
the surrounding earth, carried up to the surface with the effluent
channel between the two walls. In Fig. 60 the arrangement is similar,
but the outer wall is a dwarf wall to form the effluent-channel, and
the surrounding earth is cut back to a slope of natural repose, the
earth bank usually being sown with grass or covered with turf. It is
essential that the outer wall shall be carried up above the toe of
the earth bank, in order to prevent soil being washed down into the
effluent-channel, but a surface-water drain should be laid to take any
water that may accumulate at the back of this wall.

These methods of construction would, however, only be adopted by those
who consider it essential to provide means for lateral aeration to
the filter by constructing the retaining wall for the filter itself
of pigeon-hole brickwork. In the author’s opinion, however, lateral
aeration on these lines is altogether ineffective. It can only affect
the filter material for a distance of a foot or two from the wall. It
is true that in some cases horizontal perforated ventilating pipes
have been provided, radiating from the centre of the filter to the
outer wall and terminating with open ends on the outside. The effect
of these is, however, dependent almost entirely upon the temperature
of the atmosphere and the direction of the wind, and even if they do
induce air currents into the body of the filter the air will pass along
the line of least resistance, and therefore find its way through those
interstices which are open and not through those which are choked in
any way and thus most in need of aeration. The author believes that the
aeration of a filter is most effectively secured by the action of the
sewage itself, as it falls from the surface to the floor drawing in the
air from the top, and that if this is not effective, no amount of
lateral openings will produce the desired result. If this contention is
correct, there is no need to incur the additional expense involved in
the construction of retaining walls, and the filters may be designed on
the lines indicated in Fig. 61, where the filtering material fills
the whole of the natural excavation, and no walls are required. It is
true that this method involves an increase in the amount of filtering
material beyond what is actually in use, and the cost of this must be
set against the cost of the wall in the alternative method shown in
Fig. 62, as the cost of the excavation in either case is about the
same. Much will depend upon the cost of the filtering material in
different localities.

[Illustration: FIG. 59.]

[Illustration: FIG. 60.]

[Illustration: FIG. 61.]

[Illustration: FIG. 62.]

In both these methods it will be noticed that there is no outer
effluent channel, and that the floor slopes from the circumference
to the centre, where an effluent receiving chamber is constructed,
from which an effluent discharge pipe leads to the next stage in the
process. This is not quite so satisfactory as the arrangement shown in
Figs. 59 and 60, but it is the most convenient under the circumstances.

When it is possible to place the filter floor on or within 2 feet of
the surface of the ground, the method illustrated in Fig. 60 is the
design most commonly adopted. Sometimes the arrangement of the floor
shown in Figs. 61 and 62 is preferred. There is, however, still another
type of floor which is applicable to this case. This is illustrated in
Fig. 63, from which it will be seen that the whole of the floor slopes
in one direction, and the effluent is thus discharged over one-half
of the circumference of the filter, with the result that the effluent
channel is only one-half the length of that required in the case of
Fig. 60.


_Floors of Filters._—In all cases the floors for percolating filters
should be of a substantial character. They are usually constructed
of cement concrete. The thickness of the concrete will depend upon
the nature of the subsoil, but in any case it should be increased at
the centre in order to provide a safe foundation for the revolving
sprinkler. The surface of the floor should be smooth, so as to
facilitate the flow of the effluent, and any suspended solids it may
contain, to the outlet. From the preceding illustrations it will be
noted that the slope of the floor may be in three directions: (_a_)
from the centre to the circumference, (_b_) from the circumference
to the centre, or (_c_) from one side of the filter to the other, a
uniform slope in one direction along the diameter of the filter. It
has been suggested by some that the floor should consist of a series
of alternate V-shaped ridges and furrows, with slopes at right angles
to the general slope of the floor, but this has the disadvantage that
it causes difficulties in arranging the floor tiles and placing the
filtering material in position, and further it increases the cost of
construction without providing any real compensating advantages.

[Illustration: FIG. 63.]

[Illustration: FIG. 64.—FLOOR TILE.]

[Illustration: FIG. 65.—“NEWHAM” FLOOR TILE.]


_Sub-drainage._—It has been recommended that the floors should be laid
with a gradient of about 1 in 100, but the author believes that it
would tend to assist the free discharge of the suspended solids in the
effluent if this gradient were increased to 1 in 75, or even 1 in 50,
and that if this were done, there would be no need to make any special
provision for access to the under drains for cleaning purposes. Another
important factor in securing a free discharge of the suspended solids,
is the use of proper floor tiles or sub-drains. The old idea of laying
rows of agricultural pipes with open joints, or even of perforated
pipes, on the floor has been proved to be useless. In some cases it is
considered sufficient to have rows of floor tiles, laid at any distance
apart up to 10 feet at the circumference. There is, however, very
little doubt that the only correct method is to support the whole of
the filtering material on a complete false floor, so that the suspended
solids which are carried down with the effluent, and thus reach the
bottom of the filter at all points of its area, may fall freely into
an open space, from which they will be carried away with the least
possible obstruction by the flow of the liquid. It is certain that
these suspended solids have a great tendency to adhere to any object
with which they may come in contact, and that this can only be avoided
by providing a free space immediately above the floor and over its
whole area. The nearest approach to this acme of perfection is a
complete false floor, with openings too small to allow the filtering
material to pass through, but large enough to give a free passage
to the suspended solids. One of the simplest methods is to form the
false floor of bricks, laid flat on rows of bricks on edge. There are,
however, several floor tiles on the market specially designed for this
work. A number of these are illustrated in the accompanying figures.
The Ames-Crosta tile, Fig. 64, has a simple flat top with corrugated
edges supported on four short legs. When placed close together, the
corrugations form apertures through which the liquid passes, and the
flat tops form a table which provides every facility for placing the
filtering material in position. A somewhat similar floor tile is
the “Newham” shown in Fig. 65. In this case, however, each tile has
only two legs, and thus there is less obstruction to the flow of the
effluent. The “Stiff” floor tile, Fig. 66, is of much the same type as
the two last described. Another material which has been adapted for
this purpose is a single layer of the “Dibdin” type of slate slabs
and blocks, as shown previously in connection with the preliminary
treatment of sewage in tanks, Fig. 49. This method also provides
a complete flat false floor, and has the additional advantage of
occupying the least possible space. The slabs themselves may be split
to any suitable thickness, and the supporting blocks may be cut to any
size, as little as 1 inch in thickness if desired. The slabs vary in
shape and size, but may be secured of larger area than tiles, so that
the necessary supporting blocks are less in number than the feet of the
tiles, and thus offer less obstruction to the effluent. The apertures
for the passage of the liquid are provided by the holes formed by the
irregular edges of the slabs, where they abut on one another.

[Illustration: FIG. 66.—“STIFF” FLOOR TILE.]

There are, however, floor tiles of other shapes, such as the Candy
tile, Fig. 67, for which it is claimed that the effective depth of
the filter may be calculated from the floor, and that the height
of the V-shaped opening under the tile provides greater drainage
and ventilation openings than other tiles. Other drainage tiles are
semicircular in section. The Naylor tile, Fig. 68, and the Albion
floor tile, Fig. 69, designed and used by Mr. E. E. Ryder, Surveyor
to the Bushey Urban District Council, both have openings at the floor
level. The “Mansfield” floor tile, Fig. 70, is the type adopted for the
extensive filters at the Birmingham Tame and Rea district sewage farm
by Mr. J. D. Watson, Engineer to the Drainage Board.

[Illustration: FIG. 67.—CANDY FLOOR TILE.]

[Illustration: FIG. 68.—NAYLOR FLOOR TILE.]

In some cases, instead of providing a free space above the floor,
drainage channels have been formed in the floor, with rebates in the
sides to receive flat perforated tiles, as Fig. 71. This method of
drainage certainly leaves the floor clear to receive the filtering
material, but it has the disadvantage that it does not provide a free
space under the whole of the material; and as the suspended solids in
the effluent must of necessity travel some distance before they reach
a channel, they are, to a great extent, arrested by the material which
comes immediately on the floor, however large the separate pieces may
be, and thus tend to choke the interstices.

[Illustration: FIG. 69.—FLOOR TILE.]

[Illustration: FIG. 70.—“MANSFIELD” FLOOR TILE.]

Whatever method of sub-drainage may be adopted, it is advisable to
continue the tiles, or channels, right through to the effluent channel
or chamber in straight lines, so that the effluent may have a clear
way throughout its course under the filter, until it reaches the open
effluent channel or pipe.

[Illustration: FIG. 71.]


_Walls of Filters._—The great diversity of methods adopted to retain
in position the material of filters constructed above the level of
the ground, must be extremely perplexing to anyone who investigates
the subject. They vary from walls of such thickness and substantial
material that they would be quite suitable for resisting the pressure
of a head of water equal to the depth of the filter, down to no walls
at all. In many cases walls 9 inches thick throughout and 6 feet in
height are found to be quite satisfactory, while in a large number
of cases the material itself, when it consists of clinker, is found
standing in an almost vertical position with perfect success. The
question naturally arises, if it is possible to construct filters
satisfactorily without any retaining walls, why incur unnecessary
expense in providing walls? The answer is that clinker is not
universally adopted as a filtering material, and that the strength
of circular walls is not sufficiently taken into consideration. The
thickness of the wall will naturally depend upon its height, but
assuming this to be 6 feet, and that the wall is in all cases carried
down to a solid foundation, the extreme thickness that would be needed
is probably 18 inches at the bottom and 9 inches at the top, as Fig.
72. As a general rule, however, it will probably be found quite
sufficient to construct the wall of brickwork in cement, with the lower
half 14 inches and the upper half 9 inches in thickness. In a large
number of the most recently constructed filters, the walls have been
omitted altogether when clinker has been used for the filter. Large
pieces of this material have been selected, and carefully packed in
the form of a dry rubble wall with perfect success, even when laid
with a batter on the outside of as little as 1 in 6 or even 1 in 8.
It is obvious that this method of construction needs special care and
supervision, especially in providing a rough kind of bond between the
different layers, if it is to be successful. A rough idea of this
method is given in Fig. 73. As a kind of happy medium between this
system and a wall to the full height of the filter, a dwarf wall has
been adopted in a number of cases, and notably by Messrs. Willcox and
Raikes, Civil Engineers, who have also designed a special form of
coping, made in fire-clay or terra-cotta, as shown in Fig. 74. Again,
in several places where suitable local stone, or stones and bricks
resulting from the demolition of old houses or walls, was available at
a cheap rate, these materials have been utilised to form dry rubble
walls, similar in construction to the clinker walls illustrated in Fig.
73.

[Illustration: FIG. 72.]

[Illustration: FIG. 73.]

Occasionally the walls of filters are carried up several feet above
the surface of the filter with the idea of forming a screen to protect
the revolving sprinklers from the influence of wind. It is, however,
obvious that these measures can only have an effect upon the outer ends
of the revolving arms, and, as the action of the revolving sprinklers
is only affected by wind when the head of sewage by which they are
driven is at its minimum and the force of the wind is simultaneously
at its maximum, the necessity for windscreens seldom arises. In actual
practice, with an average head of sewage, no real difficulty is caused
by wind.

[Illustration: FIG. 74.]

So far, it has been assumed that the walls and floors of the filters
can be constructed upon solid ground. Unfortunately this is not always
possible. If the subsoil is of an unsubstantial nature it is advisable
to lay the floor of a suitable thickness and diameter, so that the wall
may be built upon the floor itself, and thus distribute the load over
as large an area as possible. In some cases the levels necessitate the
raising of the filter floor _above_ the surface of the surrounding
ground. Under these circumstances the footings of the walls should
be carried down to solid ground, and no reliance whatever should be
placed upon made-up ground even to carry the floor alone. The outer
edge of the floor should be supported by the footings of the wall, and
the remainder carried upon piers or cross-walls of brickwork, concrete
or masonry, extended down to solid ground. The piers or cross-walls
should be sufficient in number to support the floor with safety. A
smaller number may be used if steel joists are provided between them
to take the weight of the floor and filter, or the whole floor may be
of a properly designed reinforced concrete construction, supported at
the circumference on the footings of the outer wall and at the centre
on a substantial pier of concrete carried down to solid ground, with
intermediate piers if the diameter of the filter is excessive.


_Planning of Filters._—The preceding notes with regard to floors,
sub-drainage and walls, apply generally to all filters, whether
they are for revolving or travelling distributors or for sprays or
fixed troughs. The most suitable plan for travelling distributors is
naturally rectangular, but where more than one such filter is required
it will be found economical to arrange them in pairs, with a central
supply channel feeding two distributors, one on each side. This applies
equally to other types of fixed distributors, but care should be taken
to divide the total area into reasonable units. In all except the
smallest schemes (which should consist of not less than two units) it
will be found that three, or multiples of three, units form a very
convenient method of arrangement. This suggestion with regard to
the subdivision of the total area applies to filters for revolving
distributors, so far as their number is concerned, but there are
several methods of arranging the area of the filters themselves. When
the filters are separated from one another they may be placed in
regular order of one kind or another if the site is uniformly flat, or
irregularly to suit the contour of the ground if the site is uneven.
It has been considered an objection to the use of circular filters
that even if they are placed close together a considerable amount of
space is rendered useless. There may be some justification for this
objection in cases where the area of the site is limited. On the other
hand, the objection has been overcome by arranging the total filter
area on one common floor, fixing the revolving distributors so that the
circumferences of the areas they cover meet where possible, and leaving
the spaces not covered by the distributors free of material to provide
some lateral aeration for whatever it is worth, or as an alternative to
provide a convenient position for chambers to receive the effluent from
the contiguous filters. This arrangement is illustrated in Fig. 75. If
other arrangements are made for the discharge of the effluent, then the
spaces in question may be filled with material and utilised as filters
by fixing smaller suitable distributors, as Fig. 77. This has been done
at Kingston-on-Thames in converting existing rectangular contact beds
into percolating filters, while at Darwen, Lancs, the intermediate
spaces not covered by the revolving distributors are utilised as
filters and the distribution effected by means of fixed sprays.

[Illustration: FIG. 75.]

[Illustration: FIG. 76.]

[Illustration: FIG. 77.]

In other cases the filters for revolving distributors have been
constructed octagonal in plan instead of circular, as shown in Fig.
76. By constructing suitable retaining walls the whole scheme presents
a very good appearance, and the intervening spaces can be utilised
for effluent chambers or as filters with smaller distributors, as
previously described, and practically the whole area is thus utilised.


_Filtering Material._—On this subject there exists a great diversity
of opinion. Some engineers are satisfied to use any kind of material
which will not disintegrate rapidly, while others pin their faith to
one particular kind. Again, the grading of the material is a matter
upon which it is seldom possible to find two engineers in complete
agreement. The opinion is frequently expressed that true economy
consists in utilising local material as far as possible, sometimes even
to the extent of adopting a local product, even though it is admittedly
not so good as some other material which may cost slightly more for
carriage from a distance.

Undoubtedly the first consideration is to secure a material which will
not disintegrate, but this is not the only essential qualification. The
author has had many opportunities of observing the results obtained
from various materials, and, for dealing with an average sewage, he
has never seen a better material than hard-burnt vitrified furnace
clinker. This material, of the proper kind, is practically equal to
stone or gravel in its ability to withstand the various influences
which tend to cause disintegration, but it possesses the advantage over
stone and gravel of having numerous cavities, which apparently form
the most suitable means of assisting the development of the bacterial
gelatinous growth, which appears to be the essential factor in causing
the deposition of the organic matters in suspension and in solution in
the tank effluent. It would seem as if the smooth surfaces of gravel or
broken stone cannot retain this growth, and that it is washed away as
soon as it begins to form. It is true that excellent effluents are
obtained from filters of gravel or stone, but, so far as the author
is aware, only by providing a larger cubic capacity of filter than
would be required if proper clinker were used. It is thus questionable
whether the lower cost of local stone or gravel does, in fact, result
in ultimate economy, if a smaller quantity of the right kind of
clinker, at a slightly higher price per cubic yard, will secure the
same result.

It will be noticed that stress is laid upon the necessity of using the
_right kind_ of clinker. This is intentional, as the word “clinker”
appears to cover a large variety of material. House refuse, cinders,
and over-burnt bricks, as well as the products of refuse destructors,
are all considered as “clinker,” especially in cases where a contractor
finds he has taken a very low price for filter material in making up
his tender. In the author’s opinion, the only kind of clinker, indeed
the only kind of material, which should be used for percolating filters
is the extremely hard clinker from boiler furnaces, more or less
vitrified throughout, and not only of irregular shape, with a rough
surface, but possessing numerous cavities on all sides. Clinker of
this type is occasionally to be obtained from destructor furnaces, but
it depends upon the character of the original refuse, and probably to
some extent upon the method of stoking. In any case it necessitates the
exercise of experienced judgment and discrimination in selection, and
in some cases destructor clinker is so soft, and so evidently certain
to undergo rapid disintegration, that it should be rejected at all
costs.

Among other materials which are used for percolating filters in various
parts of the country are coal, broken saggars, stone of various kinds,
including granite, gravel, broken bricks, coke, cinders, coke-breeze,
and slag from ironworks, but in the author’s opinion none of these are
so satisfactory as the right kind of clinker described above.


_Grading of Filtering Material._—There is probably as much, if not
more, diversity of opinion on this point as in the matter of the kind
of material most suitable for filters. The sizes in actual use vary
from ⅛ inch to 3 inches and even 6 inches. Some engineers stipulate for
a uniform grade throughout the filter, others prefer to have different
grades at different depths, while still others are satisfied to allow
small and large pieces to be mixed together indiscriminately. In the
author’s opinion the last mentioned method is the least satisfactory of
all. One of the essential factors in obtaining the maximum efficiency
from percolating filters is a free passage for air to enter into all
parts of the filter. This can only be secured if the interstices
between the pieces of material are kept clear at all times, but when
small and large pieces are mixed together, the small have a natural
tendency to fall into the spaces between the large pieces and thus
choke them. Even if this were not of importance, the usual methods
of filling a filter do not permit of the uniform distribution of the
finer particles among the larger, so that the usual result is that some
portions of the filter consist almost entirely of fine and others of
coarse material, and the results must of necessity be unequal.

On the face of it, the division of a filter into layers, each
consisting of a different grade of material ranging from coarse at the
bottom to fine at the top, would appear to be an excellent idea, but
it is only those who have attempted to put this idea into practice
who appreciate the extreme difficulty of carrying it out. With the
exception of very small filters, where constant supervision and an
unlimited amount of labour is available, it will be found impracticable
under ordinary circumstances.

There still remains the first-mentioned alternative, viz. to have
the material as nearly as possible of a uniform grade throughout the
filter. Whether the grade should be fine, medium or coarse, will depend
upon the strength and character of the liquid to be treated, but in
either case the best practice from all points of view is, in the
opinion of the author, to provide all of a uniform grade. In making
this statement, it is assumed that there will be a layer about 6 inches
deep of coarser material all over the floor, as this is necessary to
prevent the finer grade above it being washed through in the effluent
or choking the apertures in the sub-drains. In some cases also a
modification is adopted in providing the top layer, about 6 inches to
12 inches in depth, of finer material than the bulk of the filter, in
order to arrest any suspended solids that may be present in the tank
liquor, upon the surface of the filter, from which they can be more
readily removed than if they were allowed to enter the filter. On the
other hand, some prefer to allow these solids to enter the filter,
but to provide ample clear interstices and sub-drainage, so that
these solids after treatment in the filter may be washed out as humus
or “converted products” in the effluent, from which they are easily
removed by settlement in suitable tanks to be described later.


_Methods of Distribution._—The question of distribution is the most
important factor in the successful operation of percolating filters.
At first it was considered sufficient simply to spread the liquid as
evenly as possible over the surface of the filter by any convenient
method. Gradually improvements in methods were introduced with varying
results, and at the present time a large number of different appliances
are to be found in actual use, some producing jets, others a fine
spray, and others again what is termed a “thin film.” All of these have
their advocates, and all undoubtedly can be made to give satisfactory
results. One of the essential requirements of any method is that it
shall give even distribution per unit of superficial area of the
filter. Whether the jet or the spray or the thin film is the most
efficient in this respect is a matter of opinion.

On this question of distribution, one of the most important points
to be taken into consideration in forming an opinion as to the best
method to adopt is, what happens after the liquid is discharged on
to the surface of the filter? What happens beneath the surface, in
the body of the filter? The generally-accepted theory is that the
liquid trickles slowly over the surface of the separate pieces of the
material, dropping from one to the other and ultimately falling to
the floor of the filter, and thence flowing to the effluent drain. No
definite investigations into this question appear to have been made
until the year 1909, when a series of extremely interesting experiments
were carried out by Mr. W. Gavin Taylor, M. Am. Soc. C.E., Resident
Engineer, Sewage Disposal Works, Waterbury, Conn., U.S.A. The results
of these tests were published in the “Engineering Record” of June 5,
1909, and illustrated by excellent graphic tables, which show that,
“Percolating liquids were more equally diffused in the fine material
than in the coarse; that the plotted curves of diffusion approximate
parabolas having their apexes at the surface of the filter material;
and that, in general, about one-half of the total lateral movement
taking place within a 6 foot depth was effected within the uppermost
foot of material.” These observations apply to the effect produced by
single drops applied at the surface. Further tests made to ascertain
the effect of distribution on several adjacent points at the surface of
the filter demonstrated that, “The liquid from each point of
application spread out through the material in the normal cone of
diffusion, until its cone was intersected by that from an adjacent
point of application; that there the two or many liquid films united,
as tributary sources, into minute streams which interrupted, to a
considerable degree, the continuance of diffusion and tended to descend
through the remaining material at a higher velocity and along lines of
least resistance. The streaming tendency increased rapidly as the rates
of application were made greater.” Among the final conclusions drawn
from these tests the most important is that, “The desideratum in the
application of sewage to percolating filters is to attain perfection
in aerial distribution, and that a high efficiency in sub-surface
distribution is fostered by a slow continuous rate of application
rather than by an intermittent application at a higher rate.” This last
sentence fully confirms the author’s own opinion with regard to the use
of dosing tanks which are dealt with later under the heading “Methods
of Feeding Percolating Filters.”

In the following pages a number of the many appliances which have been
introduced for distributing tank effluents upon percolating filters are
illustrated and described in detail, the various types being grouped
under separate headings.

[Illustration:

  _Sectional Elevation on Line A.B._
  _Sectional Plan on Line C.D._

FIG. 78.—“OPEN” TYPE, CANDY-WHITTAKER SPRINKLER.]

[Illustration: FIG. 79.—“CLOSED” TYPE, CANDY-WHITTAKER SPRINKLER.]


_Automatic Revolving Distributors._—One of the first, if not the
first, of the revolving distributors for percolating filters was the
“Candy-Whittaker” sprinkler in its original form. This is supplied
by the Patent Automatic Sewage Distributors, Ltd. It is now made in
three different forms: the “Open” type, the “Enclosed” type, and the
“Buoyant” distributor, as shown in Figs. 78, 79, and 80. The most
prominent feature of these sprinklers is the mercury seal joint,
for which it is claimed that it gives an absolutely watertight and
frictionless joint, that it cannot freeze, and that no renewal of
the mercury is required. It is stated that the use of the patent
“check-ring” in combination with the mercury seal prevents any loss of
the mercury, no matter what the head may be. Another feature is that
the ball-bearings are moisture-proof, due to the special methods of
construction. In addition to the above, the makers point to the special
value of their compensating arms system, by means of which their
distributors will work continuously with a very small volume of sewage
and still be capable of dealing with any larger volume of sewage that
may be required. The names of the “Open” and “Enclosed” types are
self-explanatory, but the “Buoyant” type is specially designed to
reduce the friction on the bearings. The revolving portion is supported
by a float or buoy in the form of a cylindrical tank, which floats in a
small chamber at the centre of the filter. The removal of all weight
friction on this distributor reduces the power required to rotate it to
the minimum.

[Illustration: FIG. 80.—“BUOYANT” TYPE, CANDY-WHITTAKER SPRINKLER.]

Another make of revolving distributor is well-known as the “Cresset.”
This is manufactured by Messrs. Adams Hydraulics, Ltd., and is
illustrated in Fig. 81. In this case the special feature is the joint
between the revolving and fixed portions of the apparatus. It consists
of a simple air-lock, formed by a cushion of air between two water
columns. This is clearly seen in the illustration. It is obvious that
this is an absolutely frictionless joint, so that there is no loss
of head. It also has the effect of removing the strain to which the
overhead ball-bearings are generally subject, so that the revolving
body swings freely in the true vertical line from the cross head above.
Another point is that no expense is involved, and very little trouble
is incurred, in renewing this joint whenever it may be found necessary,
but it has been maintained in perfect condition without requiring
renewal for very long periods. Special means are provided for removing
and replacing the ball-bearings in the cross-head without dismantling
the distributor.

Another type of distributor manufactured by Messrs. Adams Hydraulics,
Ltd., is their “Sypho-Jet,” as shown in Fig. 82. As its name implies,
it is siphonic in action, and combines the functions of a sprinkler
and intermitting valve in one apparatus. It may either be used in
connection with a dosing tank or connected direct to the settling tank,
from which it draws off a certain depth of water every time it fills to
a certain height.

[Illustration: FIG. 81.—“CRESSET” REVOLVING DISTRIBUTOR.]

[Illustration: FIG. 82.—“SYPHO-JET” REVOLVING DISTRIBUTOR.]

Two types of revolving sprinklers are manufactured by Messrs. Mather
and Platt, Ltd. These are illustrated in Figs. 83 and 84. The special
feature claimed for the “Pipe Arm” distributor is the overflow centre,
as this eliminates the use of joints or seals. Another point of
interest in connection with this apparatus is that there are two sets
of ball-bearings at the head, one to take the weight of the revolving
portion, the other to take lateral movement. In addition there is a set
of roller-bearings at the centre, which are arranged to prevent lateral
movement, and they can be adjusted to take up any wear that may occur.
The “Open-trough” type, Fig. 84, is provided with a special turbine
arrangement at the centre, which it is claimed gives an immediate
motion when the sewage is turned on. The distributing troughs being
open assist in aerating the liquid, and as it is possible to provide
holes in the bottom of the troughs these are able to drain completely
dry, and can also easily be cleaned. This apparatus is provided with
ball and roller bearings, as described in connection with the “Pipe
Arm” type mentioned above.

[Illustration: FIG. 83.—PIPE-ARM DISTRIBUTOR.]

[Illustration: FIG. 84.—TROUGH-ARM DISTRIBUTOR.]

Messrs. George Jennings, Ltd., manufacture a revolving distributor,
the special feature of which is that the sewage is delivered from the
central fixed column to the revolving tank, to which the arms are
attached, by means of a syphon formed in the central column itself,
but this syphon is disconnected from the supply pipe by a fixed
cylinder and port-holes, which make the syphon independent of the
pressure at the inlet to the supply pipe outside the filter bed. The
pressure acting upon the syphon is therefore the atmospheric pressure
applied direct to the surface of the liquid in the fixed cylinder,
thus operating without any loss of head. In addition to the ordinary
overhead ball-bearings, a gun-metal bearing is provided to check any
excessive lateral movement. The syphon described above when once
started remains sealed, as the outlets are trapped by the liquid at the
bottom of the revolving cylinder, and as the top lip of the revolving
cylinder is arranged to be 6 inches above the top water level in the
tanks, any flooding at the centre of the bed by the liquid overflowing
at this point is impossible (see Fig. 85). When required to deal with
fluctuating flows this sprinkler can be fitted with compensating arms.

Messrs. Whitehead and Poole manufacture a revolving sprinkler, as
shown in Fig. 86, in which a float is used to carry the weight of the
rotating parts, so that wear and tear and friction are reduced to the
minimum. The chamber containing the float is completely closed, and
this is below the surface of the filtering material, so that they are
not affected by frost. In this sprinkler the joint between the fixed
and rotating parts is made by utilising a little of the buoyancy of the
float to form an upward bearing at the neck of the float chamber. It is
claimed that this joint is perfectly watertight, with the minimum of
friction.

[Illustration: FIG. 85.—REVOLVING DISTRIBUTOR.]

[Illustration: FIG. 86.—BUOYANT REVOLVING SPRINKLER.]

[Illustration: FIG. 87.—REVOLVING DISTRIBUTOR.]

The revolving distributor manufactured by Messrs. Ham, Baker and
Co., Ltd., shown in Fig. 87, consists of a revolving trough to which
the distributing pipes are connected. The trough is supported upon
ball bearings carried upon a pillar, and the special feature of the
apparatus is that the incoming sewage is delivered into the central
trough in such a way that it strikes upon blades fixed in this trough,
and it is claimed that in this way full advantage of the initial head
of sewage is obtained. By this means also no special joint is required
between the fixed and revolving parts. A special method of gaining
access to the ball-bearings for inspection and cleaning is provided.
In connection with this distributor the makers supply an automatic
cleaning gear for the spray holes, Fig. 88. This consists of a rocking
bar attached to each arm of the distributor and provided with a number
of small fingers or pickers, one opposite to each hole. An arm or a
lever attached to the rocking bar, and suspended from same to within
a short distance of the surface of the filter, comes into contact
with a block of wood or other material fixed in the filter itself at
each revolution, and thus causes a movement of the rocking bar which
actuates the finger points and causes them to pass in and out of the
spray holes in the arms. By this means any obstruction is removed and
forced out by the pressure of water within the arm, and the holes are
kept free.

[Illustration: FIG. 88.—CLEANING GEAR FOR SPRAY HOLES.]

The “Facile” rotary distributor shown in Fig. 89 is manufactured by
Messrs. W. E. Farrer, Ltd. The special advantage claimed for this
distributor is that it has no joint, so that there is no friction to
overcome beyond that in the ball-race, upon which the whole weight of
the revolving portion is supported. It is provided with a syphonic
attachment to the central column, and thus acts as a self-dosing
distributor, and, if desired, can be connected direct to the septic
tank. A patent gauging tube is fitted, so that the volume of the dose
may be gauged whether it is fed from the dosing chamber in the ordinary
way or direct from the septic tank. Access is provided to all parts,
and the ball-race can be removed without dismantling the apparatus.

[Illustration: FIG. 89.—“FACILE” REVOLVING DISTRIBUTOR.]

[Illustration: FIG. 90.—THE “CARLTON ROTOR” DISTRIBUTOR.]

Among the later types of revolving sprinklers is the “Carlton Rotor”
distributor, Fig. 90, manufactured by the Carlton Engineering Company.
This is specially designed to obviate the use of a dosing tank, to
work satisfactorily with any variation of flow, and to prevent all
possibility of choking. It is claimed that these advantages are
secured by the use of a special water-wheel which rotates round the
central column on an inclined plane. The sewage is delivered to this
water-wheel by means of a bend connected to the central column. As
each section of the water-wheel is filled, it rotates and delivers its
contents into an annular trough, to which the distributing arms are
attached. These are, however, not perforated with spray holes, but
are provided with small bends on the top of the arms, which deliver
the sewage into distributing troughs fitted with baffle plates. These
baffle plates are so arranged that the sewage is delivered on to the
surface of the filter in a thin film and at a uniform rate over the
whole surface.

[Illustration: FIG. 91.]

[Illustration: FIG. 92.—THE “CARLTON” DISTRIBUTOR.]

A novel apparatus for the purpose of providing an automatic method
of cleaning the orifices of the arms of revolving or travelling
distributors is shown in Fig. 91. This is supplied by the Carlton
Engineering Company, in connection with their ordinary type of
revolving distributor, also shown in Fig. 92. In the distributing arm B
apertures are made in the form of elongated slots A, each slot having a
brass cleaning blade E inserted. These blades are carried on brackets
fixed by a set-screw D, on to a reciprocating rod or tube C, mounted
on runners F. Two bevel wheels G are strapped on to the arm, and are
connected by an eccentric rod to the reciprocating rod C. At every
revolution which the distributing arm makes, the bevel gearing G
comes in contact with the stop H, causing a partial revolution of the
gearing. This in turn causes the backward and forward motion of the
cleaning blades in the apertures. The apertures are thus kept entirely
free from grit or any other solid matter, and the flow is broken up
into a fine spray.

[Illustration: FIG. 93.—“SIMPLEX” REVOLVING DISTRIBUTOR.]

The “Simplex” revolving distributor is manufactured by the Ames Crosta
Sanitary Engineering Company, Ltd., and illustrated in Fig. 93. This
sprinkler revolves on a ball-bearing which runs in a bath of solid
grease, and is protected from moisture and dust by means of special
washers. The whole of the weight is carried from the top of the pillar,
no bearing being submerged in the sewage. The seal is arranged by
means of two gun-metal rings with special annular grooves turned in
them; the top ring is carried from the revolving bucket by means of a
rubber or copper washer, thus the ring carries no part of the weight of
the apparatus, and is free to compensate for any oscillation. Rollers
running on ball-bearings are placed under the bucket, and these take
up side movement due to wind and other causes. The revolving arms are
provided with a special cap at the ends, which can be removed by a
quarter turn to enable the arms to be cleaned. There is no head lost,
as the level of water in the bucket is equal to the head in the supply
chamber or channel. The apparatus is not affected by frost, and is
remarkably sensitive, as 2 inch head of water above the holes in the
arms will start the apparatus in motion.

[Illustration: FIG. 94.—REVOLVING SPRINKLER (Burn and Brown’s
patent).]

Messrs. Burn Bros. manufacture a revolving sprinkler, “Burn and Brown’s
Patent,” shown in Fig. 94, which has one or two special features. These
consist of a self-regulating apparatus for dealing with the variation
of the flow of sewage and at the same time cleaning the orifices. A
shaft carried on each of the arms, and mounted on suitable bearings, is
provided with tapered fingers fixed in a position corresponding to the
holes in the arms. By means of a float in the circular revolving tank
connected to a simple lever the shafts on the arms are given a slight
movement, which causes the fingers to enter the orifices in the arms
as the flow decreases, and to be withdrawn as the flow increases. The
result is that the orifices are enlarged or reduced as the flow varies,
and the movement of the tapered fingers tends to make the orifices
self-cleansing. It is claimed for this distributor, that it can be made
to deal with any variation in flow of sewage up to 6 volumes, with a
maximum head of less than 18 inches above the surface of the material
of the filter.

A new type of automatic revolving sprinkler is the “Hodgson” overflow
type, manufactured by Messrs. George Kent, Ltd. As will be seen
from the illustration, Fig. 95, the sewage passes first into a feed
chamber attached to the central column, from which it flows into the
distributing chamber D, to which the arms are attached. It is claimed
that by use of the weirs W in the distributing chamber, a uniform speed
of rotation and consequently a uniform distribution, is secured. The
ball-bearing upon which the rotating portion is carried is stated to
take both the side and end friction.

The revolving distributors previously described are all constructed
on the Barker’s Mill principle, to some extent if not entirely. The
Fiddian type of distributor, supplied by Messrs. Birch Killon and
Co., and illustrated Fig. 96, is based upon an entirely different
principle. It consists of an elongated water-wheel, which revolves
around a circular filter. It is pivoted at the centre of the filter
upon the supply pipe, and the outer end of the distributor is carried
upon a rail track. In the case of large filters, from 70 feet to 110
feet diameter, two rail tracks are used. The liquid passes through the
supply pipe and is delivered into the buckets of the water-wheel, from
which it falls in the form of a film, so that the water-wheel not only
drives itself over the filter by means of the weight of the liquid, but
it also distributes the liquid which drives it. A very small quantity
is sufficient to start the distributor and keep it in motion, the head
required, measured from the surface of the filter, being about 18
inches. The buckets are provided with graduated weirs, the width of
which is proportionate to its distance from the centre of the filter.
By this means uniformity of distribution is secured. It is claimed that
as there are no small holes to be choked, the apparatus can be employed
for all kinds of liquid with the minimum of attention, and that it does
not require any dosing apparatus.

[Illustration: FIG. 95.—“HODGSON” REVOLVING SPRINKLER.]

[Illustration: FIG. 96.—FIDDIAN REVOLVING DISTRIBUTOR.]


_Power-driven Revolving Distributors._—One of the first revolving
distributors in which it was decided to adopt power to drive it
independent of the sewage, was the “Scott-Moncrieff and Alliott”
distributor, manufactured by Messrs. Manlove, Alliott and Co., Ltd.,
and erected at the Birmingham Sewage Farm for Mr. J. D. Watson. In this
case a horizontal arm is attached to and pivoted upon the vertical
stand-pipe in the centre of the filter which is connected to the supply
pipe. The outer end of the horizontal arm is carried by a rail, and
is fitted with an oil engine which travels along on the outer rail
on the end of the arm. The rotating arm consists of a large main
trough or carrier into which the sewage from the central stand-pipe is
delivered, and on one side of the main trough is fitted a row of small
troughs, each of which is supplied with sewage from the main carrier
through a porthole specially proportioned to the volume of sewage to
be discharged from the trough and to its distance from the centre of
the bed. By this means uniform distribution is secured over the whole
area of the filter. From each of the small troughs the sewage flows
over the edges of the trough, and thence, trickling down the outside
of the trough, is discharged in a row of fine rain-like columns upon
the filter. It is claimed for this distributor that absolute uniformity
of distribution is secured, that it will work equally well with any
volume, that there are no holes to become choked, that it requires the
minimum of supervision, and that it will deal with any variation up to
3 times D.W.F. without attention (Fig. 97).

[Illustration: FIG. 97.—SCOTT-MONCRIEFF POWER-DRIVEN REVOLVING
DISTRIBUTOR.]

In the “Hartley” circular power-driven distributor, the apparatus
(manufactured by Messrs. Hartley, Causton and Richmond, and shown in
Fig. 98), is arranged in a form similar to that used in connection with
the Barker’s mill type of distributor, with this difference, that it
is driven by an electric motor attached to the end of one of the arms,
which is used as the distributing arm; and the arm on the opposite side
is the balancing arm to counteract the dead weight of the distributing
arm and act as a balance against wind pressure. The current is brought
to the distributor by an overhead cable, connected to a revolving
contact at the top of the central standard. The method of distribution
from the distributing arm is different from the usual type, as it takes
place through a number of sectional distributing pipes C (Fig. 99),
which are connected to the main tube H. These are arranged in echelon,
so that by opening the covers P P a cap or cleaning rod may be passed
through each tube from end to end. Each sectional distributing tube,
C, is suspended from the main tube by an attachment D. This attachment
carries a screw, G, by which is regulated a valve shown open at E and
closed at E_{2}. The flow may be increased or decreased to any extent,
or stopped altogether, by this valve. Each space between the dotted
lines on the plan is covered by one sectional tube, and the sewage flow
can be increased or decreased, or stopped upon each section. The spray
nozzles are of a special type, and by use of the deflection or spray
plates M, a smaller number of spray holes or jets are required, so
that they can be made larger, and they are thus not so easily choked.
Among the advantages claimed for this distributor are the following.
That it can be regulated to work with any variation in the volume, from
the smallest quantity to 2 million gallons per acre per day, without
increasing the consumption of power. Separate sections of the filter
may be removed or rested entirely by adjusting the valves G. The
distributor can work with any head of sewage from 6 inches to 6 feet,
or more if desired. It is not affected in any way by wind pressure; and
as the balancing arm empties at the same time as the distributing arm,
it is always in balance. The horse-power required to drive it is very
little, less than half horse-power per acre. The rate of distribution
on any of the sections of the filter can be varied from 50 to 400
gallons per yard per day.

[Illustration: FIG. 98.—POWER-DRIVEN REVOLVING DISTRIBUTOR.]

[Illustration: FIG. 99.—DETAILS OF DISTRIBUTING ARM.
(See FIG. 98.)]

This type of distributor was first erected at the Hanley Sewage
Works in 1902, in order to comply with Messrs. Willcox and Raikes’
specification fora power-driven distributor suitable for a large
circular filter, on which a Scott-Moncrieff distributor was previously
employed.

[Illustration: FIG. 99A.—VIEW OF HARTLEY CAUSTON AND RICHMOND’S
POWER-DRIVEN REVOLVING DISTRIBUTOR IN OPERATION.]

A somewhat similar type of distributor is the “Adams-Cutler” Fig.
100, manufactured by Messrs. Adams Hydraulics, Ltd. In this case the
distribution is from spray holes in the pipe arms in the same way as
the ordinary Barker’s Mill type of distributor, but instead of relying
upon the motive power due to the head of sewage, the distributor is
operated by a rope drive; and where there are a number of distributors
in one installation, each of them is operated by a cross-drive taken
from a main drive rope, so that any one or more can be cut out of the
circuit or controlled, or the speed varied as may be required. It is
claimed that a very small amount of power is required to operate a
revolving distributor, and the ease with which it is carried upon a
central column, are arguments in favour of the revolving over the
rectangular types where absolute control is essential and a natural
working head is not available.

[Illustration: FIG. 100.—“ADAMS-CUTLER” POWER-DRIVEN REVOLVING
DISTRIBUTOR.]

Messrs. Mather and Platt, Ltd., also manufacture an electrically-driven
revolving distributor of the Open Trough type, as shown in Fig. 101.
This machine is of special design, and particularly suited for large
diameter beds. The troughs are similar to those of the ordinary type
of distributor previously mentioned, but are supported on the Warren
Girder principle. There are four pipe arms, the end of each being
fitted with a special carriage running on rails fixed outside the bed
at the floor level. On one of the carriages an electric motor is fixed
and connected to the rail wheels by means of chain drives. The current
is transferred to the motor from the main at the top of the central,
pole of the distributor by means of slip rings.

[Illustration: FIG. 101.—POWER-DRIVEN REVOLVING DISTRIBUTOR.]


_Automatic Travelling Distributors._—There are several types of these
on the market at present. The first is manufactured by Messrs. Birch
Killon and Co., Fig. 102, and is constructed on the Fiddian water-wheel
principle. The design is similar to that described in connection
with the automatic revolving distributor, but it involves two rail
tracks supported on walls or piers on each side of the filter. The
head required for this type is about 2 feet from the surface of the
filter to the top water level in the carrier, and the distributor is
fitted with a syphon which feeds it from the carrier or trough. In
large schemes the trough is placed along the centre of the filter, and
supplies a separate distributor on each side.

[Illustration: FIG. 102.—FIDDIAN TRAVELLING DISTRIBUTOR.]

Another type is manufactured by Messrs. Ames Crosta Sanitary
Engineering Company, as shown in Fig. 103. This sprinkler can be
adapted to existing contact beds. It is carried from the walls of the
central channel in cantilever form, no other bearing or rails being
required. On each side of the machine a hopper is carried in which
water-wheels revolve. The water in its passage from the feed channel to
the distributing arms is conveyed on to the water-wheels, and the
power generated there is transmitted by means of a chain drive to the
wheels of the carriage, and thus the whole apparatus is moved along
the beds. When the machine nears the end of the bed a reversing lever
comes in contact with a buffer, and by this means the direction of
motion is changed, and the machine travels backwards along the bed.
Two distributing arms are carried on each side of the apparatus,
one feeding half the bed in one direction and the other feeding the
remaining portion of the bed in the opposite direction, thus giving a
regular intermittent feed to the bed. A great advantage of this class
of machine is that the whole of the machine is supported from the
central channel, and as the wheels are close together, the long lengths
of shafts with their risk of twisting, retarding and wearing are
dispensed with.

[Illustration: FIG. 103.—TRAVELLING DISTRIBUTOR.]

Messrs. Ham, Baker and Co., Ltd., have for some time supplied an
automatic travelling distributor. This is illustrated in Fig. 104, from
which it will be seen that it consists of a water-wheel mounted on
rollers and divided into sections composed of a number of buckets.
The sewage is delivered through a syphon and patent reversing
valve, in such a way that alternate sections only of the filter bed
receive sewage in each direction of travel, thereby insuring equal
inter-delivery periods.

[Illustration: FIG. 104.-TRAVELLING DISTRIBUTOR.]

[Illustration: FIG. 105.—“HANLEY” POWER-DRIVEN TRAVELLING
DISTRIBUTOR.]


_Power Driven Travelling Distributors._—The well-known Hanley
Distributor was specially designed by Messrs. Wilcox and Raikes, Civil
Engineers, for use on their schemes at Hanley, Fenton, Ilkeston and
elsewhere, as illustrated, Fig. 105. The object of this distributor
is to ensure intermittent discharge at regular intervals, and at a
predetermined rate, in the form of a thin film over the surface of fine
grade material, and in order to secure these results it was decided to
drive the distributor by electric power rather than depend upon the
variable and limited amount of power obtainable from the head of the
sewage itself. The distributor consists of a pipe extending across
the whole width of the bed and supported on wheels, so that it may be
drawn backwards and forwards by means of the wire rope attached to the
centre. The distributing pipe tapers towards the end farthest from the
supply trough, and the sewage is discharged by means of nozzle pipes
attached to the flat side of the distributor about 3 inches apart, and
connected with it near the top so that the main pipe always remains
full. The discharge of the sewage commences and stops along the whole
length of the pipe almost simultaneously, and any sediment lies in the
bottom of the main pipe where it cannot obstruct the outlets of the
nozzles. The nozzles are about ⅝ inches in diameter, and provision
is made for cleaning them by removing a plug in the top of each. In
order to secure a uniform interval of time between each dose of sewage
discharged on to any part of the filter, the distributors are arranged
to discharge when travelling in one direction only, the valves on the
outlets of the feed syphons being automatically turned off and on by
means of a lever, which is actuated by stops fixed at each end of the
iron trough or at any intermediate point desired. In this way it is
possible to arrange for any portion of the filter to be thrown out of
action when necessary for cleaning or repairs. The speed at which the
distributor travels may be varied by adjusting the gear wheels attached
to the electric motor. At Hanley it has been found that the best
results are obtained when the distributors occupy about 7 minutes in
making one complete journey, the filters being 200 feet long, so that
when the filters are working at the normal rate of 1,000,000 gallons
per acre per day, each dose represents one gallon per square yard. In
other words, the film one-fifth of an inch in thickness is applied
200 times per day, and thus a total depth of 40 inches of sewage is
distributed over every portion of the filters in operation in 24 hours.
The power required to drive the distributors is about 1½ B.H.P. per
acre of filter. Electric motors are usually adopted, but small oil or
gas engines may be substituted when electric current is not available.
The winding drums for the wire ropes are driven by belt gear of the
usual type with fast and loose pulleys. The reversal of the drum when
the distributor reaches either end of the filter is accomplished by
an automatic belt striking gear, and a float in the supply trough is
used to cut out a distributor, or continue in action at a reduced rate,
when the supply of sewage becomes insufficient to maintain the normal
rate of distribution. These distributors are manufactured by Messrs.
Blakeborough and Sons, and have now been in regular use since 1902 on
the Hanley Sewage Disposal Works, where the remarkably satisfactory
results obtained are largely due to the highly efficient system of
distribution on the filters.

[Illustration: FIG. 106.—“STODDART” DISTRIBUTOR.]

[Illustration: FIG. 107.—“STODDART” DISTRIBUTOR.]


_Fixed Distributors. Trays and Troughs._—One of the best known of the
fixed distributors is the “Stoddart,” as illustrated in Figs. 106 and
107. It will be seen that it consists of trays of galvanized sheet
iron, formed into a number of V-shaped troughs. The ridges between
the troughs are provided with diamond-shaped perforations at frequent
intervals, and in the bottom of the troughs drip points are inserted
through holes to project on the under side. The distributing trays are
bolted to specially designed supply channels, connected to a common
channel, and the whole of the troughs are mounted on adjustable chairs
(F, Fig. 106) carried on tees, which are in turn supported
on concrete or brick piers. As the liquid passes into the V-shaped
troughs, it fills them and overflows through the diamond-shaped slots
in the ridges and thence trickles down on the under side of each
trough and drops off from the drip points on the filter below in the
form of rain. It is claimed for this method of distribution that the
whole of the available fall is utilised; that it adapts itself without
adjustment to all rates of flow, however varied, that the sewage and
storm-water may all be treated on the same area, and that there is a
total absence of moving parts. This distributor is supplied by the
patentee, Mr. F. W. Stoddart, of the Western Counties Laboratories,
Bristol.

Another type of fixed distributor is fixed only so far as actual
distributing channels are concerned. This is manufactured by Messrs.
W. E. Farrer, Ltd., and illustrated in Fig. 108. The fixed troughs
or channels are laid directly over the filter-bed, and fitted at the
inlet end with concentrating chutes which receive the discharges from
a double-acting tipping trough, which is supported at its extreme
ends by heavy gun-metal bearings. If it is over 10 feet in length, an
additional support is provided at the centre in the form of a specially
designed rocker. The chutes and the channels are provided at the bottom
with ¼-inch holes at 3-inch centres, through which the sewage is
spread on to the surface of the filter, which is below the underside
of the trough. The channels are arranged at 1-foot centres, so as to
distribute the sewage over the whole area of the bed. By means of the
double-acting tipper each half of the filter is dosed alternately,
and suitable periods of aeration are thus provided. This apparatus is
suitable for a single small house with 4 or 5 inmates, and for larger
filters of 600 square feet in area and upwards.

The Ducat filter is provided with what may be termed a fixed
distributor. It consists of a series of small tipping troughs fixed
across the filter, each of which when full tips its contents on to the
filter below. The liquid is thus discharged at the same point each
time, but intermittently in small doses. As, however, the distributor
does not move it is included under this heading.

[Illustration: FIG. 108.—FIXED DISTRIBUTOR.]

A different type of fixed distributor is that manufactured by Messrs.
J. Blakeborough and Sons. This apparatus, patented by Messrs. Haller
and Machell, Civil Engineers, is used in the triple tank system of
sewage treatment. Fig. 109 illustrates the “Aerat” type, which consists
of cast-iron channels supported on wrought-iron joists carried upon
piers. Each length has six orifices on each side, through which the
sewage passes on to small trays provided with radiating grooves on
the top and dripping points on the under side at the edge. As the
distributors may be fixed within a few inches of the surface of the
filter, very little head is required to operate them.

[Illustration: FIG. 109.—FIXED DISTRIBUTOR.]

A very ingenious form of fixed distributor, Fig. 110, is supplied
by the Septic Tank Company, Ltd. From its name, “Capillary Trough
Distributor,” it will be observed that during normal and minimum rates
of flow, capillary attraction is utilised to deliver the sewage on
the filter in the form of drops. The tank effluent is delivered to
each trough distributor over brass V-notches, set in the side of the
main channel, into which the tanks discharge. These V-notches are
adjustable, and are set so as to prevent their being tampered with
subsequently. The trough distributors are of artificial stone, true
from end to end, and set level. Over both their edges copper wires are
fixed, at intervals of a few inches, each wire extending from within
a short distance of the bottom of the trough channel over the edge
and down to the underside of the trough. On a trough being filled the
liquid overflows right along the edge, and is attracted by and runs
down the wires, so that if the flow is at all excessive a little
stream will run down each; on the flow subsiding to the ordinary,
a drop-by-drop delivery commences; should the flow of sewage cease
altogether for any length of time, the wires will go on dripping until
they have drawn off the contents of the trough, almost to the end of
the wires. On a fresh supply of sewage arriving at the installation,
and the troughs refilling to a higher level, the rate of delivery
from the wires will increase until it keeps pace with the flow, thus
ensuring an even distribution on the filter throughout the day. There
is no loss of head in connection with this apparatus beyond the depth
of the distributing troughs themselves.

[Illustration: FIG. 110.—CAPILLARY TROUGH DISTRIBUTOR.]


_Spray Jets._—This type of distribution is not used in many
installations in this country, but it has been adopted in large schemes
in the U.S.A. There are a number of different types, including those in
which the sewage is forced upwards through a nozzle and falls back on
the surface of the filter, and others in which the sewage flows through
inverted nozzles, and each jet impinges upon a fixed cup or disc of
metal, or other material, and is thus spread out on to the filter.
Among the former, one of the simplest types is that manufactured by
Messrs. Adams Hydraulics, Ltd., Fig. 111, in which, by means of a
square baffle plate, provided with suitable grooves, the spray is
forced out in such a manner that it covers a square area of the filter.
It is claimed for this that it covers the whole surface and no portion
of the filter is wasted, as in the case of round jets which cover
circular areas. The same firm also supply several other types of fixed
sprays, as shown in the illustration.

[Illustration: FIG. 111.—FIXED SPRAYS.]

Another type is manufactured by Messrs. Ham, Baker and Co. This is
shown in Fig. 112, from which it will be seen that it can be adapted to
any existing installation, and to standard spigot pipes. Each jet being
in the form of a stop-valve, any individual spray may be put out of
action when necessary.

Messrs. W. E. Farrer, Ltd., manufacture a type shown in Fig. 113.
The special feature of this is a loose disc inside the nozzle, which
oscillates as the sewage passes through, and so tends to clear away any
flocculent matter which may accumulate in the orifice of the nozzle.
The sewage itself is delivered from the nozzle in the form of a very
fine film, and it is claimed that by this means any nuisance from smell
is reduced to the minimum.

[Illustration: FIG. 112.—FIXED SPRAY NOZZLE.]

[Illustration: FIG. 113.—FIXED SPRAY NOZZLE.]

Messrs. Burn Brothers also manufacture a fixed spray, Fig. 114, called
the Burn and Brown’s Patent “Whirl” Spray. The special feature of this
is that it consists of an inverted conical spreader, provided with
vanes revolving on a spindle. The sewage, as it leaves the nozzle,
impinges upon the face of the cone, causing it to revolve rapidly and
spread the sewage in the form of fine drops. Owing to the special
construction of the serrated edges of the cone, the sewage is well
distributed over the whole area covered by the spray.

[Illustration: FIG. 114.—“WHIRL” SPRAY NOZZLE.]

[Illustration: FIG. 115.—“ACME” SPRAY NOZZLE.]

The “Acme” type of fixed spray, manufactured by the Ames Crosta
Sanitary Engineering Company, is shown in Fig. 115. The special feature
of this spray is that it has a loose metallic cone, which is provided
with a helical groove, inside the nozzle. This cone rotates as the
sewage passes through the nozzle, and thus gives a maximum spray with a
small head. These sprays have a free waterway, and the rotation of the
cone tends to prevent the choking of the orifice.

[Illustration: FIG. 116.—FIXED SPRAY JET.]

A simple form of fixed spray consists of perforated pipes arranged at
suitable intervals over the surface of a filter, and provided with
dosing apparatus to give intermittent supply under pressure, and thus
deliver the sewage in the form of jets from the perforations in the
pipes. An improved form of this type of distribution is supplied by
the Septic Tank Company. This is illustrated in Fig. 116, from which
it will be seen that orifices in the top of the distributing pipes
are fitted with nozzles, set at an angle of about 45 degrees to the
vertical. The discharge from these nozzles, it is claimed, gives the
maximum efficiency of distribution.

[Illustration: FIG. 117.—“CARLTON” ADJUSTABLE FIXED SPRAY.]

A self-cleansing spray jet has been brought out by the Carlton
Engineering Co. From the illustration, Fig. 117, it will be seen that a
plug B is fitted into the orifice and supported upon a spindle, which
is attached at its lower end to a lever connected to an automatic
tipper on the outside of the nozzle. The tipper is fed by the spray
from the nozzle, and as it falls alternately on one side and on the
other it raises and lowers the plug B in the orifice, which tends to
prevent any clogging, and also varies the area of the orifice and the
rate of discharge, as well as the area of the filter-surface covered by
the spray.

A simple type of fixed spray is supplied by Messrs. George Kent, Ltd.,
and is shown in Fig. 118. In this case the liquid issuing from the
nozzle impinges upon a curved surface or cone, by which it is deflected
and spread out, leaving the surface at an angle of 45 degrees to the
vertical. One of the advantages claimed for this type is that it has no
small holes to become clogged.

[Illustration: FIG. 118.—FIXED SPRAY NOZZLE.]

A novel form of this type of apparatus is manufactured by Messrs.
Daniel Adamson and Co. Above the orifice in the nozzle is suspended
a revolving deflecting plate (Fig. 119). The vanes of this plate are
shaped in such a way that, as it revolves, the jet impinges against a
different angle every instant. None of the energy is wasted in churning
or throttling, as the plate merely directs the water in such a way that
no two drops ever fall twice on exactly the same spot. It is stated
that the diameter of the wetted area of the filter surface is more than
double the head.

[Illustration: FIG. 119.—ADAMSON’S FIXED SPRAY.]

There are a considerable number of other types of nozzles used for
fixed sprays. Several of these are illustrated in Fig. 120, where A is
the type in use at Salford; B, the type used at Birmingham; C, a nozzle
with a double spreader in use at Waterbury, Conn., U.S.A.; D shows
another type in use at Waterbury, with a single spreader, but this is
so arranged that it can be adjusted to give a smaller or larger area of
orifice. E shows the type adopted at Columbus, Ohio, U.S.A.

In connection with some elaborate studies of methods of distribution
for percolating filters, which were carried out for the Sanitary
Research Laboratory and Sewage Experiment Station of the Massachusetts
Institute of Technology, by Messrs. Winslow, Phelps, Storey and McRae,
a special series of investigations were made into the value of what
is referred to as the “gravity method of spraying.” This consists in
supplying the sewage from troughs or pipes supported above the filter,
and discharging it through orifices on the under side of the pipes
or troughs in the form of solid jets, which are made to impinge upon
concave discs fixed near to the surface of the filter. The effect of
this impingement is to cause the liquid to splash upwards and outwards
in the form of a fine spray, and, as a result of a long series of tests
with various forms of discs and various heads, it was found that the
best average results were obtained with a concave metal disc 3 inches
in diameter having a curvature corresponding to a radius of 2 inches.
With a total head of 4 feet and a head of 3 feet from disc to the pipe
above, it gave the best results obtained with that total head. With a
total head of 6 feet and a head of 4 feet from disc to supply pipe,
it gave the best results obtained by any of the forms of this type of
distribution that were tested.

[Illustration: FIG. 120.—FIXED SPRAY NOZZLES.]

A method of distribution similar to this type of gravity disc spray is
manufactured by Messrs. Glenfield and Kennedy, Ltd. The details of this
disc, with the methods of supporting same and the method of supplying
the sewage from an overhead pipe, are shown in Fig. 121.

[Illustration: FIG. 121.—GRAVITY DISC SPRAY.]

In connection with sewage disposal works where chemical precipitation
has been adopted for preliminary treatment, the tank effluent has been
distributed over fine-grained percolating filters, by means of dosing
tanks discharging comparatively large volumes intermittently over the
surface of the filter. In such cases the aim has been to deliver the
liquid at such a rate that it would flood the entire surface of the
filter in a very short space of time, and then percolate through and
leave the surface free to aerate for as long a period as possible
before the next discharge took place. This process involved the use
of small filters, in order that the area to be flooded should not be
too large for the liquid to spread over it rapidly, and the use of
very fine material to prevent the liquid from passing through too
quickly. Among the difficulties encountered by this method were the
displacement of the surface layer of the material, due to the high
rate of delivery; and the labour involved in cleaning the surface of
the filter. A further trouble which arises when effluents from septic
tanks are treated by this process is the evolution of evil-smelling
gases, which are certain to cause a nuisance if there are any houses
in the vicinity. Efforts have been made to obviate this difficulty by
distributing the sewage through open-jointed or perforated pipes laid
a few inches beneath the surface of the filter, as described later in
connection with contact beds (page 200). While this is satisfactory to
a certain extent, especially where the volume to be treated is small,
and the filter capacity is comparatively large, neither this method,
nor that of flooding the surface of the filter, can be said to comply
with “the desideratum in the application of sewage to percolating
filters,” quoted previously (page 106), nor do they avoid the
difficulties encountered in securing uniform distribution dealt with
in the following pages. The defects of these methods of distribution
may also have been aggravated by the very optimistic anticipations on
the part of the designers of the schemes as to the volume of sewage
which could be satisfactorily purified under such conditions. It is,
therefore, desirable to point out that, in the event of either of these
methods of distribution being adopted, care should be taken to secure a
very high degree of clarification (reduction of matters in suspension)
in the preliminary tank treatment, and to provide filters of ample
cubic capacity.


_Methods of Feeding Percolating Filters._—Under the heading “Methods
of Distribution” (page 106), reference is made to some experiments,
from the results of which the conclusion was drawn that “a high
efficiency in sub-surface distribution is fostered by a slow continuous
rate of application rather than by an intermittent application at
a higher rate.” This agrees in every respect with the author’s own
experience, and confirms his opinion that intermittent discharges
to percolating filters should only be resorted to in cases where it
is absolutely necessary in order to assist in securing uniformity
of distribution, or to ensure a sufficient volume for the operation
of the appliances adopted for distribution. Among the former may be
cited fixed spray nozzles and jets from fixed pipes, in which cases an
intermittent supply is useful in causing a regular variation of the
head upon the orifices, thus varying the distance to which the jets or
sprays are thrown, and producing greater uniformity of distribution
per unit of area covered. Examples of cases, where intermittent supply
is necessary in order to discharge the volume required to operate the
appliances adopted for distribution, are found in connection with
most types of fixed distributors and nearly all types of automatic
revolving distributors. In the case of the latter, it is well known
that a certain minimum head is necessary to overcome the friction due
to the resistance of the air and to the weight of the apparatus itself
on its bearings. However small this friction may be, it needs a volume
of sewage slightly in excess of that required to fill the spray-holes,
which must be large enough to take the maximum flow of sewage when
working under the maximum head. It frequently occurs, especially
in small schemes, and in schemes where the percolating filters are
preceded by contact beds or slate beds, that the rate of flow of the
sewage is at times so low that it is not equal to the minimum volume
required to operate the distributor, and, in the absence of any
arrangement to overcome the difficulty, the distributor would cease to
revolve and the sewage would trickle through the spray-holes without
proper distribution. The same difficulty arises in connection with many
of the fixed methods of distribution, and it is most readily overcome
by the use of a dosing tank fitted with an automatic syphon or valve,
by means of which the sewage is held-up in the tank until it reaches a
certain predetermined level, and is then discharged at a given rate to
the filter. When the tank is empty the discharge ceases, and the sewage
is again held-up as before.

In the case of large installations, or schemes where the whole of the
sewage is pumped and the rate of flow to the filters is thus under
control, it is not necessary to use a dosing tank for the purpose of
providing the rate of discharge to the filters necessary to keep the
distributor in motion. It is, however, maintained in some quarters
that intermittent supply is desirable in any case, in order to secure
alternate periods of work and rest for aeration. There is one obvious
disadvantage in this method of working. Assuming that during the
maximum rate of flow of the sewage the volume which comes down in 5
minutes is stored in a dosing tank, and discharged to the filters in 2½
minutes, it is clear that the rate of distribution is twice as great as
it would be if the distribution were continuous over the whole period
of 5 minutes. It is claimed that the disadvantage of the higher rate of
distribution is counteracted by the 2½ minutes of rest and aeration,
but on this point there is room for doubt, especially when the
conditions which come into play during the average and minimum rates
of flow of the sewage are taken into consideration. Taking the average
rate as equal to one-half the maximum rate, it will be seen that the
dosing tank will discharge every 10 minutes, but the time in which
its contents are delivered to the filter will still be 2½ minutes,
so that under these circumstances the rate of distribution on the
filter will be four times as much as it would be if the distribution
were continuous. During the minimum rate of flow of the sewage the
conditions are still worse, and it is difficult to accept the theory
that periods of rest for 7½ minutes will compensate for excessive rates
of distribution at four times the rate under continuous operation for
periods of 2½ min. at a time.

Those who advocate intermittent supply under all circumstances would
appear to have lost sight of the fact that very ample periods of rest
are already provided by all revolving distributors. When working at the
rate of one revolution per minute, the average time taken by each arm
to pass over any one point on the surface of the filter may be taken as
0·5 second. Before the succeeding arm reaches the same point a period
of 15 seconds will have elapsed, so that under these conditions the
ratio of the periods of work to periods of rest is as 1 to 30. In other
words, even during continuous distribution, the time allowed for rest
and aeration is thirty times as much as that during which the sewage
is actually being delivered to the filter, and there would thus appear
to be no reason for unnecessarily adding to the periods of rest by
means of dosing tanks, especially as such a course involves a greatly
increased rate of distribution at all times when the discharge actually
takes place.

There is, of course, the possibility of adopting the happy medium,
which would consist of a dosing apparatus of such a type that it would
provide a continuous supply during the maximum flow of the sewage, and
only act as an intermitting appliance during the minimum rate of flow.
This is probably the best arrangement to adopt in all cases where a
dosing apparatus is absolutely essential.

As previously stated, however, there are many cases where the
conditions render the use of a dosing tank absolutely necessary to
prevent the stopping of the distributor, and these have brought about
the introduction of several types of automatic syphons and valves, all
arranged specially for the purpose of giving intermittent discharges
to filters, contact beds, areas of lands, and so forth. There are
various methods of constructing dosing tanks, all dependent upon the
arrangement of the preliminary processes and the filters which follow
them. In nearly all cases, however, it is found necessary to reduce
the fall taken up by the dosing tank to the minimum, and the different
syphons and valves have thus been designed to work with the least
possible head. One point in the construction of dosing tanks should not
be overlooked, i.e. the provision of a washout valve for use when it
becomes necessary to clean out the tank. It may be noticed here that
neither the Fiddian type of distributor, on the water-wheel principle,
nor the power-driven distributors, require any dosing tank to keep
them in motion during the periods of minimum flow of the sewage. In
the case of the Fiddian distributor the apparatus remains standing
until the buckets are filled, and each time these discharge their
contents the distributor is moved forward a short distance. Under these
conditions, however, the disadvantage of long intervals of rest between
comparatively large discharges of sewage still remains. Undoubtedly the
most even rate of distribution under all conditions is secured by the
use of power-driven distributors, whether of the rotary or travelling
type, but these can only be economically adapted for use in large
installations or where power is available at a very low cost.

[Illustration: FIG. 121_a_.

    T = tank.               C = carrier.
    P = supply pipe.        F = filter.
    E = effluent channel.   H = humus tank.]

[Illustration: FIG. 121_b_.

    T = tank.                C = carrier.
    P = main supply pipe.    P_{1} = branch supply pipe.
    V = regulating valve.    W = washout valve.
    F = filter.              E = effluent channel.
    H = humus tank.]

Apart from the question of continuous _versus_ intermittent methods
of feeding percolating filters, there still remains the problem of
deciding whether the actual connections between the tanks and the
filters shall be in the form of channels (carriers) or pipes. For fixed
sprays and revolving distributors, which involve the use of pressure
due to a column of liquid, the actual connection to the apparatus must
be in the form of a pipe extending at any rate to the outside of the
filter. Whether it should be continued beyond this point in pipe form
or in a channel depends entirely upon local conditions, such as the
slope of the ground, the planning of the filters, and the relative cost
of the two methods. There is one thing to be said in favour of pipes,
i.e. that there is less chance of a nuisance being created by the
evolution of foul gases from the sewage than in the case of open
channels. On the other hand, channels may be, and frequently are,
covered in. Whichever method is adopted, care should be taken to
provide a valve or penstock on the connection to each separate filter,
in order to be in a position not only to throw any filter out of work
when desired, but also to regulate the rate of supply to each filter
independently of the others. Washout valves should also be provided on
the channels or pipes, in suitable positions. Where the filters are
arranged in groups, as suggested in Figs. 75 and 77, the intervening
spaces may be used to accommodate a simple receiving tank, or a dosing
tank, to which the sewage or tank effluent may be conveyed by an
elevated channel or by a pipe under pressure. The connections to the
adjacent filters may then be provided in the form of pipes leading
from this central tank direct to the distributor at the centre of each
filter. If the receiving tank or dosing tank in such cases is built
upon piers, the floor of the tank will come above the level of the
surface of the filters, and the space below may be utilised for an
effluent receiving chamber or even for a settling tank to arrest the
solids in suspension in the effluent. There are many ways in which the
supply to filters may be arranged, and the preceding notes are merely
intended as suggestions, which may be elaborated as found desirable to
suit conditions which vary almost in every case.

[Illustration: FIG. 121_c_.

    T = tank.              D = dosing tank.
    P = supply pipe.       F = filter.
    E = effluent channel.  H = humus tank.]

Figs. 121_a_ and 121_b_ have been prepared to show conditions under
which channels and pipes respectively would be suitable for supplying
the tank effluent to the filters. Fig. 121_c_ is the plan of a works
where a dosing tank is used to deliver the tank effluent intermittently
through separate supply pipes to each of the filters. If the valves on
the supply pipes are properly adjusted, each filter will receive an
equal proportion of each discharge from the dosing tank. An alternative
method is to provide an apparatus by means of which each separate
discharge from the dosing tank is delivered in rotation to each of the
filters.

In cases where the appearance of the works must be taken into
consideration, the method of arranging the tanks and filters suggested
in Fig. 121_d_ may be adopted. By this means the space covered by the
works is utilised to the utmost, and the tanks are of the form by which
uniformity of distribution and the maximum reduction of velocity is
secured. On the other hand, it is probable that the slope of the site
will only rarely suit this arrangement.

[Illustration: FIG. 121_d_.

    O = outfall sewer.    S = screen chamber.
    D = detritus tank.    T = settlement tank.
    C = carrier.          P = supply pipe.
    F = filter.           E = effluent channel.
    H = humus tank.]


_Dosing Apparatus._—Various methods of providing intermittent supply
are in use. Among these one of the first was a simple form of balanced
valve with float. This, however, has been superseded by other types,
among which the syphons are the simplest form. One of the first of
this type specially designed for the purpose in question was the
low-draught syphon, manufactured by Messrs. Adams Hydraulics, Ltd.,
and shown in Fig. 122. The special feature of this apparatus is that
it can be arranged for a small depth of sewage in the dosing tank.
It will work with any depth from 6 inches upwards. These syphons can
also be arranged to work singly, in pairs, each one coming into action
alternately, or in numbers of three and upwards, when they can be
adjusted to work in sequence or simultaneously.

[Illustration: FIG. 122.—LOW-DRAUGHT DOSING SYPHON.]

Another type of syphonic dosing apparatus is that supplied by the
Patent Automatic Sewage Distributors, Ltd., and shown in Fig. 123. In
this case a syphon is combined with a mechanical valve operated by
floats, hence the name “auto-mechanical syphon.”

A further type of syphonic dosing apparatus is manufactured by Messrs.
George Jennings, Ltd., as shown in Fig. 124. These syphons are arranged
to work in regular sequence, and are operated by means of air valves,
which are shown mounted on a frame and connected by means of air pipes
to the syphons. The sequence is obtained by means of cams, fixed in
different positions on a shaft, which is rotated by means of a float
actuated by the rise and fall of the liquid in the dosing tank.

[Illustration: FIG. 123.—DOSING SYPHON.]

A different type of dosing apparatus is the “Coleman” valve,
manufactured by The Ames Crosta Sanitary Engineering Company, Fig. 125.
This consists of a balanced valve, suspended from a hollow lever which
contains mercury, and has a branch provided with a float actuated by
the rise and fall of the liquid in the tank. As the sewage rises in the
tank, the float overcomes the resistance of the balance weight on the
lever, which is lifted, and the mercury falls to the lower end of the
lever. The combined action of the float and the mercury opens the
valve, and the sewage is discharged. When the tank is emptied, the
apparatus resumes its former position ready for the next dose.

[Illustration: FIG. 124.—DOSING SYPHONS.]

Another type of dosing apparatus is manufactured by Messrs. Mather and
Platt, Ltd. This consists primarily of a flap valve, counterbalanced
weights, and a balance vessel or drum, which is filled gradually with
liquid, the whole being pivoted about a horizontal axle. When the
measuring chamber and balance drum are both empty, the counterbalance
weights raise the latter and allow the flap valve to close, the
balance drum then being in its highest position. As the measuring
chamber fills, a portion of the sewage is allowed to pass from it
into the balance drum, which is thus gradually filled, and at a given
level its weight will suffice to overcome the counterbalance weights
and the pressure of the sewage on the flap valve, and will open the
latter, thus allowing a measured quantity of sewage to be discharged
into the troughs or pipes for spreading over the filter. The balance
drum discharges its contents meanwhile, and thus, when the measuring
chamber is empty, the flap valve closes again by the action of the
counterweights, and the chamber is ready to receive another quantity of
sewage (Fig. 126).

[Illustration: FIG. 125.—“COLEMAN” DOSING VALVE.]

Messrs. George Kent, Ltd., have introduced several different types of
dosing apparatus, which can be arranged to draw off less than 6 inches
head of water and any greater depth. In one case the apparatus, Fig.
127, consists essentially of a seating S, through which the liquid
discharges, a valve-cap C, attached to arms which rotate about the pin
P and nearly balanced by the weights W, also by floats F F attached
to a frame, also rotating about P and carrying two adjusting weights
A A. The apparatus is actuated by the rise of the liquid, raising the
floats and lifting the cap C, which returns to its position after the
discharge has taken place. For greater depths a different type is used,
as shown in Fig. 128. In this case the apparatus consists of a valve
seating S, through which the liquid discharges, two floats A and B, a
controlled device for giving a sudden release and for regulating the
levels at which the valve is opened or closed, and an alternating gear
G, which is fitted with two or more valves placed in the chamber and
operated in rotation. The action of the apparatus is as follows: When
the liquid in the dosing chamber reaches the top level, the float A has
acquired sufficient buoyancy to cause the shoulders of the plate K to
suddenly push aside the rollers at the ends of the weighted levers L L
of the control device. The lugs T T then come in contact with the
collar R, and the valve-cap C and the float B are lifted. The tank
discharges until the floats A and B have lost sufficient buoyancy to
cause the bottom shoulders of the plate to push aside the rollers and
allow the valve to close.

[Illustration: FIG. 126.—DOSING VALVE.]

[Illustration: FIG. 127.—DOSING VALVE.]

This apparatus is applied in various ways for different
purposes in connection with sewage works, and the same
firm also manufacture special valves for giving a measured
discharge under various heads.

[Illustration: FIG. 128.—DOSING VALVE.]

[Illustration: FIG. 129.—DISTRIBUTING AND DOSING APPARATUS.]

The Ames Crosta Sanitary Engineering Company, Ltd., also supply a very
ingenious method of providing intermittent supply to filters, as shown
in Fig. 129, which is equally suitable for percolating filters, for
contact beds, or for intermittent filtration on land. This machine is
arranged to give any desired timed discharge to the beds in rotation,
or to discharge measured volumes to the beds in rotation. By means of
valves or weirs, the machine is arranged so that additional beds in
the series are automatically brought into operation as the increase
in flow demands, and the additional beds brought into action go out
of action as the flow decreases. The illustration shows a machine for
delivering timed volumes on a series of six beds. The square chamber is
divided into six divisions by means of iron plates; each division is
connected to one of the six beds by means of a pipe. A circular dish or
vessel is suspended over these divisions, and revolves on ball-bearings
carried on the bridge. The circular dish is provided with a weir at
its circumference, and the sewage or tank effluent which enters the
dish from the pipe in the centre is diverted by means of the weir or
outlet to one of the divisions supplying the beds. The upright shaft
from which the revolving dish is suspended is fitted with a six-toothed
ratchet wheel, and, by the rise and fall of a float in the liquid of
the actuating tank, a pawl is moved along until it engages with the
next tooth of the ratchet wheel. When the pawl has caught the tooth of
the ratchet wheel the outlet valve in the actuating tank is opened,
and as the liquid flows out of the tank the float descends, and by
means of levers the pawl is moved one-sixth of a revolution; the pawl
being engaged with the ratchet wheel, the dish is revolved one-sixth
of a revolution, and the weir is thus brought over the next division,
and consequently the next bed receives the flow. Extra weirs can be
arranged at various levels, so that if more sewage is coming than that
required for one bed, a portion of the liquid would flow over one or
more of the weirs, and so on, to the respective beds. The flow to the
actuating tank is taken from the feed channel, and can be set to fill
the actuating tank in any desired period of time. The discharge from
the actuating tank can be conveyed either to a special plot of land, or
into the revolving dish. An ingenious device is arranged so that
any bed in the series can be instantly shut out of action should it
become overworked. In an apparatus for feeding revolving sprinklers,
arrangements are made for bringing the extra beds into operation by
means of valves controlled by the flow of sewage.

[Illustration: FIG. 130.—BALANCE-VALVE DOSING APPARATUS.]

Messrs. Glenfield and Kennedy also manufacture a balance-valve type of
dosing apparatus operated by means of floats and buckets. This is shown
in Fig. 130.

[Illustration: FIG. 131.—DISTRIBUTING AND MEASURING
APPARATUS.]

The “Uniform” automatic distributor, manufactured by Messrs. Whitehead
and Poole, is designed for the automatic distribution of measured
volumes of liquid to filters or other areas in regular rotation. This
is shown in Fig. 131, and the principles on which it is constructed
involve rotating arms carried on a float operated in a closed central
chamber. The arms are rotated by the reaction of the discharge of the
liquid, but are locked so as to remain stationary in one position,
continuing to feed one filter until the head of liquid actuating a
regulating cock and tipper releases the locking device and permits the
rotation of the arms to the next bed, where the process is repeated,
and so on to all the filters in rotation.

The “Ponding or Intermitting Valve,” supplied by the Septic Tank
Co., is shown in Fig. 132. The essential features of this apparatus
are two cylindrical vessels of cast-iron or other suitable material,
approximately of the same shape, weight and displacement. One vessel is
used as a bucket and the other as a bell. They are suspended from two
equal arms of a pivoted lever at approximately the same distance from
the standard carrying the lever. On the same end of the lever to which
the bell is hung, an ordinary lift-up valve is attached, the seat of
which is at or below the low-water level in the ponding chamber. This
valve is connected with the lever by means of a linked rod, chain, or
other suitable arrangement. The cubical content of the bell and bucket
is sufficient to ensure the displacement of the liquid, in excess of
that required to lift the valve from its seat, by rocking the lever to
which it is attached. In the bottom of the bucket a draw-off valve is
provided for the purpose of emptying same. This valve is constructed
so as to prevent the liquid passing through it into the bucket as
it rises in the chamber, and to open and allow the contents of the
bucket to escape when the water in the ponding chamber has almost been
discharged. The operation of the apparatus is as follows. The ponding
valve being closed, and the supply of liquid being turned into the
ponding chamber, the liquid will rise. On reaching the small ball in
the bottom of the valve of the bucket, the ball will float and close
this bucket valve, so as to prevent the entrance of the liquid through
and into the bucket. The liquid continuing to rise, will gradually and
simultaneously submerge the bell and the bucket. The upper edge of the
bucket is fixed at the predetermined height to which the liquid is to
rise in the ponding chamber. The bell and bucket being of the same size
and weight, and being hung so that the one is level with the other, the
displacement of both will be equal up to the time the liquid reaches
the upper edge of the bucket, and consequently no movement of the lever
will take place. On the liquid rising above the top edge of and filling
the bucket, it will cause the displacement of the bucket to be reduced
to much less than the displacement of the bell, and this difference is
sufficient to lift the valve from its seat, and discharge the contents
of the ponding chamber. As the liquid in the chamber recedes, the
bucket and the bell will both be gradually left suspended in the air.
Until the liquid in the chamber drops below the bottom of the bucket,
the latter will remain full; on the water continuing to fall, the ball
in the small valve at the bottom of the bucket will drop, allowing the
contents of the bucket to escape. On the bucket being emptied of its
contents, the weight of the bell, together with the valve and chain or
rod, will overcome that of the bucket, the valve will close, and the
liquid commence to pond in the chamber again.

[Illustration: FIG. 132.—THE SEPTIC TANK CO.’S PONDING VALVE.]

[Illustration: FIG. 133.—AUTO-MECHANICAL DOSING SYPHON.]

The special syphons manufactured by Messrs. Burn Bros., described under
contact bed apparatus, are also suitable for use as dosing syphons, and
work singly or in sequence, and give intermittent discharge to filters
or to land (see Fig. 145, page 218).

An auto-mechanical type of syphon for dosing tanks is supplied by the
Carlton Engineering Co., and is illustrated in Fig. 133.




_EFFLUENT SETTLING TANKS OR HUMUS PITS._


Reference has already been made, under the heading “Grading of
Filtering Material,” to the advisability of using coarse material, for
the reason that the converted organic matter will in that case readily
pass away in the effluent, and thus prevent the choking of the filter.
Even with finer material, a certain amount of solids in suspension will
be found in the effluent, and, in order to produce a final effluent
suitable for discharge into any stream or watercourse, it is necessary
to arrest and remove these suspended solids. This fact has been
recognised by the Royal Commission on Sewage Disposal, who recommend
the adoption of effluent settling tanks with a capacity equal to two
hours’ flow of the sewage, and provided with means for removing the
deposit.

The solids in effluents from percolating filters are rather difficult
to arrest, as they are in the form of very finely divided matters in
suspension. Many methods have been tried in various places, but in the
author’s experience he has found that the chief factor in securing
a satisfactory settlement of these solids, is the reduction to the
minimum of the velocity of the effluent in its passage through the
settling tank. If this principle is adopted, the simplest form of
tank is similar in construction to that suggested for detritus tanks,
so long as the outlet end is constructed in the form of a weir of
the greatest possible length under the circumstances. Such a tank is
illustrated in Fig. 134. By this means the rate of flow over the weir
may be reduced to the minimum; and if, in addition to this, the outlet
at the bottom of the tank is arranged in the form of a plug valve fixed
in a pocket below the lowest point of the floor proper, near to the
inlet end, and the floor is laid with a sharp slope towards this
outlet, it will be found possible, as a rule, to draw off the deposit
without discharging the entire contents of the tank, as long as it
is done at frequent intervals. As there is a tendency for a scum to
form on the surface of the liquid in these tanks, it is desirable to
provide a scum-plate of wood, slate, or other material, as shown in
the illustration, and in all cases, except the smallest schemes, these
tanks should be constructed in duplicate. It may be mentioned here that
the hydrolytic tank, Fig. 34, page 52, and the separator of the Septic
Tank Co., Ltd., Fig. 47, page 65, have both been adopted for use as
effluent settling tanks.

[Illustration: FIG. 134.—EFFLUENT SETTLING TANK.]

The deposit from effluent settling tanks, as a rule, rapidly dries
without creating a nuisance when it is spread out in a thin layer
upon a suitable draining bed similar to that suggested for dealing
with sludge from settling tanks (Fig. 58, page 83). Under favourable
conditions as to fall, the draining bed can be constructed below the
level of the sludge outlet from the effluent settling tank, and the
deposit can then be drawn off by gravitation. There is, however, still
the problem of disposing of the liquid flowing from the bed, and as
this should have a free outlet, it usually happens that the levels do
not permit of the discharge of the deposit by gravitation. Under these
circumstances the outlet should still be arranged as shown, Fig. 58,
but it should be connected to a sludge well fitted with a chain-pump,
or other means of raising the deposit to the draining bed, which may
thus possibly be situated at the same level as the similar beds for the
sludge from the settling tanks. When dry, this deposit may be spread
out on the land, or used in gardens and on farms as a manure.




_SAND FILTERS._


It would probably be more correct to use the term “fine-grain filters”
to describe the alternative methods occasionally adopted to deal with
the effluents from percolating filters, as they do not always consist
of sand. Fine clinker, ashes, broken saggars, and similar material, is
equally suitable so long as it is of a gritty nature and not wholly
dust. The term “sand-filters” is, however, used here, as it is well
known in connection with the filtration of drinking water, and the
method of construction is practically the same.

Although primarily designed for the purpose of arresting the suspended
solids in final effluents, and thus required to act simply as
mechanical strainers, sand filters have the additional advantage of
increasing the degree of purification, especially from a bacterial
point of view. The most important factor to be considered in
constructing these filters is the grading of the material. It must
not be too fine or contain too great a proportion of dust, or it will
rapidly become clogged and involve much labour and expense in cleaning
the surface layer. The nearest approach to perfection in material for
this purpose is the coarse Leighton Buzzard filter sand. As in the
case of the best material for the percolating filters themselves, it
may cost a little more than other less satisfactory kinds, but it will
generally be found to be the cheapest in the end.

In constructing filters of this type, whether composed of sand or other
material, it is essential that the bottom layer should be of a coarser
grade, in order to provide free drainage. In a general way it will be
found satisfactory to have a series of 2-inch agricultural drain pipes
laid on the floor and converging towards the outlet. Over the whole
floor should then be laid a layer 6 inches deep of gravel, broken
bricks or stones of the size of walnuts, upon this a layer 3 inches
deep of pea gravel, and at the top a layer not less than 9 inches deep
of suitable sand or other fine grade material. The surface of the
filter should be well below the level of the inlet, in order to allow
the liquid to pond up on the surface 6 inches to 9 inches in depth
without backing up to the level of the floor of the percolating filter.
One of the difficulties encountered in operating filters of this type
is to secure even distribution over the whole area. The means to be
adopted for this purpose should be simple and easily cleaned, and it is
usual to find troughs of wood or iron, glazed ware channel-pipes and
similar arrangements in use. Unfortunately these do not effectively
cover the whole area until the sand has been saturated and the surface
slightly coated, thus preventing the liquid from passing through as
fast as it comes in. When this occurs, however, the time for cleaning
the surface is not far distant, and when the filter is brought into
use again the whole preliminary process has to be repeated. The best
way of avoiding these difficulties would appear to be to arrange the
filter in such a way that the liquid must cover the whole area from
the very beginning. This can be accomplished by fixing the normal
outlet _above_ the level of the surface of the filter as suggested,
Fig. 135, where the final effluent discharge is normally from the end
of the swivel-jointed pipe when in its vertical position. When it
becomes necessary to clean the filter and drain it for purposes of
aeration, the swivel-jointed pipe is simply lowered to the floor of the
outlet-chamber, as shown in plan, and raised again when the filter is
brought into operation.

[Illustration: FIG. 135.—SAND FILTER.]

Filters of this type should never be less than two in number, so that
one may be in work while the other is being cleaned. It would probably
be advisable to have even three or four filters for schemes of moderate
size, so as to provide longer or more frequent intervals of rest for
aeration. It will be obvious from the preceding observations that
these filters must be substantially constructed and made absolutely
watertight. When dealing with a good effluent from percolating filters
or contact beds, these final sand filters may be provided at the rate
of 1 square yard for every 500 gallons of the daily dry weather flow.

Where ample fall is available, careful consideration should be given
to the advisability of operating these sand filters in the same manner
as percolating filters, i.e. by using revolving sprinklers for the
purpose of distribution without submerging the filtering material. This
applies particularly in cases where it is desirable to secure a very
high degree of bacterial purification. Recent investigations have shown
that sand filters for drinking-water, when operated in this manner, are
highly efficient and involve less expense for maintenance. In addition
to this they require less cleaning, so that a much smaller area is
thrown out of work during cleaning operations, and a smaller total
area of filter surface is needed than in the case of similar filters
operated on the submerged system. The additional fall required for the
revolving sprinklers will usually be a serious difficulty in the case
of sewage disposal works, but where it is available, and the extra cost
entailed is not of great importance, the idea deserves consideration.
Filters of this type should be preceded by an effluent settling tank as
previously described.




_CONTACT BEDS._


The almost universal adoption at the present time of biological methods
of sewage purification by means of artificial filters, is due entirely
to the original experimental work of Mr. W. J. Dibdin, at the Barking
Outfall Works of the London County Council. These experiments were
carried out with a contact bed, and during the subsequent ten years
an enormous number of works were constructed upon this principle. At
the present time, however, it is a somewhat rare occurrence to find
contact beds proposed for sewage disposal schemes of any size. It
has been stated that the principle upon which they are operated is
unscientific, that they rapidly become clogged and useless, and that,
in any case, they are not capable of dealing with sewage at the same
rate as percolating filters, or of producing such a high degree of
purification. With regard to the first point it would be futile to
endeavour to explain what is and what is not scientific. This must be
left to the scientists. That contact beds have in many cases become
clogged and useless cannot be denied, but there is also very little
doubt that this unsatisfactory result has been due to one or more
of the following causes: (_a_) overwork, (_b_) improper methods of
operation, (_c_) the use of unsuitable material for filling the beds,
(_d_) insufficient sub-drainage. It was most unfortunate that for some
years the general idea of a contact bed was that it consisted of a
simple excavation in the ground, filled with coke or similar material,
into which the sewage was discharged, held up for two hours, and then
drawn off; a very simple but crude affair altogether. It is now known
that contact beds, like other systems, can only deal with limited
volumes of sewage, the actual amount depending upon the character
of the sewage and other factors; that there is a proper method of
operating the beds, and that it must be strictly adhered to if the best
results are to be produced; that unsatisfactory material is worse than
useless, and that very ample means of sub-drainage are absolutely
essential to the continued efficiency of the beds. It is probable that
if these essential factors had been properly understood and acted upon
from the outset, there would have been very few failures to record.

It has been stated that contact beds are obsolete, but there are
engineers who even now recommend this system, and consider it
satisfactory under some, if not under all conditions. In the opinion of
the author contact beds are not obsolete, and there are cases where the
conditions preclude the adoption of any other method of purification.
Under these circumstances, it is considered desirable to describe in
the following pages the details of design and construction which have
been found by experience to be necessary to ensure satisfactory results.


_General Principles of Design._—The first point to be decided before
commencing the design of a scheme of contact beds is whether single,
double, or triple contact is necessary to produce the desired degree of
purification, and this will depend upon the strength of the sewage and
the destination of the final effluent. Single contact alone will not be
sufficient, except in a very few cases where the sewage is weak (highly
diluted), and even then it will necessitate the use of fine-grade
material for filling the beds, and consequently a tank effluent of
exceptional quality as regards the matters in suspension in order that
the fine material may not be rapidly choked. Where a sufficient area of
land of a suitable character can be procured at a convenient level for
treating the effluent from the beds, single contact may be adopted with
material of medium-grade, but even in this case special attention must
be devoted to the preliminary process in tanks, so as to reduce the
amount of solids in suspension in the tank effluent to the minimum. As
a rule it will be found safer to adopt double contact, as the primary
beds may then be filled with coarse grade material, which will be less
liable to choke, and it will not be necessary to rely so much upon the
land or any other final process that may have been provided. In special
cases, and particularly where the sewage is strong, or an exceptionally
high degree of purification is essential, triple contact should be
adopted, but the tertiary beds may consist of a set of sand filters
similar in construction to those described on pages 185 to 188. In some
quarters the question of the grading of the material is considered of
slight importance, and very little difference has been made in the size
of the material for the primary and secondary beds, but in the author’s
opinion it is absolutely essential that each series of beds should be
filled with finer material than the preceding series, and the material
in the final stage of treatment should be as fine as possible, so long
as it does not contain any dust. In making these statements, it is
assumed that the question of sub-drainage will be dealt with on the
lines recommended later under that heading.

Another factor which has an important bearing upon the general design
of a scheme of contact beds, is the method of operation which is to
be adopted. It is generally assumed that all contact beds are worked
in what is known as eight-hour cycles: viz. 1 hour filling, 2 hours
standing full, 1 hour emptying, and 4 hours standing empty for rest and
aeration. There has, however, been a tendency in the past to overlook
the fact that the periods of standing full, and of emptying the beds,
are the only sections of the cycle which are, as a rule, under absolute
control. Unless special provision is made for the purpose, the time
taken to fill each bed depends entirely upon the rate of flow of the
sewage to the works, and the period of rest empty also depends upon the
frequency with which the beds are filled, and thus indirectly upon the
rate of flow of the sewage. For example, a set of four beds designed to
receive each three fillings per day in wet weather, should not receive
more than one filling per day in dry weather. Assuming that one-half
the total flow comes down in six hours, it will be found that it takes
six hours to fill two beds in the middle of the day, or three hours to
fill one bed. During the remaining eighteen hours the other two beds
are filled, one of them in say six hours and the other in twelve hours.
The times taken to fill the four beds in this scheme would therefore
be—No. 1, three hours; No. 2, three hours; No. 3, six hours; and No.
4, 12 hours. In each case the period of filling is thus much in excess
of the one hour prescribed under the eight hours cycle. The obvious
remedy is to subdivide the total area into a larger number of smaller
beds, but if this is carried to its logical conclusion it will be seen
that there must be 24 beds if the time taken to fill any one bed is
not to exceed one hour. While it is very desirable to arrange this
subdivision in order to secure the proper cycle of operations, the
number of schemes where it is economically practicable are few, and
recourse must be had to some other method of reaching the same end.
This has already been recognised by most engineers, and provision is
now usually made for a tank known under various names, such as dosing
tank, collecting tank, equalising tank or holding-up tank, in which the
tank effluent is stored until the volume accumulated is equal to the
capacity of one contact bed, and the latter is then filled within the
regulation time of one hour. If the necessity for making provision on
these lines to ensure the proper working cycle had been recognised in
the early days of contact beds, it would doubtless have prevented the
troubles which have arisen in many places.

From the preceding observations, it will be seen that it is very
necessary to come to a decision as to the method of operation to be
adopted, before designing any scheme of contact beds. If the method
of subdivision into a large number of small beds is preferred,
the planning of the separate series, and the probable cost of the
additional work involved, must be taken into consideration. On the
other hand, if a smaller number of larger beds with a suitable dosing
tank are preferred, the extra fall required for the latter must be
provided for, even if it involves the reduction of the depth which
would otherwise be available for the beds themselves.

[Illustration: FIG. 136.]

There is still another matter which has a considerable influence upon
the general design of a scheme of contact beds, viz. the slope of the
ground upon which they are to be constructed. If it has a fairly rapid
and even slope, the tanks and beds may be arranged close together, as
shown in Fig. 136. The only part that needs special care in this case
is the cross-wall between the primary and secondary beds, which will
need strengthening, especially in its lower half, in order to resist
the extra pressure it is required to take.

[Illustration: FIG. 137.]

Where the slope is not so great, a saving in the cost of excavation may
be effected by arranging the separate tiers of beds at some distance
apart, as indicated in Fig. 137, and connecting one with the other
by pipes. The aim to be attained in arranging the beds under these
conditions is to have the entire area of the floors on solid soil, with
the walls half in and half out of the ground.

Another set of conditions occasionally met with, is where the site of
the works is perfectly flat and the position of the outlet for the
final effluent involves the construction of the secondary beds either
wholly or partly below the surface. In such cases the primary beds will
come above ground, and it will then be found economical to arrange each
set of beds in two rows end to end, with a central combined supply
channel and effluent carrier, the latter being formed in the space
between the walls which support the former, as shown, Fig. 138. If
there is not sufficient head to allow of the supply channel being made
deep enough to serve as the dosing tank, the latter may be constructed
across the ends of the settling tanks, as suggested in Fig. 139, or in
any other convenient position. A dosing tank in this position lends
itself to the method of feeding the beds by means of closed pipes
instead of by open channels, whether in sets of four, with a central
chamber for the inlets and outlets illustrated in Fig. 139, or in
series as Figs. 136 and 137.

[Illustration: FIG. 138.]

There are doubtless other alternatives, or combinations of methods,
which may be adapted to meet the exigencies of peculiar conditions
of site and fall, but the foregoing details will probably suffice to
suggest ideas to those in need of them in designing schemes of contact
beds.


_General Construction._—The most important point to be borne in
mind in constructing contact beds is, that they must be absolutely
watertight. Should they leak in any way, the sewage may pass away
untreated; or it may find its way into adjacent beds, and thus prevent
these from being properly aerated during the periods of rest when
empty. It is therefore evident that they should be constructed in
a substantial manner. The floors are usually of concrete, and the
thickness of the floor will depend upon the nature of the subsoil.
If for any reason the floors have to be laid upon made-up ground,
provision should be made by means of piers or cross-walls, carried down
to the solid subsoil to support the floor, independent of the made-up
ground which is, in all cases, absolutely untrustworthy. The walls
of the beds may be constructed either of concrete throughout or of
brickwork in cement, and they should be of such a thickness that they
will withstand the pressure of the head of water which would result if
the beds were filled to the top of the walls.

It will not be found satisfactory to place reliance either upon
brickwork or upon concrete alone to form a watertight bed, and in both
cases the whole of the floors, as well as the walls, should be rendered
with cement mortar in the proportion of 2 parts of sharp clean sand to
1 part of Portland cement. No rendering should be done during frosty
weather or during excessive heat, as in both cases it will
usually be found defective, and it is better to stop the work
altogether for a time than have to patch it up afterwards.

[Illustration: FIG. 139.]

One safeguard which can be adopted to prevent difficulties later on,
is to insist upon testing all such beds with water to the full height
before any of the filtering material is placed in position. Any slight
defects which may appear can be made good then at very little expense.
If the defects are not discovered until after the beds are filled with
the filtering material, they can only be properly rectified by removing
the material, and this involves a considerable outlay. In order that
these tests may be carried out without friction, it is necessary to
stipulate clearly in the specification for the work that each bed is to
be tested with water to the full depth before any material is placed
in the bed, and that the contractor must take full responsibility for
making the beds absolutely watertight. It is, of course, understood
that the method of construction adopted by the engineer is such that,
if properly executed, the beds will be absolutely watertight; and,
in order to prevent any misunderstanding, an item should be included
in the quantities for the contractor to provide whatever sum he may
consider necessary to allow for making these tests. It may be thought
sufficient to state simply that the contractor should make all
absolutely watertight, and to leave it to him to provide the means for
doing so. It will, however, be found more satisfactory to all concerned
to provide all means both in specification and in the quantities for
attaining the desired results. It is not sufficient even to use the
word “watertight” alone in this connection, as the interpretation of
this word may be subject to differences of opinion which are obviated
by the addition of the word “absolutely.”

The foregoing observations refer not only to contact beds but to the
tanks and other portions of the work which are required to hold water.


_Methods of Distribution._—Much diversity of opinion exists with
reference to the question of distribution in filling contact beds. The
many methods which have been tried in various places, may be arranged
under four headings, viz. (_a_) above the surface of the material;
(_b_) at the surface level; (_c_) just below the surface; (_d_) at the
bottom of the bed. Some engineers hold the opinion that distribution
over a large area of the material has no value, and that it matters
little how the bed is filled so long as the liquid finds its way into
the interstices of the material with as little disturbance as possible.
The simplest method which fulfils these requirements is to allow the
sewage to flow over the surface at the inlet end of the bed, but this
soon causes the surface at this point to become clogged, and unless
it is cleaned at frequent intervals the solids very quickly become
washed down into the bed, and in the end this portion of the material
will have to be removed and washed before it can be used again. The
efforts to avoid this trouble have resulted in the numerous methods of
distribution referred, to above.

Taking them in the order given, the idea of discharging the sewage
above the surface level (_a_) by means of elevated troughs or pipes,
has been to imitate to some extent the method found necessary in the
case of percolating filters, and thus aerate the sewage before it
enters the bed. The difficulties which arise in this case are that some
extra fall is needed, and the provision of the troughs or pipes with
suitable supports is costly. It is also extremely difficult to arrange
the distribution by these means so that it shall be uniform over the
whole area, and unless this is done it cannot be of much advantage.

Distribution at the surface level (_b_) may be provided by means
of shallow grips in the material itself, and these have the great
advantage that if they become clogged to such an extent as to prevent
the sewage from freely passing into the bed, it is a small matter for
the manager to cut a fresh grip in another direction and leave the
first one to dry up when the sludge in it can be easily removed by
hand. Another method is to use rows of stoneware channels, or wooden
or iron troughs, with their edges set level with the surface of the
material, so that the sewage may flow over the edges or through holes
or notches in the sides. This is usually satisfactory, but it is not
an easy matter to maintain all the channels at the same level, and
after they have been in operation for a time it will be found that the
material immediately under the troughs or channels is badly clogged,
and can only be cleaned or renewed by removing the channels.

Sub-surface distribution (_c_) is arranged by means of perforated or
open-jointed pipes, laid below the surface of the material and thus
out of sight. The reasons for adopting this method are: that it avoids
the unsightliness caused by surface distribution; that the surface is
kept free from obstructions, and thus allows free aeration when the bed
is emptied; and last, but not least, it prevents any nuisance arising
from the evolution of obnoxious gases in the tank effluent whenever
it is over-septicised, a not infrequent occurrence in the case of
old-fashioned schemes, or in new works where the volume of sewage for
which the tanks were designed has not yet reached its maximum. This
method has the disadvantage that when the openings in and between the
pipes become choked, more labour is involved in cleaning them than in
the case of open channels or troughs on the surface.

Filling from the bottom (_d_) is assumed to possess all the advantages
and none of the disadvantages caused by the other methods. The
distribution is certainly uniform, as the liquid first fills the
sub-drains and then rises at the same level throughout the whole of the
material, forcing out any carbonic acid gas that may have accumulated
in the lower part of the bed. As the sewage does not appear on the
surface at all, there is no unsightliness and no trouble from bad
odours. On the other hand, it is evident that the solids in suspension
in the sewage or tank effluent are retained at the bottom of the bed,
especially in the under-drains, and thus they will appear in large
quantities in the effluent. Unless some special provision is made, by
means of an effluent settling tank or sand-filter, to arrest these
solids in suspension in the final effluent, they will be liable to
cause trouble in the stream, and will, in any case, seriously affect
the results of any analyses that may be made. The usual manner of
arranging this method of filling, is to cause the sewage to flow into
an open or covered chamber at the inlet to the bed, the walls of the
chamber being provided at the floor level with openings connected to
the sub-drains laid on the floor of the bed.

Whatever method of distribution is adopted, it is desirable that the
surface of the filter material shall be not less than 3 inches above
the highest level to which the sewage will rise, so that the liquid may
not be visible at the surface.


_Sub-Drainage._—Reference has already been made to the fact that
lack of ample under-drains has often been the cause of the failure
of contact beds in the past. The general practice for a long time
consisted in placing a layer of coarse material on the floor of the
bed, and providing a few rows of ordinary agricultural drain-pipes laid
with open joints. In some few cases special perforated pipes were used,
in others the pipes were partly embedded in the concrete floor. In the
opinion of the author, however, no drains in the form of pipes are
satisfactory, as they do not leave a space _at_ the floor level as a
free exit for the solids in the effluent. Where pipes are used it will
generally be found after a few months’ operation that these solids, in
the form of black sludge, have accumulated along the sides of the pipes
and among the material at the floor level, and when this once commences
the accumulation continues to take place, rising gradually in the bed
until the interstices are choked to such an extent that the liquid
capacity of the bed is reduced to a fraction only of its original
volume. The trouble was intensified by the comparatively small number
of the pipe drains usually found in the beds. It was evidently assumed
that the matters would travel laterally through the layer of coarse
material at the floor level. Unfortunately an additional impediment to
the free flow of these matters was caused in many cases by the want of
sufficient fall on the floor itself. Very little consideration will
show that a large area of floor requires a considerable slope in order
to produce the velocity of discharge necessary to remove matters in
suspension, yet it was seldom that a gradient of more than 1 in 200 was
provided, and in a few cases the surface of the floor was absolutely
flat. Under these conditions it is difficult to see how any other
result could be expected. It may be argued that it was not properly
understood in those days that the solids in suspension (converted
organic matters, the products of oxidation) must be removed if the
filtering material is to retain its working capacity, but this fact has
long been recognised in connection with percolating filters, which have
in most cases been constructed upon complete false floors, provided
with perforations, and with a suitable slope on the surface of the
actual floor.

There is very little doubt that the question of providing ample means
of sub-drainage deserves special consideration; and, in the author’s
opinion, the floors of all contact beds should be laid with much
greater fall to the outlet than in the past, and they should, in
addition, be covered entirely with a false floor of special floor-tiles
of the kind described in connection with percolating filters (pages 91
to 94). If the usual bottom layer of coarse material is then laid upon
the false floor, it will be found that the beds will maintain their
normal working capacity for a much greater length of time than in beds
constructed on the old style. Instead of arranging the slope on the
floor from the inlet end to the outlet end of the bed, it is preferable
to construct an effluent channel with a suitable fall down the centre
of the bed to the outlet, and arrange the floor with a cross-fall from
the sides to the centre channel, which may be covered by slabs of
concrete or stone laid upon the top of the floor tiles where they abut
upon the edges of the channel as suggested, Fig. 139_a_.

[Illustration: FIG. 139_a_.]


_Material for Filling Contact Beds._—The remarks made under the
heading “Filtering Material” for percolating filters (pages 101 to
103), apply with equal force to contact beds. In none of the many
cases which have come under the observation of the author has it been
possible to obtain such satisfactory results from other material as
from clinker, when used under the same conditions as to the strength
and volume of the sewage treated. It is true that excellent effluents
have been produced by beds filled with burnt clay, broken bricks or
stones, but it will usually be found that in such cases the material
is more or less clogged, and that the volume successfully treated
per day is considerably less than that which would be dealt with by
clinker. The reasons are the same as those already mentioned in pages
101 to 103, and there is no need to say more here than to recommend as
strongly as possible the use of hard-burnt vitrified furnace clinker as
already described for percolating filters.

The foregoing remarks apply more particularly to the material for
coarse and medium-grain contact beds. Clinker of the kind described is
equally suitable for fine-grain contact beds, but it is difficult to
break it to the required grade except at great cost, and a considerable
loss in bulk due to the production of fine dust. For fine-grain beds,
requiring material specified to pass a ¼-inch square mesh,
and to be retained on a ⅛-inch square mesh, clean coke-breeze from
gas-works (not ashes) will be found to be the most efficient. The
important point to be observed in preparing material for fine-grain
contact beds is that, while none of the particles should exceed
¼ inch in diameter, as many as possible of the finer particles,
⅛ inch and 3/16 inch in diameter, should be retained, but all dust
should be removed even if it necessitates washing the material for that
purpose. Even the finer particles down to 1/16 inch diameter might be
used, but it will be found extremely difficult to arrange the sifting
process to arrest these without retaining the dust as well, especially
if the material is at all damp, as the fine meshes required for the
purpose quickly become clogged with the dust, and the sieve or screen
is rendered useless.

Among other materials which may be adopted for fine-grain contact beds,
broken saggars in the pottery districts, or slag in the neighbourhood
of ironworks, are probably the next best, but only if they are properly
graded in the manner described above. Indeed the chief difficulty in
securing this fine-grade material is the preparation and grading,
particularly where large quantities are required and the situation
of the beds involves much handling of the material after it has been
sifted. It is, however, of such extreme importance to have it as fine
as possible, without including any dust, that the stipulations in the
specification with regard to this material should be made very clear
and definite, so that the contractor may make sufficient provision
in his prices to enable him to comply with the specification in its
strictest sense.

In addition to the bottom layer of floor-tiles and coarse material
prescribed in the section relating to sub-drainage, it will be
necessary to provide an additional layer above this, about 3 inches in
depth, of a medium grade, to support the very fine material and prevent
it being washed through the interstices in the coarse bottom layer, and
special attention must be devoted to the method of distribution at the
surface in order to avoid disturbance of the fine material.

[Illustration: FIG. 140.—AUTOMATIC APPARATUS FOR CONTACT BEDS.]


_Automatic Apparatus for Contact Beds._—Although contact beds can be
operated by hand, this involves continuous and regular supervision by
a man, and, unless he is under strict control, the proper cycle of
operation may not be adhered to, and the result will, in that case,
not be satisfactory. In the early days of contact beds, several types
of apparatus were designed for the purpose of operating these beds
automatically, so that the cost of manual labour would be reduced to
a great extent, and in addition, the possibility of mismanagement
avoided. Among these appliances, one of the most widely known is that
manufactured by Messrs. Adams Hydraulics, Limited; and shown in Fig.
140. In small installations, the low draught syphons described above in
connection with dosing apparatus are used to give alternate fillings
to pairs of contact beds. Where more than two beds are in use the
automatic air-lock feed is used. This consists of a cast-iron inverted
U-pipe, both legs of which are trapped, either in self-contained
castings or in separate chambers. The sewage flows through one, the
others being charged with air. When the sewage in the bed has reached
the proper height it overflows into a small chamber in which a dome is
fixed. This dome is connected by means of an air-pipe to the cast-iron
inlet feed, and as the sewage rises in the chamber round the dome, the
air contained therein is forced up into the feed which it fills, thus
forming an air-lock, which prevents the sewage from passing through and
stops the supply to the bed. At the same time that the feed is stopped
by the transfer of air in the manner described, the compression of
the air in another small dome in the same pit has forced a water seal
on the air-pipe leading from this dome to the feed in the next bed,
and thus liberates the air-lock in the feed of that bed and allows
the sewage to flow to this in rotation. The whole operation is then
repeated with each bed in rotation, the last bed in the series, when
full, starting the feed in the first bed again. These same feeds may
be arranged to hold up sewage in a collecting or dosing tank, so as to
ensure the accumulation of sufficient sewage to fill each bed at one
dose within a reasonable time.

[Illustration: FIG. 141.—CONTACT BED APPARATUS.]

In order to ensure that the sewage is held up in each bed for a
suitable period for contact, the outlet is provided with an automatic
syphon, which is arranged in such a way that the filling of the bed
alone can start it. When the bed is full, the sewage flows through a
pipe with an adjustable orifice into the timing pit. When this pit is
full, the compression of the air in a small dome placed in the pit, and
connected to the syphon by means of an air-pipe, releases the air in
the syphon and allows it to start at the end of the desired period
of contact. A special feature of these syphons is that they are so
arranged that after they have emptied the bed they continue in action
as syphons, taking the drainings from the bed, however small in volume
they may be, and stopping off only when the bed commences to fill
again, thus ensuring a thorough draining of the material. Another
special feature of this apparatus is that there are no moving parts,
the whole operation depending upon the transfer of air by means of the
head of liquid. It is important to note that by this apparatus the
period of contact in any bed can be arranged for any particular length
of time, quite irrespective of the rate of flow of the sewage to the
works, and independent of the filling and emptying of the other beds in
the series.

[Illustration: FIG. 142.—CONTACT BED APPARATUS.]

[Illustration: FIG. 143.—CONTACT BED APPARATUS.]

[Illustration: FIG. 143_a_.—CONTACT BED APPARATUS.]

Another apparatus which has been used largely in the past is that
supplied by the Septic Tank Company, Ltd. This apparatus is made in two
forms, illustrated in Figs. 141 and 142. The chief difference between
the two types is that in the one case, Fig. 141, the period of contact
is controlled by the filling of the next bed, and is thus dependent
upon the rate of flow of the sewage, while in the other case, Fig.
142, the period of contact is “timed” for from one to two hours, and
is independent of the rate of flow of the sewage. In both cases each
set of gear is built up on its own bed-plate, and comprises the inlet
and outlet valves and the connecting pipes to and from the same. The
valves are of the simple spindle-type, and are connected by rods to
rocking levers and actuated by buckets or floats, working in chambers
or pits which are in communication with the different beds. By means
of overflows from the beds to the buckets, and other devices, the
various portions of the gear are actuated in such a manner that they
automatically fill the several beds in each set in regular rotation,
hold them full for contact, and eventually discharge the treated liquid
to the effluent drain. Full details of the method of operation can be
obtained from the manufacturers, who claim that the gear will work
satisfactorily without attention other than the oiling of the bearings
and joints every few weeks.

Another type of automatic apparatus for contact beds is that
manufactured by Messrs. J. Blakeborough and Sons, as used in the Triple
Tank System of sewage treatment, Fig. 143. In this case the beds are
arranged in sets of three, and the filling of each is effected by the
overflow from the last bed filled, the discharge being effected by the
rising of the liquid in the last bed filled. The apparatus consists
broadly of a slide-valve controlling the outlet-pipe, and connected by
levers to floating cylinders located in separate chambers. One chamber
controls the opening and the other the closing of the valve. The outlet
is provided with a rising and falling arm, which is connected by a
lever to a balance float fixed in a chamber, and coupled by means of a
pipe to the float-chamber of another bed. The method of operation is us
follows:—The tank effluent flows by gravitation to bed A, the filtered
effluent thus rising in outlet chamber A, and also in the overflow
chamber which is connected by pipes to outlet chamber A. As soon as bed
A becomes full, the filtered effluent overflows into closing chamber
A, which is coupled by a pipe to opening chamber B, and the floating
cylinders in each case are raised, with the result that the valve of
bed A closes and the valve of bed B opens, the liquid thus commencing
to run on to bed B. The same action as above is repeated when bed B is
full, and bed C is to be filled, whilst bed C in turn is coupled to bed
B, so that the triple action is repeated over and over again so long as
sewage continues to flow to the beds. As bed A fills, the liquid rises
in outlet chamber A, and, this being coupled to float chamber C, the
liquid rises to a corresponding height in float chamber C. Some time
elapses before the liquid rises to such a height in the float chamber
as to sink the mouth of the outlet pipe below the surface of the liquid
in the outlet chamber, this space of time (which can be regulated as
desired) representing the length of time that the bed is allowed to
stand full before commencing to empty. When the liquid has risen to
a given height in float chamber C, the balance float is raised, this
action tipping the lever and lowering the rising and falling outlet
pipe in outlet chamber C, thus drawing off the effluent from the top
slowly, and without disturbing the whole contents of the bed. (_Note_:
Bed C is assumed to be full). After the liquid in the bed has been
drawn off, the rising and falling outlet pipe remains stationary at the
bottom of the chamber until the next action takes place, which is as
follows:—When bed A becomes full, it is allowed to stand full until
the liquid in bed B (now filling) has risen to a given height, when it
raises the balance float in the float chamber A in a similar manner as
described above, and thus empties the bed A, at the same time emptying
the float chamber C, in which is fixed the balance float connected to
the rising and falling outlet pipe of the bed C (now standing empty),
thus raising the outlet pipe and rendering the bed again ready for use.

[Illustration: FIG. 144.—CONTACT BED APPARATUS.]

[Illustration: FIG. 144_a_.—CONTACT BED APPARATUS.]

Messrs. Glenfield and Kennedy, Ltd., also manufacture apparatus for
operating contact beds, as shown in Fig. 144. This arrangement of
the apparatus delivers the sewage to six beds. There are two valve
boxes, A, internally divided into three compartments. Three sets of
tube valves, B, on each valve box, control the inlets to the several
compartments of the valve box and also to the beds, which are connected
up to the valve boxes with suitable pipes. As the sewage collects in
the measuring chambers, it raises the float C—the float chamber being
in communication—which, through the rack-and-pinion shown, turns the
shaft D. To a sleeve, E, over the shaft D, a hammer, F, is keyed, while
a stopper catch, G, is mounted freely. Keyed to the shaft D are lifting
levers H and K. The lever K lifts the hammer F to the vertical, and,
being free to rotate with the sleeve E, it falls and strikes on one
of the copper buffers, L, in the turning plate M, causing it to turn.
In like manner the stopper catch G is thrown over by the lifter H—a
little in advance of the hammer—and, resting on the stopper plate N,
drops into one of the notches, O, thus stopping the gear at the proper
place. The turning of plate N causes the roller lever P, keyed to the
vertical shaft Q, to rotate—through the agency of the mitre gearing
and horizontal shaft—thus actuating the lever R, and raising the tube
valve B. Two valves are operated simultaneously, one on each valve box.
The valves are held open by the levers P, until, the water being
run off, the weight of the float descending puts the gear in motion
again—by returning the hammer to its original side—and moves the
roller levers P off the end of the levers R, thus allowing the tube
valves B to close and the water to collect once again. The force of
the blow of the hammer as it strikes the buffer L can be regulated
within certain limits, for, on the outer end of the sleeve E carrying
the hammer, a lever, S is keyed, which, as it works in unison with the
hammer, and is attached to the piston of the swivel cataract adjustable
oil cylinder T, has the effect of cushioning the fall of the hammer.

Another type of syphonic apparatus for contact beds is manufactured
by Messrs. Burn Bros., as shown in Fig. 145. In this case the primary
filters are usually supplied with sewage from a collecting or dosing
tank in which two or more discharge syphons are fixed, or they may be
filled from a supply channel under certain circumstances. In the former
case a syphon discharges immediately the collecting tank is full. A
“Sequela” relief apparatus is attached to each syphon, and causes
these to discharge alternately or in rotation. The relief apparatus is
divided into three compartments, and depends for its working on the
transference of oil, of a special nature, from one compartment, A, to
another compartment, C, via compartment B, in stages corresponding
with the number of syphons under control, each relief apparatus at the
commencement being set a stage in advance of the one next to it. After
a syphon has discharged, the oil which has been transferred to the
compartment C in the relief apparatus is automatically returned to the
compartment A, and the apparatus is then ready for another series of
operations. Thus the oil, which is non-evaporative and non-freezing, is
used over again and again, and as it does not come in contact with the
sewage, it remains quite pure and serviceable for years. A discharge
syphon is fixed to each filter, and, in order to ensure a proper period
of contact of the sewage with the filtering material, each syphon
is provided with a “Horometer” relief apparatus. This apparatus can
be set to give a period of contact varying from twenty minutes to
twenty-four hours. The “Horometer,” like the “Sequela,” depends upon
the transference of oil from one compartment to another, but in this
case only two compartments are necessary, A and B. As the filter fills,
the oil is forced, by air pressure, to rise in a vertical pipe from
compartment A above the level of a regulating tap, which is set to pass
the oil into compartment B in the time determined upon for the contact
of the sewage in the filter, and as soon as the necessary quantity of
oil has been transferred through the tap, the syphon discharges. After
the syphon has discharged, the oil is automatically returned from
compartment B to compartment A, and the apparatus is again ready
for use. No watertight brick chambers are required in the filter in
connection with the apparatus, thus effecting considerable economy in
structural work. It is only necessary to construct a screen in dry
brickwork or perforated iron round the syphons to hold up the filtering
material.

[Illustration: FIG. 145.—CONTACT BED APPARATUS.]

[Illustration: FIG. 145_a_.—CONTACT BED APPARATUS.]

The syphons manufactured by Messrs. George Jennings, Ltd., actuated by
air-valves as described under the heading of “Dosing Apparatus,” can
also be adapted for filling and emptying contact beds.

[Illustration: FIG. 146.—CONTACT BED APPARATUS.]

The Enock apparatus for contact beds, Fig. 146, manufactured by
Messrs. A. G. Enock and Co., Ltd., is a simple device working on the
principle of the ball valve. A float, which takes the place of the
ball, is raised by liquid entering a pit, which pit is outside the bed
or tank which has to be emptied by the valve. The valve is attached to
a vertical rod in connection with a horizontal weighted lever, at the
other end of which the float is fixed. When a tank is full, it flows
into the float chamber, and the rise of the liquid in this pit lifts
the float and opens the valve, thereby allowing the contents of the
tank to escape. The float pit then slowly empties itself by means of a
small outlet pipe, and the valve closes so that the tank is ready to
receive more liquid. This apparatus can be arranged so as to fill a
number of beds in rotation, the inlet valve to each pit being either
opened or closed as required by the overflow of liquid from each
contact bed in turn. The outlet valves to the contact beds are similar
to those already described and if the first beds are filled in
rotation, no further connection between the apparatus in the lower beds
will be required, each valve working absolutely independently of the
others.

The chief advantage claimed for this type of apparatus is, that it
can be adjusted so as to suit any required level of liquid in any
particular bed.




_CAPACITY OF PERCOLATING FILTERS AND CONTACT BEDS._


Although this separate section is devoted to the question of
calculating the capacities of percolating filters and contact beds, it
is mainly for the purpose of stating that it is impossible to formulate
any rules which admit of general application. It may reasonably be
pointed out that this last statement is a truism, and affords no
assistance to those in search of information on the subject. There is,
however, so great a tendency in some quarters, to rely upon results
obtained in one place under certain conditions as a guide in designing
a scheme in another place under possibly totally different conditions,
that it is impossible to repeat the statement in question too often.

In the first place, long and practical experience is necessary to
enable an engineer to come to a decision as to what are the conditions
under which any particular scheme is to be carried out, and which
of them will have a bearing upon the methods to be adopted in the
design of the works. A careful study of the fifth report of the Royal
Commission on Sewage Disposal, will show that they assume over 70
different sets of conditions under which percolating filters and
contact beds may be adopted. The capacity of the filters required to
produce the desired results will depend upon the strength of the sewage
to be treated, the type of tank adopted for the preliminary process of
sedimentation, the grade of material to be used, the amount of fall
available, the final destination of the effluent, and other factors, all
of which again may be affected by other circumstances, which must of
necessity be taken into consideration. As the basis for calculating the
capacity of filters may vary between 15 and 200 gallons of the daily
dry-weather flow per cubic yard of material, it is evident that there
is a wide margin for error, and the only safe course to adopt is to
allow for the worst possible conditions and thus provide a large margin
of safety. Although the suggestions made in the fifth report of the
Royal Commission with regard to the provision to be made under various
sets of conditions may be taken as a guide, to some extent, it should
be borne in mind that the figures given represent the minimum which
should be allowed in each case, and the only really safe guide in these
matters is long practical experience of a large number of works under
the greatest possible variety of conditions.

In order, however, to provide a rough guide for the purpose of making
preliminary estimates, it may be stated here that, under ordinary
conditions, with sewage of average strength, a properly designed
preliminary process, suitable material of medium grade, and not less
than 4 feet deep for percolating filters, it should be possible to
produce an effluent which will not create a nuisance by providing—

     (_a_) Percolating filters, at the rate of one cubic yard
      for every 84 gallons of the daily dry-weather volume, or

     (_b_) Contact beds, at the rate of one cubic yard in each
      series for every 56 gallons of the daily dry-weather volume.

In other words, the ratio of the cubic capacity of filter material to
the daily dry-weather volume of the sewage for all ordinary purposes
may be taken as—

     (_a_) 2 to 1 for percolating filters.
     (_b_) 3 to 1 for contact beds.

TABLE, GIVING THE RATIO OF THE TOTAL CUBIC CAPACITY OF PERCOLATING
FILTERS AND CONTACT BEDS TO THE DAILY DRY-WEATHER VOLUME OF THE SEWAGE
UNDER VARYING CONDITIONS.

    ──────────────────────────┬──────────────────────────────────────
                              │          Percolating Filters
                              ├──────────────────────────────────────
                              │           Strength of Sewage
                              ├────────────┬────────────┬────────────
                              │   Strong   │   Average  │    Weak
                              ├────────────┴────────────┴────────────
       Preliminary Process.   │           Grade of Material
       (See pages 23, 29).    ├──────┬─────┬──────┬─────┬──────┬─────
                              │Coarse│     │Coarse│     │Coarse│
                              │  or  │ Fine│  or  │ Fine│  or  │ Fine
                              │medium│     │medium│     │medium│
    ──────────────────────────┼──────┼─────┼──────┼─────┼──────┼─────
    Detritus tanks            │11·20 │  —  │ 6·72 │  —  │ 4·20 │  —
                              │      │     │      │     │      │
    Septic tanks              │ 3·73 │  —  │ 2·40 │  —  │ 1·68 │ 1·68
                              │      │     │      │     │      │
    Continuous flow settlement│ 3·73 │  —  │ 2·40 │  —  │ 1·68 │ 1·68
                              │      │     │      │     │      │
    Quiescent settlement      │ 3·36 │ 6·72│ 1·68 │ 2·40│ 1·29 │ 1·29
                              │      │     │      │     │      │
    Continuous flow chemical }│      │     │      │     │      │
        precipitation        }│ 2·58 │ 3·36│ 1·68 │ 2·10│ 1·12 │ 0·96
                              │      │     │      │     │      │
    Quiescent chemical       }│      │     │      │     │      │
        precipitation        }│ 1·68 │ 2·58│ 1·29 │ 1·29│ 0·98 │ 0·84
    ──────────────────────────┴──────┴─────┴──────┴─────┴──────┴─────

  ──────────────────────────┬───────────────────────────────────────────
                            │                Contact Beds
                            ├───────────────────────────────────────────
                            │             Strength of Sewage
                            ├──────────────┬──────────────┬─────────────
                            │    Strong    │   Average    │     Weak
                            ├──────────────┴──────────────┴─────────────
     Preliminary Process.   │              Number of Series
     (See pages 23, 29).    ├────┬────┬────┬────┬────┬────┬────┬────┬───
                            │    │    │    │    │    │    │    │    │
                            │ ×1 │ ×2 │ ×3 │ ×1 │ ×2 │ ×3 │ ×1 │ ×2 │ ×3
                            │    │    │    │    │    │    │    │    │
  ──────────────────────────┼────┼────┼────┼────┼────┼────┼────┼────┼───
  Detritus tanks            │  — │ —  │6·72│  — │6·72│  — │  — │4·42│ —
                            │    │    │    │    │    │    │    │    │
  Septic tanks              │  — │ —  │5·09│  — │4·42│  — │2·24│2·54│ —
                            │    │    │    │    │    │    │    │    │
  Continuous flow settlement│  — │ —  │5·09│  — │4·42│  — │2·24│2·54│ —
                            │    │    │    │    │    │    │    │    │
  Quiescent settlement      │  — │ —  │3·81│  — │3·36│  — │1·68│  — │ —
                            │    │    │    │    │    │    │    │    │
  Continuous flow chemical }│    │    │    │    │    │    │    │    │
        precipitation      }│  — │5·09│  — │  — │3·36│  — │1·26│  — │ —
                            │    │    │    │    │    │    │    │    │
  Quiescent chemical       }│    │    │    │    │    │    │    │    │
        precipitation      }│  — │3·90│  — │  — │2·54│ —  │1·26│  — │ —
  ──────────────────────────┴────┴────┴────┴────┴────┴────┴────┴────┴───

  KEY: ×1 = Single
       ×2 = Double
       ×3 = Triple

In the case of contact beds these figures give the cubic capacity of
each series, and they must be doubled or trebled respectively for
double and triple contact. Both percolating filters and contact beds,
constructed on this basis, would be capable of treating up to three
times the dry-weather flow in times of storm.

Having given the above method of calculation, in a form not usually
adopted in connection with sewage disposal, and bearing in mind the
misunderstandings which frequently arise in comparing the various
methods in use at the present time in different countries, it may be
useful to set out in this form the figures which, it is understood,
have been adopted by the Local Government Board, on the basis of
the fifth report of the Royal Commission on Sewage Disposal, as the
minimum which they consider suitable under varying conditions. In the
Table opposite, the figures given represent the ratio which the cubic
capacity of the filters bears to the daily dry-weather volume of the
sewage, whether it be in gallons, cubic feet, cubic metres, vedros, or
any other term of measurement.

    _Examples._—1. A daily dry-weather volume of 10,000 gallons
       of sewage, of average strength, is to be treated upon
       percolating filters of medium sized material, after
       preliminary treatment in septic tanks.

       10,000 gallons × 2·40 = 24,000 gallons
                             = 3,840 cubic feet
                             = total cubic capacity of filters

       2. A daily dry-weather volume of 3,000 cubic metres of weak
       sewage is to be treated upon single contact beds, after
       preliminary treatment in continuous flow settlement tanks.

       3,000 cubic metres × 2·24 = 6,720 cubic metres
                                  = total cubic capacity of beds

For the purpose of the above Table, the strength of the sewage is
estimated according to the amount of oxygen absorbed from permanganate
of potash in four hours, as indicated in the fifth report of the Royal
Commission as follows:—

                                      Parts per 100,000
    “Strong” sewage = oxygen absorbed 17 to 25
    “Average”   ”   =   ”       ”     10 to 12
    “Weak”      ”   =   ”       ”      7 to  8

A quick method of converting gallons into cubic feet is to multiply the
gallons by the reciprocal 0·16. This can be done rapidly (frequently by
mental calculation) by multiplying the gallons by 4, and the product
again by 4, and inserting the decimal point between the second and
third figures from the right-hand side, thus:—

       24,000 × 4 = 96,000 × 4 = 384,000 = 3840·00
                                         = cubic feet

This is useful in calculating the capacity of tanks, and for all similar
purposes.




_STORM-WATER TREATMENT._


In connection with Sewage Disposal Works, the term “storm-water” is
generally understood to mean the extra volume which reaches the works
in times of rainfall, in excess of three times, up to and including
six times, the average dry-weather flow; so that the volume of
storm-water for which provision should be made is equal to three times
(volumes) the daily dry-weather flow. Prior to the publication of the
fifth report of the Royal Commission, it was usual to provide a rough
straining filter for the storm-water, or to reserve a portion of the
land for the purpose of dealing with it by broad irrigation. In either
case the area of filter surface or land required was 1 superficial yard
for every 500 gallons of storm-water (D.W.F. × 3/500 = super-yards).
As the result of their investigations, the Royal Commission came to the
conclusion that “storm-water” filters, as generally constructed under
these conditions, were useless for the purpose for which they were
required, and this confirmed the views of most engineers of experience.
Where suitable land can be secured for the purpose, and arranged in
such a manner that it is reserved solely and entirely for treating the
storm-water, this method may still be adopted. If this is not possible,
stand-by tanks may be constructed for the purpose of receiving the
storm-water. These tanks are to be not less than two in number, and
should have a total capacity of not less than one-quarter of the
average daily dry-weather flow. The only overflow at the outfall works
from which storm-water may be discharged direct to the stream, or other
final effluent outlet, must be from these stand-by tanks, and it should
not come into operation until these tanks are full. Having regard to
these recommendations, it is necessary in every scheme to provide at
least two special storm-water stand-by tanks, with a total capacity of
¼ D.W.F.; and the drawing, Fig. 147, illustrates a simple method of
constructing these. In this case the inlets are in the form of weirs,
running the full width of the tank, so that if the channel leading to
these tanks is in communication with, and at the same level as, the
inlets to the detritus tanks, and the latter are provided with slotted
doors (see Fig. 10, page 19), the weir at the inlets to the stand-by
tanks will act as the actual storm-overflow, and, being of considerable
length, the maximum height to which the water will rise in passing over
this weir will be very small, and will thus have very little effect
upon the rate of flow to the sedimentation tanks.

As the only overflow discharging direct to the stream must be from
these tanks, and must only come into operation when they are full,
the outlet is also constructed in the form of a weir discharging into
a channel from which a pipe would be laid to the stream. A further
requirement in connection with these tanks is that they should always
be kept empty, ready to receive the excess of storm-water at any time.
From this it is evident, that these tanks must be emptied after each
heavy shower or storm which increases the rate of flow of sewage to
the works beyond three times the dry-weather flow. Unfortunately,
no directions are given as to the manner in which this is to be
accomplished. In the absence of any definite statement to the contrary,
it might be inferred that, after the overflow from these tanks has
ceased, their contents may also be discharged direct to the stream. As,
however, this would necessitate outlets at or near the bottom of the
tanks, there would appear to be a possibility of the suspended matters
deposited in the tanks being discharged to the stream—the very thing
the tanks are designed to prevent. With such an arrangement, also,
there will be a risk of the man in charge of the works, either wilfully
or by an oversight, leaving the outlets at the bottom of the tanks
open, and thus permitting the storm-water to pass direct to the stream
without the settlement which it is anticipated by the Royal Commission
(Fifth Report, page 233, par. 352) will be provided for all storm-water
arriving at the works.

[Illustration: FIG. 147.—STORM-WATER STAND-BY TANKS.]

It is obvious that these tanks must be emptied after every heavy shower
or storm, and that facilities must be provided both for drawing off the
supernatant water and for removing the deposit which will accumulate at
the bottom. In the author’s opinion, the only safe method is to provide
floating arm outlets for the supernatant water, and to discharge this
to land or to a special filter for further treatment, or, better still,
to pump it up to the detritus tanks to be treated again with the
ordinary sewage. In schemes where the whole of the sewage is pumped
at the works, the contents of these storm-water stand-by tanks should
certainly be discharged into the pump-well, as this would not involve
the provision of special pumping plant. With regard to the sludge from
these tanks, this should be drawn off by means of special outlets, and
dealt with on sludge draining beds in the manner previously described
(page 83).

The difficulties which frequently arise in designing suitable and
convenient methods of dealing with storm-water, render it desirable
that very careful consideration should be given to the question as to
whether it would not be more satisfactory, from the point of view of
both economy and efficiency, to omit the stand-by tanks, and increase
the capacity of the filters required to deal with the _ordinary_ sewage,
to such an extent that they will be capable of dealing with the
whole volume of sewage and storm-water combined up to six times the
dry-weather flow, and thus obviate the necessity for any storm-overflow
at all at the outfall works. If this idea were universally adopted,
it would necessitate greater care in the construction of any
storm-overflow required on the line of the outfall sewer itself before
it reaches the works, but there are (or should be if the sewers were
properly constructed) so few cases where the excess of flow, even
during the heaviest rainfall, ever reaches six times the dry-weather
flow, that the extra cost involved cannot be considered excessive if
the greater certainty of securing satisfactory results at all times is
taken into consideration.




_MEASURING APPARATUS._


In recent years the provision of proper means of measuring and
recording the flow of sewage at disposal works is becoming more
general, but there are still a very large number of works at which it
is impossible to obtain any trustworthy information as to the volume
of sewage treated. As long as all works smoothly, and there is no
trouble with the effluent, it is considered superfluous to trouble
about the quantity of the liquid which passes through. When, however,
difficulties arise, and it becomes necessary to investigate the cause
of the trouble, it is of the utmost importance to be in a position to
ascertain the daily volume of the sewage arriving at the works and the
variations in the rate of flow, as well as the quantities dealt with
by each separate tank and filter. It is also of great assistance, in
making investigations at such times, to have a definite record of the
volumes treated day by day during the preceding six months; indeed the
possession of a complete record of the daily flow of sewage over the
whole period during which a sewage works has been in operation, is a
valuable asset not only to those responsible for the works themselves
but also to the authorities who control the streams and watercourses,
and to investigators in search of information to be used for the
public benefit. The initial cost of suitable measuring and recording
appliances is not excessive, but, when once the works are completed
and in good working order, the local surveyor or manager has great
difficulty, and usually finds it impossible, to persuade his committee
that the outlay is justified. It is, therefore, desirable that
engineers should in all cases make provision for such apparatus in the
contract for the construction of the works, and thus make sure that it
will be available when required.

[Illustration: FIG. 148.—GAUGE WEIR PENSTOCKS.]

Various types of measuring appliances are in use. Among these the
simplest is the gauge weir penstock Fig. 148, manufactured by Messrs.
Adams Hydraulics, Ltd., from which the depth of water flowing over the
weir can be observed at any time. This, however, does not provide a
record of the volume flowing at all times. For this purpose the same
firm supply several types of recorders, among which the simplest is the
type shown in Fig. 149. This is supplied with 24-hour or 7-day
movement, and charts which register the volume in gallons per minute,
per hour, or per 24 hours.

[Illustration: FIG. 149.—RECORDING APPARATUS.]

Another measuring apparatus is that supplied by Messrs. George
Jennings, Ltd., Fig. 150. This consists briefly of a hollow copper
float attached to a brass rod, which is carried up through a hollow
column into the indicator box provided with a glass hinged door. The
brass rod is provided with pointers, which move up and down with the
rods and indicate the water level.

[Illustration: FIG. 149_a_.—VOLUME RECORDER.]

Messrs. George Kent, Ltd., supply a large number of various types of
recorders and measuring instruments, including the well-known “Venturi”
meter and others actuated by floats to indicate the discharge over a
weir. The Venturi meter type of measuring apparatus adapted for sewage,
is shown in Fig. 151. In this case the Venturi tube may form part of
the ordinary supply pipe, or it may be fitted in a chamber built in the
supply channel, the tube connected through the chamber with its ends
terminating in the channel at either side. This measuring apparatus may
be fitted with the following types of recorders. A diagram only,
counter only, or combined counter and diagram in one instrument. The
recorder may be placed in any convenient position within a thousand
feet of the meter tube.

[Illustration: FIG. 150.—MEASURING APPARATUS.]

[Illustration: FIG. 151.-THE “VENTURI” METER.]

[Illustration: FIG. 152.—MEASURING APPARATUS.]

Several types of measuring apparatus are manufactured by Messrs.
Glenfield and Kennedy. One of these is illustrated in Fig. 152. This
consists of an apparatus for recording the volume of water or sewage
flowing over a weir, a chart being revolved by clockwork, and the
volume indicated by a pen actuated by a float and cord working over
pulleys. This instrument can be fitted with cam and pen carriage to
show the rate of discharge on the chart (24 hours or 7 days), in
gallons of cubic feet per minute over a V-notch, rectangular weir, or
in an open channel of known dimensions. It is claimed that by simply
taking the area of the diagram, the total discharge for any period can
be ascertained much more quickly and more correctly than from a diagram
simply giving the height flowing over the weir.

A somewhat novel form of apparatus, recently introduced by Messrs.
Adams Hydraulics, Ltd., for measuring the flow of sewage in a channel,
consists of a water-wheel by means of which the velocity of the flow is
registered. In order to maintain the paddles of the wheel at a uniform
depth below the surface of the liquid, the wheel is carried upon a
shaft supported by two floats provided with vertical guide rods working
in brackets attached to the sides of the channel. By this means the
whole of the apparatus rises and falls with the liquid in the channel.
The wheel-shaft is provided with a bevel-toothed wheel, which engages
with another similar wheel attached to a flexible shaft, and this
drives a set of geared wheels similar to a flush-tank counter, which
thus record the number of revolutions made by the paddle-wheel.




_STERILISATION OF SEWAGE EFFLUENTS._


It has been recognised in many quarters that, although it is possible
by modern methods of sewage disposal to secure a high degree of
purification from a chemical point of view, it may be necessary in
certain cases to take steps to remove the large numbers of bacteria
present in all such effluents. Experiments have been made which
demonstrate that, when a pure culture of some specific organism is
added to a sewage in a sufficiently large quantity, it may pass through
the tanks and filters, and appear in the final effluent. This result
is not necessarily a conclusive proof that the bacteria in sewage
effluents are dangerous, as the experiments do not represent normal
practical conditions. On the other hand, it is true that, in ordinary
practice, sewage effluents contain large numbers of _B. coli_, which is
admittedly of intestinal derivation, and although this bacillus is not
a disease organism itself, its survival in an effluent is considered
an indication of the presence of sewage matter, and consequently
of the possibility of the survival of any pathogenic germs which
may be present in the crude sewage. On this basis, scientists argue
that sewage effluents are _potentially_ dangerous—that there is a
possibility of the pollution of drinking-water or shell-fish by the
bacteria present in sewage effluents. This being so, it is evident that
an additional process will, in some cases, be required to remove the
bacteria, and in a few cases sand filters have been provided for this
purpose. These, however, involve a comparatively high initial expense,
and a considerable annual outlay for maintenance, and in some quarters
it is considered that sterilisation may possibly be a means of securing
the desired result at less cost, and with a higher percentage of
removal of bacteria, and consequently with a higher degree of safety.
It is true that a number of scientists, in replying to a question which
was submitted to them on this subject by the Royal Commission on Sewage
Disposal, stated that in their opinion sterilisation was impracticable,
but this was in the year 1903, and there is good reason for assuming
that if the same question were put to the same men to-day, the replies
would in many cases be modified, if not entirely different. Practical
experiments in the sterilisation of sewage effluents have been few
and far between in this country, but in the United States of America
a large number of reliable experiments under varying conditions have
been made, and the results published. From these it is evident that
sterilisation is not only possible, but economically practicable.
Unfortunately, both in America and elsewhere, attempts were made to
sterilise crude sewage and tank effluents, and the results of these
experiments were so unsatisfactory, both in efficiency and cost, that
they gave the impression that sterilisation was impracticable.

In the opinion of the author, sterilisation should be restricted to the
destruction of living organisms, and should only be used in the case
of liquids with a high degree of chemical purity, and a low content
of matters in suspension. It is quite possible at the present time
to produce sewage effluents which comply with these conditions, by
means of properly designed, constructed and managed sewage disposal
works, as long as these include suitable effluent settling tanks
for the removal of the solids in suspension. Effluents which comply
with the provisional standard suggested by the Royal Commission, and
drinking-water supplies which are very slightly polluted, would be very
suitable.

A number of different methods of sterilisation have been tried, but so
far as the existing knowledge of the subject extends, the application
of chlorine in one form or another is generally admitted to be the most
efficient and economical process. The chlorine may be applied in the
form of a solution of chloride of lime, or as a hypochlorite of sodium,
or of magnesium. The solution of chloride of lime may be prepared by
the sewage works manager, but this necessitates a considerable amount
of care and knowledge in order to secure the correct strength at all
times, and it is, of course, essential that the chloride of lime itself
should always be of a known strength. The hypochlorite of sodium may be
produced chemically, and this can be purchased of known strength from
chemical manufacturers. For large volumes of sewage effluent, however,
it will probably be found most economical to utilise an electrically
produced hypochlorite of sodium, as this can be prepared on the works
as and when required, of a uniform strength, and at a comparatively low
cost.

In the case of the Digby process, briefly stated, this consists in
passing an electric current through a solution of sodium chloride, with
the result that the sodium chloride is broken up into its component
parts, and chlorine is liberated at the positive pole, while sodium is
deposited at the negative pole. The sodium and the chlorine are then
allowed to recombine in the form of hypochlorite, and this solution
is then ready for application to the liquid to be sterilised. Other
processes are similar in principle but vary in detail.

[Illustration: FIG. 153.—THE DIGBY MERIDIONISER.]

This process naturally involves the use of an electric current and
an apparatus for producing the hypochlorite. Of the latter there are
several on the market, and an illustration of the Digby Meridioniser,
which is manufactured and supplied by Messrs. Adams Hydraulics, Ltd.,
is given in Fig. 153. The particular feature of this apparatus consists
in the manner in which the re-combination of the anode and cathode
products are secured. Instead of the re-combination taking place in the
main body of the electrolyte, it can only take place in the special
porous compartment enclosing the electrodes. The re-combination may
take place in either the anode or the cathode compartment, the products
of the one compartment being conveyed to the other compartment. Thus
the caustic hydrate from the cathode cells flows by gravity into the
anode compartment, the two compartments being connected by a glass
pipe. Fresh water or water containing an excess of alkali is run into
the cathode cell. This process gives hypochlorite solution of low
saline content, the only salt present in the resultant liquor being
that due to diffusion, depending upon the porosity of the closely
covering compartment walls, or upon such a reaction as that covered
by the Blount hypothesis. A cross-section of this apparatus is shown
at Fig. 153, in which A is the positive lead; BB negative leads, C
outflow, D inflow, EE cathodes, F anode.

Another machine is that supplied by Messrs. Oxychlorides (1907),
Limited. This machine consists essentially of a graphite anode
of circular cross-section (except as to about 4 in. at the top,
which is left open for the free escape of gases evolved during the
electrolysis), and within it a metallic cathode of smaller circular
cross-section. The annular space between the anode and cathode is
filled with a solution of common salt, or with sea-water, through which
a current is passed from a low-potential dynamo. In either case the
ultimate result is to obtain some of the chlorine of the salt in an
active or available form, the only difference being that in the case of
using strong salt solution, a concentrated form of available chlorine
may be obtained, while with sea-water a weaker solution results. Where
sea-water is readily obtainable it is naturally more economical to
make use of it, and to employ the larger volume of less concentration;
while where sea-water is unobtainable and salt is expensive, or where
chlorides in the effluent are objected to, it is more advantageous to
prepare concentrated solutions, and to dilute them when required for
use. This machine is made in various sizes to suit varying conditions.
It was used by the Royal Commission on Sewage Disposal in connection
with their experiments at Guildford, referred to in their Report of
1908, pages 198 to 201.

Electrolytic hypochlorite of magnesium is being produced daily, by the
Borough of Poplar, for use as a disinfectant. The highly effective
qualities of this solution, as well as the low cost of production,
makes it a very valuable disinfectant, and for the results obtained
every credit is due to Dr. F. W. Alexander, the Medical Officer of
Health, who initiated the scheme, and to whose unbounded energy and
enthusiasm the success of the work is due. This solution is, of course,
equally suitable for the sterilisation of sewage effluents. The cost of
production of this solution, of an average strength of from 4·5 to 5
grammes of available chlorine per litre (·45 to ·5 per cent. solution),
is estimated at under one penny per gallon. The apparatus in use at
Poplar has been supplied by the Farringdon Engineering Co., and an
illustration of the plant is shown in Fig. 154.

[Illustration: FIG. 154.—ELECTROLYTIC HYPOCHLORITE OF MAGNESIUM
PLANT.]

It consists of four cells, each containing ten elements, consisting of
one positive and two negative plates. The positive plates consist of
thin platinum wire, wound upon slate slabs, and the negative electrodes
are of zinc. The four cells are placed one above the other, so that
the liquid passes through from one to the other by gravitation. The
feed-tank at the top contains a solution of sodium chloride and
magnesium chloride, and is fitted with a glass gauge to indicate the
amount of solution in the tank. From this tank the solution passes
through a small ball-valve cistern, so as to maintain a constant rate
of flow. The feed-tank is also provided with a plate, operated by a
chain carried over to the outside of the tank, by means of which the
liquid to be electrolysed can be stirred from time to time, so as to
secure a uniform strength of solution. The solution passes through
the four electrolysers in series, being subjected to the action of a
regulated current of 15 to 17 amperes at 230 to 250 volts, being 5·7
to 6·2 volts per cell. After the electrolysed solution leaves the last
cell it runs into a small tank, where a fixed amount of hydroxide of
magnesium is mixed with it, and it is claimed that by this means the
solution is rendered stable, a quality which should be of much value
where the solution has to be stored for any length of time.

The question as to what is the most suitable sterilising agent to use
under certain conditions, and in what proportion it should be added to
the sewage effluent, is a matter for the chemist and biologist. The
method of application is, however, the duty of the engineer. As in the
case of other chemicals, there are two ways in which it may be applied.
The solution may be added to the sewage effluent in equal doses of
varying strength, or in varying doses of equal strength. There is a
third method, involving the variation of the dose and the strength
of the solution, but while this is not impossible it is probably not
practical. The chief difficulty to be overcome is the variation of
both the rate of flow and the strength of the sewage, and the most
practical solution is to prepare the sterilising agent of a uniform
strength, and vary the doses in direct proportion to the flow of the
effluent, the minimum dose being sufficient for the maximum strength
of the sewage. This method was adopted by the author in the case of a
small scheme of sewage disposal, which he prepared for a place where
the only outlet for the final effluent was a discharge over an area of
chalk subsoil, from which the water supply of a large town is drawn. In
this case, he designed a simple apparatus which does not involve any
special appliances, and which would be quite satisfactory for ordinary
practical purposes. A new apparatus for the purpose in question has,
however, recently been introduced by Messrs. Nixon and Mannock. As
will be seen from the illustration, Fig. 155, it is based upon the
application of the Venturi principle, and involves the use of a
Venturi tube, as previously described under the heading of “Measuring
Apparatus.” In fact, the same Venturi tube can be utilised to serve
both for measuring the flow of the effluent, and for applying the
sterilising agent in direct proportion to the flow of the effluent.

[Illustration: FIG. 155.—NIXON AND MANNOCK’S APPARATUS FOR
INJECTING CHEMICAL SOLUTION.]

The apparatus consists of a cylinder C, the top of which is connected
by means of a pipe fitted with a three-way cock to the “Upstream”
end of the tube A. A similar connection is made from the bottom of
the cylinder to the “throat” B. A piston of the type used in the Kent
Standard Water Meters, and provided with a counterbalance weight,
works in the cylinder by means of the difference of the pressure on
the two sides of the Venturi tube. The chemical solution (e.g. a 5
per cent. solution of chlorine) is supplied to the underside of the
piston, and the pressure on the upper side of the piston being greater
than the pressure on the underside, the chemical is forced down by the
piston and injected through the injection tube and regulating valve
into the effluent at the “throat” of the Venturi tube. As the flow
of the effluent through the Venturi tube produces a difference of
pressure which varies as the square root of the velocity, the rate of
injection will also vary in the same proportion. The injection is thus
in exact proportion to the flow, and any variation of the flow will
automatically cause a corresponding variation in the rate of injection.

When the chemical re-agent is exhausted, the piston will be at the
bottom of the cylinder, and the pointer at zero. In order to recharge
the cylinder with the chemical, the three-way cocks must be reversed
by means of the hand lever, thereby cutting off pipes A and B, and
simultaneously connecting the top of the cylinder to the waste pipe,
and the bottom to the supply from the chemical storage tank, which is
fixed at such a height that the head will rapidly force the piston up
and re-fill the cylinder with the chemical. The three-way valves are
then reversed, and the apparatus is again in full working order. The
apparatus shown is applicable to the treatment of 1000 gallons per
hour, and will only need recharging once per day of 24 hours.

A feature of this apparatus is that it is self-starting, and should the
flow cease, the injection will also automatically stop, the static head
on both sides of the piston being equal. There is absolute immunity
from danger or over-injection of the chemical by this system, and
this is a valuable factor in the treatment of potable water. Where
absolutely necessary, the same firm can supply a de-chlorinating
apparatus. By means of the indicator, the works manager is constantly
informed of the exact amount of chemical injected, and the scale
readings can be compared with those of a Venturi Meter operated by the
same Venturi tube. This apparatus can be supplied of a larger size, and
provided with automatic recharging gear for larger installations.

Whatever method may be adopted for applying the sterilising agent, it
is essential in all cases to have a storage tank to receive the mixture
in order to provide time for the chemical to have full effect. So far
as can be ascertained at present, a storage capacity equal to one
hour’s flow of the liquid to be sterilised will be sufficient under
ordinary circumstances, but provision should be made for thoroughly
mixing the chemical with the effluent and for drawing off any deposit
which may occur in the tank without interfering with the normal working
of the plant.

Although the present volume is devoted entirely to the disposal of
sewage, it may be stated here that in the matter of sterilisation,
the suggestions that have been made apply with equal force to
drinking-water supplies. Where the water contains a considerable amount
of matter in suspension, it would be advisable to provide means for
ample storage and settlement before passing it through the sterilising
plant.

     _Note._—An apparatus for the injection of chlorine
     solutions for the purpose of sterilising sewage effluents
     and drinking water has recently been brought out by the
     Candy Filter Co., Ltd., and has for some months been in
     practical operation, dealing with 200,000 gallons of river
     water per day for an important municipal waterworks in
     the country. In this case the installation includes a
     de-chlorinating process, and it is stated that the results
     of tests in actual work show that the sterilised water
     contains neither _B. coli_ nor free chlorine.




INDEX


                               PAGE
    “Acme” spray nozzle, 151
    “Adams-Cutler” distributor, 135
    “Aerat” distributor, 145
    Apparatus for contact beds, 205
        ”     for dosing tanks, 167
        ”     for measuring flow, 232
        ”     for percolating filters, 106
        ”     for sludge removal, 40
        ”     for sterilisation, 242
    Auto-mechanical syphons, 168, 169, 180

    Birmingham fixed spray, 155

    Candy-Whittaker bacterial tank, 60
      ”      ”      distributors, 106
    Capacity of contact beds, 222
       ”     of detritus tanks, 23
       ”     of effluent settling tanks, 182
       ”     of percolating filters, 222
       ”     of sedimentation tanks, 29
       ”     of storm-water tanks, 227
    “Capillary trough” distributor, 146
    “Carlton” adjustable fixed spray, 152
    “Carlton” distributor, 123
    “Carlton rotor” distributor, 121
    Chemical mixers, 72
    Cleaning gear for spray holes, 119, 122, 125
    “Coleman” dosing valve, 168
    Columbus fixed spray, 155
    Contact beds, 189
       ”     ”   apparatus for, 205
       ”     ”   capacity of, 222
       ”     ”   filling material for, 203
       ”     ”   general design of, 190
       ”     ”   methods of construction of, 196
       ”     ”      ”    of distribution on, 199
       ”     ”   method of operation of, 191
       ”     ”   necessity for testing, 198
       ”     ”   sub-drainage of, 201
    “Cresset” distributor, 110

    Decanting valve, 49
    Detritus tanks, 23
        ”     ”    capacity of, 23
        ”     ”    Dortmund type of, 25
        ”     ”    sludge scraper for, 27
    Dibdin, W. J., 68, 189
    Digby meridioniser, 243
    Disc sprays, 155, 157
    Distribution by flooding filters, 157, 186
          ”      methods of, 104
    Distributors, 106
         ”       cleaning gear for spray holes of, 119, 122, 125
         ”       fixed, 143
         ”       power-driven, 129, 140
         ”       revolving, 106
         ”       travelling, 138
    Dortmund tanks, 25, 51, 52
    Dosing apparatus, 166
      ”    tanks, 162
      ”     ”    disadvantages of, 160
      ”     ”    necessity for, 159
    Drainage of filters and beds, 90, 201
    Ducat filter, 144

    Effluent settling tanks, 182
    Elliott and Brown, 59

    “Facile” distributor, 119
    Feed channels, 163
    “Fiddian” distributor, 127, 138
    Fieldhouse tank, 65
    Filtering material, 101
        ”        ”     grading of, 103, 204
    Fine-grain filters, 185
    Fixed distributors, 143
      ”   sprays, 147
    Floating arm, 48
    Floor-tiles, 91

    Gauge weir penstocks, 233
    Grading of material, 103, 204
    Gravity disc sprays, 155, 157
    Grit chambers (_see also_ DETRITUS TANKS), 23

    Haller and Machell, 145
    “Hanley” distributor, 140
    “Hartley” distributor, 130
    “Hodgson” distributor, 127
    Humus pits, 182
    Hydro-extractor for sludge, 81
    Hydrolytic tank, 52
    Hypochlorite solutions, 242—247

    Imhof tank, 57
    Intermitting valves (_see_ DOSING APPARATUS)

    “Kessel” tank, 63

    Lowcock, S. R., 51
    Low-draught syphon, 167

    Massachusetts Institute of Technology, 155
    Material for filters and beds, 101, 185, 203
    Measuring apparatus, 232

    Non-septic cylinder, 61

    Percolating filters, 85
         ”         ”    aeration of, 86
         ”         ”    capacity of, 222
         ”         ”    distributors for, 106
         ”         ”    floor-tiles for, 91
         ”         ”    floors for, 88
         ”         ”    general design of, 85
         ”         ”    material for, 101
         ”         ”    methods of distribution on, 104
         ”         ”       ”    of feeding, 159
         ”         ”    planning of, 98
         ”         ”    sprinklers for, 106
         ”         ”    sub-drainage of, 90
         ”         ”    types of construction of, 87
         ”         ”    walls for, 95
    Ponding valve, 178
    Power-driven distributors, 129, 140
    Precipitation tanks, 47
    Preliminary processes (_see_ TANKS, _also_ SLATE BEDS)

    Recording apparatus, 234—239
    Revolving distributors, 106
    Royal Commission on Sewage Disposal, recommendations, 3, 16, 23,
                                         29, 32, 182, 224, 226, 227
    Ryder, E. E., 93

    Salford fixed spray, 155
    Sand-filters, 185
    “Scott-Moncrieff” distributor, 130
    Screens, 3
       ”    bars for, 4
       ”    for deep sewers, 7
       ”    mechanical, 7
       ”        ”      rake, cleaning gear for, 12, 14
       ”    rotary, 5
       ”    simple, 3
    “Separator” tank, 65
    Sewage mixers, 73
    Shone and Ault, 53
    “Simplex” distributor, 125
    Skegness tank, 59
    Slate beds, 67
      ”   slabs and blocks, 68, 92
    Sludge disposal, 71
       ”   draining beds, 83
       ”   drying apparatus, 81
       ”   presses, 75
       ”   pressing plant, complete installation, 77
       ”   removal, 25, 28, 35, 71
       ”      ”    apparatus for, 27
       ”      ”    elevators for, 40, 41, 42
       ”      ”    well for, 38
    Spray jets, 147
    Sprinklers, 106
    Stand-by tanks for storm water, 229
    Sterilisation of sewage effluents, 240
    Sterilising solutions, 242
         ”          ”     apparatus for the manufacture of, 242—247
         ”          ”         ”     for the injection of, 247—250
    “Stoddart” distributor, 142
    Storm-water overflow, diverting plate for, 17
         ”          ”     fixed weirs, 17
         ”          ”     floating weir, 21
         ”          ”     movable weirs, 21
         ”          ”     swinging syphon for, 21
         ”          ”     weirs, 16
         ”      tanks, capacity of, 227
         ”      treatment, 227
    Strength of sewage, 226
    Supply pipes, 163
    “Sypho-jet” distributor, 110

    TANKS, 29
      ”  Candy-Whittaker, 60
      ”  capacity of, 29
      ”  circular, 25, 51—59, 60, 61, 66
      ”  continuous-flow settlement without chemicals, 29, 32
      ”       ”      ”       ”      with chemicals, 29, 32, 47
      ”  decanting valve for, 49
      ”  detritus, 23
      ”  Dortmund type, 25, 51, 52
      ”  Fieldhouse, 65
      ”  floating outlet for, 48
      ”  hydrolytic, 52
      ”  Imhof, 57
      ”  inlets and outlets for, 40—47
      ”  ”Kessel” type, 63
      ”  non-septic, 61
      ”  precipitation, 47
      ”  quiescent settlement without chemicals, 29, 32
      ”     ”          ”      with chemicals, 29, 32
      ”  rate of flow through, 34
      ”  rectangular, 33, 36, 37, 43—50
      ”  roofs for, 39
      ”  ”separator”, 65
      ”  septic, 29, 32
      ”  Skegness, 59
      ”  storm-water, 229
    Taylor, W. Gavin, 105
    Travelling distributors, 138
    Travis, W. O., 52
    Trays and troughs, 143
    “Triple tank” system, 145

    “Venturi” meter, 237

    Walls of filters, 95
    Waterbury fixed sprays, 155
    Watson, J. D., 93, 130
    Weirs, fixed volume, 21
    “Whirl” spray jet, 149
    Willcox and Raikes, 49, 96, 135




MANUFACTURERS OF APPLIANCES


    Adamsez, Ltd.
    Adams Hydraulics, Ltd.
    Adamson, D. and Co.
    Albion Clay Co., Ltd.
    Ames-Crosta Sanitary Engineering Co., Ltd.
    Blakeborough, J. and Son, Ltd.
    Burn Bros.
    Carlton Engineering Co.
    Enock, A. G. and Co., Ltd.
    W. E. Farrer, Ltd.
    Farringdon Engineering Co.
    Glenfield and Kennedy, Ltd.
    Goddard Massey and Warner.
    Ham Baker and Co., Ltd.
    Harriman, W. and Co., Ltd.
    Hartley, Causton and Richmond.
    George Jennings, Ltd.
    Johnson, S. H. and Co., Ltd.
    George Kent, Ltd.
    Birch Killon and Co.
    Manlove, Alliott and Co., Ltd.
    Mansfield, H. R.
    Mather and Platt, Ltd.
    Naylor Bros.
    Nixon and Mannock.
    Oxychlorides (1907), Ltd.
    Patent Automatic Sewage Distributors, Ltd.
    Septic Tank Co.
    Smith, John and Co.
    Stiff, C. L.
    Stoddart, F. W.
    Stott, S. S. and Co.
    Whitehead and Poole.
    Wolstenholme, J. and Co.


          LONDON: PRINTED BY WILLIAM CLOWES AND SONS, LIMITED,
   GREAT WINDMILL STREET, W., AND DUKE STREET, STAMFORD STREET, S.E.




ADVERTISMENTS.


                           THE
                              SEPTIC TANK
                                  COMPANY, LTD.
                           5 Victoria Street
                                WESTMINSTER

                           For the MOST
                                EFFICIENT
                                    AND
                                UP-TO-DATE
                           SEWAGE PLANT
                                OF EVERY DESCRIPTION.


                       S. H. JOHNSON & Co., Ltd.

                            _SLUDGE PRESSING
                               MACHINERY_

           SLUDGE FILTER PRESSES
                WITH PATENT PYRAMID DRAINAGE SURFACES,
                & RATCHET CAPSTAN TIGHTENING GEAR, ALSO
                PATENT HAND HYDRAULIC CLOSING GEAR,
                & PATENT HYDRAULIC OPENING GEAR.

           SLUDGE FORCING RAMS
                ORDINARY OR AUTOMATIC SYSTEM. OUR TWIN
                PRINCIPLE SAVES 50% AIR CONSUMPTION.

           AIR COMPRESSORS
                SINGLE, OR TWO STAGE, HORIZONTAL STEAM
                AND BELT DRIVEN OR FOR ATTACHMENT TO
                OIL OR GAS ENGINES. VERTICAL HIGH SPEED
                FOR ELECTRIC DRIVE.

           PRESS CAKE DISINTEGRATORS

           PNEUMATIC LIME MIXERS ETC.

                             ENGINEERING WORKS:
                         CARPENTERS ROAD, STRATFORD
                                 LONDON, E.

       Telegrams: “FILTRUM, LONDON.”      Telephone: EAST 363


                                  THE
                                STODDART
                               CONTINUOUS
                             SEWAGE FILTER

                            THE BEST FOR
                          LARGE AND SMALL
                              SEWAGE WORKS

                                OVER 500
                             INSTALLATIONS
                                AT WORK

                 F. Wallis Stoddart,
                      Western Counties Laboratory,
                                       BRISTOL.


  Telephone: 1843 AVENUE (3 lines).       Telegrams: “KEESH, LONDON.”

                           FILTERING MATERIAL
                                  FOR
                              SEWAGE BEDS

                               FRANK KEEP
                    9 and 10 St. Mary-at-Hill, E.C.


                NAYLORS’ PATENT AERATING TILES

       [Illustration: STOCK SIZE, LENGTH 18”, WIDTH OUTSIDE 12”,
                      WIDTH INSIDE 9”.]

                      USED BY OVER 50 AUTHORITIES.
                         SEVERAL REPEAT ORDERS.

  =MATERIAL=—Indestructible Salt-glazed Stoneware.
  =STRENGTH=—Tested to over 9 tons per sq. yd. without
  breaking. =COST=—Exceptionally low. =OUTPUT=—2500 sq.
  yds. per week. =SPECIAL TILES SUPPLIED TO SUIT
                                  CIRCULAR AND OCTAGONAL BEDS.=

  =EXPORT=—The tiles are easily nested, and works being near ports,
       shipping orders can be especially well executed.

                      NAYLOR BROTHERS, DENBY DALE


                         BURN BROTHERS’ PATENT

                       Automatic Sewage Disposal
                               Apparatus

                             [Illustration]

      Automatic Alternating and Timed Discharge Syphons applied to
        Contact Filters. Extreme simplicity, absolute accuracy,
           entire freedom from choking, and no moving parts.

         Awarded SILVER MEDAL by the ROYAL SANITARY INSTITUTE.

                             [Illustration]

                                 Patent
                               Automatic
                                 Rotary
                               Sprinkler

                            _Self-clearing
                             Orifices._

                           Requires attention
                              about once a
                             week or less.

              _Sewage Lifts, Valves, Penstocks, etc._

                  ROTUNDA WORKS,
                          3 BLACKFRIARS ROAD,
                 Also at EDINBURGH.        LONDON, S.E.


                             Geo. Kent Ltd.

                           Specialists in the
                             measurement of
                             WATER, SEWAGE & GAS

    are at the disposal of SEWAGE WORKS ENGINEERS who desire advice
    as to the introduction of

                              EFFICIENT &
                                RELIABLE
                    SEWAGE REGULATING & MEASURING
                               APPARATUS

                G. KENT, LTD., 199 HIGH HOLBORN, LONDON.


                           MECHANICAL SEWAGE
                          SCREENING APPARATUS

                              (Automatic)

                             [Illustration]

                            JOHN SMITH & CO.
        Telephone: 56 Sutton.     Engineers, CARSHALTON.


                   THE MANSFIELD PATENT STONEWARE
                           AERATING TILE
                           FOR FLOORS OF
                      BACTERIA BEDS AND FILTERS

                         _Manufactured by_
                            H. R. MANSFIELD
                    CHURCH GRESLEY, BURTON-ON-TRENT

       This tile has met with unqualified success owing
       to its superiority in design over all other tiles.
       Its advantages have been recognized by leading
       Engineers, and it has been largely used throughout
       the Kingdom.

HARD SMOOTH SURFACE, PREVENTING FUNGOID GROWTHS.

[Illustration]

EASILY AND QUICKLY LAID WITHOUT SKILLED LABOUR.

                       See page 93 in this book.

                _SUPPLIED TO THE FOLLOWING AUTHORITIES
                           AND SCHEMES_:—

            Birmingham Tame and Rea District Drainage Board.
                       Metropolitan Water Board.
     Corporations of Lincoln, Maidstone, Tunbridge Wells, &c., &c.
         Crieff Sewerage, Tipton, Wednesbury, Tamworth, Chard,
               Whitwick, Nuneaton, Enfield, Chesterfield,
                            and many others.

              DESCRIPTIVE BOOK OR SAMPLE TILES AND PRICES
                            ON APPLICATION.