Disposal Sewerage ctlcal SHENTON,

Disposal Sewerage ctlcal SHENTON,
H. C. H.
FS*E*t -MI*MwBiEo MRJ5*itI
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cement being removed from the
interior of newly-laid drains has been successfully overcome by
the introduction of the
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whole of
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The cement "crumbs" are
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Sewage Disposal.
F.S.E., M.I.Mun.E., M.R.San. I.
subject of Sewerage and Sewage Disposal is
one upon which many books have already been
generally speaking, they contain a
considerable amount of matter which is more or
theoretical, and much information, of methods
good and bad, from which the reader is left to draw his
The present work is intended to be more in the
nature of a practical handbook, giving definite advice,
as far as possible, to those
who have
actually to carry
methods, and leaving the theoretical side of the question
The work
is divided into numbered paragraphs, each
indexed alphabetically to enable the reader
to refer readily to any matter dealt with in the book.
The author has
endeavoured to deal with as
points as possible in as small a space as is consistent
with clearness. Thus the ordinary minimum gradients
for sewers of circular or elliptical section are given, but
elaborate tables
and methods of calculation are omitted
for the simple reason that in nine cases out of ten all
that the engineer wants to know is the flattest gradient
he can use and the velocity of the flow. For fuller
admirable text-books exist to which
reference should be made.
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Ind ex.
Adams Flushing Syphons
Adams Sewage Lift
Adjustment of Automatic Apparatus
Area Outside Sewerage District
. .
Arrangement of Sewers
Automatic Apparatus for Contact Beds
Automatic Flushing ..
Auxiliary Branch Sewers
. .
. .
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Backyards, Level of Sewers to Drain
. .
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. .
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of Filters
Level of Sewers to Drain
Cement Joints
Channels, Land Treatment
Chemical Precipitation
Choice of System of Land Treatment
. .
. .
Clay Land
Cleaning of Sewers
Combined System
Composite Joint
Concrete Sewers laid on
. .
. .
Connections, House
Construction of Contact Beds
Contact Beds
. .
. .
. .
Cost of Sterilization
. .
Capacity, Working,
Cellar Drains
Bends in Sewers
Bosley's Patent Flushing Block
Borings, Trial
Brick Sewers
Courses for Sewers, Arrangement of
Crops, Land Treatment
Curves on Sewers
Deodorization of Sewage
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234-236, 239
61, 94, 99
241, 265-272
237-253, 265-272
318, 319
gg, 141
. .
. .
Diameters for Sewers .
Dirt Boxes for Manholes
Disposal Works, Maintenance of
Double System of Sewers, Disadvantages
Doulton's Composite Joint
Drain Block
Drainage of Contact Beds
. .
. .
. .
Land Irrigation
Effluents, Sterilization of
Effluent from
. .
310, 311
Sewage Lifting used instead of
Electrolytic Hypochlorite Sterilization
Elliptical Sewers, Gradients for
Existing Sewers or Drains
Extension of Sewers . .
Common, with
. .
regard to Contact Beds
Filling Material for Contact
. .
. .
237, 238
Filter for Final Effluent
Filters, Percolating
of Material
. .
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. .
. .
Final Purification of Sewage Effluent
Float Experiments for Sea Outfalls
Flow in Sewers, Velocity of
Flushing Sewers, Need for
Streams and Springs
Methods of
Tanks for
Fish, Danger to, from Sewage
Flaps, Tidal
220-236, 273
Filtration before Sterilization
Filling Material for
Working of
Working Capacity of ..
Drainage and Aeration of
Dripping Trays
Ejector, Shone's
Electric Motors
46, 51
47, 48
Ill, 112
Flushing Sewers, Manholes for
Automatic Flushing
by Means of Water Carts
Merry weather's Apparatus
Quantity of Water required
for ..
Frequency of
with Sea Water
Some Methods for Obtaining Water
from Public Mains
Water Mains should not be connected
direct to Chambers
Flushing Chambers Above Ground ..
Advantage of Automatic Flushing ..
Adams' Syphons ..
. .
. .
. .
Augmented by Ordinary Flow
Used in
foundations for Sewers
T?orm of Sewer
Furrows for Land Irrigation
33, 63, 73, 74
Geological Formation of Ground affecting Sewerage Schemes
Grade of Material in Contact Beds
Grade of Material in Filter
Gradient of Circular Sewers
47, 48,
Grit Chambers
Gravitating Sewers
Gulley Grids
Hassall's Joint
House Connections
Hydrolytic Tank
61, 94,
Hypochlorite, Sterilization by
Intercepting Sewers
Sewer . .
Iron Pipe Sewers
Jenning's Joinder
Junctions for House Connections
62, 69
Junctions, Sewer
. .
Land Treatment
Laying Pipe Sewers, Method
Leaping Weir for Storm Overflow
Levels Required along Courses of Sewers
Levels of Sewers below Basements, Cellars, and BackYards
Liernur System
Adams' Sewage
. .
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. .
. .
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Lifting Sewage
of Irrigation
6, 7, 8,
Penstocks in
53, 62, 81-93, 159
Outfall, Position of
10, 16, 19,
Outfall, Sea
on Sea Outfall Sewers
Mechanical Ventilation
. .
Merryweather's Patent Flushing Apparatus
Motors, Various, for Pumping Sewage
Nuisance from Sludge
. .
Sewage Disposal Works
17, 18, 31,
Loans, Supplementary
Local Government Board Loans
Local Government Board Requirements
Lowering Pipes into Trench . .
Penstocks in Manholes
10, 16, 19-22, 153
Position of Sewers
Storm Water Sewers
and Levels of Sewers in Separate System
Percolating Filters
Position of Outfall
Preliminary Remarks
Preliminary Treatment for Sewage
Private Land, Sewers on
Public Roads, Sewers in
. .
288, 289
. .
Quality of Sewage
Quality of Sewage decides Gradients
. .
of Filtration
Requirements of Local Government Board
River, Sewer Crossing
Scale of Plans
..22, 153-159
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23,-Sft, 54
199, 211-217, 260
Difference of Opinion among Scientists
Sea Outfalls
Sea-water Flushing
Sections for Sewers
Separate Sewers
Separate System
Septic Tanks
Sewage Disposal
, ,
, ,
, ,
Elementary Principles ..
Typical Problems
Keasons for adopting various Methods
Degree of Purification Possible
, ,
Chief Methods of
Deodorisation and Sterilization
Early Attempts
Treatment by Settlement
, ,
, ,
. .
Upward Filtration
Septic Tank
Hydrolytic Tank
Tanks and other Devices
Separating Sludge followed by Filtration
200, 201
Sludge Removal
Chemical Precipitation ..
Sludge Treatment
Sludge Nuisance
Septic Tanks
Designs of Septic Tanks
Tank used
to break
for further treatment
Final Purification
Sprinklers, Fixed Jet
, ,
215, 216, 217
Tipping Troughs and Dripping Trays
Defects of Dripping Trays
. .
229, 230
Sprinkler Difficulties
Filters, Grade of Material
Working Capacity of Filters ..
Percolating Filters
up Sludge
Sewage Disposal,
, ,
, ,
, ,
, ,
, ;
Contact Beds
237, 238
Eelative Capacity of Filters ..
Variations of Flow
Practical Details of Construction of
Contact Beds
Automatic Apparatus for Contact Beds
Emptying Contact Beds
Requirements for Automatic Apparatus
Adjustment of Automatic Apparatus
Requirements of Contact Beds
Arrangement of Contact Beds
247, 248
Dibdin's Slate Beds
General Practice as to Dimensions of
Tanks, Filters and Contact Beds
General Practice as to Screens and Grit
Tanks ..
,, Rates of Filtration
Local Govern,,
ment Board's Requirements
Strength of Sewage
Crude Sewage which has passed
through Detritus Tanks or Grit
Treatment of Septic Tank Effluent ..
Treatment of Sewage which has passed
through Settlement Tanks with a
Continuous Flow
Construction of Contact Beds ..
. .
tion of Smell
Construction of Contact Beds, Drainage
268, 269
Construction of Contact Beds, Grade
of Filling Material
Construction of Contact Beds, depth..
Humus Tank or
Drainage and Aeration 274-276
Filling Materials
Percolating Filters
, ,
, ,
, ,
Sewage Disposal, Storm Water Tanks
Land Treatment
ment of Sewage from
288, 289
Methods of
Land Treatment
of Irri-
Irrigation Works,
Working of..
Arrangement of Land
Filtration Works
Pumped to
Area of Land Required 307-309
Sewage Effluents
Sterilization of
Choice of System
, ,
, ,
as Matter of Policy
by Hypochloritc
Filtration before Sterilization
Cost of Sterilization
Electrolytic Hypochlorite Sterilization
Hypochlorite Sterilization
Testing Sterilized Effluent
Sewer Construction
. .
Sewerage Systems, Notes on Design
of Flat Districts
Sewers, Large
Shone's System and Ejector
Sighting Rails
Site for Sewage Disposal Works, Selection of
Slate Beds, Dibdin's
Sludge Nuisance
2-5, 10-42
202, 203
Separation Followed by Filtration
. .
200, 201
Fixed Jet
Stanford Joint ..
Sterilization of Sewage Effluents
Stilfgoe's Patent Cast-iron Manhole Base
Storm Water
54, 143, 160-163, 283, 284
Strength of Sewage
Surveys for Sewerage Systems
Subsoil Drainage
4, 28
Supplementary Loans
Syphons, Inverted
36-37, 172, 173
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Systems of Sewerage
Tank Sewers
197-199, 204-217, 253, 259-284
Testing Sewers
Testing Sterilized Effluent
Tidal Flaps
60, 173
Timbering Trenches
Tipping Troughs
Trade Wastes
Trial Borings
Valley, Sewer Crossing
Velocity of Flow in Sewers
. .
Ventilation, Mechanical
Volume of Sewage to be dealt with in Sewers
Water Courses
Water in Trenches
. .
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Hydraulics, Ltd.
Adamson, D.
Councils' Accounts
Crapper, T. & Co.
Sewage Purification Plant
High-Efficiency Ejectors
Sewage Disposal App aratus
Book by F. Dodson
viii .
Departmental and Legal Decisions
Gent & Co
Lea Recorder Co., The
Lees, James..
Local Government Annual, The
Local Government Journal, The
"Loco " Draining Apparatus Co.,
. .
. .
Sanitary Appliances
Quarterly .
Sewage Recorders
Sewage Measure
Manhole Cover
Official Directory
Drain Badger
. .
" Loco "
Draining Apparatus Co.,
" Loco "
Merry weather, Messrs.
Ransome Ver Mehr
Cesspopl Exhauster
Sanitary Publishing Co., The
Septic Tank Co
Sanitary Publications
. .
Sewage Plant
Drain Cleaning Machines
Water Supply
Book by H.
H. Shenton
Small Towns.
specially suitable for this purpose.
Write for
full particulars,
with List of Users, to:
28, Deansgate,
For the most
Sewage Plant
Preliminary Remarks.
Notes on the Design of Sewerage
Systems. Physical and Geological Character of the
Ground. Rainfall Drainage. Foul Sewage Drainage.
(1) Mr. John C. Trautwine, in the preface to the first
edition of his world-famous
Civil Engineers' PocketBook," alluded to certain eminent men, authors of
standard works on engineering isubjects, in the following
He said " although their books are
scathing terms.
the productions of master minds and exhibit a profundity
of knowledge beyond the reach of ordinary men, yet their
language is also so profound that very few engineers can
The writer himself, having long since forread them.
gotten the little higher mathematics 'he once knew,
To him they are but little more than striking
instances of how completely the most simple facts may
be buried out of sight under heaps of mathematical
It is the present author's object, above all things, to
avoid coming undeo* such a condemnation, and in writing
a series of articles on sewerage an>fj sewage disposal to do
so in a simple and straightforward manner, avoiding as
far as possible all difficult technicalities and obscure
scientific expressions, and carefully explaining the meaning of all such words and expressions where their use
cannot be avoided.
A man may be a very good sanitary engineer without
knowledge of higher mathematics, chemistry, or bacteriology, each of which (subjects can only be properly dealt
with respectively by our friends the mathematician, the
chemist, and the bacteriologist, who give the results of
their labours to the engineer, but who, with all respect,
must not poach on thi* engineer's preserves any more than
To quote again from Mir.
he may .poach on theirs.
Trautwine, Let them work out the results and give them
We could afford
to the engineer in intelligible language.
to -take their words for it, because such things are their
speciality ; and because we know that they are best qualiOn the same principle we
fied to investigate them.
entrust our lives to our physician, or to the captain of
Medicine and seamanship are their
the vessel at sea.
respective specialities."
The author, <as an engineer, therefore proposes to speak
in derail of the practical design and construction of
sewerage and sewage disposal works, taking the conclusions of mathematicians and scientists as a basis, but
without 'attempting to t-ake up the position of either.
Moreover, it is not proposed to give any description
of historical matters, or to deal with obsolete methods,
or to describe discredited systems, or to treat of anything beyond what will be of immediate practical
use to Iftiose interested in (the design and construction of new work or the setting right of old and
will be the endeavour of the
defective work.
author to do this in a form that will be equally useful
and comprehensible not only to engineers and surveyors,
but also to students of sanitary engineering and to those
responsible for the welfare of municipalities as members
of governing bodies, by enabling them to judge of the
possibilities for the sewerage of new places, or to detect
serious defects and see the possibilities for improvement
It is also hoped to provide such
in existing systems.
authorities with some data which may be the means of
enabling them to see that the money spent for works of
is not wasted on imperfect work, which may
the future, or become an absolute danger to
health and prove to be an utter waste of money.
Sewerage and .sewage disposal are vital subjects, needing expert knowledge and careful separate attention and
treatment for each case ; nevertheless, certain points are
so obvious that any man of common sense can grasp them
and their importance, and, having done so, will not
readily allow money to be spent upon work unless such
points receive proper attention. It is for lack of attention
to the principles of common sense, and not to any
abstruse matters, that many failures are due. Thus,
whilst the following articles are intended to form a practical handbook for engineers and surveyors, and a textbook for students, it is also hoped that they will be of
fail in
and useful
who may have the welfare
a nature to be comprehensible
spirited person
at heart.
any publicof his district
In preparing a sewerage scheme before deciding
to be adopted, the first consideration,
upon any system
onust be the physical and geological conformation) of the
ground in the district to be drained. It must be borne
in mind, first, that the whole of the rainfall over the
district has to be accounted for in one way or another,
either by evaporation or by soakage into the ground to
an extent which will not leave the ground water-logged,
or by running it off in water courses. This rain water
must Ibe diverted into natural channels which will carry
it to streams, rivers, the sea, etc., or it can be carried to
,new sewers laid for that purpose. The excessive moisture
in the ground itself can be carried away by subsoil
To a great extent the physical and geological conformation of the ground determine what work is necessary. For instance, a district with a pervious stratum on
the surface might appear to need very little drainage at
It would seem
'all, as far as smbsoil water was concerned.
tihat surface water soaking into it would
raipidly descend
to a depth where it would be harmless. If, however, it
'were found tihat the pervious stratum was only a >few feet
thick, and that it rested on an impervious stratum, and
that whatever water fell was retained, the case would 'be
very different, and a large system of subsoil drains might
be needed to keep the district from being unhealthily
damp. Again, if a stream ran through the district it
might drain it very effectually or enable the engineer to
drain off the subsoil water with very little trouble or
A day country, with an impervious surface,
would, if it were flat, hold an immense quantity of water
on its surface, which would have to be carried to drains
of some sort.
If the country were not flat, however,
there would be natural water courses, and the surface
drainage might very likely consist for the greater part of
small drains to these courses, and subsoil
drainage might
be altogether unnecessary and impracticable.
geological character of the ground at the outfall
often be of the greatest importance. If, for instance,
the choice lies between a clay surface or sandy
ground, it
may be best k> choose the sandy site, for the reason that
the effluent could be treated much better on sand than on
Or, by choosing a site where an abundance '*
clean gravel exists, the cost of concrete walls and foundations may .be materially reduced or the gravel itself may
form a natural filter.
(3) In preparing a new sewerage scheme
it is
that the engineers' preliminary observations should include not only the particular district under consideration, but a much of the ground outside its limits as may
form part of the same natural drainage area. Surface
or sufbsoil water from the outside country may come into
the district under consideration and have to be drained
away. It may also be possible for two or more districts
to combine their sewerage systems so as to run to a
Future possibilities of this kind
common outfall.
must receive consideration as well as the probable
and possible growth of the town to be drained. Due
allowance must be made for the increase of population,
each town needing careful consideration on its own
In some cases it will not be sufficient to base
calculations upon the increase during past years, as it
may be altogether misleading.
(4) The importance of having subsoil drains has been
demonstrated many times. The unhealthiness of a saturated soil is well known, and it has been found that a
reduction in the death-rate from phthisis has followed
the drainage of the subsoil in towns, and that the deathrate from this cause is greater on a retentive formation
than on an impervious one, except in the case of seawater saturation, which does not appear to be unhealthy.
Moreover, the difficulty of laying sewers in wet ground
may be so great that subsoil drainage may -actually save
expense in constructing and maintaining the sewerage
system. They may also improve the value of land for
building purposes.
a most important fact, and one to be clearly
that the subsoil drains must never be
combined with the foul sewerage system. Where they
occur with the combined system they must be quite
In the separate system they may
distinct from it.
be separate from the clean surface water sewer or combined with it. The reason for keeping subsoil drains distinct from foul .sewers is that they would be likely to conduct sewer gas to unsuitable places, and sewage might
flow bacfk into the open-jointed sufbsoil drains and
saturate the soil under houses or elsewhere. And when
the springs were high enormous quantities of land-water
It is
might get into the sewers and increase the flow out of
proportion to the works.
The next thing to be considered with reference to the
rainfall drainage is whether any old sewers, culverts,
drains, etc., exist which can in any way be utilised.
Such old sewers, though very likely quite unsuitable for
foul sewage, may do excellently for clean surface water
or for .subsoil water.
(5) Having considered the question of the water due to
the rainfall, and ascertained the possibilities as to
whether there will be much or little to be dealt witih in
the sewerage scheme, the second thing to be borne in
mind is the question of the disposal of the whole of the
water supply of the district, which, whether used for
drinking, washing, sewer flushing, or other purposes, can
be reckoned as the amount of the foul sewage.
In doing
this care must be taken not to overlook the fact that
some of the rainfall must of necessity reach the foul
water sewers from surface water gullies in places such as
crowded streets, where the water would be too impure to
allow it to run to any clean waiter drain or natural channel ; also in the most perfect separate system it would
probably be impossible to exclude all rain water from
These matters will be dealt with later
Ordinary Method of Procedure for Local Authorities
Local Government Board Loans, Regulations, and
of Schemes
Government Board.
tion to the Local
for Presenta-
(6) When a local authority deeddes upon carrying out a
sewerage scheme, and the expense of the works proposed
is likely to prove too serious a burden to the ratepayersof the district to be benefited, it is customary to borrow
the money under the sanction of the Local Government
Board. Local authorities are allowed by the Public
Health Act of 1875 to borrow money for carrying out
works of sewerage, etc., but this money may only be borrowed with the sanction of the Local Government Board.
The money is borrowed on the security of the rates, and
is not allowed to exceed in amount the assessable value
of the district for two years.
This sum must include
all outstanding loans, if any have been previously con-
If the loan required exceeds the assessable value for
one year, the Local Government Board are not allowed
sanction the loan until one of their inspectors has held
a local inquiry. Money may not be .borrowed for a longer
time than sixty years, and has to -be paid off within that
time. It may, however, be 'borrowed for such shorter time
as the local authority, with the sanction of the Local
Government Board, determine. The local authority have
either to pay off the money borrowed by equal annual instalments, or they may in every year set apart a certain
sum as a sinking fund, and invest it. This sum must be
euch that, with compound interest, it will be sufficient to
pay off the money within the period sanctioned.
(6a) Before the Local Government Board will sanction a
loan they require to see iDlans and sections of the scheme
proposed, together with detail drawings and an estimate,
and, unless the scheme submitted is of a character of
which they approve, they will not sanction the loan. The
estimates submitted to the Local Government Board have
to be prepared upon forms supplied for the purpose by
The engineer should prepare (a) a 6-inch
the Board.
ordnance map
of the district, showing the boundary of
the particular -parish or ratable area by a single red line
(b) a 25-inch plan showing the lines of the sewers and
the positions of all works (c) sections of the sewers to
the same scale showing sizes of sewers, depths, levels, and
gradients, etc.
(d) detail drawings of sewers, manholes,
disposal works, and of any other works for which the
loan is required.
No drawings should exceed double
the estimate forms there are certain directions
and suggestions, referring
design and
details of the work.
(8) The requirements of the Local Government Board
with regard to sewerage works are not officially published, but are fairly well known to engineers who have
had much to do with the preparation of schemes for the
approval of the Local Government Board.
The Board relies upon the judgment of its officials and
inspectors in deciding as to whether a scheme is fit for
There are certain fixed requirements
approval or not.
for sewers, such a,s that a manhole or lamp-hole
shall occur at every change of gradient or direction ;
but each scheme is considered on its own merits, and the
engineer #ho presents a r>roperly designed system of
sewers need not fear criticism.
In preparing a scheme for presentation to the Local
Government Board, the engineer should make no statement which he is not able to verify at once e.g.,
he should have ready to hand detail estimates of aach
piece oif work for which a lump sum has to be given on
the L.G.B. form. He may be asked for them at the inIf he shows a retaining wall of a certain thickness he should have his calculations ready to hand.
he states that land can be purchased at a certain figure,
he should hold a nrovisional agreement with the landowner showing willingness to sell at that figure.
(9) If the estimate for the work is exceeded after the
sanction of the Local Government Board has been obtained to the loan, a supplementary estimate must be
prepared and submitted and approved before any further
loan can be sanctioned. The estimates have to be signed
by the engineer.
The Local Government Board have no liking for supplementary loans. Such loans should not be required for
extensions of the works, and are only necessary when the
cost of the work, as originally designed, has exceeded the
estimate. Extensions require separate loans, which should
be asked for before the work is put in hand.
Notes on the Design of Sewerage Systems (Continued).
Outfalls Their Position, Number, Size, etc., discussed Data Needed by Engineer in Designing New
Systems Manufactory Refuse.
(10) The position of the outfall or outfalls must be
decided upon before the actual course for the sewers of a
district can be properly planned. The lines of the natural
drainage of the land will 'be the best guide, but it
will sometimes be found worth while to follow other
courses for the sewers in order to secure good outfalls.
There are many cases where it has been found
worth while to carry sewers under rivers or through hills
or across valleys or aquoducts, or through syphons.
It is a good principle, however, to keep the main
sewers at a low level. A sewer is sometimes laid
half-way up a hill when it should 'be laid along 'the bottom
of the valley.
A sewer at the bottom of a valley will
drain everything above, whereas if a sewer is constructed,
at a higher level than is necessary it leaves areas 'below
it undrained, and (this may cause trouble in the future
when new houses are built.
(11) The natural watercourses
of a district should not
be covered or converted into sewers. If this is done, the
sizes of the sewers will probably have to be larger than
would otherwise be necessary, the natural channels for
surface water being covered up. Wherever possible, it is,
of course, cheaper and better to leave the natural channels to do their work of surface drainage.
The engineer
will require data
showing approxi-
of sewage to be expected from
various sources in the district or town, and the total
volume of sewage that may be expected at the outfall.
Such data are needed by the engineer before he can prepare a proper scheme, and they must be collected care-
mately the
Calculations as to the amount of sewage to be dealt,
with will be based on the rainfall, water supply, and.
population. The flow of sewage from houses varies in
volume considerably, according to the habits of the
people, while that from manufactories is generally discharged at all hours of the day or night. Provision has
made for this irregular flow of foul sewage, both in
the design of the sewers and in that of the disposal
works. It has been estimated that for a safe allowance
the discharge into a foul sewerage system may be as much
as 70 per cent, of the daily flow in twelve hours and 8
per cent, of the daily flow in one hour. Following this
rule, if a district had a flow of 500,000 gallons a day, the
outfall sewer would have to be of a size to accommodate
40,000 gallons per hour. The flow of sewage in flat districts, or in places where houses are far apart, will not
fluctuate to anything like the same extent as it will in
districts where the houses are close together or the sewers
The foregoing remarks refer to
laid at a steep gradient.
The surface drainage from the backs of
foul sewage.
houses and yards, and from gullies in crowded streets,
has to be carried, as a rule, to the foul sewers for reasons
already mentioned. The amount of the rain water toIt
be expected from these sources can be calculated.
should be remembered, in doing so, that the sewero must
be of a size to take all the water during the greatest
storm or rainfall.
(13) The fact that the discharge from water-closeta
?wage in a system'
does, OT does not, form part of the
to be
rmakes practically no difference in the quality of the
Of course, the drainage from water-closets
always should go to sewers, 'but there is still an ignorant
belief among some people that something is to be gained
by excluding it from the sewage of a town. Analysis has
ehown that there is very little difference in the chemical
-composition of the two kinds of sewage mentioned viz.,
that from water-closets and that from sink gullies, etc.
,That this is so may be the more easily grasped if one
realises what ordinary sewage from gullies or " slop
water" is. To quote from a modern book, "It contains
the liquid excretions of the inhabitants, the foul waters
.from the kitchens, containing vegetable and animal matters, bits of fat and other refuse, the suds from the washing of dirty linen, cooking utensils, and the people themselves, holding in solution and suspension soap, fatty
-acids, and the exudations from the human skin.
soapy slops, as everyone is aware, if allowed to stand for
twenty -four hours, become most foul and offensive. Then
there is the dirty water from the washing of floors, the
twilling of yards, the solid and liquid excretions of
animals in the streets, the drainage from stables and
pig-styes, the blood and other animal matters from
silt from road sweepings, etc." (Barslaughter-houses,
Sewage Purification.") Thus one may readily see
the fallacy of taking trouble, as was formerly done, in
eome obsolete systems, to exclude water-closet drainage
from town sewers with the idea of keeping the sewage
purer and free from disease gerrnis.
(14) Careful note should be taken of any drains
<>r sewers existing in a place which
has to be newly
also the number of houses containing water closetis, baths, or other sanitary
appliances, should be
attention should be given to the volume
from any manufactories,
In the case of breweries it should be the
breweries, etc.
object of the engineer and of the local authorities to
make arrangements that the refuse may be filtered or
otherwise treated before it leaves the works. It may also
be found well to adopt some such course with other
manufacturing refuse. Although it is possible to treat
brewery refuse with the other sewage at the purification
works, it will greatly assist if the brewers will filter it
and strain off the hops before discharging into the sewers.
This method has been adopted with success by the author,
'by using a coke filter near the brewery.
(15) Careful
and character
of the discharges
Position of Outfall Test for Effluent
Sewers Lifting sewage.
(16) Under no circumstances should foul sewage be
taken into a stream or river without careful purification.
It is even questionable whether in these days it should
be taken into the sea in its crude state. Certainly thereis no excuse for taking sewage into the sea at any place
where it will become a nuisance without purification.
Frequently it will be found possible and desirable not,
to conduct foul sewage towards any stream or river at all,
on the contrary, that it should have its
far from
river as possible.
outfall as
In these days it is quite inadmissible
.reason is clear.
that foul sewage should be allowed to run into a river
Prevention is better
always be purified first.
than cure, and therefore the further the works are from
any river the better. If the effluent is allowed to run
into a long ditch or small stream for some distance before
it reaches the river any neglect or mismanagement will
soon be apparent by the fouling of the channel. Witih
proper treatment the effluent should be so pure that the
channel is kept clean. If this ditch or channel is about
half a mile long there could be no better test for the
(17) A gravitation scheme is, no doubt, the simplest
and cheapest where practicable, both as regards first cost
and maintenance but it may often happen that there
are small districts or groups of houses situated at such a
level as to prevent their being connected directly to
the main sewers, or, if connected, it may involve making
the depth of the main sewers very great, so as to add
considerably to their cost. There are several methods
by which a small quantity of sewage may be raised by
automatic apparatus at a comparatively low cost, such
as by a small water motor, or other special machinery,
which can be obtained for the purpose from well-known
manufacturers. Such appliances are at the present time
in successful operation in many places, and will be referred to later on.
Again, such small districts may be
treated separately by means of small local sewage purification works, which can be made to act automatically.
Such works properly constructed need very little attention,
and are quite
In cases where the sewage has to be raised by artimeans, before choosing the outfall it is necessary to
consider how the sewage is to ,be raised. It may some(18)
times be well to consider the advisability of making use
of existing machinery or power, to enter into an arrangement, for instance, to use the steam from the boilers of
an existing water works, or the current from an electric
power-station, etc., by which means much expense may be
saved both in erection of machinery and in running it.
(19) That nothing is impossible in engineering is a
maxim which should be kept in mind with regard to
sewage works, and especially with regard to obtaining a
Without doubt the fewer outfalls the
proper outfall.
A mulbetter, and if possible there should be only one.
tiplication of such places, especially when sewage has to
be purified, will be likely not only to add considerably to
the cost of construction, Jbut yet more seriously to that of
The cost of the maintenance of outfall
works is sometimes a very much more important point
than the cost of construction.
(20) If possible a spot should be
chosen for the outfall
whicn the whole of the sewage may gravitate. The
outf al'l to a large town should be kept as far from houses
as possible, but there should be no necessity to carry it
to a great distance in putting in a new system.
It is
possible if the outfall works are properly designed, constructed, and attended, that there should be no noticeOf course, where crude
able smell outside their area.
sewage is discharged in great bulk it must smell at tiht
mouth of the sewer, however perfect the system. In the
case of a small town or village it is actually an advantage not to have the disposal works too far away or out
of sight.
They should be in a place where they will be
under continual public observation, and then if any
smell should arise its cause will be investigated and the
matter set right, and there will be no chance of neglect,
which has often been the only cause of the failure of
works. It car not be too clearly understood that sewage
disposal works of proper design do not smell unless they
are neglected.
The best way of proceeding is first to select a site for
the outfall, and then to see how the sewage can be conducted to the place. As already stated, it does not always
follow that sewers are best laid along the natural fall of
the ground. Great saving in expense may sometimes 'be
effected by crossing a valley or river, or by tunnelling
through a hill, or Toy carrying a sewer for some distance
falling the reverse
ing the sewer as a
to the ground level, or by carrysyphon under a river, etc.
A really good site for an outfall that will enable an
economical scheme to be carried out should not be abandoned because a small section of the district or a particular building or grouip of buildings cannot be drained
to it by gravitation.
It will be found in such cases best
either to resort to lifting the sewage of the small district
or to make a second outfall for it ebewhere.
(21) In the case of an absolutely flat district of large
extent, where a gravitation outfall is impossible, it will
generally be found bes*t to divide it into sections and let
'the .sewers of each section gravitate to a separate lifting
At each station the sewage can be lifted from
the level of the deepest sewer to the highest point of the
next section, or to a separate intercepting sewer running
to the main outfall. The number of these lifting stations
will be a matter for careful judgment.
In the one case it
onust be remembered that the greater the number of lifting stations the greater will be the cost for construction
<and maintenance, and in the other case that the fewer
(number of such stations the more sewage will have to be
Jifted at each, and the deeper will be the sewers, meaning increase cost for their construction and maintenance.
Probably in most cases it will be found that the fewer
*he lifting stations the better.
No sewage .should be allowed to run to a lifting station
if it can reach a convenient outfall by
oarefully arranged intercepting sewers and by excluding
the rainfall as much as possible the lifting may often
be reduced to a minim/um.
(22) It will be well to avoid placing the outfall and
the 'sewage disposal works near to gas works or other
With regard to sea
places producing noticeable smells.
outfalls, or outfalls near tidal estuaries, it will be well
wherever possible to avoid places where large flats of
smelling mud exist at low water, or where seaweed accuThere are many places where a strong smell
exists from natural or artificial causes, and the engineer
wiho constructs an outfall sewer near them will for ever
after be held responsible for the smell, notwithstanding
the fact that his work may have nothing to do with it.
The engineer would also do well to have samples of the
river water analysed, whenever it is proposed to discharge effluent or storm water into a river, in order to be
able to compare them with any analysis made after the
works are constructed. It is possible that the new works
be regarded as causing a contamination or chemical
in the river w.ater which is due to other and older
causes. At some places the sewage effluent will be found
to be considerably purer than the river into which it is
Notes on the Design of Sewerage Systems (Continued).
Systems of Sewerage Intercepting Sewers Preliminary Survey Best Courses for Sewers.
(23) Although there are in theory several systems for
town sewerage, practically speaking there are only two
/(I) the combined system, (2) the separate system.
In the combined system the surface water and the foul
sewage are carried in one sewer. In the separate system
they are carried in separate sewers.
With regard to the majority of cases it may even be
said that there is only one system, viz., the partially comTnned or partially separate system, for it is obvious that
the cases in which all rain-water can be absolutely excluded from the sewers must be very rare, if not im;possible.
(24) It will be 'Seen at a glance that it would be wrong
to adhere rigorously to any particular system for all cases,
or even for any particular case. The best system will
probably always be a modification of the two already men-
At the same time, speaking broadly, it is generally admitted that the separate system is not only the
most perfect, but the most economical. Though we cannot exclude all the rain water, the ibulk of it may be
kept out.
(25) In the case of separate sewers for clean and foul
water, fewer traps will be needed, and the chances of foul
There will be
gases escaping at wrong places are reduced.
no chance, for instance, of sewer gas escaping from unsealed surface gullies in hot weather if they form part
Another great advantage in
of the clean water system.
the separate system is that the quality of the foul sewage
will be more uniform, and its flow more regular facts
which help greatly in the process of purification, as will
be demonstrated later on.
Also, the amount of sewage
likely to flow through the foul sewers can be ascertained
with such accuracy that
it will not be difficult to construct
of a size suitable to the amount that must flow
If such sewers have to be made of a size
through them
to take the rainfall of the district during a storm, as
may be the case in the combined system, they must obviously be a great deal too large ever to flow sufficiently
full to be properly self-cleansing without very considerable flushing arrangements. It will be seen that if flushing arrangements are needed in the case of a small
separate sewer they will be very small by comparison.
This is an important matter in the saving of expense in
a new scheme.
It also must be remembered that in large
sewers considerable volumes of sewer gas may be accu
mulated. They are also difficult to ventilate efficiently.
The clean water sewers in the separate system will be
altogether a cheaper matter, then, to construct and maintain than foul sewers of the same size ; there should be no
need for flushing arrangements, and the construction need
not be so elaborate as for foul sewers. As already stated,
old sewers and culverts may be utilised for clean water
It may often happen that short sewers with
outfalls into existing watercourses may be constructed
for surface water, so that only a small separate sewer for
foul sewage is needed, where in the combined system a
large sewer for all purposes would have been required,
which would have been expensive to build and to maintain.
(26) It may be argued in favour of the combined system
that the rain water itself would be of use in flushing the
sewers. The mistake of relying upon any such arrangement will at once be seen when it is remembered that
during the dryest weather, when there is the most need for
sewer flushing, there can be no flush.
With regard to the com'bined system, it is sometimes
possible by the use of storm overflows to keep the sewers
of a moderate size.
It is, however, objectionable to allow
even dilute sewage to flow into rivers or the sea, for
though the water in the sewers may be clean rain water,
it may yet carry with it paper and solid matter at any
(27) It is no doubt a disadvantage in the separate system
that foul drains may through carelessness be connected to
the clean water drains, and vice versa. But on the otiier
hand, it may be argued that such mistakes should not
occur where there is intelligent supervision of work.
It would, in a perfect system, be an excellent thing if
each house in a new town had a separate system of drains
for rain water and for foul sewage where the town is
It must be owned at
sewered on the separate system.
once, however, that there is a very serious drawback to the
arrangement viz., that batli and sink wastes would in
time without doubt ibe carried to rain water heads, and
through this and other causes foul sewage would get into
the clean water drains, and it would be very difficult to
The extra cost of two
discover where it came from.
systems of drains would also be a serious outlay to private
(28) Where subsoil drainage is required with the combined system it is customary to lay the pervious open
jointed or perforated pipes by the side of the sewer in the
They are sometimes laid below the level of the
But care must be taken that this does not ultimately cause a disturbance or settlement of the ground
near the new severs or cause the sewers themselves to
(29) Sewers in either system are laid so as to slope to
their outfall, and where this is impossible they are made
to fall to a lifting station, or, if necessary, to several lifting stations, where the sewage is raised so that it may
gravitate to its outfall. There are instances where the
sewage of a town is pumped through miles of main to
reach the disposal works. There are various methods of
lifting sewage, which will be referred to later.
(30) It is often advantageous to divide a town into districts, with a sewer to intercept the sewage of each particular district, and carry it to the outfall.
Thus it may
be possible to do without any very large sewer, to avoid
the possibility of the lower parts of a town becoming
flooded, and to let the sewage from the upper parts gravitate to the outfall, and also to lift only the sewage of th*
district which lies too low for its sewers to descend to the
These seiwers, which take or intercept the sewage
from a particular district, and carry it to the outfall,
are called "intercepting sewers," and, naturally, they
generally run more or less along contour lineB.
Intercepting sewers may often be laid to carry off the
sewage of the greater part of a district to a gravitation
outfall, while the sewage of the lowest parts is raised artificially to the same outfall.
It is often much more economical to divide a town into
a number of districts, each having its own intercepting
sewer, as already described, than to let all the sewers
form one large system. Tn the former case no very large
sewers will be needed.
Large sewers will be found very
expensive to construct, and a long length of sewer of small
size will probably be found cheaper than the sewer of
If the sewage from
larger diameter thereby avoided.
the higher districts does not have to be carried to the
sewers serving the lower districts, there will not be the
same chance of floods in times of heavy rainfall that must
exist where all the sewers are connected.
The use of
intercepting sewers serving the high districts is doubly
advantageous in the case of a tide-locked outfall, as will
be explained later.
Again, in the event of a town extending, a new district can with advantage 'be served by a
separate intercepting sewer when it might be impossible
to make the original outfall sewer take so great a quantity
of sewage, in addition to the flow for which it was designed.
(31) If the country to be sewered is of such a character
it is difficult to judge, firom inspection and from tfhe
ordnance plans of the district, in what directions the
sewers may be most advantageously laid, it will be necesfiiary for the engineer to have contour surveys made of
those parts presenting difficulties as to the disposal of the
sewage. It is necessary that he should have a thorough
grasp of the lay of the land. For instance, a town on the
side of a hill with a good site for a sewage outfall at its
base would probably need very few levels to be taken in
order to ascertain the best position for sewers, it being
clear from inspection that good falls could be easily obtained. On the other hand, if the town were situated on
flat or slightly undulating ground, where a fall would
be difficult to obtain, and wheire a matter of >a foot or two
might mean having to pump the sewage or not, very
careful levels would probably haive to be taken before the
engineer could come to ia Tight decision in planning the
courses of the sewers.
(32) When the courses of the proposed sewers are
chosen, levels should be taken along their lines at every
pronounced change in the surface of the ground, and also
at the inter-sections of roads.
Care should be taken to
ascertain the levels and positions of existing water, gas,
or electric mains, tramrails, etc.
Any watercourse,
such as a stream or ditdh, which ihas to be crossed, should
have the water level and pirofile at the bottom recorded.
(33) It may also be veTy necessary in cases where the
foundations are treacherous to have trial hole borings
made along the courses of sewers, particularly so in the
A bad foundation may cause
caise of the outfall sewer.
-one route to 'be more expensive than another.
(34) It will be found best to lay isewers in public roads
in preference to private property. It is essential that a
certain number of inspection main/holes should be made
along the course of each sewer, and if these are made
on private ground it will be practically impossible to inspect and clean the drains properly when required without .serious inconvenience to the owners of property.
Sewers on private ground will probably be expensive in.
the first instance, owing to claims for compensation.
Local opposition may also be aroused by laying them in>
such positions.
(35) Sewers should be laid in the roads below the levels
of the floors of all basements and cellars, >and it is essential also that they should be constructed at such levelsthat all yards behind houses oacn be drained into them.
For this, and for other reasons easily seen, it is necessary
before designing any new scheme of sewerage that careful
levels should be taken over the district intelligently by a<
competent surveyor.
Notes on Design of Sewerage Systems (continued). Methods of Crossing Rivers, Valleys, etc. Syphons,
Lifting Arrangements, Pumps, the Shone System, the
Adams Lift, the Liernur Improved Pneumatic
System, Electric Pumping, Various Motors.
(36) When a river has to be crossed aind a bridge is
not available, the sewer may be carried under it in the
form of an inverted syphon. Such syphons aire used also
for crossing valleys, or passing under some otherwise impassable obstruction in the street (a thing which should,
They are constructed
however, be avoided if possible).
Castwith a vertical or sloping drop on either side.
iron pipes are generally used for the purpose, and, as
they will always be under pressure >and flowing full,
they should be of circular section. In the case of large
sewers they are sometimes lined. The pipes may be laid
by lowering them into position from the surface from
barges, etc., or it may be found 'better to fix caissons or
coffer dams in the river bed, and, having pumped the
water out, proceed to construct the sewers at the bottom
Tihe syphon must be weighted and held
of the river.
down to prevent its floating when empty. It should have
foundations sufficient to protect it from undermining
currents. One method of making such foundations is to
drive two rows of sheet piling .across the river, and, having excavated the material between them, to fill concrete
It is well, if possible, to make the syphons in duplicate, so that, in the event of one becoming stopped up,
the sewage can be diverted into the other, and the
stopped-up syphon can be pumped and cleaned out. "M anholes should be provided on both sides of the river, so
arranged that ,any stoppage can be removed from the
syphon. Generally there is <a catoh pit at the bottom of
one or both of these manholes to catch grit. Also there
should be penstock chambers at each end of the syphon,
so that the flow of sewage may be stopped while it is
being cleaned. Thus the cleaning manholes descend to
the level of the lowest part of the syphon, and the penstock chambers -are level with tlhe sewer at the top of the
syphon. The descending <arm should be ventilated at the
bend to prevent an air lock.
(37) Syphons should be constructed of such a diameter
that the flow of sewage through them may have the same
velocity as it should have in sewers of equal size.
difference in level of the two ends should b.> equal to the
fall required for an ordinary sewer, flowing full, of the
same diameter and of the same length as the syphon
i.e., the exact length of the pipes forming the syphon,
not the horizontal distance between its ends. The required velocity of flow may be obtained by reducing the
diameter of the pipes if the fall available is insufficient
to produce it.
As a rule, sewers run about half-full, but
the syphon always runs full bore.
If the size and the
hydraulic gradient of both are the same it will be clear
that the velocity of the liquid passing through the
syphon will be less than that in the serwer, and if the
velocity of the flow is reduced matter held in suspension
will have time to be deposited.
It is a good thing when
constructing a small single syphon to insert a loose chain
or wire which can be moved backwards or forwards from
either end of the syphon to clear stoppages when they
Sewers may also be laid under a river by tunnelling, or carried across a valley by an aqueduct, or on an
embankment, or trussed between piers.
(38) With regard to lifting arrangements, pumps of
An ordinary plunger or lift
several kinds are used.
pump may be used, or a centrifugal p-uniD, and for small
installations where plenty of power is available a chain
pump may be used with advantage.
pump is very suitable for sewage lifting, and
likely to
be more economical for low lifts than other kinds. The
wear from grit is not so harmful to centrifugal pumps as
to piston pumps, and they can be driven direct from
electric motors or engines without gearing.
of lifting stations are needed,
(39) Where a number
Rhone's compressed air system is used in a great many
In this system the sewage gravitates into a replaces.
in filling the receiver it opens a valve which
The compressed air forces all
the contents of the receiver into a sewage rising main or
The compressed air for
high-level gravitating sewer.
actuating this ejector is produced at a central station
.and carried in iron pipes to the several lifting stations.
has to be
(40) Where the sewage from a low district
raised while that from higher ground above it gravitates
to an outfall, it is possible to use the energy of this descending liquid to lift the sewage from the low-lying disThis may be done with Adams's sewage lift, in
which the drainage from the lower district runs to a
receiver, and is raised to level of the gravitating highlevel sewer by air compressed by the fall of the sewage
from the high district. Some of the high-level sewage
runs to a tank which, when full, discharges its contents
automatically into a down pipe running to an air comIn this manner the air is compressed
pression chamber.
and the sewage in the low-level sewage receiver is expelled and lifted to a higher level by the entrance of air
from the compressed air chamber, to which it is connected by a pipe. With this lift, water is more generally
In places
used for compressing the air than sewage.
where the water supply is abundant it may be used for
lifting the sewage of flat districts with advantage, the
lifts being entirely automatic.
(See also Addendum, p. 148.)
(41) The Liernur improved pneumatic system is also
well deserving of attention.
As in the case of the Shone
System, the town to be sewered is divided up into a
number of districts or sections. Each section is sewered
upon the separate system by means of cast iron pipes of
small diameter. These pipes are made to gravitate 10
an underground retainer called a district cvlinder, or if
gravitation is impossible they are laid flat, or in some
cases even uphill.
Sewage is made to flow through these
pipes at intervals by the creation of a vacuum in the district cylinder.
The flow thus induced is very rapid, so
that the pipes are well flushed out at every discharge.
The district cylinders are each connected to a reservoir
at a central station, to which the sewage flows when the
admits compressed
air is exhausted.
The whole is worked by means cf a.
at the central station.
With a carefully
designed system, in which full advantage is taken cf tbe^
natural fall available and an artificial flow or acceleration is induced by means of the vacuum pump only to
vacuum pump
the extent that is absolutely necessary, it is probable that
a very great saving could be effected by the use of this
The Liernur System has been in operation at Trouvillesur-Mer for over ten years, and in this country it has
bey<i working for some years at Stansted, in Essex.
(42) It is also possible to divide the town and district
into sections, as in the Shone system, and to instal electric motors and pumps instead of ejectors in each section to raise the sewage.
At the first glance one might
reasonably suppose that such a system would be more
economical than one worked by means of compressed
air, but it must be admitted that hitherto results have
been in favour of the ejector. This is due to several
reasons, among them being (1) The great difficulty of
keeping electrical plant in proper condition in damp
situations, and the consequent heavy maintenance expenses both for skilled labour and for renewals. (2) The
greater wear and tear of the working parts of electrically
driven pumps as compared with the ejector, in which
the moving parts are reduced to a minimum.
each case must be judged on its own merits, and the
system of lifting sewage in this manner from several
stations by electric motor pumps may have special advantages under certain conditions.
43. Steam, gas, produce gas, and oil engines, electric
motors, or turbines may be used advantageously for
driving sewage pumps, also windmills. It will always
be necessary in the case of windmills to provide for the
time when the wind fails. Also water raised to a small
reservoir by a hydraulic ram or other means may be
used to drive a motor or work a lift.
The Arrangement
Design of Sewerage Systems
Flushing Levels Gradients
Principles of
Positions of Sewers
Velocity of Flow in
After, and not until, the site for the outfall has
been chosen, the courses for the main sewers and their
branches can be decided upon. They should be laid
out on a plan of the district, and for this purpose,
if necessary, a contour plan should be
prepared. The
6 in. ordnance maps give contour lines at every 50 ft.,
besides spot levels
the 25 in. ordnance maps also have
The engineer should supplemany useful spot levels.
ment these by levels taken along the valleys and elsewhere before he fixes on the courses of his sewers.
45. To give a few general principles, all sewers should
be laid along the shortest and most direct courses. Long
branches or mains should, if possible, be avoided.
Sewers should be carried near to all houses or other
places in the district needing drainage. They should
be laid in such positions that the drains from houses,
etc., discharging into them may not be of greater length
than can be helped, and that the house connections may
have a fall of, say, about 1 in 40. Dead ends in sewers
should be avoided wherever possible. Sewers laid with
dead ends and those with flat gradients will not keep
clean without flushing, and may therefore be expensive
to maintain in an efficient condition.
If they are not
flushed they may become blocked up, and certainly will
smell badly. It is best to fix upon the positions of the road
gullies before laying down the lines of storm water or combined sewers. They are generally fixed not more than 200 ft.
apart in busy streets, and not more than 600 ft. apart in
suburban streets. They often have to be fixed much
closer together.
Sewers should be laid in perfectly
straight lines between the inspection "manholes." These
places for inspection should occur at every junction,
change of direction, or change of gradient, and also at
stated intervals along straight lengths.
On sewers of
6 in. or 9 in. diameter the manholes should not be more
than 300 ft. apart. On larger sewers the distance may be
" is
a little greater. What is called a
times used instead of a manhole on straight lengths and
at points where a change of gradient occurs.
The use
of manholes and lampholes is for inspecting and cleaning
the sewers. They will be described in detail later on.
46. The sizes and diameters of the various sewers and
the gradients to which they are laid must be, if possible,
such that they will be self-cleansing. Wherever this is
not the case flushing arrangements must be provided, aa
it is obviously wrong to lay sewers in which solid matter
will remain and cause the emanation of foul gases and
the ultimate blocking up of the sewer. The gradients
of the sewers must be sufficient to Jgoduce such a velocity
no deposit may remain behind. It has
been found that this velocity should be about 3 ft. per
second when the sewer is flowing full or half-full. Neither
is it sufficient to lay sewers at gradients which will give
this velocity unless the volume of the sewage flowing
through them is sufficient to make the sewer run at least
If the flow is insufficient to do this the velocity
will be reduced, and solid matters will be deposited.
can thus be seen that it is almost as bad to have the
diameter of a sewer too large as to have it too small, and
that care must be taken to make sewers of the right size.
They should be two-thirds full when taking their greatest
in their flow that
flow, including rainfall.
47. As a safe general statement, it may be said that
in sewers of 6 in. or 9 in. diameter the velocity should
be about 3 ft. per second, in larger sewers up to about
24 in. diameter 2 ft. 6 in. per second, and in sewers above
This is the mean
this size not less than 2 ft. per second.
velocity when the pipes are running full or half-full.
The velocity at the invert will be about 25 per cent. less.
These are statements of generally accepted facts, the
result of experience and experiment.
Thus, speaking
broadly, circular sewers may be laid to the following
gradients, at which, if they are flowing full or half-full,
self -cleansing. Unfortunately it is frequently
impossible to lay sewers at such good gradients flatter
gradients are in general use, the minimum falls being
1 in 200 for 6-in. sewers, 1 in 300 for 9-in. sowers, and
but they should not be
1 in 400 for 12-in., etc.
taken as standard by the student, though the experienced engineer may use them if compelled by the flatness
of the ground, provided that proper flushing can be
they will be
Diameter of Sewer
in Inches.
These figures give a slightly slower velocity than that
some formulae,
viz., 2.75 ft. per
diameter, and 2.3 ft. per
they are merely meant as a
rough guide. There are many excellent tables published
giving the velocity of flow in sewers of various sections
to which reference can be made, and which it is not the
author's present purpose to reproduce or add to.
practical figures the above may be of use in dealing with
some oases where highly theoretical calculations are not,
from the circumstances of the case, required.
48. As the depth of the liquid in a sewer diminishes
below half the full bore, its velocity decreases. It may
often happen that when the flow is least the sewage is
second for sewers up to
second for larger sewers
9- in.
and most likely to leave a deposit -behind.
possible, therefore, the sewers in the upper portions of
a district should have a greater slope, comparatively,
than the larger sewers below. That is to say, they should
be so .arranged that the velocity of the flow in them may
be greatter than that required to make them self -cleansing, when properly charged, as a safeguard against the
possibility of the flow of sewiage through them being fre-
quently very small.
49. Very steep gradients, however, should be avoided,
because the flow in them is generally very shallow, owing
to the rapidity of the current due to their steep fall, and
this insufficient depth of water causes solid matters to
get left behind and form a deposit in very steep sewers.
After the flow of 5 ft. per second has been reached, it is
well, if possible, not to increase the gradient.
gradient for a 6-in. sewer would be 1 in 43, for an 8-in.
1 in 57, for a 9-in. 1 in 65, for a, 10-in. 1 in 72, for a
12-in. 1 in 86, and so on. These figures are only intended
to give ,a rough idea of the steepest falls at which sewers
will work wel]
50. Sewers which receive domestic sewage only are less
likely to retain deposit than those which take storm
The heavy grit, etc., from road gullies needs a
greater velocity to carry
the house sewage.
along than that required by
The Arrangement of Sewers. Minimum Size for Sewers
Different Forms Changes of Direction Water for
Flushing Purposes Branches Connections.
It has been found by experience that it is unwise
to make a public sewer less ihan 6 in. in diameter
authorities say 9 in.
to prevent the likelihood of stoppages occurring frequently.
52. Sewers which will always flow at least half-full
should be circular, as that is the most economical form,
and gives the greatest velocity when flowing
full or halfCircular pipes should be used for all sewers up to18-in. diameter, or even up to 24-in. diameter, according
to circumstances.
Egg-shaped pipes would be as good or
better than those of circular section were it not for the
fact that no satisfactory egg-shaped pipe is to be got.
Stoneware egg-shaped pipes are untrue and difficult to
After 24-in. diameter, brick, stone, or concrete
sewers must be constructed. In the case of large sewers,
where the current is variable, an egg-shaped section is
better than a circular section, because it will give a better
rate of flow for shallow currents, the velocity being proportional to the square root of section when divided by
the wetted perimeter.
53. With regard to changes in the direction in the line
of a sewer, it is well to have no turn sharper than a right
For sewers larger than 12-in. diameter it is better
to make two manholes near together, with a flat curve
in each, and a length of straight sewer between them,
than to make an awkward sharp turn in one small manIf a sewer is of such a size that it can be entered
for cleaning it can be constructed on the curve where
necessary, as, obviously, a sewer of, say, 6-ft. diameter
could not change its direction with a very sharp curve
at one small manhole.
54. In laying a separate system of sewers, the storm
water drains are generally laid at the roadside, while
the house drainage sewer is laid at a deeper level in the
Care must be taken that the storm
centre of the road.
water sewers do not prevent the connecting drains from
the houses having a proper fall into their sewer.
55. At the top end of each branch the discharge into it
will probably be quite insufficient to give the proper rate
It will very likely be too large for its work even
of flow.
It will
if laid in 6-in. pipe, and so will need flushing.
be necessary to calculate also how soon the volume of
sewage reaching this sewer will be great enough to need
Such enlargements must take
the pipe to be enlarged.
place at manholes only.
56. When for any reason it becomes absolutely necessary to lay a sewer on a curve, instead of letting the-
curve occur at a manhole, a manhole should be built
at each end of the springing of the curve.
57. The position of existing streams, springs, or other
sources of water, natural or artificial, useful for flushing
/purposes, should be taken into consideration when planning the position of a dead end or flat gradient sewer
needing flushing. Much expense in maintenance
very likely be saved by doing so.
58. When the positions of the sewers have been plotted
-on the plan, careful sections should be made of the proThe levels
files of the streets down which the sewers run.
of the lowest cellars, basements, areas, etc., should be
shown by a dotted line on the sections. The line of the
sewers should be then plotted on. There should be a
of about 3 ft. of earth over the tops of
the sewers, which should be below the level of any gas,
water, OT electric mains. The maximum depth fox a cutting must depend entirely upon the character of the
ground. A cutting, say, 16 ft. deep is generally about
as deep as it will be found convenient to make in an
ordinary street. However, there are many examples of
sewers laid in much deeper open cuttings. It will often
be well to tunnel for deep sewers.
minimum depth
59. Where a branch joins a main sewer its centre line
should never be below that of the sewer into which it discharges. All drains, whether storm water, sewage, or
sub-soil, should be kept well below the level of cellars.
60. It is absolutely essential that sewers in a town
-should be watertight. They can be made so, and, although
it is by no means an easy matter to ensure them, they
are to be had if the work receives proper supervision.
They should be capable of standing a carefully made
water test, which should always be applied before the
trench is filled in. The lower end of the sewer should
be plugged, and the sewer itself filled with water.
engineer will probably have to overcome very considerable opposition in order to get the sewers quite watertight, but there can be no doubt as to the absolute need
that they should be so. Obviously it is wrong to allow
sewage to saturate the ground and find its way under the
houses along the road where sewers are situated, or to
permit land water to enter the system.
House connections should always join the sewers
wherever possible
however, ithey have
at points in between they should join by
junctions in the case of small
of large sewers.
Junction pipes may be built into a pipe .sewer if the
connection to a house is to be laid forthwith, otherwise
it is best not to put in any provisional junction piece,
which will be probably forgotten or lost. House connections are generally made after the sewers are laid, and;
in that case, if they cannot 'be made at a manhole, it
is best to use what is generally called 'a
saddle junction." For this a hole has to be carefully cut in the pipe
sewer of the size of the inlet with a cold-chisel and
hammer. The saddle junction is a stoneware pipe with
a saddle, whidh fits against the main pipe, so that a
sound cement joint can be made. There is no great
difficulty or risk of cracking the pipe if the work is
properly done. The advantage is that the sewer remains
undisturbed. The practice of inserting a Y pipe and
disturbing the sewer is not to be recommended. Saddle
junctions similar to those described are regularly used'
for making private connections to sewers in some of our
most important towns, and have been used by the author
in his ordinary practice.
and by T junctions in the case
Where for any reason it is absolutely necessary toleave a junction pipe in the sewer not connected with
a house drain, the socket of the junction pipe should be
carefully sealed off with a piece of slate fastened in.
place with a fillet of cement. This can easily be cut
out when the connection is made later on.
house connections join a brick sewer, what
This is a
is generally used.
stoneware block for building into the sewer wall, having
a hole through it of the diameter of the house drain and
a socket to receive the last pipe of that drain.
called a
"drain block"
Iron pipes should be used for sewers wherever they are
under pressure or where the foundations are
untrustworthy. They should be coated to prevent rust,
and may, in the case of a large sewer, be lined.
likely to be
If a long length of sewer is laid at a great depth it
be well to make one connection to it from
an auxiliary branch, laid >at a higher level, to which the
house drains may be connected to save the trouble of
making a number of very deep and difficult trenches.
may sometimes
Wherever it is possible to do 'So, house connections
should run to manholes. Hidden junctions of any kindshould always be avoided as far as possible.
The Construction of Sewers. Details of Construction
ateri als Forms.
63. To pass on to the actual construction of sewers.
In the first place it is essential that all sewers should
have good foundations. They should be so constructed
that 'there is no chance of a settlement, for, as will
readily be understood, the least settlement will be likely
The sewers must be strong enough to
to produce a leak.
support any weight which may come upon them front
above, which, in the case of a shallow sewer, may mean
the full impact due to a heavily-loaded wheel of a traction engine dropping several inches into a hole in the
road surface; or, in the case of a very deep sewer of
large diameter, may be due to a live load plus the great
weight of the earth above the sewer, and may be very
considerable. The necessity for a good foundation exists
with regard to small pipe sewers quite as much as for
the larger kinds. A dip in a sewer is most undesirable.
64. In constructing sewers their centre lines should be
pegged out on the ground, and the levels for their gradients set up in the following way.
Horizontal sighting rails should be set up ,at each end
of a trench at such levels that the line of sight between
the two rails may be parallel to the gradient of the
sewer. Thus, if a sewer had to >be laid at ia fall of 1 in
500, and its trench were 250 ft. long, the sight rail at
the top end would have to be fixed at a level six inches
higher than the sight rail at the bottom end of the
same trench, giving .a fall of 1 in 500 to the line of
Slight 'between the two rails. By standing at either end
of the trench it will be easy to see whether the top of a
rod, cut to the proper length, and held on the bottom of
the trench at any point, or on the pipe itself, is projecting above or comes below this line of sight. In this,
way the bottom of the trench can be formed, the concrete foundation laid, and the sewer itself put in to a
perfect gradient. This is the simplest and surest way
of laying pipes at proper gradients.
The levels thus set up must be frequently checked. In
such positions the rails may easily sink or get shifted.
65. Pipe, sewers, even of the smallest sizes, should
always be laid on concrete. If the earth is soft they need
support, and if the ground is hard rock they need
bedding on to it. The pipes must not be laid so as to
on their sockets only. If a sewer passes near trees
the concrete will be needed as an additional protection
against ithe roots, which, if they once find an entrance,
will soon fill any pipe.
66. It may not be out of place to describe one method
of laying pipe sewers. There may .be variations to the
method described which would not alter the soundness of
the work. The following is a description of the general
practice in laying pipe sewers in the author's experience.
The trench is excavated carefully to 'the exact gradient
of the sewer, and, if the ground is firm, a layer of at
least 3 ins. of concrete is put in also to the exact graIf the ground is treacherous, stronger foundadient.
tions must be constructed as required. Bricks should
then be laid on this concrete (one brick laid flat or on
edge coming behind each socket) to raise the pipes to a
height at which a man can easily reach the tinder -side of
each joint. It is very important that there should be
plenty of room for the pipe jointer to make the underside of each joint. The pipes should be laid with their
sockets uphill, beginning at the lower end. They can be
supported (temporarily with small props of concrete to
prevent rolling. The first pipe should be bedded and
tested for position and level, then the next pipe should
be put into position and tested in the same way, and a
joint of neat cement made. Any cement projecting on
the inside of the joint should be carefully removed with
a ecrapeT or mop. One good method for doing this is to
have a disc of heavy indiarubber, slightly larger than
the sewer, bolted between two smaller wooden discs ; it
can be drawn through each pipe as a joint is made. It
is well to have two such discs, one following behind the
other. The process, as described above, is repeated for
eadi pipe ias it is laid. The cement should be mixed, not
too wet, in small quantities, and none should be used
which has once begun to set. The very greatest care
must be taken not to disturb or shake any joint after it
is once made. The slightest movement will mean a crack
in the cement. Each joint should be finished off with a
trowel to a smooth face, the joint should be the same all
round, with no projecting piece of cement on the underside and no roughness. It is at .such places that leakages
occur. The joints should be protected from the sun or
frost while setting. After the joints have set for three
days the sewer should be plugged at its lower end and
Each joint should be carefully exfilled with water.
amined, iand the level of the water at the top end
It is as well to make sure that
does remain constant, because it sometimes happens
that a pipe leaks through a crack on the under-side, and
the water does not show at once in the trench. Defective
joints should be made good by cutting >away the cement
as far as possible and re-making 'the joint. If a broken
pipe is discovered it must be, of course, taken out; to
do this it will be necessary to remove at least three pipes.
Joints should be quite sound, and should not sweat.
Such joints can be easily made by any man used to the
work, and those interested in sewer laying should not
be satisfied till they get them.
should remain constant.
67. The cement used for joints should be exposed to the
air in a 6-in. layer on the floor of *a dry shed for a week
or so before being laid. If this is done there will be no
If it is not
difficulty in making the neat cement joints.
done, the cement will be apt to crack, or otherwise to
give trouble by bursting the pipe sockets. When a length
of sewer has been laid it should be possible to see through
it from end to end, and to see the full circular section
throughout. Sometimes as many ias three pipes may be
jointed in cement outside the trench, and after they have
set for a week or more may be lowered into place. This
is perhaps rather too risky a method to recommend, but
it has been done with success. When the sewer has been
tested and found to be quite watertight, concrete should
be filled in round the pipes up to the middle all along
the trench. This should under no circumstances be done
Great care should be
till the work has been passed.
taken in filling the trench with earth that no damage is
done to the sewer.
68. There are very many kinds of joints used for stoneware pipes, some of which will be mentioned in detail
They may be described as the ready-made composition joint, the same with a cement joint added, and
the bitumen joint. Asphalt joints are used extensively
in Germany and elsewhere, and are generally well spoken
The ready-made bitumenous joint may sometimes be
used with advantage, but there can be no doubt th'aft
when it has once been made there is nothing to beat
the cement joint for stoneware pipes. Composition joints
are apt to perish in time.
The author has used socket joints into which hot
bitumen was poured, and butt-jointed stoneware pipes
having a joint made with a steel clip and hot bitumen
with good results in wet ground.
Some people like to mix sand with the cement for pipe
joints. If this is done at all, the sand should be added in
a very small proportion
say one part of sand to three
parts of cement. If the sand used for joints is in too
large a proportion or too coarse, the joints will be
porous. Tarred yarn is sometimes caulked into a socket
before making a cement joint. It serves the purpose of
keeping the pipes concentric and of preventing cement
from getting into the sewer. There is, however, the
cnance of an end of yarn projecting inside, which is more
difficult to geit rid of than cement. Such ends, when they
occur, have to be burnt or cut off. In very wet positions
a cement joint may sometimes be made by wrapping a
piece of light canvas or muslin tightly round the joint
it is setting.
will be
found convenient to use stoneware pipes
made in 3-ft. lengths instead of
number of joints is then reduced.
lengths as the
Construction of Sewers (continued). Iron Pipe
Sewers Patent Joints Danger of Water in Trenches
Ferro Concrete
Foundations Larger Sewers
Elliptical Sewers and their Gradients Inverts
Special Joints for Pipes.
iron pipes are used they generally have
and sockets with caulked blue lead and yarn
joints, or else turned and bored joints are used, which
consist of a close-fitting spigot and socket with machined
faces these faces are smeared with red lead and the pipe
driven into the socket either with a malleit or, in the
case of large pipes, by using one pipe to drive the other
home, a wood buffer being used between the two.
more expensive flange joint i s also used under exceptional
circumstances. With regard to yarn in joints, it has been
pointed out that in the event of a serious epidemic,
disease germs might find a home in the yarn after the rest
of the sewer has been cleansed. A more serious objection,
perhaps, is that in time the yarn will probably decay and
leave a space to be filled with foul matter. As it is quit
easy to do without the yarn by substituting a cold lead
packing before pouring in the hot lead in the case ol
iron pipes, and by leaving the yarn out in stoneware
pipe joints it is best not to use yarn in new work.
Concrete pipes for large sizes are made and used with
success; the joints are made with cement, or with a
steel clip .and bitumen,
Ferro concrete pipes are also
made ; these and Ferro concrete sewers generally are
largely used in America and elsewhere.
Pipes may be lowered into a trench conveniently
rope and hook. The rope is passed through tine
pipe and the hook is caught into the rope so as to hold
the pipe in a horizontal position.
71. It is desirable, for several reasons, that water
should be kept out of the trenches during the construction of sewers. This is effected by means of subsoil drains
or by making temporary inlets so that the new sewer
may carry off any water accumulating in the trench
(these holes are afterwards sitopped up), or by pumping
from sumps made to take the water, or by cutting the
trench through to same point where the water may find
a natural outlet. In some cases turned and bored iron
pipes have been laid actually in the water.
72. Care must be taken that trenches do not fill with
rain water during a storm or through the land wiater
If a trench containing a pipe sewer
rising in them.
fills with water, the pipes, being water-tight and full of
air, will float, and so break their joints. Whenever water
mains cross a trench they should be very carefully supported with chains or struts, not with ropes. If a waiter
main should break over a sewer trench, the greatest
harm will probably be caused.
If the ground upon which sewers are constructed
very unstable, more extensive foundations must be
provided. The width of the concrete bedding can be increased, and sometimes, for pipe sewers, longitudinal
planks or longitudinal bearers supporting cross boards
are laid to take the concrete.
74. With regard to large sewers, whether of brick or
concrete, it may be readily understood that <a circular
or egg-shaped sewer needs a solid foundation, with a
In ordinary firm ground ia plain concrete bedlevel base.
ding may be made of a width generally greater than that
of the sewer. If the ground is very unstable, this width
can be increased. If the ground is marshy, or of running;
sand, it is customary to lay planks or hurdles on the top
of heavy limbers, placed longitudinally, so as to break
If necessary, a
joint, as a foundation for the concrete.
close ^boarded raft is made, and it is a generally accepted
fact that, with sufficient width and depth given to the
foundations so put in, a heavy sewer may be carried over
Pile foundations are also frequently
the worst ground.
used for large sewers.
Another method is to construct
concrete piers, carried down to a firm stratum, and having bridged between them, to build the sewer on this
constructing a brick sewer it is often
is called a cradle upon which
to build it.
The cradle is generally made of timber,
sometimes with wrought iron ribs.
The cradle is made
of the size and shape of the outside lower part of the
found useful to have what
75. In the matter of timbering the sides of trenches it
will be found cheapest in the end to err on the safe side,
and to take no great risks.
Large sewers are constructed of brick, concrete,
and ferro-concrete, even large timber sewers
made on the barrel principle are sometimes constructed
abroad, lal&o sowers have been cut in the solid rock and
used without lining in this country.
They are of various
The circular form is used for a large constant
flow, and the egg-shaped sewer for a variable flow.
very large sewers it is sometimes necessary to make the
shape altogether different, the width may have to be
greatfcer than the (height, because the depth is limited,
owing to there being very little fall in the sewer. Sewers
with vertical sides and flat roofs are sometimes constructed of necessity for storm water for this reason.
There is no reason why the roof of a sewer should not be
made flat in many cases where arches are used, and where
the construction of a steel and concrete roof would be
cheaper, and would allow a better fall to be obtained in
the sewer.
For this purpose sewers of an egg-shaped
section, but without 'the top semi-circular arch, are some76.
stone, iron,
times used.
Another form worthy
substituted for the arch.
of notice is that in which
there is a footway above the invert level on one side of an
The object of this is to
otherwise egg-shaped sewer.
make the inspection work easier.
Small brick sewers are constructed in cement mortar,
and rely upon their jointing, and on the imperviousness
of the bricks themselves, for keeping watertight.
they are constructed of two ring work or more, a lin. space
:611ed with cement can be made all round between the
collar joint," and is for tlhe purrings. This is called a
pose of making them quite water-tight. As it is well
known how difficult, or even impossible, it is to miake
brickwork watertight, owing to the fact that bricks themselves are likely to be porous, it is clear that sewers
which are for foul sewage must be constructed with some
such provision for making them watertight.
stoneware pipes are made up to very large
be found well to use them for large
sewers if they are regarded as a lining only and enclosed in a protecting mass of concrete.
Large iron
sewers .are generally lined. A good example of a modern
sewer was one laid in Edinburgh, near Leith Docks. It
wias eggnshaped, and its dimensions were 9 ft. 3 ins. by
7 ft. 2 ins., and it was made in segments. The lower
part of it was lined with concrete, the flanges of the
joints being inside. The upper part of it had the flanges
on the outside, and was not lined. An iron sewer such as
the one described has many advantages it cam be laid
with rapidity through the worst ground, and will be
quite watertight and of great strength.
78. Circular
-and it
79. Elliptical sewers are generally made of one or two
sections, for which the ordinary tables of velocity of flow
in sewers have been calculated. These sections are called
" -and
" new form." The
respectively the "old form
new form " is of smaller diainvert of the sewer of
meter than that of the " old form." The velocity of flow
in such sewers should not be less than 2 ft. per second.
will be safe for sewers of either
The following gradients
Size of Sewer.
2 ft.
2 ft.
2 ft.
by 1
3 in. by
6 in. by
9 in. by
by 2 ft".
4 in
1 ft. 6 in. ...
1 ft. 8 in. ...
1 ft. 10 in....
1 in 540\
1,, 640 (Velocity of
1 ,, 750 -flow about
850i'2'25 ft. per
80. It is impossible in a short article to deal at all
fully with the subject of large sewers. One point needing
careful attention is that of the invert of a sewer. It has
been found that the scour will very often wear through
ordinary brickwork, and for this reason inverts in sewers
are generally constructed of some hard material, such as
Staffordshire blue brick, granite, or vitrified stoneware.
The glazed abonewtarre invert is aipt to be slippery to walk
upon, and some difficulty has been experienced in keep-
ing the joints watertight. A simple method of forming
a good invert is to use a half-channel pipe bedded on
Where sewers have
to be laid at very steep gradients
that they should be of iron, to withstand the
SCOUT, and to resist the great pressure which mray at
times come upon them.
is welil
Access to Sewers for Inspection and Cleaning. Junctions
Manholes Various Forms and Details of Construction.
should be arranged in such a way that the
whole system may be clear and easy to understand. The
81. 'Sewers
complicated it is the better. The object of the engineer should be to construct straight lengths of sewer
between inspection manholes, so that access may be possible to every part by means of rods or other cleaning
There should be no sewer junctions except at
manholes. Wherever possible, the drains from houses,
surface watei* gullies, etc., should be connected to manholes in preference to the sewer. No doubt a hidden
junction of any kind is an unsatisfactory thing, but
unfortunately in the case of house connections they cannot be avoided very often without considerable expense,
and it is the general practice at the present time to take
house drains into the sewers by junctions made in the
sewer itself, except where a manhole is close at hand.
It must be seen that it is very unsatisfactory to have to
disturb the main sewer every time a new house has to be
drained, and in designing a new scheme, an effort should
be made to prevent this where possible. The practice
of laying new sewers with junction pipes provided, for
connections which are not going to be laid at once, is not
to be recommended for two 'reasons viz., as soon ais the
sewer is finished and the trench refilled, the position
of the provisional junction will, without doubt, be
and very soon its existence will be forgotten,
and, even if it' is remembered, it is not possible to make
an extensive excavation whenever a new house connection
made, in order to find the junction provided.
dead junctions along a sewer will be apt to retain
filth, and cause the sewers to smell badly, however well
they may be flushed or cleaned. It is as well to state,
before leaving the subject of drain junctions with sewers,
that, in the case of pipe sewers, the Y junction pipe
junction pipe
should be set to rise sharply
from the sewer, and in the
case of brick sewers the
junction should be kept
above the ordinary water
level in the sewer.
It is necessary that
the whole sewerage system
should be under perfect control for cleaning or testing.
It should be so designed that
the engineer can at any time
tell without trouble whether
any part is quite clean or
sound. For this reason inspection manholes have to be
the flow
likely to be
places where
the sewage is
changes of direction, and
places where sewer junctions occur.
Special open
channels, with a fall greater
than that of the sewer, to
make up for the extra friction, due to the bends, etc..
are constructed in the manholes.
Manholes are also
placed at such distances
apart on straight lengths
may be passed through from
one to another. Before going
further, it will be well to
describe the construction of
manholes and lampholes.
Manholes may be
built of brick, stone, conor
material, and they must be
of such a size that a man
can descend and work com"~
fortably at the bottom.
shape should depend upon
number and
sizes of the sewers run-
and from it. The
accompanying diagrams will
The inside
explain this.
line of the walls is shown
in plan in each case, and
ning into
sewers are shown by dashes
and dots.
In Fig. 1 the
oblong form of manhole is
shown. This shape is most
suitable for manholes on a
length of sewer.
Length is needed rather
than width in such a case. Any work which has to be
done in it will be along the line of the sewer, such as
pushing a cleaning rod down and screwing fresh lengths
to the rod as it is pushed into the sewer.
For small
sewers a length of 4 ft. and a width of 2 ft. is quite
the minimum .size it will 'be found possible for a man
to work in.
In Fig, 2 a manhole of the same shape is
shown, with a slight bend in the channel. It would be
well in such a case to increase the width a little. If the
manhole were constructed as in Fig. 3 the work of cleaning would be easier. If the manhole were deep the extension might take the form of a pocket that is to say, the
extra bay need not be carried up to the ground level. A
height of 3 ft. or so would be enough for it. The same
shape, with a longer bay or Docket, is suitable where
sharp bends or junctions occur, as in Fig. 4. In Figs.
5 and 6 manholes square and circular in plan are shown.
These shapes may be used for deep manholes of all
kinds, and have the advantage that movement in all
directions is possible to the same extent.
There are
i'mjportant existing examples of manholes only 3 ft.
The square and circular forms are more suitsquare.
able for manholes where changes of direction or junctions occur than the oblong shape.
It is a matter of
opinion entirely which of these shapes <shown in Figs. 1
to 6 is the best. Local conditions, the material available,
decide the point.
c/blong form, with
pockets made .at the bottom of a size to make access to all
sewers easy, is perhaps as jsatisf actory as any. If necessary, it can be lengthened to -any extent where many
junctions occur.
84. In the accompanying detail drawing of an
manhole on a, pipe sewer, the manhole is shown constructed for its full length and width for a height of
Sec// 0^7
A &
above the sewer invert, with a semi-circular arch
above that (a flat roof would do .as well). The main sewer
runs into a half-pipe channel, which is tedded in the
manhole. It is equally good if this channel is formed of
cement worked to a polished face, but unless the engineer
is quite sure of obtaining men
capalble of making suoh
channels properly it is best to put in glazed stoneware
channels in manholes. These channels can be procured
4ft. 6in.
from makers. In the case
the manholes can be (made
of large sewers the inverts in
of (blue brickwork bedded in
cement. It is as necessary that the manhole should be
For this
watertight as that the sewer Should be so.
reason the channels should be bedded in cement, and
the pipes passing 'through -the walls of *he manhole should
The weight
also be carefully bedded in
manhole wall should not be allowed to
An arch turned over
come upon the pipes.
them, or a stone bedded above to take the weight
overcome the difficulty.
What is called a
drain " chute
is sometimes used at manholes, on the
sewer at the outgo end. This is .a pipe with an enlarged
mouth, and it is useful on small sewers to facilitate the
working of rods, and also to prevent the checking of a
In the accompanying drawing it
flush at the .manhole.
will be seen that the side junction drain discharges over
the side of the main channel through a curved channel.
This arrangement is to assist the flow of the sewage and
to prevent any 'back-flow up the branch.
Sewers of equal
size 'may, if necessary, join on the same level, but smaller
sizes should always come dbove the nunn sewer. Vertical
walls are built above each channel to a level of 3 ins. above
the top of the main sewer, and the top of the benching is
sloped at an angle of 30 degrees up (to the manhole walls.
The vertical walls of the channel and the sloping tops
are rendered with cement worked to a polished, trowelled
face. The rendering is carried up the manhole walls to a
height well above any possible sewage level. This rendering is a matter deserving the most careful attention. By
means of it the bottom of each manhole should be made
so that there are no rough places or projections to which
sewage can adhere. There should not be any small ledge
left at the top of each channel pipe, or where a pipe nrojects into a manhole. The rendering should be made flush
with all such projections, so that there is absolutely
no place where sewage can be held. By careful attention
to .such matters a cause of smell in sewers will be avoided
The benching is sloped at an agle of 30 degrees to ensure
all water falling rapidly back from it into the channel,
and also in order that it may afford no resting-place for
raits to nesit upon. If the slope is made greater than 30
degrees it is difficult for a man to obtain a footing at the
'bottom of the mianhole. It is not necessary to build thniianhole ap to the surface of the ground for its full size
In the accompanying drawing a square shaft equal to tha
width of the manhole is shown. It is, however, essential
that plenty of space should be left for a man to work at
the bottom, tand, therefore, in all cases it is well not to
begin diminishing the size of the manhole rtill a height of
about 5 ft. above the benching is reached.
After this
height, in the case of circular manholes, it is customary
to begin reducing the diameter, giving them a conical
form in the case of deep manholes, and a domed form
where it is necessary to reduce fehe diameter quickly. In
order to take the cover care should be taken that the form
of the dome is not such as to make it difficult for a man
to enter and climb down the manhole on the
step irons.
The curve of the dome should, therefore, be flat, and if
the shallow depth of the manhole renders this impossible, a manhole of rectangular form should be used.
Where manholes have
to be constructed in wet
very important that they should be made
watertight above the highest possible water-level in the
ground. This will 'sometimes mean making them waterIn
tight >by means of rendering up to the ground level.
such cases it is weJl to lay first sail-cloths, tarpaulin,
or linoleum on the 'bottom. On this should be placed a
it is
platform of at least 6 to 1 strength,
through which water can enter.
platform should then be rendered with cement
mortar worked to a polished face. On. the platform thus
made the brick walls should be built, and two or more
iron weeping-pipes should be built in the bottom courses.
These pipes will relieve the water pressure behind the
brickwork, while the rest of the work is setting. It will
then be quite easy to build up the 'brick walls and to
them down to
the floor rendering.
cement work has had time to set hard say in ,a week
or a fortnight's time the iron pipes should be carefully
plugged, first with wooden stoppers driven well in, and
then with neat cement.
After this the benching ant
channels can be finished. If by any chance water should
leak in at any crack or bad place in the cement, it is
quite useless to try to patch it with cement in the ordinary way. In such a cast* a hole should be made and an
iron weeping-pipe inserted well below the leak to relieve
After this pipe has been, set in
the water pressure.
cement so that no leakage can take place round it, and
after the water has been lowered to a level below the
leak which has to be repaired, the bad place can easily
be made good, after which the weeping-pipe can be
plugged as already described.
This simpler method should be noted by contractors,
great loss in their attempts to make good
leaking manholes, ejector chambers, and tanks. The
author has seen much money wasted in such attempts,
and has used the method described many times with in-
who often incur
variable success.
86. In places where the ground is very bad, the Stilfgoe patent cast-iron manhole base, made by Messrs.
Ham, Baker, and Co., may be used with advantage.
This is the lower part of a, manhole made' of cast iron,
which can, of course, be fixed in the worst ground, and
cannot possibly
The circular concrete manhole
worthy of
notice, and may be found economical in construction.
It consists of a concrete base on to which complete conThe
crete cylinders with cement joints are lowered.
concrete cylinders are of great strength, and will probably be used more extensively in the future.
88. Iron covers of various shapes are used for manOf these
They are square, oval, and circular.
the circular is probably the best .shape for most cases
where manholes occur in the road, because there is no
trouble in placing it, as is the case with others, which if
placed askew as regards the line of the road look bad.
The circular cover also cannot through carelessness be
dropped into the manhole through its frame, as is the case
with the oval or square patterns. It has also been found
that the oval cover has a greater tendency to
that the circular. -Covers should fit closely into their
frames a loose fitting cover will often rise in its frame
through stones getting in, and a rocking action will be
caused when wheels pass over it. For manholes on the
footpath hinged square covers are generally used, and a
grating is provided under the cover, which can be kept
shut when the other is open, to prevent passers-by falling
89. Entrances to manholes should be kept on side walks
wherever possible, so as to be out of the way of the wheels
of vehicles,
and .also in narrow
streets to avoid obstructing
In the case of large or deep sewers it is generally possible to do this by having side entrances to the
sewers connected with a descending shaft on the footway.
In the case of small, shallow sewers, the same thing can
be often arranged by keeping the pipes on one side of the
It is not safe to fix iron manhole covers in the
roadway on steep hills, where skids are used, which will
be likely to get caught in the cover and damage it, or even
lift it out of place. Covers in the roadway should be strong
and heavy, to resist the weight of heavy loads, such as
team rollers or traction engines. Light covers on roads
are very likely to break.
They should be heavy also that
they may not be easily lifted by jars or by unauthorised
90. Covers may be either perforated with holes for
yentilation or closed, according to their position and the
requirements of the case, a subject which will be dealt
with later, under the heading of " Ventilation."
covers should present a sure foothold for horses.
may be accomplished by having a specially rough iron
surface, or by inserting blocks of wood into the cover?
made to receive them, or by filling the spaces with other
suitable material.
One good way is to fill them with
broken granite and cement. This will, however, add to
the weight of a cover, which may, in the case of one
already heavy enough, be a drawback. The covers should
be fixed a quarter of an inch below the road surface,
and should be surrounded with granite pitchers or other
suitable material. Great care should be taken to give
these granite pitchers a firm foundation, otherwise they
will fall away from the cover and give considerable
They should Hbe bedded on concrete, which
should take its bearing on the solid earth or on the
manhole roof or walls.
In the case of perforated covers it is customary to
dirt boxes to catch the road material which may fall
through. It is questionable, however, with a large sewer,
having a good gradient and flow, whether this is quitenecessary, as the dirt falling through would soon be carried away. Also it is very probable that the dirt box will
be neglected in some places. Covers must have openings
large enough to admit a man and buckets, etc.
Lampholes are of very little use. The object of a
is that a lamp may be lowered through it into
the sewer, so that by looking through the sewer from the
nearest manhole it may be seen whether the sewer is
blocked up or not. Also, if the sewer is blocked beyond a
lamphole, the liquid can be seen standing in the sewer
from the top. It is obviously no good putting in the ordinary kind of lamphole anywhere where a real place of
Sometimes places may
inspection and access is needed.
exist where there is a very slight change of direction or
gradient in the sewer, and where, owing to the fact that
manholes are
at hand,
no necessity for
The Local Government Board's regulations
demand a manhole or lamphole at such a place. It is
generally best to put in manholes instead of lampholes.
The lamphole is generally a vertical pipe taken out of
the sewer with a Tee junction. The weight of such a pipe
would be liable to break the sewer and its own sockets if
Some authorities, therefore, preit were not supported.
fer a small vertical brick shaft, constructed just like a
small manhole. The vertical pipe can, however, be supround it and a
ported, with, say, a 6 ins. ring of concrete
mass of concrete at the base. It is generally made about
9 ins. in diameter. The vertical pipe should stop in a
pit below the level of the ground, and the lamphole cover
should rest on the walls of this pit, so that loads coming
upon the cover are not transmitted to the pipe or sewer,
with which the pit walls should have no connection. The
remarks made about manhole covers apply generally to
lamphole covers, with the exception that lamphole covers
are smaller.
for sewers large
enough for a
through take the form of an inlet shaft only, or inlet
shaft and tunnel, the bends or junctions being carefully
constructed in the sewer itself. Step irons are built into
the walls of manholes to allow a man to climb down. They
are generally spaced 15 ins. apart vertically by 12 ins.
horizontally from their centre lines.
If this is done the manIt is wrong to bury a cover.
hole it belongs to will most probably be forgotten, and
will then be a cause of obstruction rather than a means
of access to the sewer.
94. Where house connections or drains from surface
water gullies run to a manhole they should be made to discharge into properly constructed curved channels. No junctioms should be made so as to discharge sewage with a
vertical drop into the manhole, as this will produce
If necessary, a vertical pipe with a bend at
the bottom may be carried down to the right level outside the manhole, and the drain itself continued in a
straight lino above into the manhole for cleaning
Junctions made with sewers should never be carried
through so as to project within the sewer, <as they will
cause an obstruction, and filth will be .always liable to
adhere to such projections and cause the sewers to smell
also great care should be taken to make good all such
junction places inside the sewer.
If proper means of access to sewers is provided it will
never be necessary to break them, at any point for
95. Speaking generally, it will be found best to use
the plain stoneware spigot and socket joints wherever
possible, as they are cheaper and cannot be beaten as to
lasting qualities and general trustworthiness when proThere are, however, many occasions when,
perly laid.
owing to a variety of circumstances, it is desirable to
use pipes which can be laid more quickly and with less
labour than the ordinary stoneware, cement- jointed pipe.
Iron spigot and socket pipes with blue lead join/ts and
turned and bored pipes, and, in special cases, flanged
pipes, should be used where the circumstances make it
worth while to incur the extra expense i.e., where the
cost of the labour in laying other pipes makes the total
cost of the work greater than it would be if iron pipes
were used, or where great strength is required, as in crossing a stream or passing through a treacherous bank.
The most common form of patent joint is that in
wlhich the outside of the spigot and the inside of the
socket of the ordinary stoneware pipe is coated with a
composition which generally consists of tar, sulphur,
This is generally known by the name the Stansand.
There are many
ford joint, and has 'been Largely used.
composition joints of this kind.
Owing to the fact that the patents have elapsed,
there is a danger of the Stanford joint, the HassaH'e
and others which come under the heading of
composition joints," being made of inferior composition which
will perish.
It is therefore very important that the
engineer should satisfy himself not only as to the design
of the joint, but as to the quality of the composition used
in making it.
With the ordinary Stanford joint the
direct advantage lies in the fact that the pipes require
very little labour in jointing. If, however, the composition perishes, it is clear that there is nothing to prevent
the joint leaking badly.
The author has on various occasions discovered joints which have so perished.
Fig. 9
shows the improved Stanford joint, in which the composition forms the first part of the joint at the bottom of the
socket, and a cement joint is made beyond this. It is no
doubt safer with the cement, but it seems hardly worth
while to make half a cement joint in this way. If it is
possible to work the cement in the trench and form a good
cement joint outside the composition, it certainly would
be quite possible to make a good cement joint throughout,
which would be far more trustworthy than the composition joint.
Fig. No. 10 shows the ordinary form of Has-
This joint is used perhaps more than any
other for sewers in wet trenches, and, owing to the fact
that a ring of cement is formed, or supposed to be formed,
sall's joint.
Fig. 10.
between two rings of composition,
great confidence is
placed in its watertightness. When the joint is properly
made the cement grout is poured in at one hole at the
top of the socket, and finds its way round, filling up the
whole of the space and driving the air before it, until
the cement appears at the other hole at the top of the
socket, thus proving that the entire ring is full of cement.
There can be no doubt that this is a very good joint, provided the composition is right, and provided the work is
properly looked after. But there is always a possibility
of a careless workman pouring cement into both holes at
once, in which case the joint would appear to be quite
sound, and would stand a water test, owing to the two
composition joints; but the cement ring, which is trusted
It is therefore
.as a third line of defence, is not there.
necessary to guard against a false sense of security when
using joints of this description. The supervision of the
work should not be relaxed because patent pipes are being
As an improvement
grouted composite joint
of this joint, the Doulton's
especially designed for use in
Fig. 11.
wet trenches. To quote from the catalogue
In earlier
joints of this class the grout has been poured into a closed
cavity in the socket, of small dimensions, and having no
adequate provision for the escape of air or superfluous:
Fig. 12.
the operation therefore invariably having been
attended with risk of imperfect joints. In the grouted
composite joint the sealing chamber is of ample dimenThesions, offering no hindrance to the flow of the grout.
is closed by a band of porous fabric of a texture
such as to retain solid particles of cement while it allows
the escape of air and superfluous water. It is this feature
of the joint which makes it unique. Risk of faulty joints
through confinement of air or water is entirely absent,
since both air and water filter away through the pores in
Fig, 13.
the canvas. An absolutely solid and continuous ring of
cement around each joint can thus be relied upon and,
moreover, its presence is actually visible from the outside
of the completed joint, an advantage not possessed by
any joint used hitherto."
This joint should be very usebut the porous fabric must be of a quality to allow
air to pass through it easily even when wet.
In the Brandram joint (shown in Figs. 11 and 12) the
pipes employed are plain butt- jointed stoneware pipes,
or sometimes pipes with a rebated joint contained in the
thickness of the pipe, are used.
A wide steel band (b),
nearly equal in length to the circumference of the pipes
to be jointed, coated with Dr. Angus Smith's composition,
with each end curling over to form an ey (e), is dipped
into hot bitumen. The pipes to be jointed are generally
wrapped round with a piece of special bitumenised paper
or canvas, and the wide steel band is drawn closely round
the joint, as shown by means of wire handles, while a
small covering plate (c) bedded in bitumen is placed under
its ends to complete the metallic circle. Hot bitumen (b)
is poured over the joint while this is taking place.
screw-cramp is then applied and the band tightened until
the bitumen begins to exude at the edge of the band all
round the pipe.
Staples (f) are then inserted in the
eyes of the band from each side, and further bitumen is
poured on, covering the staples, and the joint is complete.
Fig. No. 13 is from a photograph taken in a trench
showing some Brandram jointed pipes laid under the
author's supervision in very wet ground, the foundation
being running sand. Among the advantages of this joint
are that it can be made quickly and tested at once. Pure
bitumen being imperishable, the joints should le
altogether superior to the composition joints already mentioned, which composition generally consists of a mixture
of tar, sulphur, and sand. A pipe can be readily removed
to insert a junction, or for any other reason, when Brandram joints have been used. This joint is very useful for
large concrete tube sewers, and has been used by the Portsmouth Corporation for jointing 42 ins. concrete tubes in
running sand.
Messrs. Thos. Wragg and Sons, of Burton-on-Trent.
are makers of what is known as the "Jennings joinder."
It is an ordinary junction pipe with a capped socket,
designed for use in the construction of stoneware pipe
sewers, where it is required to leave junctions for future
connections of branch drains. The general feature in
or capped sockets lies in the cap and
socket being in one piece, with a groove formed outside
the socket immediately behind the cap, so that the cap
may be easily cut off with a chisel when it is required to
make a junction, leaving the socket ready for the inser
tion of the spigot of the drain pipe.
By using these cap
in new sewers laid in wet ground,
sockets or
leakage of subsoil water into them through the junction
pipes is rendered impossible. The extra expense of the
joinder is a very small matter, whereas the saving of
money in using them may 'be very great.
Messrs. Doulton also supply a similar junction cap.
The cap fits over the outside of the socket, and is secured
with an ordinary cement joint, and, like the "Jennings
joinder," the cap is grooved on the outside; it is also
grooved on the inside, so that the outer disc can be
readily cut off with a chisel, leaving the interior of the
socket ready to take the branch pipe.
There are other methods adopted by pipe makers
whereby the junction pipes may be securely sealed, and it
will always be well for the engineer to specify some such
method of insuring that the dead junctions do not irive
trouble when laying the sewers in wet ground.
junction pipes are frequently a fruitful source of trouble,
and great expense is sometimes incurred in finding and
making good leaks which take place through them.
Surface Gutters Gullies Trapped and Untrapped Retainers Often Useless Gulley Tanks.
100. GULLIES should be fixed at all places where surface
water is likely to accumulate
thus it is necessary to
place them along the sides of roads to take the water
from the surface gutters. These gutters have necessarily
to follow the profile of the roads to a certain extent, and
the gullies" have to -be spaced accordingly. The surface
gutters are constructed always with a fall towards the
The fall should, if possible, not be less than 1 in
On porous ground it is often possible to keep the
gullies further apart than in paved streets.
They should
be fixed at the bottom of every depression, and .should be
so arranged that there may be no open gutters crossing
It is well to avoid placing a gully at the corner
of a street, where the wheels of vehicles taking a sharp
turn will come over it, or at any other inconvenient
grids of gullies should be of such a shape and
water from the gutters may be easily
siae that all the
received, and that they may not be easily choked with
leaves or rubbish.
This is particularly the case where
they are situated on steep hills, where the water has a
tendency to overshoot the gully. In such a place it is
essential that the grid should be long enough, and for
this purpose two grids are often fixed together.
with diagonal bars are often fixed at such positions.
A gully is an inlet to the sewer covered by a grating
or other arrangement for protection and for straining the
water passing into it. Such a gully may be trapped, and
may have a pit beneath its outlet for intercepting deIt is essential
tritus, according to the needs of the case.
that the gully should be of a simple form, so that it may
be easily cleaned, and also that the drain from it may be
accessible for cleaning.
102. Gullies are rmade of stoneware and iron, and they
also be built of brick, stone, or concrete.
should always be watertight. As an example of a small
economical gully, witlh a retainer for detritus, it may be
of use to mention that a stoneware T pipe is sometimes
The 'bottom of the pipe is made watertight with
concrete and cement, and a grid is bedded over the top,
while the branch acts as the outlet.
103. The simplest form for a gully would be one for a
separate storm-water sewer. All that is necessary in such
a case is a small basin with a curved bottom, having an
If the gully were conoutlet pipe running to the sewer.
nected to a foul sewer in a place where ventilation was inadmissable a trap, and a cleaning eye for access to the
outlet drains would have to be added. If, again, the erully
were in a place where a great deal of sand would be
washed into the drain, a large and deep basin, well below
the outlet pipe, would have to be added. It is well that
the outlet from a gully should be larger, if possible, than
the drain to which it runs, so that this drain may run
full bore, and that there may be no check to the flow from
the gully.
a separate sewer for surface water
and where surface water
gullies discharge into a sewer which is not likely to smell,
and are in positions where such ventilation would be unRetainers for
objectionable, there is no need for a trap.
detritus are very often fixed at places where they are
There is no object in fixing a retainer
quite unnecessary.
to a gully unless it is in some place in which there will
no need
for a trap,
be a considerable amount of sand or gravel washed into
On paved roads they are not needed where the sewers
are well laid. It is also questionable whether it is not
If they are
better to allow silt to go into the sewers.
well laid to iproper gradients silt will probably be carried
away with the sewage. As an instance, it may be mentioned that at every storm many tons of sand <are washed
through the London sewers and deposited at the outfall
works at Barking, where vast quantities of it may be
It will probably be cheaper to remove the sand
remaining in the sewers than to have to clean out all the
It is sometimes well to have
gullies after every storm.
catch pits for sand at the manholes on the course of a
storm-water sewer. Another point to be remembered is that
after a gully retainer has been filled beyond a certain
point all the detritus brought in will be carried into the
sewer as if no retainer existed. Two more points with
regard to gullies should also receive careful attention,
first, that the trap is very likely to become unsealed
through evaporation in dry weather when it is most
and next, that the matter retained in gullies,
particularly in those situated in paved streets, becomes
very foul in the hot weather, and is likely to smell
badly as gully retainers are often insufficiently cleaned
they are likely to smell, whether full or not. For these
reasons it seems well to omit the retainer wherever possible, which will be in most cases, and also to avoid
putting in traps whe.re they will be useless.
105. There are positions, such as at the foot of a
In such
steep gravelly hill, where a retainer is essential.
places the retainer should be of ample size to take all
the material likely to come into it during the greatest
storm. It should be remembered that a great rush of
water will disturb all the sand to a depth of, say,
1ft. 6in. below the outlet, so
depth will be essential.
What is called a gully tank is suitable for such positions.
For this a watertight underground tank is built of brick
or concrete, rendered, with a grid over it and an outlet
drain to the sewer.
106. Gullies can generally
manholes on small sewers.
be arranged to discharge into
Where a small amount of detritus only is expected, an
iron gully with a retainer which can be lifted out may
be used with advantage. The gully is then more likely to
be properly cleaned out, and is less likely to smell.
107. Gullies are sometimes placed under the footway,
with an inlet opening under the kerb and an iron cover
over the gully. It is well to put a vertical grid at such a
place if the opening is of a size to admit small animals.
Sewer Flushing.
Hand Flushing Flushing by
Streams Flusliing Tanks Portable Flushing Valves
Flushing with Sea Water.
108. WITH regard to flushing arrangements, it must be
remembered that at the commencement of a sewer there
is, as a rule, .practically no flow of sewage, and also that
sewers may exist which have self-cleansing gradients
wihen run/niing half-full, but which generally have a
much smaller flow, and are, therefore, not self-cleansing.
Dead ends of sewers are sure to need flushing, except
under exceptional circumstances, also sewers with insufficient flow, and those with gradients too flat to make the
sewers self-cleansing. It will be seen that small branch
sewers are more likely to need flushing than main sewers
laid at proper gradients.
109. Sewers may be flushed by several methods.
It is
sometimes possible to use a natural stream for the purA small stream may be dammed off and diverted
into a sewer for a certain time every day, or it may b*
made to fill an underground tank, the water from which
is discharged suddenly into the sewer.
The water from
the mains of the district may be used to supply auch
a tank. Also water from carts or bath waste water may
be stored, and used for flushing. Some of the sewage
itself may be held back, and allowed to discharge at "a
head into the lower sewers. Under some circumstances
the subsoil water may be also used for flushing purposes.
110. In flushing sewers less than, say, 24in. diamerter,
or even larger, it is well that water should, if possible,
completely fill them from invert to soffit. The value of
a flush depends more upon its sudden discharge than
upon the quantity of water used. That is to say, the
quantity of water which suddenly discharged will make
a pipe flow full bore will be more useful for cleaning
the sewer than ten times that quantity discharged at
such a rate that the sewer will only run quarter full.
Thus, it is clear that it will be better to store water in
a tank and let it off suddenly into the sewer than to let
it run in through a hose, or by any slow process.
111. Flushing tanks are built underground of brick or
concrete rendered, or of iron. They are made, if possible,
so that water in them may be a few feet above the level of
the sewer, so that the head of water may make the veloThe head should be
city of the flow at the start rapid.
as great as possible, but it must be limited by the amount
of pressure the sewer will stand.
In the case of small,
well-laid pipe sewers a head of 6ft. or 8ft. may be given
with safety, but in the case of large sewers greater care
must be taken. The effect of a too rapid flush may be toburst the .sewer, in the same
way that an
excessive rain
storm will sometimes burst a large brick sewer.
112. Flushing tanks are often made in conjunction with
manholes, and sometimes the manhole itself is used for
a flushing tank.
A flushing tank may be a simple rendered pit with an outgo at the bottom connected with a
manhole. A plug is inserted in the outgo, and the tank
Upon removing the plug the contents will be discharged into the sewer. A syphon trap is fixed on theoutgo pipe to prevent the sewer being ventilated into
the tank when it is empty. A sluice valve or penstock isused instead of the plug if that is more convenient. The
outgo from the flushing tank should be at least as big as
the sewer into which it discharges.
It will probably befound a more convenient arrangement if this tank is made
as part of the manhole, so that the water may be discharged more directly.
113. Speaking, generally, the flushing tank as described'
is unnecessary, except where
special discharging apparatus is used. The simplest method for sewer flcctaliff
by hand is, generally, to fill a manJhole with water and
discharge that into the sewer. It is well to proceed by
stopping all inlets and outlets to the manhole, but this is
not absolutely necessary. It will be enough sometimes tostop up the outlet only. The following things must be
borne in mind, however: (1) The water will have tofill all branches that are not
plugged off, ajid this may
mean using a great deal of water.
(2) It is necessary, in order to get the required quantity and pressure,
to let water stand at a depth of some feet in the manhole.
(3) The water must not be allowed to rise to such
a level that it will be above any inlets on the branch
drains running to the manhole, otherwise, instead of
manhole, the water
the areas of houses, etc.
filling the
be finding an exit in
114. Penstocks are frequently fixed in manholes for
stopping off drains for flushing purposes. There are
many kinds, and they are very generally used. It does
not, however, appear that they are altogether satisfactory for the purpose, for the reason that they afford
places where foul matter may accumulate, and it is inconsistent to fix valves of a kind which will tend to
collect dirt in manholes, after taking elaborate measures
to avoid such accumulations of filth. As a rule, it is
not difficult to insert a plain plug of some kind into a
sewer which has to be stopped for flushing. A conical
piece of wood covered with india-rubber and with a chain
attached will answer the purpose, or, better still, a port-
patent flushing block.
If the sewer is very large it may be partly dammed off
to allow water to accumulate to a certain height, which
is then released for flushing.
Where sewers are situated
near great tidal rivers it is sometimes possible to build
-able flushing valve, e.g., Bosley's
large underground tanks, which are filled by the rise of
the tide, after which the valves are shut and the water
stored for flushing purposes.
115. To do away with the difficulty and expense of
flushing sewers by hand, as already described, various
automatic apparatus have been invented, and are largely
used, being found, as a rule, more economical than
other methods.
The more common form is the syphon.
The automatic syphon is fixed in a flushing chamber
near, or built in conjunction with, a manhole. As soon
as the tank is full of water the syphon discharges its
contents. By regulating the supply the tank can be
to discharge at
interval required.
116. The meithod of flushing sewers by means of waiter
carts is important. It is possible to carry a large quantity of water in a cart, which should be discharged into
the sewer through an outlet as large as the diameter of
the sewer itself, if it is to be used as a direct flush.
117. Messrs. Merryweather's patent hydraulic apparatus
for cleansing sewers is one in which a number of small
jets of water are used on a hose which is put into the
sewer to be cleansed. It is said that a great saving in
water is effected by this apparatus viz., that it takes
only one-tenth of the water needed to flush sewers by
ordinary methods.
This, in the case of a scarcity of
water, or where an increase in the flow is of serious consequence at the outfall, is very important; also for
'flushing large sewers through which a man can walk its
advantages are obvious.
118. The quantity of water needed for flushing a sewer
will depend upon the size of t!he .sewer, its gradient, and
its length, and also upon the head of the water in the
tank. It is a matter which will need careful consideration in each case. Bath waste water, rain water, etc., is
sometimes stored in a gully tank, which discharges by
means of an automatic syphon for flushing.
119. Sewers will need flushing probably at least once
in twenty-four hours.
large flush once a day or once in
two days will probably clear the sewers -better than two
or three smaller flushes given during the same period.
Sewers regularly flushed will not need anything like the
same amount of water as those which are not cleansed in
this manner till they show signs of becoming foul or
120. Sea water is used for flushing low lying sewers by
allowing them to fill with water at high tide, and letting
it discharge at low tide.
The objection to sea water for
flushing is that it corrodes ironwork. It is also said to
cause solids to deposit and to produce putrefaction in the
sewage, which renders it poisonous to fish when discharged into the sea, whereas fresh sewage would not be
eo harmful.
Sewer Flushing (Continued).
by Automatic Syphons
Hand Flushing Flushing
Length of Sewer Flushed by
There can be very little doubt, speaking generthat where a sewer needs systematic flushing, an
automatic apparatus which discharges a flush regularly
at stated intervals is better than any hand flushing
arrangement can be. The quantity of water used can
be regulated to a nicety, and whatever water is so discharged into a sewer does useful work. It must always
be a difficult matter where sewers are flushed by hand ito
.prevent a great deal of water being wasted. The valves
<are very likely to leak and allow a quantity of water to
run into the sewer uselessly.
122. The following sketches show some of the various
methods for flushing sewers. In Fig. 14 a hand flushing
arrangement is shown. A hydrant pit is made next to a.
flushing pit, which has a trapped drain running to a
manhole. An earthenware bend, carried up nearly to
the ground level and covered with an iron cover, will do
instead of the git. When the sewer has to be flushed*
all sewers are stopped by plugs or penstocks at the manhole, and a bent copper pipe is connected to the hydrant
by a union, and the manhole is allowed to fill. When
full, the water is discharged into the sewer to be flushed
by opening the valve or "removing the plug. It will be
seen that great C are is taken that there may be no possible connection between the sewer and the water main.
Fig. 14.
Of course, the 'bent copper pipe is only attached to thehydrant when the filling is going on. In Fig. 15 an
arrangement is shown which is used in a case where the
supply of water for flushing purposes has to be measured by a meter. A small main runs to an open gully,
into which it discharges. There is a meter and stop
It is
cock in a small pit on the course of this main.
very necessary that the water main should be disconnected from the sewer in the open air, and not in a
closed gully, or as is sometimes done, by dipping it into
a pit of water which has an outgo to the sewer.
careless arrangements cannot be sufficiently condemned.
It is by no means uncommon to find water mains running
directly into flushing manholes, owing to the ignorance
or carelessness of those who designed and carried out the
work. It is almost needless to say that it is the duty of
.all those responsible to see that such arrangements are
It is possible that sewer gas, disease germs, and
-even sewage may find their way into waler mains connected to a sewer manhole. If a water main should
prove to be empty when the valve is opened, it will readily
.'allow sewer gas to enter the main, and in the case of
a bad stoppage in the sewer it will allow sewage to enter.
It may be said that such a case is not likely to occur,
Fig. 15.
ought not to be even remotely possible, and any
engineer or other person who is connected with the management of an old or unknown sewerage system should
look out carefully for such defects, and also see that they
do not occur in new work.
*but it
123. For practically the .same reasons water mains
should not be connected to flushing chambers. These
'Chambers, though forming no part of the sewer, have a
direct connection with it, and tnis connection is often
not trapped, but is kept closed with a valve. If it is
trapped there can be no guarantee that for various
reasons the trap may not at some time become unsealed.
'Therefore water mains should never foe carried direct to
'the flu'shin^ chamber unless the water in them is not
used for drinking purposes. It is quite easy to disconnect them in the open air by some such arrangement
shown in Fig. 15.
Flushing chambers are sometimes
above the ground at suitable places.
as is
125. The advantages of automatic flushing; for sewers
are so :great that it will be well to give a few examples'
of some of the syphons most generally in use. The regularity of the flushing, perfect control, economy of water,
and saving of all labour make the use of automatic
apparatus very desirable in most places.
126. Fig.
16 shows the working and fixing of one of
It consists of a central syphon-
Fig. 16.
with a deep water seal, which is surmounted by a dome.
The apparatus is fixed in a flushing chamber, as shown
in Fig. 16. As water rises in the chamber it locks a
quantity of air in the dome between the water in the
deep .trap of the isyphon and the water in the dome.
This air is gradually compressed by the water rising
higher and higher in the flushing chamber, till at last
a pressure is produced sufficient to expel the water from
the syphon trap into the sewer, and to set the apparatus
discharging the whole of the contents of the chamber.
It will be seen that no discharge can take place till
the water has risen to the proper level, after which the
whole of the water will be discharged Tapidly.
These syphons are generally placed at dead ends of
sewers, and may discharge directly in/to a manhole, as-
in Fig. 16.
127. Sometimes a flush may continue to be useful for
a great distance along a sewer. At first it may be sufficient in volume to clean the sewer for its full diameter,
and later it would decrease in depth to such an extent
as to be useless, were it not for the fact that the ordinary
flow of sewage increases in volume the further the flush
travels, so that the flush water and the flow of sewage
together may produce something like the volume and
velocity of the flush at its start.
128. In cases where a sewer is flushed by hand and
the supply of water is .small, balls of various sizes are
sometimes floated .through the sewer, behind which the
water backs up whenever the ball is stopped by any
substance, till the pressure is sufficient to remove it.
If the sewer is very much fouled a small-sized ball is
floated through first, .and then one a size larger, and!
so on till a ball of a diameter say lin. less than the size
of the sewer is floated through.
Whenever the ball stops
the water passes all round it, and so scours the sewer.
As the ball floats the .greatest scouir will be along thesewer invert.
Objects of Ventilation of
Steep Sewers Influence of AtmosphericChanges Prevention of Pressure or Vacuum, Essen-
The Ventilation
of Sewers.
The objects to be attained in ventilating a system
of sewers are, first, that no pressure of air or gas may
occur in any place; next, that fresh air may be freely
admitted to aill parts of the system ; and lastly, that all
foul air may find a ready outlet at places where it will
at once be diluted by plenty of fresh air, and where therewill be no risk of its being a danger to health, and where
also there can be no objectionable odour, harmless or
otherwise, experienced by anyone. The subject is a difficult one, owing to the fact that the most densely crowded
districts, where houses are close together and streets
narrow, will naturally have the largest sewers and greatest
flow of foul sewage, needing a great deal of ventilation,,
and will possess the fewest places where foul air can*
safely be given off.
The great difficulty of ventilating sewers in such
and the undoubted objections, scientific and prac-
which can be justly made to any of the methods at
present used for ventilating such sewers, has led to great
differences of opinion upon the subject of sewer ventilaSuch objections, if examined, will often prove to
i>e not so much objections to the methods of ventilation
as to the existence of sewage at all. No doubt it is not
an ideal state of things that a large quantity of foul
sewage should be carried in a sewer through a crowded
district, pipes from which are connected to every dwelling-house. Unfortunately, as nothing better has yet been
devised, it is necessary to make the best of a bad job and
protect ourselves in every way possible. That this can be
done with great success and safety has been conclusively
131. The ventilation of sewers must be always a matter
of common sense rather than of rules and methods. It is
often urged that a well-laid sewer does not give off harmful sewer gas, because all foul matters are carried away,
and do not remain in the sewer long enough to putrefy.
This being so, it is then urged that open ventilators can
be fixed at all manholes and lampholes along the road or
elsewhere, so that an abundance of fresh air may enter
the sewer. This system, on the whole, is a good one, but
it can only be applied in certain places under certain
It must be remembered that even fresh
sewage may smell very badly, and, whether the odour is
dangerous to health or not, it may be very objectionable.
If fresh sewage flowed down the road, for instance, in an
open channel, it would be offensive. Again, it is possible
for sewage to accumulate in the best-laid sewer at times
when the flow is insufficient, and 'there is also the probability that a certain amount of foul matter will adhere
to the sides of the sewer in time, and this will certainly
decompose and form sewer gas. Also, the engineer has
to provide against possible neglect, and even damage, to
the sewer in the future, when accumulations will be likely
to occur and smell badly.
For these reasons outlets for
sewer air should not be put near houses, in narrow roads,
or in confined places.
It is well to consider every inlet
ventilator as a possible outlet, there being no doubt but
that at certain times sewer air will come out there.
132. In order to consider the principles upon which
sewers are ventilated, it is well to take the simple case of
a long sewer receiving the discharges from a number of
house drains. Assuming, first of all, that it is not ven-
it will be seen that any discharge, such as
running into it suddenly will be likely to
^compress the air and unseal the traps in the house con-
tilated at
a large
nections, thereby forcing the sewer to ventilate into the
house drains, as it must be remembered that the pressure
needed to unseal such traps is probably between one and
two ounces on the square inch only.
Next, let us
consider the same sewer with open grids to manhole
covers at frequent intervals.
As soon as a large enough
to compress the
air flows down the sewer,
expelled, and then drawn in from such places, as the
water runs out of the sewer. The same will happen in a
less degree for every sort of flow that may pass down the
It is probable that the air in such a sewer is
seldom still. The flow of sewage will tend to make a current of air down it. The air inside will often be much
hotter than that outside, and it will then rise out of the
The wind passing over or blowing into the
ventilators is likely to produce a current, etc. It is fairly
clear, then, that a sewer under these circumstances would
be well ventilated and, provided that the ventilating
covers could be fixed in places well away from houses, the
arrangement would be satisfactory. If this could not be
done without objection, it would be necessary to rut in
ventilating shafts, carried to a height and position where
sewer gas could be safely discharged. It might be ima^
gined that such a sewer could be ventilated easily with
one inlet point and one outlet point at the far end, where
a fan or other apparatus could be used for drawing out
the foul air. This is not so, however. A fan extracting
the air or forcing air into a long sewer would be likely
to force the traps of the house connections and the road
also, the existence of any unsealed trap, opened
cover, the sewer running full bore, etc., would be likely
The impossibility of vento entirely upset the working.
tilating a large system of sewers with a strong mechanical
draught of any kind may therefore be seen at once. The
effect would, of course, be that only a very small part
would be ventilated at all. The same objections apr>ly to
the method of ventilating a system of sewers by a furnace at the bottom of <a high brick shaft. These methods
of producing ventilation can only be useful in the case
of a sewer without branches, such as a long outfall sewer.
Sewers should be so arranged that none of the
takes longer than twenty-four hotfrs to reach the
After this time sewer gas is produced, as distinfrom the smell which is present with fresh
sewage. They should be so well laid, cleaned, and flushed
that there may be no accumulations of foul matter. Very
steep sewers should be avoided, for the reason that the
matter flowing through them is likely to be churned up,
when it will smell very badly
also, as already mentioned, such sewers are likely to become foul with solid
matter left behind.
134. In steep sewers there is a tendency for the air contained to be carried past any openings for ventilation,
along the course, with the consequence that there is ant
undue discharge of sewer air at the highest ventilator on
For thesethe sewer wihich may be very objectionable.
reasons it is customary to break the line of sewer into
short lengths on a steep hill. To do this the number of
manholes may have to be increased. The sewer is laid in,
The top mansteps or ramps in the following manner.
hole is made deep, and the sewer is carried from it at a.
gradient as steep as can be given without producing bad,
results to a point where the sewer is as near the surface
of iahe ground as is safe.
(The question of the maximum
gradient for a sewer was dealt with in Article No. 4.)^
When the sewer has been carried as far as possible a.
small manhole is made, and a short, very steep length of
sewer is taken to a larger manhole close by. The small
manhole referred to may be omitted if the sewer is carried on straight to the deep manhole at the ordinary
slope, for cleaning, while the steep branch descends tc
the invert level in the manihole. The steep sewer should,
enter the manhole curved at the bottom, so as to avoid
Another metJhod is to carry the sewer to the
deep manhole, and allow the fall to take place in it with
a carefully formed curved channel.
It is not possible tomake a great difference in the level of the sewer in this
way. In both these methods flaps are fixed, if necessary,
on the inlet pipes, and the manholes are ventilated, and'
also the end of each sewer just above the manhole, sothat a number of short lengths of sewer are made with an
inlet at the lower end and an outlet at the top end.
modification of this system of steps will prove to be useful!
in the case of steep sewers, which are more liable to give
trouble than sewers at ordinary self-cleansing gradients.
135. In order to arrive at any general conclusions on,
the subject of sewer ventilation it is necessary to bear in
mind that, in addition to the facts already mentioned,,
sewer gas is generally heavier than air, and that some-
times the atmospheric air is hotter than that ill the
and that there are often periods when there is no
wind all tihese things tend to produce a sewer full of bad
air, which has no tendency to move or leave the sewer
except from the flow of sewage, which at such times will)
probably be at its lowest. The fact that a sewer is full
of air polluted by fresh sewage is not as satisfactory as
might be, but, as has been already pointed out, some
smell must exist, and even if the sewer were open along
its whole course and a wind blowing through it this could.*
not make foul sewage sweet. When the air from a welllaid sewer which is properly kept is discharged into the
open, it is generally believed that there is no risk tohealth. If, however, there is any risk of sewer gas entering a dwelling house, the danger to health is believed
This is the great danger, and whatever
to be very great.
else is done, the ventilation and arrangement of sewers
must prevent the possibility of sewer gas finding its way
into houses. It is better to have an offensive ventilating
manhole in the roadway than to have a slight pressure
of sewer gas in the sewer, which, though perhaps unnoticed, will occasionally force the traps on house connecting drains and allow sewer gas to get into house
drains, where, perhaps, another step of the same kind
will take it straight into the houses. Thus the first thing
to do is to arrange that the sewer may have ventilatorsIn pracfor inlet or outlet of air at frequent intervals.
tice 600 ft. apart is considered the maximum distance
at which ventilators should be placed. It is better to have
them much
closer together say, 300 ft. apart.
The circumstances of the case must always make a very great
If pressure of air inside the sewer at any
point is made impossible, the fact that at times there is
no current of air along a sewer and that there is no
discharge of sewer gas other than an outflow from the
top of shafts does not necessarily constitute a danger, and
may be necessary in crowded localities, where ventilators
at ground level are out of the question.
The Ventilation
of Sewers (continued)
of ventilators Ventilators near flushing chambers
Ventilators in crowded localities The use of shafts.
Having decided that frequent openings for ventilation are essential on a system of sewers, and that
must be sufficient to prevent any pressure of air
from taking place, and any vacuum or partial vacuum
being formed in the sewers, it is next necessary to consider at what places and in what form they should occur.
In the first place, a ventilator must always be
provided where any large flush enters the sewer. Thus
the manhole which receives the water from a flushing
chamber must be ventilated. The same would apply to
any other large discharge. This is done to prevent a
vacuum which would otherwise be formed. Again, as
any sewer may at some time flow full bore, the necessity
of frequent ventilators for the same reason is clear.
138. In parts of London it Is becoming more and more
the custom to avoid ventilators at the ground level, and
long shafts to allow the sewer air to escape at
as far removed ias possible from buildings and
other places where their discharge would be a nuisance
or harmful.
There can be no doubt that the system is
.a good one.
It has, however, been pointed out that in
cold weather the column of air in a long metal shaft
will be chilled and heavy, and tend to prevent circulation
also, that when the outside air is still and moderately warm, sewer air does not rise in the shaft, and, in
fact, that under certain conditions of the atmosphere
the air will not circulate in a sewer ventilated with long
These defects, however, and other theoretical
considerations are outweighed by practical results.
alternative of having open ventilators in narrow, crowded
a method not only exceedingly unpleasant, but
also dangerous to health
and the other alternative of
closing any ventilators, and thereby creating a pressure
of gas in the sewer, are both of them imposible arrangements.
139. It is, then, clear that in the present state of our
knowledge the use of long shafts to convey the bad air
from sewers to safe places is often essential. This does
not mean that a system of sewers ventilated throughout
with shafts is to be recommended, although the objections generally raised to it are more theoretical than
practical. It is cheaper and better to ventilate the sewer
through an open manhole or lamphole cover wherever
It must be noted that, although the long shaft
will at times produce a better circulation of air than
would take place at an open manhole under the same
-circumstances, this advantage is outweighed by the fact
that a long shaft is just as likely at other times to
the circulation, as already stated.
is based on these
being clearly understood that, though a constant
circulation of air is desirable, the fact that it does not
occur at all times need not be considered a vital objection
so long as no pressure occurs in the sewer.
The modern ventilation of sewers
facts, it
141. In dealing with an ordinary sewer it is best tohave a ventilating cover at every manhole or lamphole
where possible, and it may often be well worth while to
use the road gullies for ventilators that is to say, to
have no trap to these gullies. This can, of course, only
be done under circumstances where the engineer can feel
confident that the flow of sew-age is of a character that
an offensive smell at the ventilators, and
when the gullies are in such positions that a possible
As an
smell from them would not be a nuisance.
example, it is not unusual to find manholes with ventilating covers at rthe road side, and sometimes trapped
Such methods are
gullies occur close to such ventilators.
It is well, also, in considering the posinconsistent.
sibility of using road gullies as ventilators, to notice th<e
many open ventilators situated even in crowded places,
which we very likely walk over every day, and from which
we experience no nuisance. The fact that in certain
places a very great nuisance occurs, owing to open manhole covers, probably points to the fact that the sewer
It is, perhaps, worthy
there is defective or badly kept.
of note, also, that some of the places where the worst
smells occur are not the most crowded. Open ventilators,
for instance, exist at Westminster in roads which are by
no means very wide, from which there is not any very
noticeable smell.
On the other hand, there are open
manholes in certain sewers in more or less open situations
in the suburbs which are all the year round a nuisance
to those living near them. There is abundant evidence to
prove that a well-laid, properly kept sewer does not smell
badly at the ventilators.
will not create
The idea of untrapped
gullies as ventilators
good sewer in an open place is, therefore, well worth
taking into account in new work, and may save expense.
143. Storm water overflows generally act as ventilators
to the sewers.
Care should be taken that their position
is suitable for the purpose.
There is also likely to be an
inlet draught at the outf alH, which may need to be controlled.
144. Where sewers cannot be ventilated at the manhole
covers an air pipe may be carried from the manhole or
lamphole to a 'convenient place. This pipe is laid as if
it were a sewer, and should take a straight course.
It may
necessary at can be connected to a vertical shaft.
be possible also to carry a vertical shaft up direct from
the sewer.
Ventilation shafts for sewers
are generally not
than 6 ins. in diameter and circular in section, as
rthat shape gives the least friction, and consequently a
better draught. All bends are avoided as far as possible,
it being estimated that a square bend will reduce the
Such shafts axe
velocity of the draught by one-half.
very generally made of iron, though there is no doubt
that if the inner surface of the pipe were of lead or other
metal not likely to corrode the result would be better. It
-is not at all an uncommon thing to discover a ventilation
pipe choked at its foot with rust, which has formed on
the inside of the pipe and fallen to the bottom, making
the shaft useless.
146. Where an iron shaft, therefore, joins an air pipe
in connection with the sewer there should be a small pit,
with an inspection door, so that if rust should fall it
not block up the pipe, and that it may be possible
to clear away any accumulation. Where iron ventilation
pipes exist in old work such pits should be added. Galvanised iron pipes should always be used in preference to
plain iron pipes, as rust not only tends to block the shaft
up at its foot, but also, by making the interior surface
rough, retards the flow of air.
147. The tqps of shafts should be protected with wire
Cowls to
caps, so that birds cannot build in them.
extract the air are useless in calm weather, when a
draught is most needed. Lamp-posts are sometimes used
as ventilator shafts.
148. Mechanical apparatus for ventilating an ordinary
sewerage system has not proved to be a success, partly
for the reasons already given, and partly because the cost
of the apparatus and its working is too great.
however, in the case of the Shone system, compressed air
mains are carried to various parts of the system to raise
the i&ewage, it may be- found worth while, where necessary, to create an artificial draught in a shaft by means
of the compressed air.
It goes almost without saying,
however, that the method of ventilating sewers should be
as simple as possible, and that, except under very
exceptional circumstances, mechanical apparatus will be un-
149. It is necessary to refer to the system of ventilating
sewers by allowing all house drains to run untrapped into
them, and thus using the vent pipes of the house drains
to ventilate the sewers.
Of course, there would be no
objection to this system if all the house drains and vent
pipes were sound, and well arranged with a perfect
system of sewers to discharge into, but inasmuch as such
perfection is extremely unlikely .to exist, or to remain
.perfect, the method of ventilating sewers through house
^drainage systems cannot be considered safe. Moreover,
:this method is, .at its best, very unnecessary. If the sewers
.are fit to be ventilated through the house systems, there
can be no reason for not ventilating them at many points
in the streets, say at the road gullies, which would be a
;much better method than allowing them to ventilate near
the houses, within a few feet of bedroom windows or
chimney tops.
150. The tops of sewer vent shafts should be kept as far as
^possible from all windows, chimneys, or dwelling-houses.
Unfortunately, in the crowded parts of London this is
very difficult. It is, however, unpardonable to place
them, as is frequently done, close up against houses,
within three or four feet of the tops of chimneys, where
;a down draught can. at any time carry the gas emitted
into bedrooms. Glaring examples of this exist in London
at the present time. It is certainly not only worth the
extra expense and possible unsightliness to keep the tops
-of sewer ventilation pipes at least 20 ft. away from windows or chimneys, but to do so should be absolutely obligatory.
Probably the most serious defect in the ventilation of
is the careless way in which vent pipes are placed
against buildings.
151. Common sense and a careful study of existing sys'tems can be the only guide in the difficult subject of sewer
ventilation, and it is the object of the present article to
point out that, speaking generally, new sewers may be
safely ventilated in the roads at open manholes and
lamphole covers, and .sometimes at gulLes, and that,
where this cannot be done safely, shafts must be connected to the sewers and carried to safe places, and that
if such arrangements prove to be a nuisance it points to
the fact that there is something very defective in the
sewer itself.
152. The practice of building a vent pipe up in a wall
=and so hiding it is to be condemned if the position is one
in which a leak of sewer gas into the wall would be dangerous. There have been many cases of such shafts which
have allowed sewer gas to enter into
especially where glazed stoneware pipes have been used.
Hidden work of any kind is always liable to be a source
of danger in the future.
Sea Outfatts.
153. Crude sewage should never be
discharged into the
sea without purification, unless the existing conditions
it certain that it will be carried
by the currents
right out to sea, and not taken along the coast to be
blown or washed ashore. In choosing a site for an outfall sewer the greatest care must be taken that the
position is one from which the currents flow in the right
direction. For instance, if the current sets from west to
east it would be the greatest folly to place the outfall;
west of the town, as in that case the sewage would all be
carried past the town in front of it, and very likely deposited on the beach. It is obvious that in such a case
the outfall should be at the east end of the town.
154. Before deciding upon any position selected, very
careful observations should be taken and experiments
carried out at all states of the tides and currents to ascertain what will become of sewage discharged at the point
The direction of the currents may be ascertained with floats, which must be of such a character
that the wind will not be able to blow them in a direcat the same time, the
tion different from the current
action of the prevailing winds upon floating matters must
not be lost sight of. The floats used for such experiments
are sometimes made of long strips of wood, weighted at
one end, so as to float vertically in the water, the
greater portion being submerged ; also a small float, from
which a bucket is suspended, can be sometimes used with
advantage. In either case the float will be very little
affected by the wind, and the direction of the current will
be made clearer. The currents at different depths should
be tested in this manner those close to the shore, also
the currents further out.
Under the most favourable conditions the
generally carried out to sea to the lowest low-
sometimes a vertical dip into
generally necessary th<a/t the outlet
Where conditions are not soshould be submerged.
favourable the outfall sewer may have to be carried far
out into the water, the work being done by divers.
Sewage should not be discharged against the current, or
the flow will be interfered with. If possible, the outfall
should be parallel with the currents, so that they may
aid the flow.
It is necessary to arrange that the end of
the outfall sewer may not become blocked up with sand
or stones, or be undermined by the action of the currents,
or sink into the sand, etc.
It is also important that the
sewage discharged may not settle in front of the sewer,
so as to in any way block up the outlet.
It may often
happen that an outfall sewer has to be constructed at such
a level that during a great part of the tide it is full of
water, and the pressure of the sewage is insufficient tocause any discharge. Also it may happen that there are
objections to discharging the sewage with a rising tide.
In such cases the sewer is made of a sufficient size to store
all the sewage flowing into it.
'Such sewers are generally
called tank sewers.
It is generally very necessary to provide them with efficient storm overflows, to prevent the
flooding of the lower parts of the town.
water mark,
the water.
156. It can well be understood that if the tank sewer is*
a level below high-water mark the sea water must not
be allowed to enter it, or the lower sewers of the town
will be likely to overflow.
It is customary in such cases
to have a flap valve fixed on the sewer to keep back the
water, and a penstock to retain the sewage in the tank.
flap valve is fixed as a safeguard in case the penstock is not closed when the tide rises to its level.
penstock should be shut and remain closed till the sewage
can be safely discharged without the sea water running
into the tank sewer. It is customary to fix a tidal flap on
most outfall sewers, unless sea water is intentionally
admitted into them for flushing purposes. This flap i&
not necessarily fixed on the end of the sewer, where it
for excluding water
will be likely to become useless
through getting stuck open, and where it is in an in-
It is better, if possible, to arrange
accessible position.
to have the tidal flap valve fixed in a special manhole,
the cover of which is above high -water mark, where it
can be attended to and kept in order. The retaining penAnother
stock should be fixed in a similar chamber.
method by which the sea water may be excluded from
and the contents
tank sewer,
discharged at
water only is that of the floating arm outThis consists of a float in a chamber, fixed to a
pipe connected to the sewer, with a movable joint at the
As the tide rises this floating outlet is always
kept above water level, and after the liquid level in the
tank sewer is reached no discharge can take place till
the tide falls.
157. It is said that fresh sewage discharged into the sea
not harmful to fishes, but that, on the other hand,
putrifying sewage is very harmful to fish life. Sea water
admitted to sewers is said to produce rapid decomposition.
This, therefore, is a further reason for excluding
ea water from sewers in certain cases.
158. That part of an outfall sewer which is in any way
likely to be exposed to the action of the sea is generally
constructed in cast iron, or sometimes in steel.
Stoneware or brickwork sewers may be used in r>laces where
they are at sufficient depth to be well bedded and protected in a mass of concrete, but it is necessary always
to bear in mind the fact that a sewer on the foreshore is
liable to be uncovered and undermined by the action of
the waves. It is, therefore, necessary to protect sewers
Thus it is well to support a
against this possibility.
sewer, laid in sand, not only underneath, but also at the
This will
sides, with piles or other suitable protection.
generally be necessary up to a depth of 3 ft. or so,
On a rocky shore it is
according to the circumstances.
sometimes well to lay the sewer in a trench with
and to fill
substantial foundation of concrete,,
the entire trench with concrete, which should follow
the general conformation of the rocks at the surface, so
that there may be no unsightly gash in the rocks, as frequently may be seen in works on the foreshore, when ex-
posed at low
159. When an outfall sewer comes above the ground
level it should be well protected against vertical or lateral
movement. This is generally done with substantial timber piling on both sides of the sewer, the pipes themselves being held by walings above and below them.
Flange pipes are generally fixed more easily than
spigot and socket pipes for this work, because a
if necessary.
joint may be made under water,
should be made on the course of the sewer in the tidal
water. These generally consist of an inspection hole in
the iron pipe, the cover of which is bolted down and made
that the outfall
watertight. It is necessary to remember
sewer should t f-j eonstructed that there may be no danger to boats from the cubm-srged piles
With regard
to storm overflows, their need arises
a great advantage to get rid of the
excessive quantity of clean water, which flows into a
system of sewers during a storm, which, if 'Carried to the
outfall, would make it necessary for sewers to be of very
great size, and would very much increase the bulk of the
foul sewage to be dealt with in one way or another.
very small sewer may be quite sufficient for the ordinary
flow, but if all the storm water is to be carried to the outThere will also
fall a sewer of vast size may be necessary.
be chances of such a sewer bursting and flooding the lower
The smaller a sewer can be kept in size the
greater chance will there be of it having a rapid flow, and
from the fact that
it is
keeping clean.
161. In designing storm overflows it must be remembered that when a storm occurs after a long drought the
first rush of storm water through the sewers washes away
any deposit left in them during the dry weather, and
thus it may be more foul in character than the ordinary
sewage. It is very clear, therefore, that the sewers must
be of a size to take this first flush, and that none of it
must escape at the storm overflow outlets. As soon as the
sewers are clean the flow of storm water following the first
flush will be clean enough to discharge into any natural
Storm outlets can
watercourse, or river, lake, or the sea.
be, therefore, carried to any .suitable point at hand where
the water may be safely discharged, or it may be possible,
as stated in an earlier chapter, to discharge the storm
water into some older and otherwise disused system of
sewers, or into a disused outfall sewer, or it may be discharged into a separate sewer made for the purpose.
These storm outlets from sewers must be taken from the
main sewer at such levels that storm water only can
The arrangement shown in the accompanying
is copied from Baldwin Latham's Sanitary
sketch (which
Engineering) shows how the principle of the leaping weir
(by which waterworks engineers manage to intercept the
clear water in a storm for use, and let the flood water pass
away) may be used for storm water sewers. In this case
the liquid which flows with a low velocity in small quantities is
the foul sewage, and
it is all
intercepted by pass-
through an opening in the bottom of the upper sewer
and falling into the sewer
.below, which rune to the outa .great current of clean storm
water in the upper sewer it will leap over this opening
and run to the storm outlet shown. In constructing this.
interceptor the opening through which the foul sewagemade smaller than is necessary, and
afterwards enlarged to suit the requirements of the
case as ascertained by trials.
idrops is generally
Great care must be taken that the storm outflowssufficient size to carry off the flow due
and sewers are of
to the greatest rainfall.
The storm outlets should discharge in places wihere
a possible ventilation of sewer gas will not be dangerous or objectionable. A flap valve may be fixed at the
outlet, and this is needed particularly in the case where
the storm outlets are 'below the water level of high tide.
The Maintenance of Sewers.
164. The proper maintenance of sewers is a matter
needing very careful and regular attention, without which
the best-designed system is always liable to fail in one
way or another.
It is, however, very often neglected, with
the result that sewers become a nuisance and a danger,
and various troubles arise.
165. If a sewer ventilator should give forth foul _
first thought of a great many persons is that it should
either be stopped up, or that the foul gas should be conducted elsewhere. It goes without saying that if such a
state of things occurred in a properly designed system
the remedy would ibe to remove the cause of the smell
viz., the foul matter adhering to the sides of the sewer.
If, on the other hand, the sewers are so defective that
they are not self-cleansing, it is clear that steps ought
to be taken to alter the defect.
166. The maintenance of ,sewers consists chiefly of keep,
ing them quite clean and well ventilated, seeing that all
valves and apparatus are in an efficient state, and that the
sewers are sound.
The sewerage system should be inspected frequently
and regularly, and the more often the work is inspected
the less chance will there be of any very considerable
work having to be done. If the system can be inspected,
say, once a fortnight, it will be found well worth while.
The inspection should include looking
to see that they are clear, and that no stoppage
exists in the sewer to dam the sewage back, and to ob-
exists due to a deposit
All flushing chambers should be examined,
together with the automatic flushing apparatus.
ehould foe ascertained that the syphons discharge properly, and do not dribble into the sewer continuously
instead, in which caise they will not flush the sewers.
All gully grids should be inspected and cleaned out if
The gully catch basins and retainers should
stopped up
and cleaned out whenever
foe inspected once a week,
necessary they should also be cleaned out after every
severe rainfall. If they do not receive frequent attention they will be useless for intercepting road grit.
Gully basins and retainers should be cleaned with a hose
.and brush, after the material has been removed, and if
they smell they should be washed out with a deodoriser.
serve if
any particularly foul smell
of sewage.
169. Manhole dirt boxes should be emptied, and the
open gratings of manhole and lamphole covers should
If any
!be cleared wherever necessary at the inspection.
foad smell other than that caused by fresh sewage is
{found to exist in opening any manhole or in inspecting
any ventilator the cause should be ascertained and
170. Hand flushing from manholes, lampholes, and
gullies will be found advantageous under particular circumstances, in addition to the ordinary flushing arrange-
171. Iron vent shafts should b inspected occasionally
to see that they are not choked, and the rust pits at the
foot of each should be periodically (inspected, and cleaned
out if necessary. No iron pipe with a bend at its foot
tehould be allowed to ,go uh tested, as the chances are
'that the bend will be found to be blocked up with rust
from the rpiipe.
172. If inverted syphons under streams, etc., occur ut
the system they should be frequently inspected to see
that they are working properly. If necessary they can
be cleaned by means of a chain passing through them,
as described in a previous article. If a catch-pit occurs
at the foot of the descending arm of the syphon it can
be cleaned by means of a ladle at the end of a long pole
without emptying the syphon. If the syphons exist in
duplicate, of course, the sewage can ibe diverted to one
syphon while the other is thoroughly cleaned out.
173. Any part of a small sewer found to be broken or
cracked should be relaid. Sewers which exist in places
where the soil is porous and where a leakage would be
dangerous to living houses near, should be periodically
tested with water to see that they are quite watertight.
It is an excellent thing to test the whole system where
The maintenance of sewage disposal works is a suband is a matter of the greatest importance.
ject in itself,
Space does not allow of a detailed description of the work
Failures are generally, if not
required being given.
It is well that the
always, due more or less to neglect.
disposal works should be placed in some conspicuous position, such as at the side of a railway or near a public
road, so that if they are neglected and a smell should
arise the matter may attract notice and the defect be
remedied before serious harm is done.
The most usual way of cleaning a small sewer much
These rods are generally
is by means of rods.
made of cane, and are fastened one into another, some175.
blocked up
times with a screw joint and sometimes with a spring
They are very like the rods a sweep uses for cleaning a chimney. A number of tools and brushes of various
kinds are sold for use with the rods. They may be worked
from one manhole and passed through the sewer to the
next, so that the sewer can be thoroughly cleaned. Dirt
so removed must be collected at the manholes and carted
176. A disc with cords attached to each side of it is
sometimes pulled through a sewer for cleaning it. A cord
is attached to a point near the edge of the disc, and this
cord pulls the disc into the sewer edgeways. The other
cord is attached near the edge of the disc at three or four
places, so that when it is pulled the disc travels so as
It is generto fill the sewer and scrape out any deposit.
move it backwards and forwards, cleaning a
To get the cord
short length of the sewer at a time.
through the sewer in the first instance it may be floated
through, or, when this is impossible, a ferret may Be seat
through the sewer with a string attached, a rat being let
loose in the sewer first.
ally best to
177. The method of cleaning a sewer by floating balls of
various sizes through it has been described in a previous
chapter sometimes, also, an egg-shaped ball, with a cord
attached, is used, with a flush of water behind it, for the
purpose of cleaning a sewer.
178. Sewers of 2 ft. diameter and upwards can be
cleaned by sending a man through them. It should be
remembered that prevention is better than cure, and
that, therefore, it is better to prevent the accumulation
of deposit in sewers by flushing than to have to clean
them by hand. No deposit should be allowed to systematically accumulate in the sewers with the knowledge
of those in charge.
179. If ashes or other improper substances are found to
cause a deposit in the sewers, the source from which they
'come should be carefully ascertained and the cause re-
extension of the sewerage system should be
carefully considering how it will affect
the existing work, and all new work should be carried!
out as well as the old work generally, it will be neces180.
made without
sary to carry
out better.
Sewage Disposal.
subject of sewage disposal is one which presents endless and fascinating problems to the scientist.
For him there is an endless amount of useful work to do
he is ever seeking to discover better and still better
methods, and to give the engineer exact reasons for the
results discovered in practical working.
To him sewage
disposal is a problem.
For the engineer, however, the case is different, and
without any disrespect to the scientist, it may safely be
said that the engineer can approach his work in a very
different spirit, knowing certainly that there is no insuperable difficulty in purifying any ordinary sewage.
In fact, where conditions are satisfactory, it is a comparatively simple matter to the experienced engineer.
The scientist very properly seeks to ascertain the exact
causes of purification, to find the best and simplest treatment for sewages of various kinds, to tabulate results
obtained, and he is continually experimenting with new
methods, with the object of finding out such things as
how to treat sewage on tne smallest possible area, how
to eliminate sludge, how to treat the sludge thus extracted so as to get rid of it without nuisance, how to
produce an effluent of absolute chemical and bacterial
All the
purity, or how to prevent clogging of filters.
results obtained have their uses, and the engineer avails
himself of them, but, owing to the vastness of the work
before the scientist, he occasionally becomes extremely
pessimistic, and tells the world in general that he has
but touched the fringe of the subject, and that the great
sewage disposal problem remains unsolved.
Though this may be true from the scientist's point
of view, it is, practically speaking, an absurd statement,
as the many excellent works constructed of late years
182. Unfortunately, there has been a very strong tendency for individuals to urge the merits of the particular
system on which they have been working, to the exclusion of other methods of equal worth upon which they
have had little or no experience. The scientist whose life
work has been the perfection of the X system does not
hesi taste to tell us that the Y system is dead and useless
similarly, the eminent inventor or discoverer of the
system does not hesitate to condemn the
system with
Not only so, but each has been in the habit of
bitterly opposing all other methods of sewage purification, so that the effect on the lay mind has been that
the whole subject of sewage purification is an unsatisfactory problem, the proper solution to which has yet
to be found. People have even taken sides at municipal
and " anti
elections, calling themselves
in the
actually happened
West Country, and serves to show the foolish way in
which people have taken sides over the matter.
By practical men, with knowledge of sewage disposal,
these differences among scientific men are looked upon as
purely academic discussions, useful, no doubt, but by
affecting the undoubted fact that ordinary
sewage can be purified, and is being purified satisfactorily and economically at all works properly designed,
constructed, maintained, and attended to, of which works
there are not a few in existence at the present time.
The causes of failure, where they occur, are almost
always perfectly obvious to the experienced engineer.
no means
183. To start at the very beginning, it is an undoubted
fact that if a bucket full of the foulest sewage were
poured out upon the surface of a loamy, ploughed field,
it would soon be purified.
The liquid would soak away into the ground, and soon
become harmless, and the solids would disappear in a
'short time. This would happen in the same way that all
foul matters lying on the surface of the ground have,
since the world began, been resolved into harmless constituents by natural processes.
184. It is clear, then, that, given sufficient 'area of
land and proper distribution and attention, crude sewage
can be purified on land. Whether the method is practiand
cally possible, economical, unaccompanied by smell
so forth, is a separate matter, which can only be considered
when the
conditions of the
185. It is acknowledged that sewage passing through
sand or loam or most earths is thereby filtered and purified to a degree, and the scientist tells us that the purification effected is due largely to the action of certain
micro-organisms present in the land and in the sewage.
186. Experience proved that certain land was better
than others for the purification of sewage, and efforts
were successfully made to prepare land so that it might
possess the characteristics that were found to give the
best results. Hence, acres of porous, sandy, or gravelly,
under- drained land were prepared also with success, so
that the area could be reduced.
This, again, led to the
construction of
of various kinds.
187. It is not too much to say that all modern methods
of purification in general use consist of filters of one kind
or another under many different names, and that after
all has been said and done the undisputed fact remains
common to all systems that for its final purification
sewage must be brought into contact with some medium
on which the micro-organisms necessary for sewage puriHow this may best be done is a
fication can exist.
matter needing very careful consideration in each case.
The quantity and quality
The possibilities are endless.
of the sewage, the available fall, the possibility of sanedl
being a nuisance, the cost of land, and a hundred other
considerations will guide the engineer in designing his
will need all the assistance that can be obworks.
tained from the chemist 'and bacteriologist, and it may
here be demonst rated that the conclusions of the scientist
will rrove of little worth, and even be .apparently falsified by 'bad engineering. Unless the works axe properly
designed, constructed, and managed the results confidently expected by the scientist will not be forthcoming,
This has happened only too
and he will earn discredit.
often where apparently the scientist h<as been his own
It is with the design, construction, and
management that the engineer
concerned, and failure
only possible where 'these matters get into inexperiTo condemn any system
enced or incompetent hands.
of .sewage purification as altogether worthless is wrong.
One mi spit just as well condemn the wheel-barrow because of the automobile, or the steam engine because of
The system which may be essential
the electric motor.
to success under the special conditions of one place may
be perfectly unnecessary, useless, or even harmful at
It is, therefore, necessary for the engineer who
wishes to take up the important work of sewage disposal
to study every known method so as to be able to a.pply
his knowledge to meet and overcome all difficulties as
they arise.
188. With regard to the various cases where sewage
treatment must differ the possibilities are endless but
in order to demonstrate how widely different the condi;
may be at different places it will be instructive to
consider a few typical instances.
small town or village on the sea coast where it
possible to obtain an outfall at a point where a strong
current will carry the sewage discharged right out to sea,
where also there is no likelihood of contaminating oyster
beds or fisheries, or of otherwise causing a nuisance.
small town or village the outfall from which must
In this case the degree of puritidal river.
fication required will often depend on the nearness of
It is clear that in, a large
fisheries 'and oyster beds.
river like the Thames the dilution will be great, and
that df the outfall were carried out into the main, current
it would be possible, so far as nuisance was concerned,
to discharge an effluent that would not be fit to go inito
the same river aibove the locks.
An eflluent tihat would,
in fact, be only partially purified would certainly cause
no 'annoyance so far as sight or smell were concerned,
but, on the other hand, the possibility of contaminating
oyster beds might make it necessary to produce an, effluenl
of very high* bacterial purity.
It might be necessary
to take every possible precaution even to the sterilisation
of the effluent.
run into a
(c) A town situated on a river .above locks- might in
some cases have to discharge an effluent of a very high
standard of purity if the riveir water weTe used for
This may seem a.n exdrinking purposes lower down.
treme case, but it is not.
The water of the Tn<ame'8
and Lea are used for the supply of London, and an
enormous quantity of sewage effluent is discharged into
these rivers above the intakes neither is it possible to
avoid doing so, for assuming that it were possible to lay
sewers alongside the rivers <and all tributaries receiving
sewage, or to go elsewhere for a water supply, manyyears must elapse before such works could be brought to
The problem is therefore a very grave onecompletion.
(d) A town situated on a river above the locks where
water was not used for drinking purposes.
Such a case
differs from case (c), inasmuch as all the purification
that is really required is such as will render the effluent
unobjectionable to sight or 'smell, so that it doe not in
The enormous
any way spoil the river or hurt the fish.
dilution and the purification which takes place in the
river itself may be taken into consideration if the river
authorities will permit.
(e) A small village situated far from any river where
the effluent must pass into a ditch with a small flow and
travel perhaps for .a mile or more toe-fore it reaches
In such, a case the effluent is
larger stream.
subjected to a test as severe as can be given, and if it
is not quite pure a
very serious nuisance may result.
Therefore, it will very likely be necessary to treat the
sewage in a, manner that would be altogether unnecessary
for a much larger and more important town situated as
in case
189. These instances are
typical of many others, and
will serve to show how the conditions of each case must
carefully considered before one can come to any conclusion as to the degree of purification
Then, again, with regard to methods of purificasewages vary in quality and strength at different
places and at different times at the same place, so that
the area of a filter which may be quite sufficient to
produce an excellent effluent without clogging or exhaustion
of the filter at one place, may be
altogether inadequate to
purify the same volume of sewage of different character at
Sewage may be affected by large quantities of
subsoil water in the case of old sewage systems, causing
enormous dilution. Trade waste may give it a peculiar
character. Septic action may take place in large systems,
or in systems in which lifts, ejectors, or pumps are used,
and many other causes may' affect the sewage, and till
they are all known it will always be impossible to decide
the method of treatment exactly.
Also at some places
the disposal works can be constructed at a site where a
certain amount of smell caused by the discharge of the
sewage and tank liquor may be of little account. At
others it may be a matter of the greatest importance to
reduce the smell to a minimum. There may be facilities
for dealing with sludge, or, on the other hand, it may be
almost impossible to remove sludge, or treat it on the
works at all. There may be unlimited area at the engi190.
or space
be absolutely restricted.
Likewise the available fall, the character of the subsoil,
the materials available, the price of the land, the rights
of private owners, and innumerable other conditions, will
have to guide the engineer in deciding as to the be'st
method and details of the work.
Royal Commission on
Sewage Disposal very justly states that sewage may be
purified to any degree required, either by land treatment
or by artificial filters.
This clause, however, would be
very misleading if one did not read the other conclusions,
which the Commissioners clearly state that
effluents from soils which are not well adapted for the
purification of sewage may often be very impure." In writing on the subject o;f sewage disposal-it would be impossible
to omit the extremely important conclusions arrived at
by the Commissioners after so many years of work, the
report having been published felicitously while the present
articles on sewage disposal were in course of preparation.
The present work, however, applies more particularly to the sewage disposal of small towns and districts,
while the Royal Commissioners deal with the subject
generally, and perhaps more particularly with the problems presented by very large towns and cities, where the
methods of disposal must differ in detail considerably
from those suitable for the vast majority of works,
which are, of course, small. A verbatim report of the
Summary of the Conclusions and Recommendations of the
Commissioners appeared in a recent issue of this journal,
so it will be unnecessary to give them again in detail, but
reference is made to them and to statements in the body
of the report in order to elucidate the subject.
192. It is not the author's intention to describe in
detail every known process of sewage disposal. There are
a large number of special processes where special che-
micals, filtering materials, and apparatus are used which
may have their own special merits for special conditions,
but, broadly speaking, the methods are as follows:
(a) The treatment of crude sewage on filters, contact
beds, or land.
(b) The treatment of crude sewage by a preliminaryprocess, whereby grit and suspended matters are more
or less removed, the liquid afterwards being treated OR
filters, contact beds, or land.
The Royal Commissioners are of opinion that such
preliminary treatment is generally desirable before
attempting to treat sewage on land or filters. It should
never be forgotten, however, that the mere removal of
suspended matters results in the accumulation of sludge,.
and that
.sludge has always been the great difficulty of
sewage disposal, and that this sludge has then to be
satisfactorily purified at the works, or removed and purified elsewhere.
Many reports and interesting papers
have been written to describe how best to purify a clear
tank effluent, but very little has been said of the sludge.
On visiting the works described, it is not at all uncommon to find masses of putrid sludge exposed in a most
primitive manner. It is begging the question merely to
remove the worst part of the sewage and to purify the
193. The methods (ct) and (b) may again be sub-divided
under various 'headings, and in doing so it will be best to
follow the statements made in the Royal Commission
Report as far as possible. There are also two other processes which, under special conditions, have their uses
when applied in connection with the above, which deserve special attention viz., the deodorization of tank
effluents and the sterilisation of filter effluents, both
being possible within limits of reasonable expense, the
former being well spoken of by the Royal Commissioners,
and the latter by the Board of Agriculture of the United
States, in a report issued recently dealing with the
194. The earliest attempts at dealing with the subject
of sewage disposal as a whole without any preliminary
process for the 'removal of suspended matters consisted
Such treatment
of discharging crude sewage on to land.
is by no means an impossibility, but the area of land
required or the amount of labour involved in producing
good results was found to be enormous in most .cases.
The nuisance caused by the solid matters exposed on the
surface of the land, and the unequal character of the
sewage at different times, proved the necessity for some
other treatment as a preliminary.
195. As a result, tanks in which the sewage could mix
and settle were used, and this settlement or precipitation
of solids was promoted by the use of chemicals, with the
result that a clear effluent, liable, however, to secondary
was discharged on to the land. This
method, at one time considered perfection, of course, resulted in the accumulation of enormous quantities of the
most objectionable matter, and this sludge had then to
oe removed in carts, spread out to dry, buried in the
land, hopelessly stored
pressed into cakes.
more successful method, and one which undoubtedly got rid of some of the sludge, was that of upward nitration. The crude sewage was passed into the
bottom of a tank in which gravel or other filtering material was supported, two or three feet above flow level,
on an iron grid or other similar arrangeni' nt. The liquid
passed up through the filter, and the sludge remained
below. It was found that some o.l the sludge thus intercepted was liquefied and broken up. The bulk of it, however, had to be removed and dealt with as before.
a tank with .submerged inlet
197. Next the septic tank
and outlet holding' about a day's flow was found to produce liquefaction and breaking-up of the sludge. From
the fact that such tanks will often work for years without emptying or removal of sludge, it was argued that
the whole of the sludge was liquefied. As a matter of fact
fact it has been proved by experiment that a considerable proportion of the sludge is actually liquefied. The
remainder, in cases where the sludge is not removed, is
broken up before it is discharged, but as this sludgy
effluent put sprinklers out of action, and as it also
clogged up filters of inadequate size, a storm of condemnation was aroused against this otherwise useful
198. The hydrolytic tank, patented by Dr. Travis, was
the outcome of the upward and downward submerged
The sewage is passed
and the septic tank.
through various sections of the tank, some being filled
The sludge is thoroughly
with slabs of material.
eliminated by this tank, and a very good effluent is produced suitable for subsequent filtration, but in order to
obtain this result, the sludge thus separated has to be
withdrawn regularly and dealt with. As in the case of
the septic tank, the quantity of sludge was lessened
owing to liquefaction in the tank.
199. Every effort has been made, by the use of tanks of
various forms, to eliminate the suspended matters
tanks with conical bottoms, with special arrangements
tanks in series, upward and downfor drawing off sludge
ward and vertical submerged filters in tanks chemical
precipitation tanks ; and, in fact, a large number
in order
special devices have been used from time to time
to remove as much sludge from the sewage as possible,
so as to render the subsequent filtration easier.
200. Most of the experiments aiidi research
during the
last ten years have been in the direction of separating
the sludge from the sewage and in filtering the effluent
on the smallest passible area. Each exiponent has done
his best to prove that by his particular method a greater
number of gallons per cubic yard of filter can be purified
than by any other. They have not failed to tell us that all
suspended matter must be first removed. The clearer
the tank effluent, the smaller the filter and the better the
working of the sprinkler. The arbsurdity of such reasonTo carry the arguing scarcely needs demonstration.
ment one step further, one reaches the fact that if one
could only extract all impurity by the preliminary process and produce clear water, the filter would take an
unlimited quantity of liquid per square yard and the
sprinkler would not be needed.
201. Practically nothing by comparison has been done
in the direction of finding out how to purify OT gt rid
of the sludge itself.
Seeing that by the ibest processes
about 75 per cent, of the impurities present in the crude
sludge are removed as a preliminary to filtration, and
that these matters remain unpurified, it will be seen
that the remarks made above are not without justification.
A liquid is discharged from the settling tanks containing
only 25 per cent, of the original impurities present in
sewage, and this weak liquid can, of course, be easily
dealt with on filters.
We are asked to forget the remaining 75 per 'pent, of foul matter remaining on the
ground. It is this fact which has caused a general feeling of uneasiness and distrust of every known method
Local authorities and others who
of sewage disposal.
are considering new schemes should ascertain
(1) how
much sludge is likely to be removed by the preliminary
process, and (2) how it is proposed to deal with this
202. It is necessary to consider in detail the methodsgenerally adopted for the removal of suspended matters
from sewage before filtration, for the reason that, whether
it is possible to do without such treatment or not in the
majority of oases, there will probably always be special
where such preliminary treatment is the only practical possibility.
Before, however, describing thesemethods, the author wishes to draw special attention
to the fact that latterly the object of scientists, and also
of many engineers, has been to reduce the area of the
filter to a minimum. The reasons for this effort are by
no means clear, seeing that filters are not expensive to
construct or to maintain, and that if they are of sufficient area they will deal with any sewage in which the
solids are broken up, and that, moreover, with filters
properly designed and worked, the smell can be reduced'
to a minimum. On the other hand, the removal of sludge
which is an essential preliminary where filters are of
inadequate size is very expensive, very objectionable,
altogether unscientific, and in most cases would be altogether unnecessary if a larger filter area were provided.
Moreover, it is no question of the relative costs of two
methods it is the comparison of the system which purifies the whole of the sewage with the system which purifies part only.
In this there should be no confusion of
there can be no doubt as to the comparison between the system which purifies and gets rid of the
sludge and that which leaves it unpurified and lying
about in heaps.
it is
sense to start with the as-
sumption that any system of sewage disposal worthy of
the name must be one which will purify ox at least get
rid of the whole of the sewage.
does not necessarily follow that any system
is incomplete.
Provided that some
satisfactory provision can be made for the treatment of
For insuch sludge, the system would be complete.
stance, there are some irare cases where all the sludge
can be carried out to sea in barges where there is some
some real use for it or where it can be buried without
nuisance or danger. Such cases are the exception, and
not the rule.
203. It
which produces sludge
204. The processes for the removal
matters are generally as follows:
There are two types of settling tanks:
sedimentation tanks, or those in which
the sewage
stands for some time absolutely at rest; and (b) continuous flow sedimentation tanks, or those through which
the sewage flows slowly, but continuously. In the first
case a tank is filled with sewage, and after it has stood
long enough for the solids to settle probably about two
or three hours the liquid is drawn off, generally through
a floating arm outlet. This sort of tank has the disadvantage that the sludge must be removed every time the
liquid is drawn off, as fresh liquid entering would otherwise stir it up. It is, therefore, necessary to have duplicate tanks in this system. The cost of so frequently removing the sludge is a drawback.
In the case of a tank with a continuous flow through
everything is done to retard the flow as much
The sludge is continuously deposited,
as possible.
and is drawn off without emptying the
There have been many
designed, such as the Kinebiihler tank at Dortmund, in
which the sewage descends through a tube to distributors
fixed low down in a circular tank with a conical bottom.
The sewage rises very slowly from the distributors, leaving the solids at the bottom. The sludge is removed by
pumping, and the clarified effluent overflows at the top.
The sludge has to be removed from such sedimentation
tanks before fermentation is started, which would cause
a great deal of solid matter to float, and would render
both sludge and sewage very foul. For this reason sedimentation tanks have to be of such a design that sludge
may be removed frequently it is obvious that in the
case of quiescent sedimentation tanks the sludge must
be removed between each filling if the fresh sewage is
not to stir up the old sludge, and that in the case of
continuous flow tanks the sludge must be removed at
least every two or three days to give the best results.
205. The capacity of a tank or set of tanks must be
such that they will take all sewage that requires such
treatment without allowing any overflow to take place.
The Royal Commissioners advise a period of flow through
continuous flow tanks of from ten to fifteen hours where
the tank liquor is to be treated in fine filters, and advise
also that they should be cleaned out at least once a week.
The precipitation
of solids
be accelerated in
sedimentation tanks by the use of chemicals, and this is
generally done in the case of very strong sewages or where
is thought desirable to neutralise an acid sewage.
The Royal Commissioners advise an eight-hours rate of
flow through tanks in the case of a continuous flow and
a two hours' rest in the case of quiescent sedimentation
but they very justly point out that no rule for general
application can be given.
Generally the chemicals are mixed with the sewage on
its way to the tanks. That no proportion of chemicals
Trill do much more than clarify the sewage has
.proved, so it is best to do with the least amount that will
Lime has been found to be an
produce precipitation.
economical substance for this treatment, and it is thereiore largely used, generally in conjunction with other
chemicals. It is reduced to a milky state before adding
to the sewage, and it is sufficient by itself to clarify
sewage. It appears, however, that a large quantity is
needed, comparatively, to produce the result, and this is
very objectionable, for the reason that though an excess
of lime produces rapid precipitation and a fairly clear
though apparently clear and without
very likely to become putrid later on,
owing to the fact of the lime dissolving a large amount
of offensive matters held in suspension in the sewage
which will afterwards putrify.
effluent, this effluent,
Thus, lime and chloride of lime, lime and sulphate of
alumina, lime and phosphoric acid, lime and proto-sulphate of iron, -lime and alumino ferric, etc., are used at
different places, also strange mixtures of alum, blood,
magnesia, manganate of potash, burnt clay,
chloride of sodium, animal and vegetable charcoals, and
magnesian limestone have been used, also the complicated
material ferozone has been found to give good results as
a precipitant.
The Royal Commissioners state that
for the precipitadomestic sewage of average strength alumino
ferric is by far the most common precipitant, and that
the average quantity of the precipitant may be put at
about 10 grains per gallon, and that the average cost of
the chemical is about
2 5s. per ton.
207. The Royal Commissioners also state that chemicals are most effective when added in the form of a solution, but that in practice it is sometimes more economical at small works to use it in solid form, owing to
the fact that it is advisable to vary the quantity of the
solution with the varying strength of the sewage.
cases a solid block of the chemical is placed in the
sewage channel so that the top of it projects above the
level of the sewage.
In this way the largest quantities of
the precipitant are dissolved with the greatest flow. The
sewage should be well mixed immediately after the addition of the chemical by means of baffling boards or other
mechanical apparatus.
208. Such tanks as those described above have to be
emptied of sludge, as a rule, by hand. The nuisance
experienced is great, while the labour is of a most
unpleasant nature, and no man ought to be expected todo it at ordinary workmen's pay.
209. It is here necessary to state what methods
have been devised for getting rid of the vast quantitiesof sludge remaining behind after the process of precipitation is complete.
This sludge, consisting, as it does, of
the foulest matters present in sewage, is a very objecAs already stated, it is
tionable substance to move.
sometimes pumped into vessels and discharged out at sea.
At small works it is sometimes possible to get farmers to
buy it to manure their fields with, or they may be induced
to take it as a gift. It is more probable, however, that
little reflecthey will have to be paid for taking it.
tion will enable anyone to understand that it is impossible to apply manure to fields at all times of the
It has long been recognised by those who are
brought face to face with the practical difficulties in
dealing with isludge that it is useless to regard
material as a valuable manure, but that, on the conwhich
trary, it is merely a highly offensive substance,
must be got rid of without delay at the least possible
Its disposal is often likely to cost as much
as 2s. or 3s. a ton in practice, and the process of removal
Sludge is sometimes intercepted \yy
highly offensive.
A mixrunning the sewage through ash refuse heaps.
ture of sludge and ash is formed, which is carried to the
It is sometimes mixed with house refuse, and
The process of digging sludge
burnt in a destructor.
into the ground has been found effective at some places.
The ground is genet ally trenched for the purpose.
Another method is to compress the sludge into cakes in
what is called a filter press. The solid matter is refoul liquid is strained
tained, and a large amount of very
out through canvas when the pressure is applied.
The cakes produced
is thus made less in bulk.
the best
in the sludge press have then to be got rid or by
available means, and the foul liquid runs to he ordinaryi
210. It may be well understood, therefore, that any
system of sewage disposal in which sludge is accumulated
presents a very horrible state of things, to be avoided
^wherever possible.
septic tank receives all the sewage and retains
the solids. This is effected by having the inlets and outSolids either sink to the bottom and
lets submerged.
form sludge or float and form a thick scum on the surThe object of having the inlet submerged is to
prevent disturbance of the scum, and the object of having
the outlet submerged is to prevent solid matter escaping
with the effluent.
212. In recent work the plan most generally adopted
has been to allow the .sewage entering the tank to flow
over a weir extending for the full width of the tank; a
scum plate, or dipping-board, or submerged arch extends
in front of this weir for the full width of the tank, and
dips two or three feet below the surface of the liquid.
There is a similar arrangement of scum-plate and weir at
The inlet and outlet weirs are necesthe outlet end.
The tank is generally not less
sarily at the same level.
than 7 ft. deep, though there does not appear to be any
reason why such tanks sihould not be made (as, indeed,
they are sometimes made) considerably deeper its length
is generally at least two or three times as great as its
'breadth, or more.
213. The Local Government Board generally require
that septic tanks shall be made in duplicate, and also
that they shall have one or more sludge outlets for cleanSuch tanks generally hold from one to
ing purposes.
two days' dry weather flow, and are sometimes covered
with a brickwork or concrete arch, or with a lighter roof,
and are sometimes open. In the author's opinion septic
tanks should always be covered, generally with a light
roof the thick scum which forms on the surface of the
tank will otherwise be disturbed by the wind and rain,
and a great nuisance will be experienced also when the
scum is thus disturbed it is likely to be given off in large
lumps at the outlet end.
214. In considering the design of a septic tank it isnecessary, first of all, to have a clear idea as to the work
one wants it to do. There may be said to be two distinct
(a) If the tank liquor is to be distributed over
a percolating filter by means of a sprinkler or over a fine
filter it will be necessary first to see that no thick effluent
or broken up solid matter likely to clog the sprinkler
or filter are discharged ; and, secondly, to avoid as far
as possible the smell which is generally inseparable from
septic tank effluent sprayed or spread out in the open'
The periodical removal of sludge from the tank:
will be inevitable in such a case. The object to be aimed
at will therefore be to construct the tank in such a
manner that sludge may be easily withdrawn, and that
the liquid may not remain in the tank long enough to
become very foul, or, as it is sometimes termed, " over
septicised." Up to a certain point there can be no doubt
that the longer sewage remains in a septic tank the
worse it smells. It is not the author's intention to deal
with the purely scientific side of the subject of sewagedisposal, which matter must be dealt with by the chemist
and bacteriologist, but it seems reasonable to suppose
that while the solids require a long period of septic
action, a considerable amount of the liquid sewage might
well pass on to the filters at once without being subjected
to the action of anaerobic organisms.
Thus, if the tank
is large enough to treat the solids to the extent required,
and if sludge is to be removed, it follows that a septic
tank of the kind here described may be of a smaller
capacity than in a case in which the object is to avoid
sludge removal. In such a case as that under consideration the tank might hold, say, half the dry weather daily
American practice gives results in
flow or even less.
favour of such small tanks. They differ from quiescent
sedimentation tanks in that sludge is not removed so
Good results
frequently, and they are therefore better.
have been obtained by making such septic tanks in sections capable of being used altogether or with one or.
more sections cut out. These sections are connected by
valves, and the floor of each section has a sharp fall,
The tank
to a sludge outlet valve.
of 45
should be, say, 10 ft. deep to allow of a large accumulation of sludge, and the outlet should be, say, 3 ft. orr
more below the
surface, to avoid the escape of
scum or
building the tank in this
possible to
it is
by experiment, the size of tank or duration of flow
through the tank which will produce the best result,
and it will also be possible to draw off the sludge easily
The ordinary septic tank with flat
as may be required.
floor and perhaps one sludge outlet cannot be cleaned
but when
satisfactorily without draining off the liquid
the tank is divided up into sections, as described above,
the process of sludge removal without emptying the tank
becomes possible. In the author's opinion, this is not
the most satisfactory method of treating sewage under
ordinary conditions, but it is a practicable method of
producing a tank liquor fit for distribution by the ordinary sprinkler or fit for treatment in a fine filter.
215. (b) In the second case we have the septic tank
used not to eliminate the sludge, but to break it up so
that it may be treated on filters and ultimately purified.
That a tank of this kind is quite unsuitable as a preliminary for the treatment of sewage to be afterwards
sprayed through sprinklers should be thoroughly understood.
Much discredit has been brought upon both the
septic tank and the sprinkler by the strenuous efforts of
engineers and chemists to force solid matters through
small holes in sprinkling apparatus intended for liquid.
Also a septic tank which has a capacity sufficient for the
breaking up of the solids, and for producing the greatest
amount of liquefaction, will undoubtedly under ordinary
circumstances produce a most evil-smelling effluent, which
when sprayed in the open air will cause an intolerable
nuisance. On the other hand, a septic tank in which the
solids are broken up and discharged in the effluent is an
excellent preliminary to contact-bed treatment or to
filtration in filters of large
area, over which
sewage is distributed in channels. Filters of the latter
kind are not common those of the Baldwin Latham type
are, however, an example where the filter is of large
area, with a layer of fine material on the surface, by
means of which the sewage is distributed.
Contact beds, on the other hand, are largely used, and
if they are of sufficient capacity and not overworked they
will certainly purify the sludgy effluent discharged by a
septic tank of the second type. The author has not the
slightest hesitation in making this statement, having
carried out the process and observed it closely during the
last six years at several places.
Speaking generally, this
system, whereby all sludge is purified, has not received a
fair trial.
Either the septic tank has been emptied,
owing to counsels of panic on the part of the authorities,
or the capacity of the contact beds has been altogether too
small for the work required.
216. Assuming that the filters or contact beds are sufficient, a septic tank may be allowed to discharge a thick
effluent, and thus get rid of its broken up sludge without
causing trouble.
Consequently, it will generally not be
necessary to remove any sludge from such a tank. Also,
if the liquid is not sprayed in the air there is no great
objection to its being foul, as liquid in distributing
channels does not smell to the same degree, and such
channels can always be covered up if required.
Thus the tank may be made of sufficient size to enable
maximum amount of septic action to take place,
and in consequence more sludge will be liquified than in
case (a), and what solid matter is not liquified will be
broken up by the violent fermentation which takes place
in such tanks, and will be delivered with the effluent.
The Royal Commissioneis appear to have grasped this
fact to some extent when they state in Clause 40,
that in the case of works constructed to serve
to 10,000 it would be best that septic
tanks should be allowed to run, without being cleaned
out so long as the suspended matter in the tank liquor
showed no signs of affecting the filter detrimentally,
provided that the tanks were never allowed to become
more than one-third full of sludge. This is a very
cautious statement, but it suggests for one thing leaving
the tanks alone.
Obviously, such advice does not apply
to any place where a sprinkler is used.
any population up
217. In case (b) the septic tank will be very much
more economical than any precipitation tank. There
should be no sludge removed at all, because the tank, if
will clean itself.
Violent fermentation will
take place at times, and the whole of the sludge and
most of the scum will come out with the effluent in a
broken up form. The filtei's will be hard worked for a
time, but in case of, say, triple contact beds filled only
once a day, holding material of the right grade, tholeft alone,
roughly drained and worked with regularity, the whole
of this sludge will certainly be .purified under ordinary
It is objected that in course of time the beds will lose
their capacity. This is quite true, and one must honestly
face the fact that in such a case the filtering material must
be washed or renewed. In the author's experience this
has been found to be necessary with a very strong sewage
once in five years. Now, it is a fact that the material
thus washed away is by no means the same thing as
If the contact beds are in proper order it will
not be foul at all, but simply of a black, earthy character.
The removal of sludge matter at long intervals is not a
very serious thing, and compared with the cost, trouble,
.and annoyance of chemical treatment, and the constant
removal of sludge, is a trifling matter. Compared with
the septic tank of type (a), the advantage is clear, for in
the one case the frequent removal of sludge has to be
faced, which is an intolerable nuisance while in the case
{b) no sludge is removed, but a more generous allowance
of filter area is provided. To compare the relative expense is difficult, but in one case the whole of the sewage
is purified and in the other part only.
Septic tanks of class (b) must have a capacity of
least a day's flow or more, and otherwise there is no
special difference in the construction except that they
need not be divided up into sections, the floor need 'not
have the sharp fall, and a depth of seven feet is generally
sufficient. It will do no harm to have sludge-emptying
valves, provided they are not opened till the need arises.
In the author's experience it has not been found necessary to open on during six years of constant working.
218. Having briefly described the preliminary processes for sewage treatment it is now necessary to consider the methods whereby final purification may be
First of all it can be -clearly stated that under ordinary
conditions there is only one way in which this final puriThere
fication can be effected, and that is by filtration.
are, however, many methods of filtration, starting
the simple irrigation of natural land with sewage and
ending with the very latest percolating filter or contact
Neither is it possible to draw any hard and fast
line between what is generally known as natural filtration and the artificial processes.
There are many ways
of preparing land for the treatment of sewage, which
vary according to the conditions of the case and the
character of the ground. In one case a few irrigation
channels and general preparation of the surface by levelling may produce the required result, while in another
special areas of land may have to be carefully prepared
for a depth of three feet or more with an elaborate
system of sub-drains. It is difficult to see that such a
natural filter permits in any way a more natural method
of dealing with sewage than a filter which has to be built
up artificially. In fact, it seems wrong to call any process natural whereby sewage is conducted through pipes,
channels, and other artificial works over a filtering area.
As a rule, the ground needs very careful preparation if
good results are to be obtained, and more subsequent
attention than an artificial filter, or so-called. It would
be better to call every method of filtration natural or
every method artificial without distinction, since there
is no essential difference in the work done by land or by
and according to the Royal Commissioners
there is no essential distinction between effluents from
land and effluents from artificially constructed filters,
and also because " it is practicable to purify the sewage
of towns to any degree required, either by land treatment or by artificial filters, and there is no essential
difference between the two processes."
219. Thus there can be only one general method for
final purification
and that is filtration. The problem
as to how this filtration may best be accomplished inany case is a matter which can only be decided by the
engineer when the
exact conditions of
that case
There are endless possibilities sometimes one method may be best and sometimes another,
frequently a combination of methods will be necessary
laid before him.
endless scope for ingenuity, which, if properly
applied, will produce works that will not only fulfil their
useful purpose to the benefit of a whole community, but
will do so in the most economical manner possible.
220. 'The recommendations of the Royal Commissioners with regard to filters do not leave the reader
with any very satisfactory idea of how best to proceed.
The first conclusion with regard to filters proper, as disthere
is as follows
For pracand assuming good distribution, the samepurification will be obtained from a given quantity o
coarse material, whether it is arranged in the form of a
deep or of a shallow percolating filter, if the volume of
sewage liquor treated per cubic yard be the same in each,
tinguished from contact beds,
tical purposes,
With regard to percolating filters of fine material, if
the liquid to be purified were absolutely free from suspended or coloidal solids, and if thorough aeration could
be maintained, the statement just made for filters of
coarse material might possibly hold good for filters of
In practice, however, these condifine material also.
tions can scarcely be maintained with large rates of
flow, and we think that the greatest efficiency can be got
out of a given quantity of fine material, arranging it
in the form of a shallow filter rather than of a deep
But we are not in a position to make an exact
quantitative statement as to the difference in efficiency
of the two forms."
a percolating filter is
through which the sewage percolates, and it is obvious,
that if such a filter is composed of coarse material the
"good distribution" mentioned by the Royal Commissioners is absolutely essential, as otherwise the sewagewill run through it in a stream at any point where it
may be so delivered on the surface of the filter, and
under these circumstances it is desirable at the outset
to consider what appliances are available for producing:
"good distribution."
The Royal Commissioners
offer comments on four
types of sprinklers used for this purpose, which may be
quoted at length as the matter is of the most seriousimportance.
Type No. 1 As regards moving
make the following observations:
we may
"(a) Unless driven by applied power, which must obviously increase the cost of distribution considerably,
they are seriously affected in their distribution by wind,
and by severe frost and snow. In the case of light revolving sprinklers, they are not infrequently brought
to a standstill by the wind.
Where the filter is a small
one, this disadvantage may, to a considerable extent, be-
overcome by building wind screens round the filter or
by raising the retaining walls, but on large works we
should think that this would be impracticable, and with
large filters not very effective."
Whether the Commissioners have quite grasped
From the statement
open to question.
quoted above it would certainly seem that the ordinary
moving sprinkler is a very unsatisfactory arrangement
hut most engineers and sewage works managers must
have been surprised to read this sweeping condemnation
of an appliance which is in very general use, and in the
author's opinion it would be well for any engineer protrue facts
posing to use a revolving sprinkler to gather facts for
himself from the many works where such sprinklers are
However, assuming that the Royal Commissioners have had an exceptional experience with their
sprinklers one cannot altogether lose sight of the fact
that with such a method of distribution there is always
the possibility of the sprinkler stopping, in which case
the sewage will leave a coarse filter absolutely untreated.
No such risk is run with filters which are not dependent
for their proper working on a sprinkler.
Report continues as
(b) Automatic revolving sprinklers, when used
intermittently (i.e., when fed from a flushing tank) are
capable of adjusting themselves to large variations of
flow, but when used continuously, without a flushing
tank, their rates of working can only be varied within
somewhat narrow
would be more, exact to say that when used with a
storage tank the tank will, if large enough, deal with the
varieties of flow.
It cannot be said that the -sprinkler
ever adapts itself to these variations, except within the
narrow limits mentioned, and it may be taken that
storage sufficient to deal with the inequalities of the
sewage flow is generally, if not invariably, necessary.
are next told that " (c) The forms of sprinkler
tank liquor is delivered to the bed through a
number of small holes need a considerable amount of
The arms require to be brushed out at least
once a day, unless the tank liquor happens to be exceptionally free from suspended matter, or not likely to
This is a cautious statement,
produce fungoid growths."
fout indicates the need for cleaning the sprinkler, which
in which
is generally admitted.
The Commissioners next comment on
"Distribution by means of persprinklers as follows:
forated pipes or nozzles laid over a filtering area is almost
always rather unequal, and this would be a disadvantage
except in the case of fairly deep beds constructed of fine
or medium material, or deep beds (say eight feet or onore)'
or coarse material.
This method of distribution, moreover, needs a considerable head of liquid for propel
working; and also requires constant attention to keep the
pipes and distributing holes clean and free.
At Birmingham, where the nozzle form of distributor
is in use on a large 'scale, for each acre and a half of
filter one 'man is constantly employed night and day in
The constant walking over the
cleaning the nozzles.
surface of a filter is a distinct drawback, but assuming
that the occupation is not dangerous to the workman, we
think that this method of distribution will, in many
and cheap."
that the distribution is
rather un"
constant attenequal," next that the (sprinkler needs
tion," which involves injury to the surface of the bed by
a .man "constantly employed night and day" in trampling it down, and finally that this method of distribution
If the condemnawill be found
efficient and cheap."
are -accurate it follows that the method
cannot possibly be efficient or cheap for the majority of
cases, be
first told
227. The commissioners deal with tipping troughs and
dripping trays under one heading, as follows
troughs and dripping trays are, in our opinion, more
suitable for small than for large installations.
They can
adapt themselves to large variations of flow, and in the
case of small or medium-sized works, where constant
supervision is not available, this is a great advantage.
We may
add that these trays are better suited for the
distribution of a large volume of weak liquid than a small
volume of strong liquid.
The 'distribution is seldom perfect by either of these
forms of distributor, and, consequently. It is advisable to
have deep beds where they are adopted.
Further, owing
to the fact that the delivery of liquid in both cases ia
more or less constant, mediunnsized or coarse material
should be used to prevent ponding.
amount of supervision is necessary, as a tray
blocked or a trough may fail to tip. The
trays would require to be looked at about once a day, we
think, and tipping troughs about twice a day, but beyond
The distribution in
this little attention is required.
both these forms is -divided up into a number of units,
and it is very unlikely that more than one or two units
would get out of order at a time, and that the distribution would be bad over a whole filtering area.
That tipping troughs are liable to be completely put
-out of action by severe frost we have seen exemplified at
Little Dray ton, and, although we have had no experience
of the same kind with regard to 'the trays, we should
think that these were also liable to be affected by severe
distribution would never
frost, though in their case
cease altogether."
may become
228. The commissioners do not, however, point out the
worst defect of the dripping tray, which is that sewage
is discharged drip by drip on to certain fixed points, and
"these points soon become foul, also the sewage thins discharged passes through the coarse filter along a certain
narrow track, which is then overworked. A foul track is
formed, and the surrounding material in the bed is not
The .sewage passes quickly down the foul track and
is apt to issue in a worse condition than when it left
the sprinkler. For this reason alone the moving sprinkler
much to be preferred. Also the fixed trough must have
rigid supports; if allowed to merely rest on the surface
of the filter it will sink in places, causing unequal distribution and overflowing.
This point is of the greatest
229. The concluding remark of the commissioners with
regard to sprinklers is that all forms of distribution or
to give rise to nuisance from
smell, if a strong-smelling liquor has to be dealt with.
In the case of trays or tipping troughs the ismell is confined, more or less, to the immediate vicinity of the filter,
and it may be largely done away with at -small works
by providing an inexpensive form of cover for the filter.
The nozzle form of distributor is, naturally, the most
liaTble to give rise to smell, but moving sprinklers are
almost equally bad in this respect."
230. It may be seeivfrom the above that the commissioners tell us that every form of .sprinkler is defective.
and if they do not actually advise us not to use them
damn with faint praise " praise so exceedingly
so qualified that their remarks must be reall
in the light of a serious warning against
Drms of distribution in percolating filters."
>are told, in the first place, that we must
no way
of satisfactorily obtaining
told that "percolating
filters are better adapted to variations of flow than contact beds," but this cannot be possible unless the sprinkler
will isupply the varying quantities.
contact bed or a
filter worked without a sprinkler will take sewage in a
sudden rush, if required, but no sprinkler will do so.
good distribution,"
It does not follow that there is no way out of the difficulty, however, and as the problem of unequal flow has
long ago been dealt with by means of a tank used to feed
the sprinkler, it is extraordinary that the commissioners
should make such a point of the difficulties of unequal
flow, and it is not desirable or necessary, within limits,
that any sprinkler should distribute liquid at unequal
rates on a filter.
The ordinary revolving sprinkler is probably the best
for imost cases, and the fixed jet has its uses.
In the
author's opinion the defects of the rotary sprinkler are
less serious than the commissioners' report would lead
one to suppose.
The discussion as
to the grade of material to be
used in filters has gone on for many years, and will proSome authorities recommend fine
bably continue.
material, and point to results which justify their conclusion, while others obtain good results from filters constructed of large blocks of clinker. There can be no doubt
"that when very fine material is used the good results could
not have been obtained without considerable preliminary
treatment, for it is perfectly obvious that a thick sewage
would choke a fine filter if discharged upon it in large
quantities. Assuming, however, that the sewage can pass
through the filter properly, it is clear that a mass of
fine material must provide more surface for contact than
an equal mass of coarse material, and it is common
experience that the finer the material the greater the
degree of purification and the greater the tendency to
clog also that the coarser the material the less will be
the amount of purification and the less the tendency to
232. It must therefore always remain a matter which
can only be settled by the engineer after weighing all
considerations whether the niters shall be coarse or fine.
be that filtering material is very dear, and that
very valuable, and that sludge can be extracted
from the sewage and removed without nuisance. In such
a case, with a clear tank effluent to treat, a
very fine
filter may treat a very
large quantity of sewage per
cubic yard most satisfactorily.
In another case where filters can be constructed cheaply
and where there is ample area for the works, it may ot
well to treat a thicker tank effluent on coarser filters,,
with secondary treatment on finer filters to follow.
At other places it will be desirable to purify the whole
of the sewage, including the sludge, and this may be
done by the use of the septic tank, followed by contact
beds or filters, or by the slate bed, followed by contact
beds or filters. In such cases the first filters will probably
be best constructed of coarse material, and with secondary
of a
finer material.
233. There can be no hard and fast rule for the grade
of material in filters. It is merely a matter of size and
A very fine filter would treat the thickest
sewage if the quantity distributed were only small enough,
while the weakest tank effluent may sometimes be treated
advantageously in very coarse material, iseeing that very
and an enormous quantity
of liquid per cubic yaTd may have to be passed through.
little purification is required,
234. There is much confusion of ideas with 'regard to
the work that can be done by a filter per cubic yard. One
hears of one filter dealing with 500 or 600 gallons of
sewage per cubic yard per diem, while another of similar
construction can only be worked at the .rate of, say, 60
gallons per cubic yard. These statements are valueless
unless one knows (1) the strength of the sewage, (2) the
nature of the preliminary treatment, (3) the degree of
purification effected, and (4) the life of the filter.
understand that wihere sewage is
and diluted with an enormous quantity of land water, and is then submitted to elaborate
tank treatment, and afterwards passed on to a filter
witih, say, 75 per cent, of the impurities removed, ah
enormous quantity can be treated per cubic yard because
the work required from the filter is, in that case, very
Also it is possible to make a filter purify for a
It is quite easy to
weak at the
time a much larger quantity of sewage than, it can go
This is particularly the case
on treating indefinitely.
with contact beds, which will often produce a good
These results do not
effluent when much overworked.
dhow normal working possibilities.
235. Similar errors are due to the results obtained from
small experimental installations, which do not reproduce
The only true guide
tihe conditions of ordinary working.
to the engineer is the result of actual working at sewage
dispo&al works which have existed under normal condiSuch results are to hand all over
tions for some years.
the country, and from them can the only true practical
data be gathered.
236. The Royal Commissioners say that "For practical purposes, and assuming good distribution, the same
purification will be obtained from a given quantity of
coarse material, whether it is arranged in the form of a
deep or of a shallow percolating filter, if the volume of
sewage liquor treated per cube yard be the same in each
With regard
to filters of fine material, owing to pracdue to suspended and colloidal solids
insufficient aeration, the commissioners consider
that the greatest efficiency can be obtained from a given
quantity of material arranged in the form of a shallow
filter rather than of a deep filter.
not the author's purpose to criticise bhe
report, but in dealing with the subject of sewage disposal in the order adopted by the Royal
Commissioners it is impossible .always to agree with
The Commissioners consider contact beds and
filters under one heading, and although it is convenient
in many ways to do so there is such a wide difference
in the working of the contact beds and in that of the
percolating filter that it is unwise to regard them always
in this manner.
Moreover, it is due to this careless
method of regarding: the contact bed as a filter that bad
results have been obtained at many places.
237. It
Royal Commission's
238. Nothing is more common than for a contact bed
to be allowed to fill at a very slow rate.
This method
of filling or of distributing sewage is quite satisfactory
with the filter, but is calculated to put the best contact
bed out of working, and neither the Royal Commission
nor the Local Government Board appears to have grasped
this all-important fact.
Another point which should be clearly grasped is that
where sewage is allowed to stream through a coarse contact bed the purification effected is nil, for the reason
that in such a case a stream of sewage short circuits down
through the bed and into the sub-drains, and is at once
discharged, with the result that no purification is effected
and the part of the bed through which the sewage runs
is altogether overworked and soon becomes exceedingly
For instance, one finds such statements as the
In the experiments carried out for the United
States Board of Agriculture at Marion, Ohio, in the
sterilisation of contact bed and filter effluents the result
was found to be better in the case of the sand filter
effluent than in the case of that from contact beds, but
we are told that " during the night the contact beds are
This is exactly what is done at
operated continuously."
many other places, and the practice cannot be sufficiently
If the contact beds are allowed to act as
streaming filters during any part of their period of work
not only will a bad effluent be the immediate result, but
the beds will 'become clogged and foul, so that they will
not work properly at other times, and not only will this
take place, but it will be hard to induce the manager to
work the beds regularly at all, with the consequence that
foul sewage will be allowed to pass through the beds
frequently so as to make them absolutely unfit for their
Unless a contact bed is so arranged that it cannot be
used as a streaming filter in this manner, it will be sure
to produce an unsatisfactory result.
The Royal Commissioners state that,
into account the gradual loss of capacity of contact beds,
of a pera cubic yard of material arranged in the
colating filter will generally treat about twice as much
tank liquor as a cubic yard of material in a contact bed."
This conclusion, however, is based upon experiments
which are by no means conclusive. In the first place
the question of the
the Commissioners admit that,
relative merits is one of some difficulty, as very few
' '
strictly comparative experiments on a large scale have
been made." The contact beds and filter* used by the
Commissioners from which they drew their conclusions
were very small indeed, and from evidence contained in
the report they appear to have been filled with material
liable to disintegration, and to have been insufficiently
drained. General experience does not bear out the conclusion that contact beds are only half as efficient as
percolating filters, and any engineer who had obtained
results from the best type of contact bed would be very
unwise in assuming, under ordinary conditions, that he
could deal with the same quantity ol sewage on a percolating filter of half the size. Until recently the Local
Government Board required percolating filters to be of
the same cubic capacity as contact beds.
If the same
care were taken over the construction and the filling and
the emptying arrangements of contact beds as is generally
taken in the case of the percolating filter, there is no
doubt that the old rule would be a very safe one. In
the author's opinion the conclusions of the Royal Commission can only be justified by taking into account the
fact that contact beds are generally badly constructed
and improperly worked. Both the percolating filter and
the contact bed have their special uses, and it is a mistake to condemn the contact bed because it is not, as a
rule, properly understood.
The Commissioners
tell us that
percolating filters
are better adapted to variations of flow than contact
It is impossible to agree with this conclusion.
The percolating filter is 'generally fed by means of a
sprinkler, and it is clearly not practicable to increase the
rate of flow to any great extent in such a case, seeing
that a considerably increased head of liquid would be required to produce the effect, which is generally quite out
of the question, and the only means 'by which
great variations of flow can be dealt with is by
putting additional
sections of filter into use, or by
storage in a tank, which
is equally possible witJi either contact beds or filters.
the other hand, it is possible to increase the rate of
filling the contact bed to any extent, seeing that, practically speaking, it is simply pouring so much liquid into
a tank, and increased rate is of no
contact bed must be so arranged
within a stated time and at any rate,
and at works of moderate size this regular filling can only
be obtained by the use of a tank the liquid capacity of
which is equal to the capacity of the contact bed. Only
one such tank will be required, and the ordinary sewage
or tank effluent must flow into it until it is sufficiently
The liquid must then be discharged
full to fill one bed.
till the bed is full, when the flow must be shut off.
can be done by hand, but, if so, constant attention will be
required, and, practically speaking, automatic apparatus
Unless this dosing tank forms
is therefore essential.
part of the scheme and automatic apparatus can be install^d, it is as improbable that good results will be obtained with the contact bed as would be a case if a
percolating filter were installed without a preliminary
tank in which the liquid could head up and without some
form of automatic distributor.
In the
first place, a
it will fill
CONTACT BEDS (Continued).
242. The automatic apparatus required for the proper
working of contact beds is of such importance that it is
necessary to deal witJi the subject separately as a preThe automatic apparatus for filling
liminary imatter.
the first contact beds should be of such a kind as to make
it possible for a works manager to set it to discharge
tihe full contents of the storage tank (described in the
preceding paragraph) or part of the contents only, as
required, so bhat in the first place it may be
possible bo make the apparatus fill 'the beds up to a
certain level wilihouit any fear of over-filLng, and in Hhe
next place that it may be possible for the manager to
alter the quantity discharged from the tank ;to a nicety,
so that as the beds lose their capacity they may not
over-fill ; absolute control of the discharge is required.
The apparatus as a whole must be capable of filling the
'beds in rotation, and must be such
(a) That all flow
may be
into any bed
be absolutely shut
off directly
the bed
required level (b) that this flow may rethat
till -the bed is quite empty .again
is, that no liquid may flow into the (bed and so through
it while the bed is emptying
(c) that there may be absolutely no possibility of liquid rising in tihe bed albove a
certain stated water level. In .addition .to these requirements, it may sometimes ibe desirable to have tihe apparatus so arranged that in case of an excessive flow due
to an abnormal rush of storm water, when all the first
contact beds .are standing full, the overflow may cause
the opening of the automatic valves so as to produce
streaming action (throughout each set of beds until the
excessive flow ceases. This requirement is more in tihe
nature of a safeguard against accident, such as the
bursting of a large water main, than for ordinary workIt, in short, prevents the possibility of the works
ever being flooded.
is filled to the
so shut off
243. The automatic apparatus for emptying contact
bed in first, second, or third series, sihould be capable of
holding the liquid in contact in the beds for any time
required say, between five minutes and six hours
Wlhen the liquid has stood in contact for tihe time required, the liquid contents of the bed must be automatiThe time taken for emptying the beds
cally discharged.
generally be about one hour, and sometimes
It is important that the discharge from the first
beds Should not be too violent, or the undigested solid
matters will (be washed out. It is to be remembered that
the work of the contact bed is done whfile it is empty
rather tihan while it is full, and that the matters in
suspension should first come to resit on the filling material in the body of the bed, after which tihe liquid should
be drawn off in sudh a manner as not to disturb tihem.
The solid matter will then be liquefied in the firsft bed,
while it is standing enupity. This remark applies particularly to slate beds, which often dischairge much undigested matter owing to the quick rate of discharge.
The engineer should give this point very careful attention in the design and construction of the bed, and also
Wlhen the beds
in his choice of automatic apparatus.
are empty the apparatus should permit them to dirain
for at leaslt half an hour, and if the beds are clogged a
longeT period of drainage will be desirable.
It Should be possible for the manager to be able
easily to adjust the apparatus so that one set of beds,
consisting of a first, second, and third bed, if so many
are installed, may be set out of use for rest or for cleaning, without in any way upsetting the automatic tilling
and emptying of the otlbers.
The apparatus should be capable
working with
the smallest possible flow, or with the main sewex running full bore without over-filling or under-filling the
contact beds.
The apparatus in the contact beds should also be capable of being sett easily, to discharge wihen liquid stands
at any particular level to be dfibsen ; that is to say, the
apparatus should be capable of working with the beds
filled up to the top water level, or to any level below it.
For it is obvious that if the first beds lose their capacity
quicker than the second beds as may well occur they
will not deliver enough liquid to fill the second beds, and
unless the apparatus in the second beds can be set to discharge their contents wheoi water stands at a lower level,
In such a
it is clear that they will not work properly.
case the second bed will not empty, but will sltand, say,
three-quarters full of liquid till the next discharge
In the meantime,
occurs, when they wiH be over-filled.
the second bedis will have been fouled and temporarily
This miserable condition of affairs is very
common, and largely accounts for the prejudice against
tihe contact bed.
Thus it is absolutely essential that the
apparatus of the second beds must be capable of being
easily adjusted so as to empty them, however low the
water level may be. First contact beds will often lose
capacity and recover, and it is most uneconomical to
clean them whenever they lose capacity.
The same remark also appilies to the apparatus in the
nhird beds, which beds very often have a greater liquid
capacity than the second beds. The automatic apparatus in the tin*ird beds must, therefore, be capable of
similar adjustment.
important that the adjusting of the apparatus
should not be a difficult matter, needing special knowledge, seeing that if the works are to be kept in the
best possible order and give the best economical results
small adjustments will frequently be necessary. Thus
245. It is
should be well within the power of an intelligent
manager to set the apparatus and adjust it to a nicety,
and for this reason simplicity of design should be taken
into account.
Moreover, it ought not to be a lengthy
process making it necessary to put the beds out of use
while it is being done. It is well, in addition to the
automatic working, to have complete control of the in-
by means of hand valves.
These requirements may sound exacting, but there
is automatic apparatus on the market supplied by more
than one firm which will comply with them, and unless
such automatic apparatus is installed it will, in the
majority of cases, be unwise to 'have contact beds.
the other hand, if the contact beds are properly designed
and constructed so that they fill promptly, stand full
without leaking, discharge at a proper rate, and give
their proper time of contact, and cannot be used as
streaming filters for sewage, they will deal with the
whole of the sewage in a manner which has not as yet
been equalled by any other type of filter if freedom from
nuisance and absence of the sludge difficulty may be taken
Unfortunately, the vast majority of existing works do not conform to this standard, and so give
into account.
indifferent results.
The author has found that
in order to obtain a
a strong sewage, such as is dis-
really good effluent from
charged from a septic tank which is never emptied, it
is necessary to provide a much larger area or filter or
contact bed than is generally allowed if the clogging of the
beds is to be avoided. Unless the sewage is very strong,
double contact is generally sufficient, but it will always
be well as a safeguard to provide triple contact beds, or
fine filters if the effluent is to be of the highest possible
It is a
error to
assume that contact
beds, if filled only once a day by the dry weather flow,
will produce a good effluent, taking no account of the
grade of the filling .material or of whether the beds are,
arranged for single, double, or triple contact.
Assuming that it is possible to arrange the filtering
material in a triple contact system, it is clear that for
each cubic yard of material in the first bed about fiftysix gallons of sewage can be treated per day with one fillThis is true, but the sewage will be only partly
purified by the process, and it must then pass into a
second contact 'bed of similar capacity. This bed will
generally be sufficient, but in the case of a very foul
sewage a further step will be required, viz., triple contact.
Thus, for the fifty-six gallons of sewage purified,
three separate cubic yards of filtering material will have
been required, so that the quantity of material required
will 'be roughly one cubic yard for every nineteen gallons
This may sound an impossible allowance, but
the author has found it to be absolutely necessary in
one case, at least, after trials extending over several
Ordinarily for second contact the allowance for
dry weather flow will be about thirty-eight gallons per
cubic yard, but not more if the sludge difficulty is to be
really overcome.
Whether it is more economical to remove the
sludge before filtration is a matter for very careful consideration, but the author has found it economical to
construct works on the lines suggested above in the case
of small flows, of, say from 30,000 to 40,000 gallons per
diem. If such an allowance of contact bed or filter cannot be made it is practically impossible, with an
ordinary sewage, to avoid having to remove the sludge.
With regard
to the sludge difficulty, the engineer
another and probably a better alternative tham.
the septic tank and contact beds to deal with the sludge,
The Royal Commission are
viz., the Dibdin slate beds.
at the present time carrying out a series of careful
observations in order to set forth exactly the results
which are to be obtained from this bed.
The principle of the elate bed is the same as that
of the contact bed, but it is filled with horzontal layers
of slate. Crude sewage is run into the bed, and is held
there, and the solid matters are deposited on the shelves
of slate. These shelves of slate become covered with a
growth of living creatures, these being not only bacteria,
but small worms, and other life visible to the naked eye.
As the bed stands empty these organisms feed upon the
and break them up and tend to resolve
liquid, and into harmless chemical constiThe result is that with careful working the
liquid discharged contains no solid matters, but onlv
In the author's
earthy matter more or less inodorous.
opinion, the weak point -about the slate bed is that it is
generally allowed to discharge with such violence that
solid matters, such as pieces of .paper, are discharged at
solid matters
them into
This could be overcome, no
times with the effluent.
doubt, and the only other possible drawback to the slate
bed is that it might possibly sludge up. It does not
seem at all probable, however, that it would sludge uip,
and it therefore seems that if more care were given to
the emptying arrangements that the slate bed would be
the best and least objectionable method of dealing with
The bed lias now been working at several places
with good results for some years, and is being generally
adopted, and it is to be hoped that the report of the
Royal Commissioners will confirm the good opinion which
If it
many engineers have formed of this .process.
should prove to be successful there can be no doubt that
it will form the 'best preliminary treatment for sewage,
whatever the final process may be.
Assuming, however, that the slate bed will not
deal with all the sludge, it will obviously deal with most
If the bed clogs it can easily be cleaned by flushof it.
ing, and the sludgy matters washed out can be conducted
to a sump and dealt with by one of the usual methods
of sludge disposal.
If the effluent is too thick to be
passed through a sprinkler it can be passed through a
settling tank from which the humus or solid matters, if
any, can be withdrawn. At the worst, the quantity of
sludge would have been greatly reduced, and, therefore,
facing all these possibilities, engineers have not hesitated
to instal the slate bed at a great many places.
The engineer must decide whether he is prepared to face the disposal of sludge removed from settling
tanks or whether it will be best to deal with the whole
If he is prepared to
of the sewage, sludge and all.
eliminate the sludge by a preliminary process, it is clear
that he may fill his contact beds several times a day.
and obtain a pure effluent according to the degree of purification effected by the preliminary process, or he may
make his (percolating filters of correspondingly small
area, but if he is not prepared to remove the sludge he
his filters or contact beds of
amount of
size, or be (prepared to do a considerable
If these facts are thoroughly grasped the chief
cause of disappointment in sewage disposal works will
have been removed.
As a result of the report of the Royal Com253.
mission, certain generally accepted rules may be said to
exist and to have been adopted by the Local Government
Board, but it is not to be imagined that such rules have
more than a general application, and, although they are
helpful, they do not in any way alter the fact that each
case has to be dealt with according to its particular conditions.
The Local Government Board generally require
that all sewage should be screened, and two or more
detritus tanks are demanded below the screen chamber,
the capacity of each tank
of the dry weather flow.
being about one-hundredth
Septic tanks are generally demanded in duplicate, at least, and their total capacity is generally taken
as equal to the daily dry weather flow.
Chemical precipitation tanks for quiescent treatment
are generally not less than eight in number, each tank
having a capacity equal to two hours' dry weather flow.
Chemical precipitation tanks for continuous flow treatment are generally not less than two in number, with a
capacity of at least eight hours' dry weather flow.
will be safer as a rule to have more than two tanks.
tanks for quiescent sedimentation
generally at least eight in number, each of which tanks
should have a capacity of about two hours' dry weather
Settling tanks for natural precipitation with continuflow treatment should not be less than two in
number, with a total capacity
hours' dry weather flow.
from ten
With regard
to the total cubical contents of
required for treating sewage, the rates of filtration
given by the Royal Commissioners are rates which, according to their conclusions, can be doubled in wet
The Commissioners state that where it is
necessary to deal with threfc times the dry weather flow,
it would only be necessary to provide one and a-half
times the capacity of filters required for the dry weather
Thus the
flow (see par. 293, page 209, Fifth Report).
rates of filtration given by the Commissioners are
reduced by one-third to arrive at the basis for calculating the size of filters for three times the dry weather
The Commissioners give figures for sewages of
various strengths and for filters of different grades, and
for sewages which have been treated by various pre^
Thus, following their conclusions,
timinary treatment.
it is by no means easy to make any short statement as
to the capacity of filters.
Moore and
Silcock, in their book,
Engineering," give a table which summarises the requirements of the Local Government Board as based
upon the Royal Commissioners' conclusions, and a
similar table has also been issued by private publication.
Moreover, it is generally accepted that the
L.G.B. have certain requirements regarding percolating
filters and contact beds, and that these requirements
vary according to the strength of the sewage and the preliminary treatment chiefly, though other conditions
must also have due weight. These requirements should
be carefully studied by the engineer, but should not be
too readily accepted as the sole guide in
preparing a
scheme for submission to the Local Government Board.
In the following remarks as a rough guide the
working basis is the figure for oxygen absorbed from
thus the
strong permanganate in four hours at 80deg. F.
strength of sewage may be roughly classified as follows
Strong sewage, 17 to 25 parts per 100,000.
Average sewage, 10 to 12 parts per 100,000.
Weak sewage, 7 to 8 parts per 100,000.
It will always be safest to assume that the sewage will
be strong unless samples can be properly analysed.
One cubic yard of coarse material that will not
pass through a one-inch mesh arranged as a percolating filter is considered to be capable of treating
not more
than fifteen gallons of strong sewage
If this
per day.
sewage is of average strength
the allowance may be increased to twenty-five gallons
per cubic yard, and if the sewage is weak, one
cubic yard is considered as being capable of dealing with
If one cubic yard of material is arranged
forty gallons.
in the form of triple contact beds that is to say, with
one-third of a cubic yard in each bed it will treat
twenty-five gallons of strong sewage per cubic yard per
diem. If the cubic yard is arranged in the form of double
contact beds, it will treat twenty-five gallons of average
sewage per diem, or thirty-eight gallons of weak .sewage.
For the treatment of septic tank effluent
reckoned that one cubic yard of coarse material, that
will not pass through
a one-inch mesh, arranged
it is
as percolating filters,
will treat
of strong sewage per day, but the allowance is reduced
to thirty-five gallons if the material is from lin. to ^ in.
in size. The obvious reason for the reduction is that the
small material is more likely to clog.
For sewage of average strength it is reckoned that
seventy gallons per cubic yard can be dealt with, the
material being over lin. in size, while for weak sewage
the allowance is 100 gallons per cubic yard, the material
being either coarse or fine, the minimum grade being -^ in.
With contact beds, however, the allowance is only thirtythree gallons per cubic yard, the material being arranged
With average sewage the allowin triple contact beds.
ance is thirty-eight gallons per cubic yard, arranged for
double contact. For weak sewage the allowance is seventyfive
gallons per cubic yard, the material being arranged
for single contact only, and with weak sewage the allowance is sixty-six gallons per cubic yard, the material
It appears to the
being arranged for double contact.
author that the* chief reason why it has been considered
necessary by the Commissioners to allow a larger capacity for the contact bed treating tank effluent than for
percolating niters treating the same effluent is that in
the case of the percolating filter a sprinkler is generally
used which will clog up if the effluent is too thick. This
is a safeguard as far as the filter is concerned in that
there is every reason to suppose that the works manager
will take steps to clear the tank, so as to keep the effluent
In the case of
moderately free from sludgy matters.
contact beds, however, the tank is less likely to be cleaned,
because, however sludgy the effluent may be, it can flow
readily into the bed, and thus there is not much chance of
the tank being cleared until the bed becomes clogged.
The Commissioners have apparently thought well to allow
a larger area for contact beds, so that this tank effluent
can be treated, and it is difficult to believe from the results
of actual working that the percolating filter will deal
with a larger quantity of tank effluent than the contact
bed, provided that the effluent in each case is of exactly
similar quality and that the drainage arrangements in
the contact bed and the grade of the material are equal
to those of the percolating filter.
In this case the allowance for percolating niters
and for contact beds is exactly the same as that for septic
tank effluent.
For Effluent from Quiescent Settlement Tanks.
With a strong sewage the allowance for percolating filters
50 gallons per cubic yard, the material being above
in grade
40 gallons per cubic yard, the material
in. in grade
and 25 gallons per
ibeing from 1 in. to
cubic yard if the material is under 1 in. in grade. For
sewage of average strength the allowance is 100 gallons
cubic yard, the filtering material being above 1 in.
in grade, and 70 gallons per cubic yard, the material
For weak sewage the allowbeing under ^ in. in grade.
ance is 130 gallons per cubic yard for coarse, medium, or
For contact beds the allowance for strong sewage is 44
gallons per cubic yard where the material is arranged for
for sewage of average strength 50 gallon^
triple contact
per cubic yard where the material is arranged for double
-contact, and with weak sewage 100 gallons per cubic yard
where the material is arranged for single contact.
Treatment of Sewage from Chemical Precipitation
Tanks with Continuous Flow. For percolating niters the
allowance is 65 gallons per cubic yard, the material being
over 1 in. in size; 50 gallons per cubic yard, the material
For sewage of average strength
being under in. in size.
the allowance is 100 gallons per cubic yard, the material
toeing 1 in. in size, and 80 gallons per cubic yard for
material under 1 in. in size. For weak sewage the allowance is 150 gallons per cubic yard for material over 1 in.
in size, and 175 gallons per cubic yard for material
under g in. in size.
For contact beds with strong sewage
the- allowance is 33 gallons per cubic yard, the material
being arranged for double contact for sewage of average
ctrength 50 gallons per cubic yard for double contact,
and for weak sewage 133 gallons p^r cubic yard for single
Treatment of Sewage from Precipitation Tanks
with Quiescent Settlement. For strong sewage the allowance is 100 gallons per cubic yard, the material being
over 1 in. in size, and 65 gallons per cubic yard, the
material being under ^ in. in size.
For sewage of average strength the allowance is 130 gallons per cubic yard
ior coarse, medium, or fine material, for weak sewage the
allowance is 170 gallons per cubic yard for material over
^ in. in size, and 200 gallons per cubic yard for material
under ^ in. in size.
For contact beds with strong sewage
the allowance is 43 gallons per cubic yard for double contact, for average sewage 66 gallons per cubic yard arranged for double contact, and 133 gallons per cubic yard
for weak sewage in single contact beds.
It cannot be said that the'se results appear to be altogether consistent, but the chief point which is quite clear
is that niters and contact beds must be made of
ample size
^or the sewage with which they have to deal.
With regard to the construction of contact beds it
remember that the design of the first beds in
particular must depend largely on the local conditions.
essential to
desired to treat a strong septic effluent at a
is undesirable, special steps must be
taken in order to prevent the liquid being exposed to the
if it is
where smell
several works where
special steps have been taken to prevent smell, and the
process (that has proved mo^t satisfactory has been to distribute the liquid about 9 ins. below the surface of the contact
bed in covered channels in some cases the author has
constructed distributing walls composed of slate layers
throughout the bed for the full depth, so that the liquid
can be conducted to all parts of the bed without appearing
It will be necessary to take special
at the surface at all.
steps in order that the liquid may never be able to rise
above the surface of the bed. This can easily be arranged
by putting in an overflow slightly above the level of the
liquid, so that in the case of tihe liquid beginning to rise
too high in the bed it may flow into a pit and open the
The filling material in the
outlet valve automatically.
bed is then laid to a level of 9 ins. above the overflow
The author has constructed
It is also, in the author's opinion, essential to good
working (that the beds should have a false bottom for
drainage purposes similar to that of a waterworks filter
or percolating sewage filter. This has generally been
constructed of dry brickwork, and also in certain cases by
of a layer of slate similar to that used in the
slate beds in the author's practice.
necessity of having automatic apparatus has already been
mentioned, and it is difficult to see how contact beds
can be satisfactorily worked at small installations by
means of hand-valves
contact beds can be filled with various
but it is certainly of importance that the
material used shall <be unlikely to disintegrate. Clinker,
of very good quality, will disintegrate, and
reason it is better to use an imperishable
The author has used gravel, broken stone, and
quartzite in preference to clinker where possible.
will generally be necessary in course of time to cleanse
the filtering material in the contact beds it is certainly
well to use some material which will not break up during
the process. It certainly has not appeared from practical
experience in working that there is any special advantage
in using coke or clinker.
It is often maintained that the
structure of 'these materials affords a better home for
bacteria, but seeing that each lump of coke or clinker
becomes in time covered with humus and gelatinous
growth, each piece of material appears to present a face
similar to that of broken stone, and under these circumstances the amount of contact given by each piece of
the filtering material cannot depend upon its fine structural formation, seeing that the interstice's are generally
filled up with matter which does not come in contact
with the sewage.
it is
for this
A curious result has been noticed in one case
where the contact bed was filled with broken flints. In
this case the contact bed has cleared itself at intervals
and has retained its capacity in a manner quite superior
to that of the clinker beds working alongside under precisely
similar conditions.
The amount
humus and
sludgy matter discharged on to the surface of the second
beds was greater in the case of the broken flints than in
the case of the clinker, but it is obviously better to have
bo remove this material from the surface of the second bed
than to have to wash the material in the first bed. If
the effluent discharged on to the second beds is of a
quality which is likely to cause a nuisance from smell it
will be well to place covers over the distributing channels
and also to arrange matters so that the liquid standing
in the bed does not rise to the surface of the filtering
material. This can be done in the manner described in
the case of the first bed
The drainage arrangements
should be similar to those in the first bed. In the case
of the third contact beds, the liquid can be discharged
over the surface without fear of causing smell.
contact beds should be drained in the same
described in the -case of the first contact bed.
The third
manner as
For ordinary
author's practice to fill the first contact beds
with material of such a size that it will pass
and be retained on a
through a 1^ in. mesh
For second contact beds the material
| in. mesh.
used has been capable of passing through a f in.
mesh and of being retained on a \ in. mesh.
On third
contact beds the material has been capable of passing
in. mesh and retained on a ^ in. mesh.
through a
is very important that all dust or fine material should be
excluded from the beds.
These sizes of material have
been used for the treatment of sewage which has passed
through a septic tank, and which is of sludgy quality at
If a preliminary treatment for removing the
sludge by sedimentation or by emptying the septic tank
were given it would be possible to make the grade of the
material in the first and second beds finer. Probably in
this case the third beds would not be required.
In general practice contact beds are not as a rule
made less than 2 ft. 6 ins. deep or more than 6 ft. in depth.
The chief reason why it is undesirable to have beds of
less depth than 2 ft. 6 ins. is that frost is apt to put a
largo portion of the filter out of use at times if the beds
are too shallow. If they are more than 6 ft. in depththere is very great difficulty in removing the material for
As a matter of fact it is generally undesirable
to have beds much over 4 ft. 6 ins. in depth.
In the case of double contact effluent or of
from percolating filters experience has shown
(that a sediment tank for humus or a fine straining filter
ihas generally been required before the effluent can be
(safely discharged into any ordinary stream or river of
email volume, and it should be observed that this is parrticularly the case at certain seasons of the year when
the filters discharge large quantities of humus and sometimes foul matter.
Where third contact beds or filters272.
containing fine material are used such tanks are generally unnecessary, as the humus aind other matters are
intercepted on the surface of the fine material.
different series of contact beds are generally made of equal
culbical contents.
In the remarks made by the Royal Commissioners
upon the construction of percolating filters, it is demonstrated that the main consideration in laying out an
area of filters is to arrange so that portions can be put
out of work without interfering with the general working
of the plant. Thus, for instance, at small sewage works,
if the whole of the sewage had to be dealt with on one
percolating filter fed by one sprinkler, the stoppage of
that sprinkler would result in the temporary discharge
of a large quantity of unpurified sewage into the watercourse, unless there were some other means of disposal
This point is extremely important.
In many cases the most economical form of construction involves open sides, but this arrangement does
not appear to have any effect upon the aeration of the
There does not
filters, especially in those of large size.
appear to be any special advantage, therefore, in -having
open sides. Many persons, however, appear to be of the
opinion that there is some special benelit as to aeration
in the open-sided
In constructing the base of a percolating
necessary to remember that large quantities of suspended matter will work their way down through the
able to go
filter, and that if this suspended matter is not
away with the effluent the bottom layers will be clogged,
and this will interfere with the proper working of the
aerafilter, both mechanically and also by preventing
Thus, it is necessary to make provision for free
passage of suspended matters passing through
material into the effluent drains. This statement refers
particularly to coarse filters.
it is
In the finer
washed out material will be intercepted on the upper
part of the filter, and so the need for drainage is not
quite so great at first, but ultimately there will be
every probability of this material finding its way to the
bottom and choking the filter if it cannot get away
A bottom layer of large pieces is therefore
generally allowed to rest upon the false bottom made of
tiles, and these tiles rest upon a sloping platform of
concrete. This concrete is sometimes made to fall evenly
to the channel at the circumference of the filter, or
sometimes it is made to fall to a central sump whence
the effluent drain runs, and in other cases the whole
floor has a fall to one side.
Drainage arrangements are
of the utmost importance.
filtering material must be retained by a wall
of some kind, and though this wall is
frequently constructed of large lumps of clinker or other material of
which the bed itself is formed, it is, nevertheless, the
The only other alternative is to have a
.retaining wall.
bank sloping at the natural slope of the filter material,
but it is obvious that such a slope is not an economical
In constructing the walls of a
percolating filter
necessary to remember that they have to withstand a certain amount of thrust, and there are instances
of the retaining walls
cracking and giving trouble, which
emphasises the importance of making the walls of
sufficient strength.
With regard to the
depth of percolating filters, it is
suggested by the Commissioners that the minimum
depth should be three feet, but no reference is made to
the maximum efficient
It is generally found,
however, that the maximum economical depth is somewhere between six feet and nine feet.
Filters may be constructed above or below
ground or half in and half out of the ground. They may
have a containing wall of brickwork or
masonry or drv*
rubble or clinker.
Sometimes they are made with a
brick retaining wall
surrounding them for part of the
height, the remainder being built of
material inside and above the
retaining wall proper.
i 2
The grade and quality of the filling material
used must depend upon the particular conditions of the
The size of the material available, the depth o
the filters, the convenient area, the character of the
sewage, and the methods of distribution vary so
that it is impossible to lay down any general rules^
which may be applied to all cases. The Royal Com"
the size of the filtering material
missioners say that
should depend very largely upon the amount and character of the suspended matter in the liquid to be filtered."
The decision as to the most suitable size of the material
can only be arrived at when all the facts of the case are
It may be said that materials of all sizesknown.
are used at various places. Clinker is the material that
has been used most generally and there are advocates of
very coarse material and very fine material, but it is
clear that a very good tank effluent can be treated very
satisfactorily on material of fine grade, whereas a tank
matters will need treatment on a very coarse material.
Likewise the primary filters will be of a coarser material
than the secondary filters. It is important to have the
material throughout a percolating filter of even grade,as otherwise the smaller pieces will tend to fill up the
interstices in between the larger pieces and prevent aeration.
can be worked continuously,
will generally be better to work them on the
principle of the slow distribution of a given quantity
over a particular area than on the principle of the
quicker distribution of the same quantity over the samearea in intermittent doses with periods of rests between
the doses.
As a general principle it will be found much less
trouble to extract the humus or solid matters from the
effluent by means of a humus tank or small fine filter
than to attempt to remove it from the surface of a percolating
The Commissioners
state that as a general rule
with the storm water lies not
o much in the actual quantity flowing over an area as
the result of rain, as in the rate at which it flows off. The
old method of treating storm water on rough filters was
undoubtedly wrong, .and the practice has been abandoned,
#nd the Local Government Board generally refuse now to
sanction loans for such work. The Commissioners state
that " storm water filters as generally constructed and
used do not effect the purification in proportion to their
cost." Such filters receive sudden rushes of exceptionally
foul sewage which passes through them quickly without
receiving any purification other than straining.
difficulty in dealing
The Commissioners suggest that one and
times the area of filter, which would be necessary for the
treatment of the sewage if only the dry weather flow had
to be treated, should be provided, and that special stand-by
tanks, two or more in number, should be provided at the
works and kept empty for the purpose of receiving the
excess of storm water which cannot properly be passed
through the ordinary tanks. It is assumed that ordinary
tanks are capable of dealing with three times the normal
dry weather flow. It is recommended that the storm
overflow should be so arranged that it would come into
operation only when the stand-by tanks were full.
sewage intercepted in the stand-by tanks would then be
treated on the ordinary filters, which should be made
The Commissioners
sufficiently large for the purpose.
vstate that in most cases it would
probably suffice to
provide stand-by tanks capable of holding one-quarter of
the dry weather flow, and it would not be necessary to
provide for filtering more than three times the normal
dry weather flow. The chief difficulty which presents
itself with regard to this arrangement is that in most
cases there is not sufficient fall at the works to allow for
the emptying of these storm water stand-by tanks on to
the filters by gravitation, and it will, therefore, be neces-
sary sometimes to pump the contents on to the niters. In
such a case, however, as that in which a dosing tank is
provided for filling contact beds it seems that a smaller
tank might be provided for taking the storm water, and,
moreover, the closing tank itself would in some cases
actually hold .one-quarter of the daily dry weather flow,
and would, therefore, certainly intercept any sudden
lush of the sewage.
Where there is sufficient area of suitable land available
the Local Government Board generally allow treatment
of storm water in excess of three times the dry weather
flow by irrigation provided that detritus tanks are provided.
Under favourable conditions, treatment of sewageon land is a very satisfactory and efficient method. Theground must 'be of such a quality and of such an area asto form a natural filter for the sewage.
Hence clay
land or soil which is impervious is unsuitable for sewage
It should be noted that ordinary clay, clay mixed
with sand or clay mixed with gravel, are used in ordinary
practice for the formation of watertight linings and'
puddle trenches for storage reservoirs, yet this same
material, viz., clay, has been used in the past for sewagefiltration in broad irrigation schemes, and it is to thiscause chiefly that so much trouble has been experienced at
sewage farms within the area of the London clay.
Sewage farms have failed owing to the unsuitable nature of the ground, but also owing to the negleot
of those in charge, and to the fact that the production of
good crops /from the land has received more attention
than the proper disposal of the sewage.
considerations must always be subservient to the object
for which the works are intended, viz., the disposal of
Land treatment was used in the past, and is still
used for the purification of crude sewage, or tank effluent,
but unless the land is very light and gravelly it will generally be necessary to purify the crude sewage by tank treatment as a preliminary, and it will often be economical to
treat the sewage also by filtration to a greater or lesser
degree before it is applied to the land, using the land, in
fact, only as a final filter.
If the land is of a really good quality it will
deal with an effluent such as one might expect to get from
a settling tank under its worst conditions, but if the
surface should be impervious owing to clogging there will
be a nuisance immediately.
It is impossible to avoid a
very bad smell wherever a strong septic effluent is discharged in the open, and for this reason alone it will be
seen that treatment in sedimentation tanks or slate contact beds will generally be a better preliminary than
the septic tank.
Where the land
is of first-class
quality and very
light and gravelly, and where the sewage is not too
strong, crude sewage can be satisfactorily treated on the
land, .but it should ibe clearly understood that this can
only be done satisfactorily under the best conditions.
To attempt to treat crude sewage on a limited area of
land or on land which is anything less than a natural
filter, allowing the liquid to soak away rapidly at all
times, will be to count disaster.
Similarly, to attempt
to purify even a sewage of average strength by passing it
once only through a few feet of soil -will mean an unsatisSewage
factory effluent unless the area is enormous.
farms exist all over the country, where it was at firstt supposed that the land would deal with crude sewage, and
when bad results were obtained it was supposed that removal of the sludge 'by precipitation would settle the
and where
been found necessary
before turning the
This is quite a normal state of
affairs, and the moral is that if crude sewage is to be
treated on land an area much larger than that generally
allowed is required.
Hence, speaking generally, land
treatment alone is not economical, but as an auxiliary
for secondary treatment land is most valuable.
it ha.s finally
to use settling tanks
effluent on 'to the land.
chief drawbacks to land treatment are tlio
of the Avorks required, the heavy cost, the
amount of attention needed, and the ease with which a
short cut to the outfall or stream is made by the sewage.
On the other hand, as a set-off against expense, there is
the value of the crops produced. There also remains the
great difficulty of making the agricultural considerations
subservient to sewage disposal. It has sometimes been
found that the crops from a sewage farm sell well for a
year or two, and then some animal dies, and the local
reports say that its death is due to the effects of eatimr
The story goes
vegetables grown on the sewage farm.
round the country, with the result that the sale of vegeThus
tables grown on the farm is seriously injured.
those farms which at the beginning show a profit frequently fail to do so afterwards. Sewage farms are sometimes let to farmers, but in such a case the disposal of
sewage becomes a secondary consideration, and the
chances of failure are largely increased.
In the very best cases, where circumstances are
favourable, the land is sometimes capable of dealing
with both sludge and sewage, the liquid soaking away
into the ground and the sludge being left on the surIn this case the sludge generally has to be dug into
the ground.
293. Generally
speaking, however, there are two
of land treatment:
(1) Intermittent downward
filtration and (2) broad irrigation.
In either system the
object of the engineer is to make the sewage percolate
With intermittent downward filtration
through the soil.
the sewage filters vertically through the earth to the
subsoil drains below
with broad irrigation the sewage
is purified by allowing it to flow over the surface of the
land so that the top soil is saturated with the sewage,
which filters through it laterally to drains or surface
There are also cases where, owing to the character
ground, the sewage will soak straight into the
ground and descend to the ordinary water-bearing stratum
of the district.
Thus sewage farms on the chalk and ir
of the
deep sandy districts frequently have no sub-irrigation
the sewage is merely discharged into them and
allowed to soak away into the ground. This arrange;
very satisfactory as far as getting rid of the
concerned, and the sewage works manager has no
but it must always be
trouble as to quality of effluent
remembered that where sewage can percolate into the
halk or other water-bearing stratum it is liable to pollute
the water supply of the district.
Land must always receive the flow of sewage intermittently in either system that is to say, the land must
have frequent intervals of rest. A loamy, porous soil is
the most suitable for sewage purification. It is sometimes possible to do with very few sub-drains, as many
being put in as will keep the water level in the soil
below a certain depth being found sufficient. As a rule,
however, the giound will become waterlogged if a regular
system of subsoil drains is not provided, and the number of these drains can only be decided when the character
of the ground is known.
Irrigation is generally adopted where the ground
not very porous. It is often very difficult to adopt such
a system because it is difficult to find sufficient land surface suitably situated. The surface must present a gentle
slope, so that the sewage may travel forward slowly in
a lateral direction, and thus admit of the surface being
regularly saturated and of the liquid draining off rapidly
and of the ground drying again. Land has to be divided
up into sections, and the sewage is discharged on to each
section in turn.
There must be no hollows in which
sewage can be held. -Sometimes even clay land has been
made to give a good result by preparing a porous stratum
about 12 ins. thick on the surface, by mixing the top
soil with some light material such as ashes.
It should
bs clearly understood, however, that it is not the clay
which does the work It is useless to mix ashes with
stiff clay, and it is only where top soil is available that
a good result can be obtained in this manner, the clay
itself being quite useless for this purpose.
Ashes are
laid on the comparatively porous top soil and the ground
is ploughed.
The gentle slope required for broad irrigation is
sometimes obtained by breaking the land up into a series
and furrows the carriers from which the sewage
discharged run along the tops of these ridges, which
may be 30 ft. or 40 ft. apart. The furrows at the foot
of each incline have a slight fall towards the effluent
drain or channel. It is not to be supposed, however, that
a very high degree of purification can be accomplished in
this manner, unless the sewage passes through two or
three such systems of ridges and furrows. In the author's
experience it has been found that sewage will pass very
rapidly through land which has been thus prepared, the
made ground being of a loose and porous character at
the start. On the other hand, it is necessary to keep
the land in a porous state by ploughing it.
Crops can
be grown on such irrigation land298.
The most general plan with board irrigation is to
allow the sewage to travel forward over the surface of
the land from long carriers made at right angles to the
slope, the distance which it travels depending upon the
character of the land.
Within limits, the greater the
distance the more thorough the purification. The chief
danger of the system is that the sewiage is apt to form
channels and run across the land without percolating
through it. The working, therefore, requires considerable attention.
The laying out of the land is a very important matter, and should receive the greatest care.
Subsoil drains may be required in the broad irrigation
system to prevent the land becoming waterlogged by
water or by sewage percolating downwards, but it should
be observed that it is worse than useless to put in subsoil drains in the case of clay land, as the clay is sure tocrack in dry weather and the sewage will short-circuit
into the sub-drains without any proper purification.
Drains and surface channels must be provided to take
the effluent after it has flowed over the surface of the
Wherever possible the effluent from one plot of
land should be made to pass on to a lower plot before it
Double or even triple filtrais discharged into a rdver.
tion may sometimes be obtained in this way.
of ridges
most important that land should not beoverworked. It requires long periods of rest, and sewage should never be allowed to pond on the surface or to
form a scum, as the ponding will cause a nuisance and
the scum will prevent the sewage percolating through the
Wherever a scum forms the ground should bebroken up.
can only be carried out where the soil is very porous.
Great care must be taken in the preparation of the surface.
The land is divided up into level sections, and the
As miuch use
sewage is discharged on to each in turn.
is made of the area of the land as is possible in this
system by using the soil as a filter for a depth of about
3 ft., instead of using the top lew inches of the soil only,
The sewage passes
as in the broad irrigation system.
vertically downwards to subsoil drains, the effluent from
It is, of course, well
which may be run into a stream.
in this case also to pass the sewage through land a
second or even a third time in the same manner.
If the flow of sewage is slow and the ground
very porous each section is laid out in furrows, so that
the sewage may run equally on to all parts.
furrows are sometimes very close together, say two or
three feet apart.
M, however, the discharge of sewage
is rapid the whole area of the section is covered by it
at once, and the furrows are then unnecessary.
In either system of land treatment it is as well
to avoid artificial-built channels as far tas possible, as
they are expensive, and interfere with the cultivation of
the ground. Of course, the main carriers for t'he sewageto be built, and, in fact, as many of the
fevill have
channels as are essential for conveying the sewage to the
various sections must be of a, permanent nature, but the
other channels for distributing the sewage over any particular section can be formed by cutting them in the
customary to grow crops on the ground in
both broad irrigation and intermittent downward filtration systems, but in some oases where the filtration area
It must
is limited and hard worked no crops are grown.
always be remembered that the crops are quite a*
It is
secondary consideration, and that they should never be
allowed to interfere with the proper use of the land for
sewage disposal.
It is as well to hear in mind when examining the
effluent from sewage farms that the naturally clean subsoil water may form a large part of the effluent earning
from the subsoil drain, iand thus give a very false idea
of the amount of purification effected by the land.
The choice of
a system depends chiefly upon the
-character of the ground and the area available. Whether
any particular land is fit to deal with sewage and how
much preliminary treatment is 'required before sewage of
a given strength can be applied to land is a matter which
can only be decided after careful consideration of the
circumstances in each case.
With regard
to land-treatment schemes, it should
be remembered that the sewage must be applied regularly and in suitable doses, and where a. day's flow is to
be pumped up on to the land in two or three hours it
will obviously be necessary to have some intermediate
tank between the land and ith sewage. For instance,
under good conditions the Local Government Board would
allow 30,000 gallons of sewage to be treated upon one
acre of land.
Now, irrigation, land is generally divided
up into sections, and the whole sewage is not distributed
over the whole area at one time. Thus, in any ordinary
pumping scheme the sewage would be raised more quickly
than the land could take it^ and at such a rate that it
would be practically impossible to distribute it in ordidary channels over the comparatively small area of land
in use, and it would quickly flow over it, and would then
find its way into the neighbouring ditches or water-
With regard
to the area of land required for treatabsolute rule. The Local Government
Board have in the past required one acre of suitable land
ment there can be no
for every 150 head of population with broad irrigation,
and with intermittent downward nitration one acre for
The Royal Commisevery 1,000 head of population.
sioners, in their fifth report, make suggestions for the
area of land required under different conditions, and they
make the following statements:
(1) A strong sewage which has passed through,
chemical precipitation tanks can be treated at the
rate of 25,000 to 30,000 gallons per acre per day, i.e.,
at the rate of from 5 to 6 gallons per square yard.
(2) Strong sewage which has passed through
settlement tank can be treated at the rate of 20,000
gallons per acre or 4 gallons per square yard.
(3) Strong sewage which has passed through a septic
tank will be liable to create a nuisance if discharged,
over land, and no recommendation as to area is-
(4) Sewage of average strength which has passed
through chemical precipitation tanks may be treated
at the rate of from 30,000 to 40,000 gallons per acre,
or at the rate of from 6 to 8 gallons per square yard.
(5) If it has passed through natural precipitation
tanks sewage may 'be treated at the rate of from
25,000 to 30,000 gallons per acre, or from 5 to 6gallons per square yard.
With regard to sewage which has passed
through a septic tank, the remarks made in the case
of strong sewage apply equally in the case of average
(7) For weak sewage which has passed through a,
chemical tank the area of land required is one acre
for from 30,000 to 40,000 gallons, or from 6 to 8gallons per square yard.
If weak sewage has passed through a natural,
settlement tank from 30,000 to 40,000 gallons per
acre, or from 6 to 8 gallons per isquare yard, are-
(9) If weak sewage has passed through a septic
tank, 30,000 gallons per acre, or 6 gallons /per squareyard, are allowed.
It is fairly clear from these statements that for
intermittent downward nitration the old allowance of
1,000 persons per acre is a fair figure to assume, in the
case of good land.
Where the land is of inferior quality
and the sewage is to be filtered on the surface by broad;
irrigation, a
larger area is required, which
anything according to the nature of the ground, but the
low figure of 150 persons per acre will be useful as a
Where sewage has been filtered, of course
rough guide.
the allowance of land will depend upon the quantity
which can be passed through it, and the amount of puri
ficaticn already effected by filters.
It is to be assumed
that the Local Government Board follow the advice of
the Royal Commission.
In considering the large area of land which has
to be allowed for the treatment of sewage, one must not
overlook the fact that owing to the need for resting the
land and for growing crops, it is not possible to treat
the sewage over the whole area at once, bat that on the
contrary a large portion of the land must generally lie
310. There can ibe no doubt that the effluents from
ordinary sewage purification works are bacterially impure.
Though proper treatment will produce a clear, inodorous
effluent which under the best conditions may look like
drinking water, and be subject to no after change, yet
this effluent is often comparable to crude sewage from a
bacterial standpoint.
Dr. Houston, in his report to the Royal Commission on
Sewage Disposal, states that effluents from septic tanks,
contact beds, and trickling filters contain enormous numbers of bacteria, and that though they may be reduced in
number by the treatment, the relative abundance of the
different kinds of bacteria are much the same in effluents
as in the crude sewage. Furthermore, and this is a very
important point, Dr. Houston showed that of undesirable
bacteria such as B. coili, proteus-like germs, spores of B.
enteritidis sporogenes, and streptococci the effluents contained almost as many as the crude sewage.
The Royal Commissioners have also pointed out
that none of the ordinary methods of sewage disposal are
sufficient to make it possible to discharge sewaige effluents
near to shell fish layings, and recommend that in such
cases the isewage should be discharged elsewhere, or the
layings closed.
It is perfectly clear, therefore, that if for any sufficient
reason it is impossible to keep sewage effluents out of drinking water rivers or shell fish layings, some further treatment is necessary in order to remove the harmful bacteria.
It is often urged that to drink water which is known to
be sewage polluted, or to allow sewage-contaminated shell
fish laying to exist, ought not to be countenanced, and on
the face of it, without exact knowledge of facts, one might
readily agree to the contention. But as a matter of fact
this is a counsel of perfection not to be attained under
existing conditions. For on the one hand we have a shell
fish trade of immense value which cannot ibe done away
with, and on the other hand the polluting 'authorities
in some cases have not the funds required to take their
sewage elsewhere. We have an ever-growing population
which must cause pollution to the sources of supply, to
the rivers and springs and wells, and we have an ever-
demand for pure water.
Of course, in many cases the dilution is so great
that no bad results are apparent or are even likely to
occur, and it may well be that in most cases natural
processes remove the harmful germs after the sewage or
sewage effluent has been discharged into the sea or river.
It is impossible for the (engineer pure and simple to say
whether a given effluent under given conditions is dangerous to health when discharged. In this we must look
to the bacteriologist and medical officer to tell us whether
the danger does or does not exist, whether the shell-fish
are ^contaminated or are likely to be contaminated, and
whether the water drawn from the river or spring for
drinking purposes is pure or not. But where danger is
found to exist there is a possible remedy, and that is
There is, however, another case in which the
engineer may be called upon to decide whether it may or
may not be politic to sterilise the sewage. It is only too
well known that a river or an oyster bed may be polluted
from causes quite outside the sewage works where the
fault is attributed to the sewage, however unlikely sewage
There are cases on record where
pollution may be.
authorities have been fined for such pollution where the
contamination may have been due to other causes. Ii>
such a case the engineer will do well to sterilise all
effluent passing from his works, and then it will be
possible to prove that no effluent
containing disease germs
issues from his works, and this will be better evidence in
a court of law than the pious opinion of the most eminent
scientific witness to the effect that the dilution
ought to
have been .sufficient to remove all harmful germs.
Sometimes also it will be necessary to prove to nervousopposing authorities that a new system will be quite safe,,
and here again sterilisation may be found useful.
Various methods of sterilisation have been tried,
but the method which has hitherto proved to be efficient
and economical is that of mixing a solution of hypochlorite with the effluent after filtration. Plain chloride
of lime mixed with water has 'been used largely, but it
is probable that for large works
hypochlorite, prepared
by electrolytic methods, will eventually be found to be
the most economical and trustworthy method.
315. Early experiments, as well as those of later date,
have been imade with crude sewage, tank effluent, and
These have flailed for the simple
poor filter effluent.
reason that the amount of sterilising agent required was
enormous, and then not effective as a rule. The energy
of the sterilising agent was used up in dealing with solid
matters and organic miatters in suspension, and bacteria
contained in the solid matters were not affected. Later
experiments, both with water and sewage, all point to
the fact that sterilisation can only be economically and
satisfactorily accomplished under ordinary conditions
after the solid matters and organic matters in suspension
have been removed as fiar as may be possible.
316. The process is really so simple that it is surprising
that it is not more readily understood. Having filtered
the sewage a liquid remains which is clear of matters in
suspension, but contains harmful bacteriia. If the liquidis really well filtered a very small quantity of hypochlorite will deal with the bacteria, in some cases one
part iper million of available chlorine hias been found to
suffice, and where the water is really well filtered, say,
through sand, the quantity required is sometimes much/
If, on the other hand, the effluent contains organic
matters in suspension, the quantity of chlorine bas to be
increased in proportion.
and careful application of the
work lies. The experiments hitherto have generally been carried out by
chemists and scientists with very crude appliances, but
latterly engineers have taken the matter up and are perfecting dosing apparatus, electrolysers, and testing ar317. It is in the nitration
sterilising agent that the engineer's
Early experiments were made with electrolytic
hypochlorite at Guildford and Maidenhead in this
country, while in America experiments with chloride of
lime solutions made at Boston, New Jersey, and Baltimore have proved after years of careful experimental
work that sterilisation of ordinary percolating filter
effluents can be effected on a large scale satisfactorily and
319. The following figures were given in a report published by ths Massachusetts Institute of Technology. The
cost per million gallons, including maintenance and
operation of plant, for sterilising sewage or sewage
effluent with chloride of lime, based on a capacity of
5,000.000 .gallons a day, is estimated as follows:
obtained at Boston land Baltimore, for instance, it was
found that three to four parts per million of available
chlorine would successfully treat the effluents of percolating filters of a quality which cannot 'be considered firstrate.
better effluent
would need
less chlorine.
achieved by the American experimenters with plain chloride of lime and probably the
difficulty of obtaining the necessary machinery and plant,
have, in the author's opinion, caused them to overlook the
advantages of electrolytic treatment. By means of an
electrolyser it is possible to prepare a sterilising agent
which possesses many advantages, the reasons being that
electrically produced hypochlorites can be more econo-
mically produced under favourable conditions than solutions of chemically produced hypochlorites, such as
bleaching powder, and not only is the cost of production
and labour less, but the resultant available chlorine is
much more efficient than the chlorine of the bleaching
This has been carefully demonstrated by the
Ledboir, Messrs. Cross and Bevan,
Commission, Dr. Rideal, Dr. Alexander,
of Poplar, and others have supported this fact. It should
be noted that Messrs. Cross and Bevan determined that
the superior efficiency was as great as two to one. Bleaching powder is subject to great fluctuations in price, and
though it contains from 33 per cent, to 37 per cent, of
available chlorine, this is apt to be very misleading,
because the chemical rapidly loses its efficiency when
open. Thus, it is often necessary to deduct something like
from 15 per cent to 18 per cent, from the efficiency of the
available chlorine in the chemical at its best, and when
actually used. Of course, loss of efficiency will in a great
measure depend upon the care which is exercised. It will
be seen also that two-thirds of the bleaching powder
added to the water will be useless, and this is apt to
accumulate and, in a minor degree, it will become a
The means adopted for sterilisation are extremely
Suppose a flow of 100,000
simple. To give an example
gallons a day has to be treated with two parts of available chlorine per million. If ordinary commercial bleach-
is used and steps are taken to prevent loss of
by exposing it to the air too soon, it will possess
about 33 per cent, of available chlorine.
Thus, three
times as much bleaching powder will have to be used as the
ing powder
of available
One hundred
thousand gallons equals one million pounds, therefore two
pounds of available chlorine will be required and six
pounds of bleaching powder. This powder is then dissolved in any quantity of water that may be convenient
to facilitate the dosing.
Suppose, for example, it is dissolved and mixed with 100 gallons of water, then one
gallon of solution must be added to or mixed with every
thousand gallons of sewage effluent.
Suppose, for instance, the effluent passed through a tank of 10,000 gallons
capacity, ten gallons of solution must flow into this tank
with the sewage. Short contact is sufficient, but, as will
be seen by the table quoted above, a longer period
of contact will sometimes reduce the quantity of sterilisIt is in the arrangement of this
ing agent required.
dosing that the skill of the engineer is required. It will
sometimes be best to add the sterilising agent to the
effluent as it flows away, at other times it may be best to
provide sterilising tanks, and so on.
322. The mixing of the chemical solution is not a difficult matter.
It is generally mixed in a small tank, and
With elecfurther diluted and settled in larger tanks.
trolytic hypochlorite the process is equally simple.
which electric power is, of course, reThere are
quired, supplies a liquid already diluted.
several types of electrolyser on the market, but in the
simplest form common salt is the only chemical required,
and where sea water is available this can be used instead
In the electrolyser an
of the sodium chloride solution.
electric current passes through the solution of salt and
water, with the result that the chemical constituents are
divided and recombined, so as to form a hypochlorite of
greater efficiency than the chemical hypochlorite.
electrolyser, for
a very simple matter, and quite
within the power of an intelligent works manager to test
the effluent. A simple colour test is employed, whereby
all that the manager has to do is to add a chemical to a
sample of effluent. If the effluent contains an excess of
chlorine the harmful germs will have been destroyed, and
In both cases
it is
the colour of the sample will be violet. If the violet
colour does not appear it shows that a larger dose of
If the colour is too deep
sterilising agent is required.
then the dose may be diminished.
While the author does not suggest that
generally desirable or necessary,
he does emphasise the fact that the process is available
wherever required, and that in some cases it is necessary,
and if adopted would save much public money and pos-
tion of sewage effluents
A difficulty with pneumatic ejections has been the dealing with the ndght flow. It ha been necessary either to
keep the compresscHts at work all niight ox to allow the
sewage to lie in the sewens. On tha (Joounbs system., however sufficient compressed air cam be stored to deal with
the niight flow withoutt worfdnug the air compressor. Such
an a-mamigemettft has beem installed at Husiigerford.
The Coombs Plneumatic recomipir'esiSii.nig system is a halfway system between, the Store and the Leernnir. In this
sysitem the sewage is ,sucked in (thus the ejectors can b<e
arrantged above the sewens) by a pairtial vacuum ip'roduced
by the comtppeiStsor, and is forced out by cotnuprteed aitr.
The compnessed ad-r, after dischangdmg the ejector, exipands
the compressed cylinder.
The comipres &ed
air working expansively is theKeiore
mo-re ecomomically used ihaji in the ordinary pneumatic ejector, where it exhausts dialect into the atmo!
The system bedmg
a closed cycle, there
no offence
frtom the exhaust.
ejectors can, be
worked singly or
a-rrangemetnt is completely
worked by eXectrdc nafotows or any
automatic, and can be
other motive power.
Patent Automatic Sewage Disposal Apparatus
Automatic Alternating and Timed Discharge Syphons applied
simplicity, absolute
freedom from choking, and no moving parts.
Tipping Apparatus and Distributor
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for Sewage Trickling Filter.
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"Coombs" 300
gallon Pneumatic Ejector.
are also Makers of Centrifugals and
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to His Majesty the King.
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No. 499.
No. 532c.
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