escaping through the gulleys were not rendered less poisonous by being
given off in the middle of the road or carriageways, but the constant
passing of carriages over the gratings had the effect of mixing the
gases of the sewer more quickly with the atmosphere of the street. Thus
their noxious qualities were not so much observed, but the effect of
these gases on the public health by this arrangement has not been as
satisfactory as many imagine.
When the carriage traffic is suspended during the night, the effluvium
from these gratings is now similar to that experienced from the shoots
or gulley shafts in 1830. This opinion is formed by comparing the report
with testings taken at the gratings last year.
The last improvement suggested to give a better system of sewer
ventilation in the City is the erection of shafts at the sides of the
buildings where possible. A resolution was passed last year by the City
Commissioners of Sewers to erect these shafts where practicable. The
effect of this alteration will be to remove the nuisance from the street
and carry it to a higher level. If it is intended to work these shafts
in conjunction with the open gratings, the effect will be that in some
streets the whole of the gas from the sewers adjoining will be pouring
out of one shaft. In the summer time its density will be increased by
the friction in the shafts, and the high temperature of the atmosphere
will cause a more rapid decomposition in the sewage.
In low buildings or warehouses there is nothing to prevent the poisons
from the sewers from being conveyed down the chimneys into the
buildings, and the employés taking a zymotic disease whilst engaged in
their work and at night taking it to various parts of the suburbs.
It is better to let the gas escape at a low level, where it could be
purified when it is a nuisance, and especially in the case of an
epidemic, than discharge gas with a greater density at a higher level.
The General Board of Health in 1848 issued the following minutes of
information with reference to sewers and house drains—
“Make proper provision for the ventilation of sewers and drains in such
a manner that there may be a free current of air in them in the
direction of the sewage flow.”
It was also recommended “that the stack pipes should be connected with
the sewers without the intervention of traps, in order to assist the
ventilation, and there should be no trap between the trap at the inlet
and the sewer.”
This system was found far worse than the open gulleys or shoots in the
City in 1830, as at Croydon (which was one of the first towns to carry
out works of sewage under the General Board of Health) no sooner had the
sewers been in use before an outbreak of fever took place.
Dr. Niell Arnott and Mr. T. Page, C.E., were appointed by the Home
Secretary to report on the outbreak at Croydon, and Mr. Page in his
report states—
“Whenever a water-closet even with the best form of siphon trap is
introduced into a house, it will be well to provide an escape into the
open air. When several soil-pans or sinks from the apartments of a large
house are discharged into a common soil-pipe or vertical main, the main
should be continued up to the roof and to the open air, and if
practicable it should be carried near the chimney. Pipe sewers must also
have ample ventilation provided at all available points. If the air is
confined it is most dangerous when it breaks forth, which sooner or
later it will do, such evils would have been avoided.”
It is quite evident by these remarks that the system of ventilating the
City sewers in 1830 was more perfect than at Croydon, and had Mr. Page
tested the sewers and sanitary fittings by the compression of gases in
them, he would have found that the action of the water in the pipe
prevented the gas from being confined.
In 1858, Mr. Goldsworthy Gurney made experiments in the neighbourhood of
the House of Commons by connecting a number of the sewers near with the
furnace of the clock-tower, and this was reported on by Sir Joseph
Bazalgette as follows:—
“I find that the furnace of the clock-tower of the House of Parliament
was supposed to have been connected with the adjoining district to the
extent of about a quarter of a square mile, and with about 6½ miles of
sewers, but that the ventilation had in reality been intercepted by a
flap so that the benefit supposed to be derived therefrom was purely
imaginary. Having come to that conclusion, the next thing I directed my
attention to was, supposing the whole of the air extracted by that
furnace was produced from the sewers, and supposing that all the
intermediate channels could be stopped, and that it could be directed
from the most remote ends of each of the sewers, and distributed over
those sewers with the most perfect theoretical accuracy, so as to have
uniform currents passing through each of the sewers towards that
chimney, still the effect on those sewers would be nothing, and the way
in which I prove my statement is this: the total area of the 6½ miles of
sewers now connected with the furnace is 713 feet, the total area of the
channel through which the air has to be brought from them is 8 feet,
that is about the ninetieth part of 713; the air was passing at the rate
of 542 feet per minute through the 8 feet area. Therefore, if I could
divide that over the whole district, the velocity in all those sewers
would be 6 feet per minute or ⅕ of a mile per hour. But we have shown
already that there exist in the sewers from other causes velocities
amounting to 100 feet per minute and upwards; and 6 feet per minute is
practically speaking stagnation and not ventilation.”
This experiment was undoubtedly one that if it had been continued
(instead of being abandoned), and the errors corrected, would have led
to a more practical result. The area of the air space to the furnace was
8 feet, and the current 542 feet, or equal to 6 miles an hour. If this
current had been the same in the sewers as in the channel, the suction
produced on the water-traps of the small drains attached would lift the
water in each trap a little more than 3 inches. But as the ordinary trap
has only an average 2-inch dip, the weakest would have been at once
sucked and the experiment a failure. Had the dips of the traps been 4
inches, the drains would have remained sealed except at the intended
inlets. The air being supplied at the ends would have gone through the
sewers without breaking the water-seal, providing that the air space
between the crown of the sewer and the sewage was not in any way
blocked. If the current in the sewers had been less than 200 feet per
minute, the ordinary trap would have effectually sealed the various
inlets.
Had the average area of gas space above the sewage been 8 feet, the
whole of the 6½ miles of sewers would have been emptied of its gas and
supplied with fresh air in about an hour. The different areas should not
have been considered, but the total quantity of gas taken.
The velocities of 100 feet here mentioned, is accounted for as follows.
Should the sewage in any part of the sewers lower itself, causing an
additional gas space in that part of the sewer, the rush of gas in the
sewer to fill the space would cause this 100 feet per minute current.
These hitherto unaccountable currents in sewers and drains are produced
by the variation of the gas space above the sewage, the result of water
being thrown in at the various inlets.
The gases of a sewer may be passing backward and forward in currents
varying from 100 to 300 feet per minute, and not any ventilation would
take place except at the gratings, and this would be very little indeed
when the gas in the sewer was of a heavier gravity than the atmosphere.
Speaking on the same subject, Colonel Haywood says, “a down draught so
complete as to be superior to the diffusive power of the gases, you
cannot start with a velocity of less than 2 miles an hour, and suppose
the whole district has been so arranged as to have a sufficient
exhaustive power, the mere opening of a water-closet, or the enlarging
or the putting in of a new drain into a sewer, or the making of a hole a
foot square, or a servant taking up a bell trap in a sink, or a
sewer-man lifting a side entrance covering, would very much destroy the
power of the furnace, and unless you had a gigantic power sufficient to
guard against these casualties the system could only be a failure.”
What is here meant is, suppose that if the whole of the 6½ miles of
sewers were emptied of their gas at a velocity of 2 miles an hour, the
poisons from the sewage would not be noticeable or injurious in the
atmosphere of the drain. If each inlet to the sewer was trapped, the
opening of a water-closet or the opening of a bell trap in the sink
would not have affected the 2 miles an hour current in the sewers.
Had the currents in the sewers near the furnace of the clock-tower been
kept at 2 miles an hour, the traps being tight, and attention been paid
to the compression of the gases in the sewers by the water entering
them, the experiment would have been in a measure a successful one.
From 1855 to 1872, Sir R. Rawlinson, C.B., Dr. A. Miller, and Sir Joseph
Bazalgette were carrying out experiments with charcoal trays and
screens, and a committee of the Metropolitan Board of Works in their
report, say:—
“The results were sufficiently favourable to warrant the use of charcoal
ventilators in connection with such air-shafts as were sources of
annoyance and complaint, but their adoption had also the effect of
diminishing the upward current of foul air through the shafts and of
confining it to the sewers, thereby endangering the safety of the men
working in them, so that it is necessary that such ventilators should be
cautiously and not generally applied.”
These experiments in sewer ventilation were the most valuable of any yet
made to solve the question, but their failure could be attributed to the
following results.
The working of charcoal in the extraction of poisons given off from
sewage in its transit through the sewers, and which poisons become mixed
with the gas, and the placing of charcoal in layers or baskets so that
the gas should pass through the interstices of the charcoal, is without
a doubt the best method of dealing with charcoal as an agent in
arresting or picking up the poisons from putrid matter which is
contained in the gas. But the obstacle this gives to the supply or
exhaust of air to the sewer is greater than the power of the water-trap:
consequently traps are sucked or forced, and the inlet of fresh air
takes place through them into the sewers.
The gas from the sewer escapes through the weakest trap into the house,
thus nullifying the effects of the charcoal trays and rendering them
almost useless. The passing of the gas over the tray would prevent this,
but experiments prove that charcoal has not the power to attract and
retain the poisons from the sewage, and which is retained in the gas.
Thus these poisons pass over the charcoal through the grating into the
street.
These experiments prove that if the poisons from the gas are extracted
at the outlets, and the drains into sewers trapped with a water-seal,
and the siphoning or forcing of traps are prevented, sewers can be
ventilated without being a nuisance or prejudicial to health. But it
must be borne in mind that the instant the current of air in sewers,
drains, soil-pipes, or sanitary fittings exceeds a velocity of 3 miles
an hour, even if they have open ends, no trap with a 2-inch seal is safe
from being siphoned.
Many surveyors state that sewer gas is uncontrollable. This is an error.
The gases of a sewer are as controllable as the atmosphere of a room. It
is the compression of the gas caused by an increase of water in the
drain, and the temperature of the atmosphere on the surface of the
ground at the various points where the gratings are fixed, which makes
the currents of gas in drains so uncertain. The sudden lowering of the
sewage in a drain will stop the nearest gratings or shaft from working
as inlets.
What led engineers to form this opinion was the failure of experiments
with motive power to get certain results in ventilating sewers, the same
as in a building. It is impossible to get ventilation to sewers through
open gratings except the inlets from the house drains are sealed with a
water-seal.
The unsatisfactory results obtained from many experiments in sewer
ventilation have not been the fault of the plan or the appliances used,
but arose from the wretched manner in which house drains have been
connected to the sewers.
I shall never forget the testing of some sewers with a view of improving
the ventilation of them. They were public sewers and almost new ones,
and as far as the sewers themselves were concerned you could not have
had better, both as regards a good fall and tight joints. But the manner
in which the connections were made to them was something astonishing.
Untrapped gulleys at the sides of the streets, drains from the sewers
into the kitchens of houses without a single trap. In some cases two or
more traps were fixed according to the whim of the owner of the house.
Rain-water pipes from the flats of windows on the ground floor were
connected to sewers without traps, closets with no ventilating pipes,
and a dozen other imperfections were found on the branch drains and
fittings.
Before completing the tests of these drains and fittings, I suggested
that all faults and errors found in connection with the branch drains
should at once be remedied, and each drain be connected to the sewer
with a trap, or else it would be of little use to improve the
ventilation of the sewers. I was quietly informed that this could not
for one moment be entertained, as where these evils were the worst was
on property belonging to members of the Local Board, and any attempt to
pass a resolution to compel this to be done would be futile, and the
necessity of doing it attributed to the zeal of local sanitarians.
I am glad to say that cases like this are exceptional, but there are
many towns where similar evils will remain until an epidemic breaks out
and the authorities are compelled to have them remedied. The amount of
air passing through the sewer is no indication of the ventilation or the
amount of air that is being admitted, or the quantity of gas charged
with poisons that is given off. I have tested the gas in a sewer and at
the ventilating shafts, and have repeatedly found the gas in the sewer
flowing at a high velocity whilst the air in the open ventilating shaft
was perfectly stagnant.
I have not been able to make experiments to know how long gas will
remain in a sewer, but from observations of its gravity and working in
different localities I believe that the poisons from sewage matter are
retained in the lower strata of the air of sewers in some cases for
months, and when open gratings are only 50 yards apart. The quantity of
air taken in at the gratings at this time is a little more than the
displacement caused by the water, and when gases are released to any
extent it is through atmospheric influences.
If you measure the amount of air going in and out of say twenty open
gratings in the same locality, the small quantity would astonish those
who had not previously tested it.
Repeatedly has it been written and said that if you put a shaft or
grating at the top of a hill, or sewer, it will take off the impure gas
of a district, but I have had men working for days at the top of a sewer
150 and 200 feet higher than the lowest grating, and no trap
intervening, yet during this time not a particle of gas left the drain
at this the highest opening, but at times a good inlet current would
take place down the drain.
I find that one of the most fatal mistakes to make in sewer ventilation
is to introduce a large quantity of fresh air into a sewer at a high
temperature. An atmosphere at from 90° to 100° thrown into a sewer will
rapidly decompose sewage matter and produce results exactly opposite to
that intended. It is when the hot atmosphere of a summer’s day comes in
contact with the sewage in the sewers that the worst poisons are
generated and given off, and the gases which come from the gratings are
the most noxious. This rapid decomposition is more particularly felt in
the suburbs, where the drains are of stoneware and the sewage has to be
carried through them for a considerable distance.
Common sense teaches us that all matter of a nature like that passing
through sewers will decompose more rapidly and reach a higher state of
putrefaction in an atmosphere of a high temperature, even above ground,
than in any other condition. Experiments confirm this to be the case
whether it be applied to matter in sewers, vaults, or tanks. The best
experiments that I have made in sewer ventilation is in keeping the
temperature as low as possible, admitting into the sewer sufficient air
to prevent any action taking place on the water-seal, and what gas came
out of the sewer by compression to purify it at the gratings, extracting
the poisons that it had taken up from the sewage.
Many gases which are found in sewers have an affinity to water, and will
make their way to the water-seal and become absorbed in the water of the
trap to some extent, but not to the extent many persons imagine, or to
become detrimental to health.
When a disinfectant having a greater attractive power than the water is
used in connection with the ventilation, this attraction to the
water-seal will not take place.
If necessary, in hot weather or in large sewers, or those of an easy
gradient, cold air of a very low temperature could be introduced, which
would prevent decomposition taking place to any extent during the
transit of the sewage. If the whole area of a system of sewers was
charged four times a day with air at a temperature of 30° we should have
no complaints of sewer gas.
The details of such plans would be out of place here as they are the
subject matter of several patents, but sewage can be carried through
districts without its gases being injurious to health or without sewage
being subjected to rapid decomposition almost as easily as meat is now
transmitted from New Zealand to England without any decomposition taking
place during its transit.
Before leaving the question of sewer ventilation it will be well to note
the results obtained by the valuable experiments I have quoted, and
which have been made from time to time by the City Commissioners and the
Metropolitan Board of Works. One cause of the failure of these
experiments has been conclusively proved to be due to the wretched
condition of branch drains and house connections attached to the sewers
on which the experiments were made, and these are in a measure out of
the control of the officers of these Boards; and until a proper survey
of branch drains and house connections has been made, and traps placed
in proper positions, the ventilation of these sewers will be almost as
imperfect as in 1830.
The sanitary survey which is now being made in this country by the Local
Government Board will fail in one of its most important objects unless
it insists on every surveyor knowing the condition of drains under the
surface of the ground. A cursory glance at the plans of a district, or a
surface sanitary survey will not do very much in arresting zymotic
disease, and the staff stated to be employed on this survey is totally
inadequate for the work.
To successfully deal with sewer ventilation it should be divided into
two sections: (_a_) That of the sewers and branch drains which are
directly under the control of the officers of the various Boards, (_b_)
Those drains immediately attached to houses, including the soil-pipes.
The necessity for ventilating a sewer is, that in an unventilated sewer
or drain the instant a compression of the air in a sewer, between the
sewage and crown of the sewer, takes place to 1/300 part of its bulk,
gas is forced through the weakest trap according to the displacement of
water, and as the water is lowering in the drain fresh air will be
admitted into the drain through this trap. Thus if a drain or sewer is
not ventilated it will ventilate itself.
Should a sewer be ventilated with open gratings in the centre of the
roads, the placing of gratings at a moderate distance apart would
diffuse the gas equally in a flat district if the temperature on the
surface of the ground at each grating were the same, but the variation
of the temperature in streets is such that the heat of the street at one
grating will be a sufficient motive power to extract the gas from many
sewers through this one grating, the others only forming inlets while
the increased heat lasts.
In hilly districts, where the drains are of necessity of a steep
gradient, the quick flow of the sewage will cause the gas to pass more
rapidly when much sewage is flowing in the drain, the worst gas coming
out at the lowest grating, generally a grating before a junction, or
where two drains meet of different gradients: but when scarcely any
sewage is flowing, the gas will flow to the highest grating. Thus, in
putting gratings on steep gradients (if no method of purifying the gas
is used), the gratings should not be placed in regular distances apart,
but where the gas can be discharged in the most open space.
In hot weather, although ten times the amount of air passes through
sewers of steep gradient than in a flat district, the gases from sewers
on a steep gradient are far the most noxious. If open gratings only are
used on a system of sewers, the gas but not the sewage should be trapped
off into districts, not only as the means of preventing the gas rushing
in volumes to certain points, but for preventing germs of disease
travelling in the gas of a sewer from an unhealthy to a healthy
district, which is the case under the present system, by leaving sewers
for miles without any gas check.
We have in sewers and drains a power created by the influx of the sewage
which is greater than any mechanical means that can be used in the
ventilation of them, and it is to get the best method of applying this
power that sanitary engineers must direct their attention.
The difficulties that are met with in ventilating sewers with open
gratings are so great, that I am convinced that as soon as engineers
study the question more fully, the system will be abandoned.
The method of carrying off the soil by water through sewers has proved a
good and convenient one, and scarcely any defect can be found in any
drainage scheme except that of the ventilation, which is at present one
that is condemned by the inhabitants of most towns as a nuisance,
especially in hot weather, and by the medical profession as being most
prejudicial to health.
As a remedy for this, we must profit by the experience of the early
experiments I have quoted, which points conclusively to the fact that if
we extract from the gas (which by compression must of necessity leave
the sewers at openings) the noxious and disease-producing poisons
contained in it, at the gratings, and by those means prevent a rapid
decomposition of the sewage taking place, and without putting any undue
pressure on the water-seals to houses, we have overcome the greatest
difficulty in the work.
In old drains or sewers it will be the work of some time for the
surveyor to know that each drain from the house to the main sewer is
trapped with a good water-seal, yet this is the most important factor in
providing good ventilation.
Some persons have hastily condemned the water-trap, having found out
that they have been siphoned by the transit of the sewage. This is a
mistake. A properly constructed water-seal or trap that clears itself at
each flushing is the best seal for sewer gas. It will not keep out gas
if it is forced in bulk by excessive pressure, any more than coal gas
can be kept out of houses if a greater pressure is put on at the works
than the resistance of the trap in the chandelier. In manufacturing gas,
either experimentally or otherwise, water is the seal always used, but
we do not attach anything to break that seal when dealing with gas for
illuminating purposes, the same as is done in many cases with sewers.
Where new drains are laid, no difficulty in getting a good seal to
branch drains need be experienced, and no drain from the house to sewers
should be laid without its being completely disconnected or cut off as
near the soil-pipe as possible.