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THE
- VENTILATION AND FLUSHING OF SEWERS.
13 Y
ALMON D. THOMPSON.
THESIS
For the Degree of Civil Engineer,
COLLEGE OF ENGINEERING,
UNIVERSITY OF ILLINOIS1897 .
VENTILATING AND FLUSHING SEWERS.
By A. D. Thompson, City Engineer, Peoria, 111.
The principal objects sought to be attained by ventilation
and flushing of a sewerage system are—
(1) To prevent the formation and accumulation of disagreeable
and unhealthful gases;
(2) To carry off in a manner without detriment to health or> 'annoyance to citizens the odors which are unavoidably given off by
the sewage;
(3) To prevent the filling up of small sewers.
One of the most important and difficult considerations in the
design of a system of sewers is to provide for the above objects
in an effective manner that will not be obnoxious or dangerous to
health. Various plans for these purposes have been adopted and
usedjby different municipalities, the majority of which have
given but indifferent results. The most successful methods for
providing ventilation are—
(1) By perforated manhole and larnphole castings giving surface
vent ilation;
(2) By open, untrapped storm-water inlets from the street gut
ters, in addition to the perforated castings on the manholes and
lampholes;
(3) By special ventilation shafts as used in mines, together with
chimney shafts, in each of which a fire or other heat furnishes a
draft;
I *
(4) By pipes or shafts without heat.
European engineers and municipalities have made extensive
experiments and investigations of these different systems, and
have reached various conclusions regarding their efficiency.
The Metropolitan Board of Works, of London, has given this
question careful consideration for a number of years, and through
special committees has conducted extensive investigations to de
termine the best methods for ventilating and flushing sewers. On
July 13, 1877, the Board appointed a special committee upon "Clean
sing and Ventilation of Sewers," who made a thorough study of the
methods of ventilation in use, and experimented with various de
vices designed to maintain a constant circulation of air. The
committee also obtained information from the thirty-nine Districts
of the Metropolis regarding the care, flushing and ventilating of
the sewers, and on Jan. 12, 1886, submitted an extensive report of
their conclusions, which were based on the results of the experi
ments and investigations conducted by themselves and the previous
special committees. The advantages and disadvantages of the sev
eral systems of ventilation and deodorization are so well stated
in the report that they are given as an appendix hereto.4
The committee recommended that provision should be made for
effectually flushing the head ends of branch sewers, and other
places where there was not a sufficient supply of water from the
connections to carry the sewage in suspension.
It was considered necessary to provide some means for effec
tively ventilating the sewers in order to dilute the gases and
— 3—
prevent the nuisancey and danger to health therefrom. They found
that ventilation by means of shafts from the sewer to the surface
of the street may cause complaint from odors. Such surface venti
lators should be placed from 50 to 60 yards apart, with air open
ings in the gratings equal to 60 square inches.
The committee concluded that ventilation by large special
shafts, with or without heat, would be effective only in excep
tional circumstances. Ordinarily such a system would be of lit
tle benefit, and in no way commensurate with the expense of con
struction and maintenance. The system of surface ventilation
assisted by pipes affixed to buildings was recommended as giving
the best results.
Charcoal ventilators, and other appliances in ventilators
for deodorizing gases, was stated as undesirable, because they
obstruct ventilation and are costly and troublesome.
In 1871 the City of Liverpool attempted to ventilate the
sewers by means of Archimedean screw ventilators six inches in
diameter, 1046 of such ventilators being placed in position at a
cost of about ten pounds each, the annual cost of maintenance
being about five shillings each. Drs. Parke and Sanderson re
ported in 1071 unfavorable to these ventilators, stating that
they exercised no practical influence in preventing the escape
of air into the streets and houses. For this reason but few
ventilators of this pattern have been added since 1871, the num
ber in 1890 being reduced to 976. Later the principal ventila-
--4--
tion has been secured by placing ventilating girds and perforated
manhole covers at intervals of 80 yards, and having a clear opening in each of not less than 63 square inches. Since 1871 the
Archimedean screw has been placed mainly at confined places where
open ventilation was found objectionable. The ventilation of all
the sewers was completed in 1882. The cost of ventilating sewers has been about 7 d. per yard run of sewer.
At Brighton, England, the upper part of the sewers is venti
lated by a shaft 110 feet high, in which a fire is kept burning
continuously. All sewers are provided with charcoal ventilators.
The City of Bradford, England, with a population of about
216,000, in 1891, has a combined system of sewers, which are ven
tilated at intervals of 100 yards by shafts composed of nine
inch earthenware pipes, which extended from the top of the sewer
to the street surface. This plan is reported as very successful.
The experiment has also been made of assisting the ventilation
by connecting in a few instances the main sewer with the furnaces
of factories, but the plan was only fairly successful.
At Carlisle, England, the entire system is ventilated by
smoke flues and rain-water leaders. All street gullies and open
ings are ventilated by charcoal ventilators.
Frankfort, Germany, has a combined system of sewers in which
ventilation is accomplished by the air entering through grated
openings in the street and passing out through high ventilating
shafts placed at the highest portions of the city. The air also
passes out through vertical pipe openings of the houses. This
— 5 —
system is reported as almost perfect in its operation.
In Hamburg, Germany, the surface water enters by untrapped
direct connection gullies provided with iron grates. These con
nections are about 135 feet apart, and serve as ventilators to the sewers.
The Sewerage Commission of the City of Berlin made a number
of trials of the efficiency of the various methods of ventila
tion. They attempted to secure this end through large factory
chimneys and specially constructed air shafts, with suitable ifurnaces, but soon abandoned the system because the strong
draught from the furnaces opened the traps of the dwellings in
the immediate vicinity and some explosions were experienced.
They next attempted to obtain ventilation through the house
rain-water conductors, and soon pronounced this plan a failure,
as a great many connections operated only a portion of the time,
and others did not operate at all, because of the different
height of the houses, while the gas escaped in large quantities
from the joints of the conductors and permeated the rooms of the
dwellings.
The traps were then connected with the smoke flues of the
dwellings and the draught assisted by ventilators of light mo
tion, but this plan proved unacceptable, because of the difficul
ties encountered and because the gases in most cases were brought
directly into the rooms of the dwellings.
After the failure to burn sewer gas by conducting it into
the boiler fires of furnaces, the attempt was made to use it for
— 6 —
additional street lights. New lamp posts were erected at all
corners where sewers intersected, and were connected by conduits
to the sewer. This plan failed, because the sewer gas escaped by
the strong draught and entered the dwellings.
The plan which the Commission considered had proved most successful is as follows: In all new houses a large round stone
pipe is placed in the inner wall between two smoke flues. The
joints are well cemented and the pipe is solidly walled up and
extended above the roof a sufficient distance to be above the
neighboring houses. This pipe is connected with the trap on out
side of building, and is provided with a charcoal filter in the top.
The street sewers of Berlin are provided with charcoal venti
lators, the Rawlinson Charcoal filter being much used.
American engineers have generally secured ventilation
through perforated manhole and larnphole castings, spaced from
350 to 400 ft. apart, supplemented either by untrapped storm
water inlets, or by omitting the intercepting trap from the main
soil pipes of buildings.
In order to determine some of the principles governing the
air currents in a sewer ventilated by the first method, the
writer made a series of experiments, as shown in Table No. 1,
with the wind in different directions and the air outside the
sewer at various temperatures. A Birams 3-inch Anemometer was
used in obtaining the velocity of the air currents. Pig. 1 shows
a general pla^=£f_t^e_^ort!ion=ofthe sewerage system upon which the
— 7 —
experiments were made. Ventilation is provided by perforated
manhole and lamphole covers located not more than 400 ft. apart,
and also by untrapped storm-water inlets situated about the
same distance apart, thus giving as free a circulation of air as possible.
. The stations marked on the plan show the points at which
the experiments were made, together with the distances from the outlet.
The territory from the outlet to Station 7, 6720 feet there
from, is comparatively flat, the surface of the flow line at
that point being only 50 ft. higher than the flow line of the
sewer at the outlet. This station is at the foot of an abrupt
bluff which rises 90 ft. to Station 9, a distance of 1150 ft.
After reaching the top of the bluff the territory is again com
paratively flat. The flow line of the main sewer at Station 10,
9120 ft. from the river, is 142 ft. above the flow line at the
outlet.
In experiment No. 1, with the temperature inside and outside
about the same, and the wind blowing directly away from the mouth
of the sewer, thus tending to draw the air out of the sewer, there
was no current 280 ft. from the outlet, while above this point
there was a strong current up the sewer.
In the second experiment, with the conditions about the
same as before, except that the air outside was cooler, the current
continued up the main sewer until the summit at Station 10 was
reached, when the air was forced out of the sewer through the
— 8—
untrapped storm-water inlets. In both these tests the air was
drawn into the sewer through the inlets at Stations 5, 6 and 9.
Experiment No. 3 showed the same conditions to exist in the
sub-mains, the air being drawn into the sewer through the lower
inlets and given out with considerable velocity through the
inlets at the upper ends of the system.
. In experiment No. 4 the wind was blowing into the sewer at an
angle, and a current was maintained up the sewer from the outlet,
the temperature inside and outside being about the same.
In experiment No. 5 there was no appreciable wind, but the
outside temperature was from 20 to 30 degrees higher than the
temperature in the sewer, with only a moderate amount of mois
ture in the atmosphere. The heavier air in the sewer, therefore,
naturally fell to the outlet, and a strong current was maintained
down the sewer and out the mouth, the air entering through all the inlets.
These experiments were not sufficiently extensive to defi
nitely determine all the points desired, but they have fairly
established the following general conclusions:
(1) That the direction of the wind in relation to the outlet
has no effect upon the general currents of the air in the sewer;
(2) That the currents of air in the main sewer do not control
the currents of air in the laterals;
(3) That untrapped storm-water inlets produce more ventilation
than is required for sanitary purposes, the numerous openings
creating a strong draft in the sewers;
-9--
(4) That the direction and velocity of the air currents in large
sewers are ordinarily determined by the relative temperature and
weight of the atmosphere within and without the sewer.
Untrapped storm-water inlets having open areas so much
larger than the openings in the covers of manhdles become the
principal outlets for the air within the sewer. The combined
areas of such inlets enclosed within a very small portion of the
entire sewer district ordinarily equals the area of the main
sewer, while the sum of the areas of all the inlets is several
times the area of the main. The consequence is shown by the
results of the experiments given in Table No. 1, where the air
enters certain inlets, rushes a short distance through the sewers,
and finds access to the outer air through other storm-water open
ings.
In experiment No. 1, above, if the air enters the inlets
from Station 0 to Station 6 and passes out at Station 10 and
vicinity, having an average velocity of about 112 ft. per minute,
the air in the main sewer would be renewed every 55 minutes.
Taking experiment No. 5 in the same manner, with an average veloc
ity of about 163 ft. per minute, the air in the main sewer would
be renewed every 37 minutes.Provision should be made in all cases for sufficient ventila
tion to prevent the accumulation of gases, which may force the
traps in buildings, and also to preserve the air in a condition
sufficiently pure to allow workmen to enter same without danger
— 10—
to health. To renew the air in sewers as rapidly as this wide-
open plan accomplishes increases the danger to health and exposes
the citizens to an unnecessary nuisance. The storm-water inlets
are located usually near the curb corners, at the street or alley
intersections, and the foul air rushing from the sewers comes in
immediate and direct contact with pedestrians. If such air were
always entirely free from disease germs, it is sufficiently dis
agreeable to condemn such a method.
Baldwin Latham, in his 1878 Edition of "Sanitary Engineer
ing," protests against wide-open ventilation in the following man
ner:
"At the present time there appears to be a very strong feel
ing to extend the system of open and unprotected sewer ventilation
to such a degree as would lead to the almost continuous opening of
sewers. The author would point out that extremes are often danger
ous. It has been shown that the multiplication of open ditches
containing foul matter has militated against the health of parti
cular localities. On the other hand, it is believed that hermeti
cally sealed sewers cannot be used for conveying decomposing
faecal matter without danger to the health of the district using
them. Those who would advise the construction of our sewers en
tirely open, would carry us back to the period of old foul ditches^
or to adopt such systems as may be seen in operation in most conti
nental towns, where it receives, however, daily attention from
scavengers; yet there can be no doubt, looking at the health of
the places where such systems are in operation, that it militates
— 11 —
against the health of the people. Security to health with regard
to ventilation is only to be had where there is ample ventila
tion without undue exposure, which would bring into operation
the agencies whereby decomposition and noisome air are increased."
The plan of omitting the intercepting trap from the main
house drain and allowing the air to enter from the sewer and
pass out a soil pipe extended above the roof has been extensive
ly used in many cities, and the method of extending a pipe to
the surface of the ground between the trap and the sewer has
many advocates. The cure is more dangerous than the disease in
either case. In the first instance the air from the sewer, laden,
possibly, with disease germs, is given free circulation through
out the building by faulty joints, while in the second such air
is thrown out directly in front of the building at the surface
of the ground. *'The investigations and experiments conducted,as briefly
noted, have caused the writer to conclude that under ordinary
conditions the safest method of sewer ventilation is through
openings, of sufficient size and number, at the surface of the
street and in the center thereof. Where it is not possible to
provide a sufficient number of such openings to give the amount
of ventilation necessary, the system should be supplemented by
pipes on the outside of buildings extending above the roof. In
exceptional cases a special ventilating shaft will be the most
effective means for the escape of the foul air from the sewers.
It is only within the past ten or twelve years that American
— ,— — — =— — ----------- — -------------- — — — --------- -— ..................... ......— — ------ ----— ------ -------- -
- 1 2 -
engineers have given the question of systematic flushing the
attention the subject demands. There are places in every system
of sewers where a light grade, or the small amount of water in/
the sewage, causes a deposit of solid matter, which soon fills
up the sewer at such points, or maintains a deposit of foul sew
age that decomposes and gives off offensive and unhea'lthful gases.
Deposits occur most frequently at the upper ends of the sew
ers, where the grades are light, sewers small, and the amount of
sewage a minimum. When the sewers at these dead ends are suffi
ciently small to allow effective flushing, it has been generally
conceded that the cleansing can best be secured by automatic flush
tanks, fed with a continuous supply from the water main and set
to operate as often as may be necessary. This method is more
economical and more certain of results than any yet devised.Previous to 1382 the sewers of Paris were flushed entirely
by sewage water, but at that date flushing reservoirs of 280 to
350 cu. ft. capacity were established. These reservoirs are fed
by a continuous supply from the water mains and furnished with
automatic flushing siphons. The supply is regulated so they
operate from two to three times per day. In 1386, 367 of these
reservoirs were established, and in 1895 this number was increased
to 1700.
Information from 224 towns in England, ranging in size from
2,000 to 500,000 inhabitants, showed that nearly all used flushing
of some kind. About one-third reported some form of patent auto
matic siphon in use with flush tanks. Flushing was held to be
— 13—
satisfactory. Four forms of siphons are used, the Adams, Bodins,
Doultons and Fields. The prices paid for water vary from 5/ to
50/ per 1,000 gallons.In order to determine the American practice regarding the.
use of flush tanks, the writer sent letters, during 1896, asking
for information, to the City Engineers of 360 cities in the
United States, with populations from 2,000 to 60,000, and 201
replies were received. 143 cities, or 71/> of the total reply
ing, flush the sewers by some means, and of this number 123, or
86/o, use automatic flush tanks, while 20, or 14>, flush by
hand. The information given by the cities using automatic tanks
is given in detail in Table No. 2.The total number of flush tanks in use in the 123 cities
is about 5,353, the number in each city ranging from 1 to 350,
while the maximum time flush tanks have been used in these
places is 16 years. The amount of water discharged per tank
varies from 50 to 2500 gallons, the usual amount being from
250 to 400 gallons. There is a wide variation in the frequency
with which the flush tanks are operated. Some cities report
that the tanks discharge once every half hour, and others once a
week, or "as often as necessary." This variation indicates eithe
a great difference in local conditions, or a lack of knowledge
as to the frequency of flushing necessary to prevent stoppage
and to maintain the laterals in a sanitary condition. In 118
cities, which is 96 > of the total using flush tanks, the tanks
are filled by a continuous supply of water through a small pipe
— 1 4 -
conn ected with the water mains, and in 5 cities, or 4 they are
filled by some other method.The principal siphons in use are the Rhoads-Williams, Miller
and Van Vranken. The first named siphon is used in 62 of the
cities, the second in 53, and the third in 27, while the Field-
Waring is used in 18 cities.The diameter of the siphons vary from 4 to 8 inches.
Very little data was obtained regarding the amount of
leakage, or cost of repairs, as these questions have apparently
received but slight attention from the officials in charge.
The diameters of the sewers into which the tanks discharge
discloses another wide range in custom, the sizes varying from
6 to 36 inches in diameter.52 of the cities, or 42/i of the total, own the water works,
and consequently water used for flushing purposes is furnished
free in such places. In 37 cities, or 52/« of the remainder, the
water is furnished free of charge by the water companies under
private control. 11 cities pay a rate per tank per year, the
price varying from $4. 60 to $30. One City— -Riverside, Cal.—
pays a water rate of $3. per month per tank, while Houston, Tex.,
pays 25 cents per day per tank for water furnished. In 12 cities
the rate is based upon the number of gallons of -water used for
flushing purposes, the rate varying from 5^ to 30/ per thousand
gallons. 2 cities pay the regular meter rates, and 5 cities pay
a lump sum for water to be used in flushing sewers.Table No. 3 gives the results of experiments to determine
t
— 15—
the efficiency of flush tanks with different sizes of sewers,
laid at various grades. The experiments were conducted upon
lines laid with no more care than the remainder of the system,
but which contained no sewage. The average velocity between
stations was obtained by dividing the distance by the number of
seconds required to flow the given distance. If it were necessary
to entirely fill the sewer to obtain good results in flushing,
as some authorities claim, these experiments clearly prove that
flush tanks of the sizes given are not effective on sewers
having a greater diameter than ten inches, and even on the sizes
smaller than ten inches, the pipes are filled for only a short
distance from the tank.In experiment No. 3 (B) small holes about 1-1/2 inches in
diameter were cut in the tops of the pipes at Stations 1, 2 and
3, as shown on sketch, with table and observations of depth made
at these points to determine the distance from the flush tank
that an 8-inch pipe on the grade given is completely filled.
These observations showed that at a distance of 81 ft. from the
tank the pipe was practically filled, and the depth of water
was reduced to 5-3/4 inches at 130 ft., 4-3/4 inches at 190 ft.,
and 3-l/2 inches at 380 ft. Under ordinary operating conditions,
with the sewer partly filled and containing some deposits, the
pipes would be filled for a greater distance than these experi
ments indicate, but with the tanks operating once in 24 hours,
it is hardly probable that the pipes would be completely filled
for a greater distance than 200 ft.— - • - • "•••• ~ '
- 1 6 -
Tables Nos. 4 and 5 give the results of experiments upon
flush tanks of two different makes and sizes to determine the
time required to discharge and the amount of water thrown out at
each discharge. Each tank was discharged three times. The
average discharge of 24 tanks, 4 ft. in diameter, fitted with
6-inch siphons of the Rhoads-Williams pattern, was 200 gallons,
while the average time of discharge was 22 seconds. The average
discharge of 5 tanks, 4-1/2 ft. in diameter, with 8-inch siphons
of the above pattern, was 295 gallons, and the average time of
discharge was 18 seconds.The average discharge of 14 tanks, 4 ft. in diameter, fitted
with 6-inch Miller siphons, was 267 gallons, and the average time
of discharge was 20 seconds. The average discharge of 4 tanks,
4-l/2 ft. in diameter, fitted with 8-inch Miller siphons, was
404 gallons, with an average time of discharge of 50 seconds.
Many authorities claim that it is necessary for the water
to entirely fill the sewer in order to be of any benefit. The
portions of the system requiring flushing are seldom filled with
sewage, and tanks should be considered to eltectually cleanse
a sewer so long as the water therefrom maintains a cleansing
velocity and fills the sewer up to the height attained by the
ordinary flow of sewage. In case of stoppage, a head is formed
and the pipe is entirely filled. This distance varies with the
size and grade of the line, but the results of the experiments
given in Table No. 3 would indicate that Hushing is efiecuive
upon 8 to 12 inch pipe lines laid to grades of 1 in 230 to 1 in
--17--
250 for a distance of from 400 to 700 feet.A large number of siphons have given satisfactory results
in discharging the water, but the method of filling the tank, whic
is practically the same in all, has been the source of great
dissatisfaction, and the cause of prohibiting the use of tanks
in many places. The ordinary method when the supply of water is
continuous is by connecting a service pipe from l/2 to l/4 inch
in diameter, upon which is fastened within the tank a stop
cock. It has been found in most cases a difficult matter to
control the flow by this means so as to fill the tank once in
24 hours, and almost impossible to fill it once in 48 hours, or
longer. This is due to a number of reasons, chief among which
are too heavy or too light a pressure on the water mains, or too
much sediment in the water. Before flushing by tanks can be a
success, with an automatic feed, some method must be devised for
absolutely controlling the flow from the feed pipe under all
conditions.Whether it is better to fill the tanks by a continuous
supply from the mains, or by men with a hose, is controlled by the
comparative cost of the two methods, the controlling fea^uies
being the cost of pumping and cost of labor.The frequency of flow necessary to maintain the sewers in a
sanitary condition and prevent stoppage is controlled largely
by local conditions. In sewers laid to no less grade than
the minimum recommended by the best authorities, and in which
only ordinary sewage flows, flushing once in 48 hours should
— 18—
easily obtain the results demanded.The writer is led to the following conclusions regarding
sewer flushing as the results of the investigations and experi
ments given above:(1) The upper ends of branch sewers from 6 to 12 inches in
diameter should be regularly flushed;(2) That under ordinary conditions this end is best attained
by atitomatic flush tanks;(3) That such flushing with tanks of the ordinary size is
effective on 6 to 12 inch pipe lines, for a distance from 400
to 700 feet;(4) That once in 48 hours is sufficiently frequent to flush
such sewers;(5) That some method for controlling the supply of water must
be devised before tanks can be filled automatically from the
water mains;(6) That local conditions govern the question of how tanks
shall be filled.
A P P E N D I X .
The following complete discussion of the several systems of ventilation and deodorization was included in the Report of the
special committee on Cleansing and Ventilation of Sewers, sub
mitted in 1886 to the Metropolitan Board of Works, London:
"The objection to surface ventilators is rather on account
of the gases escaping from them being more directly in evidence,
than to their being more dangerous to health than in other
methods of ventilation. Although the improvement effected by re
moving the point at which the sewer gases escape from the side of
footpaths to the centre of roads was very great, both in reducing
offence and in minimising danger to health, there can be no doubt
that at times the system is, as at present carried out, a serious cause of nuisance.
The efficacy of surface ventilation depends upon the number,
size and construction of the ventilating shafts, the size of the
openings for air in the gratings, and the relation of both shafts
and openings to the size and character of the sewer to be venti
lated. Your Committee find that the construction and size of the
surface ventilators varies widely in the several districts of the
Metropolis, and also in the various towns in the Kingdom. For in
stance, this method of ventilation is adopted either solely or in
combination with other means in all parts of London, but the dis
tance from each other at which the ventilating shafts are placed,
varies in different parts of the Metropolis from 17 yards to as
much as 600 yards. The average distance apart in London is 120 •
yards. In other towns the distance apart of the ventilators var
ies from 30 yards to 200 yards, the usual distances being from
50 to 100 yards. In answer to the question addressed by the
Board to the Vestries and District Boards of the Metropolis—
"Do you consider that it would be advantageous to increase the
number of surface ventilators?" 29 districts say "Yes." Of these,
six districts approve of the ventilators being placed at from
20 to 50 yards apart; twelve districts recommend them to be
placed from 60 to 80 yards apart; seven districts consider that
they should be between 80 and 100 yards apart; three districts
say the distance should be from 100 to 150 yards; and one district
thinks the ventilators should not be more than 200 yards apart.
Two districts do not approve of surface ventilation, two are
doubtful, and one district has no experience on the stibject.
One measure of the effectiveness of any ventilating appli
ance is the size of the opening through which the escaping air or
gas must pass. Every ventilating shaft should be as straight as
possible, and large enough at its smallest part to pass the neces
sary quantity of air or gas. Sewer ventilating shafts connected
with surface-ventilators usually answer these requirements, but
the gratings do not. For instance, an 18-inch circular shaft
has an opening for ventilation of 254.47 superficial inches, but
the openings in the gratings above such shafts do not often ex
ceed 48 inches— less than one-fifth of the capacity of the shaft.
The openings in the surface gratings are, however, usually
smaller than 48 inches. it will be seen in the appended return
that in 16 districts of the Metropolis they vary from 27 to 40
inches; in 9 districts from 41 to 50 inches;-in 6 districts from
51 to 60 inches; in 2 districts from 61 to 70 inches; and in
only 2 districts does the ventilating capacity of the gratings
reach from 70 to 80 inches. The surface gratings to the recently
constructed sewers have ordinarily been placed at a distance of
from 50 to 60 yards apart, and such gratings have air openings
equal in area to 60 square inches, and your Committee consider
that the number and size of many of the ventilators on other
sewers in the Metropolis should be increased.
It has already been pointed out that some amount of road
grit passes through the ventilating gratings and forms a deposit
in the sewer at the foot of each ventilating shaft, and that this
deposit causes nuisance by arresting the flow of the sewage,
which often forms a stagnant and offensive accumulation near to the
ventilator. This may be obviated by placing under each grating
a deep tray or catch-pit, care being taken that an air-passage,
at least equal in size to the area of the openings in the grating,
is left outside the tray; and the shaft should be enlarged at
this part for the purpose. This tray should be emptied periodi
cally, and the openings in the gratings should at the same time
be cleared of mud.
The second method of ventilating sewers with which it is
necessary for your Committee to deal, is that in which heat in
some form is used to extract the air from sewers through shafts.
The shafts may be the chimney shafts at factories; large special
shafts, with a fire burning in their base; or smaller special
shafts; but the principle is the same with each, viz.,the expan
sion by heat of the air in the shaft, which, in becoming lighter
in consequence of such expansion, is rapidly driven upward
through the shaft by the colder and heavier air from the sawer
with which the shaft is connected. As already pointed out, this
method is successfully adopted in the ventilation of mines, and
the principle of it is quite sound, as it embodies the natural
law which governs the movement of air. The principle is in opera
tion in every house, as an ordinary chimney, when the fire is
alight, draws from the room and discharges into the external at
mosphere a very large quantity of air. A heated ventilating
shaft attached to a sewer is of course equally effective in draw
ing air from the sewer and discharging it into the external atmos
phere; and there may be cases where it would be desirable to
ventilate a particular part of a sewer in this way. Such would
be the case with a sewer where surface ventilation proved objec
tionable, and where the sewer could be connected with the chimney
shafts of factories. In such a case there would be no expendi
ture for fuel, and the expense of connecting the sewer with the
chimney shaft would be repaid by the amount of ventilation afford
ed. This has been done in a few cases, both in London and else
where, with benefit to the sewers within 50 or 60 yards of the
shaft. Your Committee, however, do not consider that the system
can be much extended. In the first place, very great difficulty
has been experienced with owners of factories, who object to
allow their chimney shafts and furnaces to be made use of for
such a purpose, and only in one or two cases has the necessary
consent been given. The fact that separate shafts with fires
in them solely for the purpose of ventilating sewers would, if
generally adopted, entail an enormous expenditure, both in first
cost and in maintenance, renders the adoption of that system
undesirable, even if it could be made effectual.
The Board have constructed three special shafts in the Green
wich District, over the Southern Low Level Sewer, in which shafts
coke fires are maintained, for the purpose of drawing the gases
from the sewer and destroying the offensive effluvium. But the
system of passing the gases through the fire which is adopted in
these shafts has not been found to answer so well as the method of
allowing the bulk of the gases from the sewer to pass into the
shaft over the fire, very much in the same way as the air of
a room passes into a chimney. The mechanical obstruction to the
passage of air offered by the furnace bars and by the fire it
self is so considerable, that, unless a large and fiercely burn
ing fire is maintained in the shafts, the quantity of air which
passes from the sewer is small. The estimated proportions of
the air passing into the chimney in the case of an ordinary open
fire are, to one volume of air which passes through the fire,
nine volumes pass into the chimney over the fire. From this
data it would follow that for any given quantity of coke con
sumed, a much greater amount of ventilation would be attained
by making the air pass from the sewer to the shaft around and
over as well as through the fire. Having regard to first cost,
cost of maintenance, and to the limited amount of work they do,
special shafts and furnaces have not proved successful in the
ventilation of sewers, and the Board have, on the recommendation
of Sir Joseph Bazalgette, decided not to increase the number.
With this view your Committee entirely coincide. They consider
that this system cannot be applied to the ventilation of sewers
generally, and that it can only be locally used with advantage
in exceptional cases.
The objections above stated apply with equal or greater force
to the ventilation of sewers by means of a large number of small
shafts, such as the ordinary lamp-posts, in which it is suggested
heat should be used as a means of drawing out and then destroy
ing the offensive air in the sewers. A number of suggestions of
this character were made many years ago, and are described in
the Engineer's Report of the 6th June, 1866. These suggestions
were not of a practical nature, and were not seriously considered,
and a more recent plan submitted to the Board, called "The Pat
ent Hygienic Sewer Furnace System," comes within the same
category. This system provides for the extraction of the air
in sewers by means of gas furnaces, but as it is only claimed
that from 800 to 1,000 feet of air will be drawn from a sewer per
hour by the consumption of from 7 to 10 feet of gas, the cost of
the maintenance of such a system would be enormous. The Board
have already decided that such a system is not practically appli-
cable in either the main or local sewers, and with this view
your Committee agree.
The third method of ventilating sewers which your Committee
have to deal with, is ventilation by means of pipes or shafts
carried from the crown of the sewer to the side of the road, and
thence up houses or buildings. This method is gradually coming into greater use, more or less in combination with surface venti
lators, and it is therefore necessary that its advantages and
disadvantages should be carefully considered.
The plan usually adopted is to connect a stoneware pipe with
the upper part or crown of the sewer to be ventilated, and to
carry this pipe to the foot of an adjacent building, where it is
connected with a metal pipe which is carried up to and above the
roof. This pipe sometimes acts as an inlet for fresh air into the
sewer, but ordinarily it acts as an outlet by which the gases in
the sewer escape into the external atmosphere. About twenty
years ago the Board put up a number of such pipes for assisting
in the ventilation of the main sewers in the neighborhood of
Greenwich and Deptford. These pipes were of 4, 5 and 6 inches
diameter, and were described in a report made to the Board by
the Engineer on the 31st May, 1866, as having if of sufficient
area the same effect on the sewer as ordinary surface ventilators.
The method was not, however, adopted to any extent with the
Board's sewers, mainly owing to the great difficulty that was
experienced in obtaining the consent of householders to the pipes
being carried up their houses, as it was generally thought that
there was danger of the sewer gases entering the houses by open
windows or by chimneys. ~
Pipe ventilation has, however, been extensively adopted in
many towns, and to some extent in the Metropolis, by the Vestries
and District Boards. Some of the Vestries, in complaining of the
present system, suggest pipe ventilation as the remedy. The Ves
try of Tooting-Graveney, in their memorial to the Board on the
subject, say that "several persons have had shafts from the sew
ers carried up the sides of their houses, at their own expense,
which have answered in their immediate neighbourhood; but to be
successful the scheme should be general;" and they suggest, that,
as no power exists to compel owners to consent to such pipes, the
Board should obtain Parliamentary powers to carry out that or any
other plan of ventilating sewers which may prove efficient. This
view has been supported by the Vestries of St.George, Hanover
Square; St.James, Westminster; and St.Mary,Islington.
The Commissioners of Sewers of the City of London, in the
appended return, state that 137 pipe ventilators have been con
structed in the City, but the ventilators are described as not
having helped materially in reducing the nuisance from surface
ventilators. In 22 out of 37 other districts in the Metropolis,
pipe ventilators have been to some extent used; and the total
number of such ventilators provided by the Local Authorities in
London appears to be 582, of which Lewisham has 149, the City
137, and Wandsworth 100. The result appears to have been satis
factory in the majority of the districts. In twelve districts
the pipe ventilators are described as having abated nuisance, es
pecially when used in connection with surface ventilators. In
five districts the pipe ventilators, it is stated, have not
abated nuisance. Fulham states that the pipes have not material
ly abated nuisance, "as in certain atmospheric states they are
inoperative." St.Saviour, Southwark, states that the ventilating
pipes had to be abolished, as they were a nuisance to the inhabi
tants; and St.George, Hanover Square, states that owners would
not consent to pipe ventilators being attached to their houses,
and. those erected had to be removed in consequence of repeated
complaints. In Bethnal Green and in the Strand District it was
not found practicable to obtain the consent of owners to the
fixing of the pipes. Hampstead and St.James state that they are
unable to decide whether the pipe ventilators have materially
reduced the nuisance or not. Mile End Old Town states that the
pipe ventilators do not reduce nuisance "beyond the parti
cular surface ventilator to which they are attached." Lewisham
has the largest number of pipe ventilators of any district of the-
Metropolis, but the experience of that district is not conclu
sive, as the information as to their use which has been furnished
is "where the surface ventilators have been closed the complaints
have of course ceased. "
Elsewhere than in London the system has been more extensive
ly tried, the city of Coventry having erected 1,500 pipe venti
lators, while other towns have also used them in considerable
numbers. Out of 32 towns of which inquiry was made, 20 had tried
this method of ventilating their sewers, but in eleven cases only
to a small extent. The results were stated to have been satisfactory in 14 cases, and unsatisfactory in three cases. In the
three remaining cases the result was not ascertained.
The advantage of pipe ventilation over surface ventilation
consists in the discharge of the sewer gases at a point more re
mote from general observation than the centre of the public road
way. Whether or not the gases discharged through the pipes are
more likely to be injurious to the inhabitants of the houses to
which the pipes are detached, depends entirely upon the position
of the mouths of the ventilating pipes relatively to the openings
of such houses. If the pipes can be carried to points distant
from windows and chimneys then the danger would probably be not
greater in.one case than in the other; but if the pipes discharged
at a few feet only from the openings of houses, then the danger
would be greater from pipe ventilation than from surface ventila- t ion.
The effectiveness of pipe ventilators in carrying off the
gases from sewers depends on the capacity, construction, and
number of the pipes. In most of the Metropolitan districts in
which this system has been tried the capacity of the pipe venti
lators is small. In five districts the average capacity of the
pipe ventilators is returned as 10 square inches, in six dis
tricts as 20 square inches, and in seven districts as 30 square
inches. In the remaining districts there are 6 ventilators of
30 inches, 4 of 66 inches, 1 of 96 inches, and 1 of 144 inches.
In the three districts in which pipe ventilators are mo.st
largely used, the City of London, with 137 ventilators, states
that the average capacity of each is 2o square inches; Lewisham,
with 144 ventilators, returns the minimum capacity as 28.27
square inches; and Wandsworth, with 100 ventilators, states that
the capacity varies from 12 to 20 inches. It is probable, there
fore, that the average capacity of pipe ventilators in the Metro
polis is about 20 inches, which is insufficient.
Your Committee believe that pipe ventilators can be used
with advantage in aid of the present system. Pipes should be
carried up to the ridges of the roofs of adjacent buildings,
where the owners’ consent can be obtained, and they should dis
charge six or eight feet above the ridges, and as far from
chimneys and windows as possible. These pipes should be of as
large an area as possible, and angles should be avoided, as the
pipes will do far more work where easy bends are substituted for
angles. Such pipes would ordinarily make efficient up-cast
shafts, the nearest surface ventilators becoming inlets for air or down-cast shafts.
The question of deodorising sewer gases before they escape
through ventilating shafts or surface gratings has received much
attention from drainage authorities and others interested in the
subject. If, without materially interfering with the ventilation
of the sewers, the gases could be readily, economically, and
effectually deodorised before reaching the external air, one of
the greatest difficulties in connection with the subject would beIf
solved. This object has been attempted in various ways, either
by means of fire, by the agency of chemical substances, or, more generally, by means of charcoal.
Wood charcoal, when it is fresh and dry, has the peculiar
property of absorbing the offensive properties of sewer gases,
and many authorities, including the Board, have been at great
trouble and expense in the endeavour to utilise this quality in
charcoal in overcoming the nuisance from sewer ventilators. It
will be unnecessary for your Committee to describe the experiments
with charcoal made by the Board and by other bodies, as whenever
that substance has been used under the conditions that must al
ways govern its use in sewer ventilators, it has totally failed.
While it has remained dry, charcoal has for a few days absorbed
the effluvia from the small quantity of gas that has succeeded in
passing through it, or rather through the interstices between the
small pieces into which the charcoal has to be broken. But direct
ly the moisture in suspension in the sewer has saturated the
charcoal; it ceases to a great extent to act upon the effluvia.
The only means of preventing this would be by constantly changing
and re-burning the charcoal; and that might possibly be done, at
great trouble and expense, if there was no other objection to the
use of charcoal. But the opinion of the authorities who have most
experience of its use is that placing a charcoal tray in a sewer
ventilator is almost equivalent to closing that ventilator alto
gether. In six of the appended returns from the Vestries and Dis
trict Boards charcoal is described as having been used with ad
vantage, but that is probably due to the fact that little or no
effluvia has, since its use, been noticed from the ventilating
shafts in which it was placed. Out of 19 Vestries, &c. , who
have used it, 13 say it is useless on account of losing its vir
tue through wet, and through obstructing ventilation.
The system was, however, more thoroughly tried at Croydon
than perhaps at any other place. The ventilating shafts of the
Croydon sewers were fitted with a spiral tray, in which broken
charcoal was packed, and the gases before reaching the external
air had to pass between the layers of charcoal; the object of
this arrangement being to bring the gases into contact with the
charcoal with the minimum of mechanical obstruction to its es
cape. After thoroughly trying it for 10 years, during which time
the charcoal was regula_rly replaced every 3 weeks, the Croydon
Sanitary Authority abandoned the system because the charcoal could
not be kept dry, and because it obstructed the ventilation of the sewers.
Deodorisation of the escaping gases by means of the heat
obtained by burning coal gas in small furnaces attached to the
ventilating shafts or pipes of sewers has been proposed, but the
first cost and cost of maintenance xvould, apart from other consi
derations, be so enormous as to render the system impracticable.
There remains to be considered the question of whether a
remedy for the nuisance from sewer gases cannot to some extent be
found in arresting putrefactive action within the sewers by means
of some chemical agent, and the Board's recent experience in
dealing with the sewage in this manner indicates that such a
remedy may be successful. During the colder months the conditions
are unfavourable to putrefactive action in the sewers, and no
special treatment of the sewage may be.at such times necessary,
but during the hot and dry weather decomposition is rapid, and
your Committee are of opinion that it is then desirable to im
prove the condition of the sewage, and as far as possible prevent
the formation of offensive gases.
The remarkable effect produced by the action of permanganic
acid in destroying the foul odors emitted from decaying matters
and in arresting their further putrefaction, suggests the means
of attaining the desired object. The result of past experience
has been to demonstrate most clearly that the quantity of re
agent required to destroy existing foul odors, and stay the
production of others, is exceedingly small, viz. : from one to two
grains of the crude manganate of soda, supplemented with about
one-third that quantity of sulphuric acid, to each gallon of
sewage. The ready manner in which this mixture has been applied
by the Board, with such successful results to the whole of the
Metropolitan sewage during the past summer is evidence of its
adaptability for the prevention of the serious nuisance arising
during the hot weather from sewer gases. To obtain the maximum
effect with the minimum of expenditure, it is advisable that the
points of application should be multiplied to the greatest extent
found practicable. With this object Local Boards should under
take the treatment of the sewage in the sewers in their respective
districts, and householders, in addition to the flushing re
commended in this report, should be urged to employ some method
of deodorisation, in order that, as far as practicable, the forma
tion of sewer gas may be effectually prevented. Your Committee
are not prepared to recommend any particular form of apparatus
for effecting the above purpose. There are several kinds nowbefore the public, which are well
desired object.Madapted to fulfil the
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