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

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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-

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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

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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

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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

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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

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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;

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(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

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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

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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

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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

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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

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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

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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.— - • - • "•••• ~ '

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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

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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

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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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