VOLUME OF SEWAGE.
The total quantity of sewage to be dealt with per day can be
ascertained by gauging the flow in those cases where the sewers
are already constructed, but where the scheme is an entirely
new one the quantity must be estimated. If there is a water
supply system the amount of water consumed per day, after
making due allowance for the quantity used for trade purposes
and street watering, will be a useful guide. The average amount
of water used per head per day for domestic purposes only may
be taken as follows:--
DAILY WATER SUPPLY
(Gallons per head per day.)
Dietetic purposes (cooking, drinking, &c.) 1
Cleansing purposes (washing house utensils,
clothes, &c.) 6
If water-closets are in general use, add 3
If baths are in general use, add 5
Total 15
It therefore follows that the quantity of domestic sewage to be
expected will vary from 7 to 15 gallons per head per day,
according to the extent of the sanitary conveniences installed
in the town; but with the advent of an up-to-date sewage
scheme, probably accompanied by a proper water supply, a very
large increase in the number of water-closets and baths may
confidently be anticipated, and it will rarely be advisable to
provide for a less quantity of domestic sewage than 15 gallons
per head per day for each of the resident inhabitants. The
problem is complicated in sea coast towns by the large influx
of visitors during certain short periods of the year, for whom
the sewerage system must be sufficient, and yet it must not be
so large compared with the requirements of the residential
population that it cannot be kept in an efficient state during
that part of the year when the visitors are absent. The
visitors are of two types--the daily trippers and those who
spend several days or weeks in the town. The daily tripper may
not directly contribute much sewage to the sewers, but he does
indirectly through those who cater for his wants. The resident
visitor will spend most of the day out of doors, and therefore
cause less than the average quantity of water to be used for
house-cleansing purposes, in addition to which the bulk of the
soiled linen will not be washed in the town. An allowance of 10
gallons per head per day for the resident visitor and 5 gallons
per head per day for the trippers will usually be found a
sufficient provision.
It is, of course, well known that the flow of sewage varies
from day to day as well as from hour to hour, and while there
is no necessity to consider the daily variation--calculations
being based on the flow of the maximum day--the hourly
variation plays a most important part where storage of the
sewage for any length of time is an integral part of the
scheme. There are many important factors governing this
variation, and even if the most elaborate calculations are made
they are liable to be upset at any time by the unexpected
discharge of large quantities of trade wastes. With a small
population the hourly fluctuation in the quantity of sewage
flowing into the sewers is very great, but it reduces as the
population increases, owing to the diversity of the occupations
and habits of the inhabitants. In all cases where the
residential portions of the district are straggling, and the
outfall works are situated at a long distance from the centre
of the town, the flow becomes steadier, and the inequalities
are not so prominently marked at the outlet end of the sewer.
The rate of flow increases more or less gradually to the
maximum about midday, and falls off in the afternoon in the
same gradual manner. The following table, based on numerous
gaugings, represents approximately the hourly variations in the
dry weather flow of the sewage proper from populations
numbering from 1,000 to 10,000, and is prepared after deducting
all water which may be present in the sewers resulting from the
infiltration of subsoil water through leaky joints in the
pipes, and from defective water supply fittings as ascertained
from the night gaugings. Larger towns have not been included in
the table because the hourly rates of flow are generally
complicated by the discharge of the trade wastes previously
referred to, which must be the subject of special investigation
in each case.
[TABLE NO. 4.
APPROXIMATE HOURLY VARIATION IN THE FLOW OF SEWAGE.
Percentage of Total Flow Passing Off in each Hour.
-----------+------------------------------------------------
| Population.
Hour. +-----+-----+-----+-----+-----+-----+-----+------
|1,000|2,000|3,000|4,000|5,000|6,000|8,000|10,000
-----------+-----+-----+-----+-----+-----+-----+-----+------
Midnight | 1.0 | 1.0 | 1.2 | 1.3 | 1.5 | 1.5 | 1.8 | 2.0
1.0 a.m. | 0.7 | 0.7 | 0.7 | 0.8 | 0.8 | 1.0 | 1.0 | 1.0
2.0 " | nil | nil | nil | nil | 0.2 | 0.2 | 0.3 | 0.5
3.0 " | nil | nil | nil | nil | nil | nil | nil | 0.2
4.0 " | nil | nil | nil | nil | nil | nil | nil | nil
5.0 " | nil | nil | nil | nil | nil | nil | nil | 0.2
6.0 " | 0.2 | 0.2 | 0.3 | 0.5 | 0.6 | 0.5 | 0.7 | 0.8
7.0 " | 0.5 | 0.5 | 1.0 | 1.5 | 1.6 | 1.7 | 2.0 | 2.5
8.0 " | 1.0 | 1.5 | 2.0 | 2.5 | 3.0 | 3.5 | 4.0 | 5.0
9.0 " | 3.5 | 4.5 | 4.5 | 4.8 | 5.5 | 5.8 | 6.0 | 6.5
10.0 " | 6.5 | 6.5 | 6.8 | 7.0 | 7.5 | 7.7 | 8.0 | 8.0
11.0 " |10.5 |11.0 |10.5 |10.0 | 9.6 | 9.3 | 9.0 | 8.8
Noon |11.0 |11.3 |10.8 |10.3 | 9.3 | 9.5 | 9.2 | 9.0
1.0 p.m. | 6.0 | 5.5 | 6.0 | 6.7 | 7.0 | 7.2 | 7.3 | 7.5
2.0 " | 7.0 | 7.3 | 7.0 | 7.0 | 6.5 | 6.5 | 6.2 | 6.0
3.0 " | 6.8 | 6.5 | 6.5 | 6.5 | 6.5 | 6.3 | 6.3 | 6.0
4.0 " | 7.5 | 7.5 | 7.3 | 7.0 | 6.7 | 6.5 | 6.2 | 6.7
5.0 " | 6.5 | 6.5 | 6.5 | 6.3 | 6.0 | 6.0 | 6.0 | 5.8
6.0 " | 4.5 | 4.5 | 4.7 | 4.8 | 5.0 | 5.0 | 5.0 | 5.2
7.0 " | 6.5 | 6.2 | 6.0 | 5.8 | 5.5 | 5.5 | 5.5 | 4.7
8.0 " | 6.2 | 6.0 | 5.8 | 5.5 | 5.5 | 5.3 | 5.0 | 4.8
9.0 " | 5.0 | 4.8 | 4.7 | 4.5 | 4.5 | 4.5 | 4.5 | 4.0
10.0 " | 4.8 | 4.6 | 4.2 | 4.0 | 3.8 | 3.5 | 3.0 | 3.0
11.0 " | 4.3 | 3.5 | 3.5 | 3.2 | 3.2 | 3.0 | 3.0 | 2.8
-----------+-----+-----+-----+-----+-----+-----+-----+------
Total |100.0|100.0|100.0|100.0|100.0|100.0|100.0|100.0
-----------+-----+-----+-----+-----+-----+-----+-----+------
ANALYSIS OF FLOW]
Percentage of total flow passing off during period named.
---------------------+----------------------------------------------------------------+
| Population. |
+-------+-------+-------+-------+-------+-------+-------+--------+
| 1,000 | 2,000 | 3,000 | 4,000 | 5,000 | 6,000 | 8,000 | 10,000 |
---------------------+-------+-------+-------+-------+-------+-------+-------+--------+
7.0 a.m. to 7.0 p.m | 77.3 | 78.8 | 78.6 | 78.7 | 78.5 | 78.8 | 78.7 | 75.2 |
7.0 p.m. to 7.0 a.m | 22.7 | 21.2 | 21.4 | 21.3 | 21.5 | 21.2 | 21.3 | 21.8 |
Maximum 12 hrs. | 84.0 | 83.6 | 82.6 | 81.7 | 81.0 | 80.6 | 79.7 | 78.2 |
" 10 " | 72.8 | 72.8 | 72.1 | 71.4 | 70.0 | 69.8 | 69.2 | 68.5 |
" 9 " | 66.3 | 66.6 | 66.1 | 65.6 | 64.5 | 64.8 | 64.2 | 63.3 |
" 8 " | 61.8 | 62.1 | 61.4 | 60.8 | 59.5 | 59.0 | 58.2 | 57.5 |
" 6 " | 48.8 | 49.1 | 43.1 | 47.5 | 46.8 | 46.5 | 46.0 | 45.8 |
" 3 " | 23.0 | 28.8 | 27.11| 27.3 | 26.8 | 26.5 | 26.2 | 25.8 |
" 2 " | 21.5 | 22.3 | 21.3 | 20.3 | 19.3 | 18.5 | 18.2 | 17.3 |
" 1 " | 11.0 | 11.3 | 10.8 | 10.3 | 9.8 | 9.5 | 9.2 | 9.0 |
Minimum 9 " | 3.4 | 3.9 | 5.2 | 6.6 | 7.5 | 6.9 | 8.8 | 10.0 |
" 10 " | 6.9 | 7.4 | 8.7 | 9.8 | 10.7 | 10.4 | 11.8 | 13.0 |
---------------------+-------+-------+-------+-------+-------+-------+-------+--------+
The data in the foregoing table, so far as they relate to
populations of one, five, and ten thousand respectively, are
reproduced graphically in Fig. 12.
This table and diagram relate only to the flow of sewage--that
is, water which is intentionally fouled; but unfortunately it
is almost invariably found that the flow in the sewers is
greater than is thus indicated, and due allowance must be made
accordingly. The greater the amount of extra liquid flowing in
the sewers as a permanent constant stream, the less marked will
be the hourly variations; and in one set of gaugings which came
under the writer's notice the quantity of extraneous liquid in
the sewers was so greatly in excess of the ordinary sewage flow
that, taken as a percentage of the total daily flow, the hourly
variation was almost imperceptible.
[Illustration: Fig 12 Hourly Variation in Flow of Sewage.]
Provision must be made in the scheme for the leakage from the
water fittings, and for the subsoil water, which will
inevitably find its way into the sewers. The quantity will vary
very considerably, and is difficult of estimation. If the water
is cheap, and the supply plentiful, the water authority may not
seriously attempt to curtail the leakage; but in other cases it
will be reduced to a minimum by frequent house to house
inspection; some authorities going so far as to gratuitously
fix new washers to taps when they are required. Theoretically,
there should be no infiltration of subsoil water, as in nearly
all modern sewerage schemes the pipes are tested and proved to be
watertight before the trenches are filled in; but in practice this
happy state is not obtainable. The pipes may not all be bedded as
solidly as they should be, and when the pressure of the earth comes
upon them settlement takes place and the joints are broken. Joints
may also be broken by careless filling of trenches, or by men
walking upon the pipes before they are sufficiently covered.
Some engineers specify that all sewers shall be tested and
proved to be absolutely water-tight before they are "passed"
and covered in, but make a proviso that if, after the
completion of the works, the leakage into any section exceeds
1/2 cubic foot per minute per mile of sewer, that length shall
be taken up and relaid. Even if the greatest vigilance is
exercised to obtain water-tight sewers, the numerous house
connections are each potential sources of leakage, and when the
scheme is complete there may be a large quantity of
infiltration water to be dealt with. Where there are existing
systems of old sewers the quantity of infiltration water can be
ascertained by gauging the night flow; and if it is proved to
be excessive, a careful examination of the course of the sewers
should be made with a view to locating the places where the
greater part of the leakage occurs, and then to take such steps
as may be practicable to reduce the quantity.