THE STORAGE AND DELIVERY OF WATER.
The methods of storing and delivering water will vary with its source.
In rural districts, deep wells and springs are the best sources of
supply; but in large towns they are found to be insufficient for
the wants of a rapidly-increasing population; and they can only be
multiplied in a given district within certain limits, as every well
drains a large surrounding area. The supply from surface wells in
gravel or sand beds or in chalk districts is liable to fail in seasons
of drought; but deep wells in oolite or chalk formations, and in the
new red sandstone, generally yield a constant and abundant supply.
When the water is supplied from upland surfaces, springs, or small
streams, =a collecting reservoir= is required. This is generally
a natural valley below the level of the source of supply, but of
sufficient elevation above the place supplied to allow the water to be
distributed by gravity, without any pumping apparatus. The reservoir
should be large enough to hold five or six months’ supply, and its
embankment should be perfectly water-tight, and of great strength.
When water is collected from upland surfaces, it is important to know
the amount of rainfall to be reckoned on. If we know the area of the
surface which drains towards the reservoir, and the average rainfall,
the _total_ rainfall is easily calculated. This will, however, differ
greatly from the _available_ rainfall, owing to the losses from
penetration into the ground, evaporation, and other causes. The amount
lost will vary, according to the season, from one-half to seven-eighths
of the total rainfall; and according to the soil (page 68). The
proportion of percolation in the chief water-bearing strata surrounding
London varies from 48 to 60 per cent. (Prestwich). It is less when the
ground is steep and the rainfall rapid, and usually less in winter than
in summer.
Water collected near its actual place of fall, and from uncultivated
districts, is always purer than that collected further from its source,
and from cultivated land.
From the collecting or impounding reservoir, water is carried by the
aqueduct or conduit either directly into the service-pipes, or when the
pressure is too great, into a second =service-reservoir=, resembling
the impounding reservoir in general structure, and capable of holding a
few days’ supply.
This must be high ground, above the level of the highest houses to
which water has to be supplied, as water cannot rise above its own
level. When this cannot be arranged, the water is pumped into tanks at
a higher level, and distributed from them.
The greatest hourly demand for water being double the average hourly
demand, the water-mains supplying a town must have double the
discharging power that would be required, supposing the demand was
uniform. The first requisite of a supply of water is that it should
be abundant, and sufficient in amount for any extra strain on its
capacities. Water ought to be laid on to every house, and to at least
two floors of the house. Anything preventing free access to water,
militates against cleanliness.
=Cast-iron= is the most serviceable material used in the construction
of the main water-pipes; it is coated with pitch, or Dr. Angus Smith’s
varnish, or with magnetic oxide of iron (Barff). The service-pipes to
each house are generally made of lead, and the ease with which this
material can be bent and curved, and carried to the different floors
of a house, makes its use very convenient. Lead pipes, furthermore,
can be easily obliterated in case of bursting, and so any waste of
water and flooding of the house minimised. Some kinds of water,
unfortunately, act on and dissolve lead; this is especially true of
soft waters and those containing organic matter. Shallow wells, being
very liable to organic pollution, ought never to have the supply-pipe
of their pumps made of lead. With hard waters, lead pipes may generally
be used safely. When the quality of the water renders lead pipes
objectionable, the use of iron, tin, zinc, tinned copper, earthenware,
gutta-percha, and other materials, has been suggested. Of these, cast
and wrought-iron pipes are the most serviceable, or pipes composed of
an inner lining of block-tin and an outer of lead, a layer of asbestos
intervening to prevent galvanic action between the metals. According
to Rawlinson, “supply-pipes of wrought-iron are cheaper, stronger, and
more easily fitted than service-pipes of lead;” but it is urged against
them by Perry, that with soft water they become choked by rust in a few
years. If galvanized they are more durable. Cast-iron pipes are rusted
less easily than wrought-iron.
When the water-supply is from a river, filtering beds are needed, in
addition to the parts of a water-service hitherto described. Moreover,
since the river is usually at a low level, the water, after passing
through the filtering beds, requires to be pumped into raised tanks,
from which it is delivered.
In laying down water-pipes, in the streets and to houses, it is
very important to make the distance between them and all drains and
gas-pipes as great as possible. Suction of gases or liquids may occur
into leaky pipes, even though these contain water, and still more
when they are empty; and disease has occasionally been traced to this
source. Thus if sewers and water-mains are laid in the same trench,
foul matters which have escaped into the soil from the former may be
sucked into the latter. This may happen if the water-mains are leaky,
even when they are running full. Experiments have shewn that the flow
of water causes a partial vacuum and insuction at the defective points.
During intermissions of supply when the mains are partially or entirely
empty, the danger of leakage into them is still greater. Coal-gas has
been similarly sucked into water-mains.
The pipes bringing the water to a house may be kept constantly filled
with water, or only for a limited time once or twice a day. The
intermittent system of supply necessitates the provision of cisterns
or water-tanks, in which water can be stored in the intervals of flow
of water. With a sufficient and properly-distributed public supply of
water, no cistern ought to be required.
=Cisterns.=—Cisterns for the supply of potable water may be made of
iron, slate, stone, glass, glazed earthenware, or brick lined with
Portland cement. Other materials have been used, as timber, lead,
and zinc. _Timber_ is inadvisable, as it easily rots; _lead_ is very
objectionable, owing to the possible solvent action of the water on it.
_Zinc_ or galvanized iron cisterns are also acted on by soft water;
but they may be used with most waters. Galvanized iron is iron coated
with a thin layer of zinc. Iron cisterns soon rust; but this may be
prevented by giving them a coating of boiled linseed oil before they
leave the foundry. _Stone cisterns_ are too heavy for use, except in
basements. _Slate cisterns_ are good, but are apt to leak; the points
of leakage have occasionally been stopped with red lead, which is
attacked by the water, and thus lead poisoning results. If the slate is
set in good cement (not mortar, as this makes the water hard), it is a
good material for a cistern.
Every cistern should have a _well-fitting lid_, always kept closed, to
avoid the entrance of dust of various kinds, or even dead cats, birds,
etc. Noxious gases may be absorbed by the stagnant water.
The cistern should be _easy of access_. If it is indoors, the cistern
room should be well ventilated; and in any case the cistern should
be periodically visited and cleaned out. When the cistern is full, a
ball-tap prevents any further flow of water; and if this does not act
properly, an overflow pipe carries off the excess of water.
Cisterns badly arranged or neglected have been in the past a common
source of disease. (1) The _overflow pipe_ should not pass into any
part of the water-closet apparatus or the soil-pipe, or into the supply
pipe to the water-closet.
Where the overflow-pipe discharges into the soil-pipe or closet pan,
foul gases or even solid particles may find their way into the cistern.
(2) _No water-closet_ ought to be supplied from the same cistern as
supplies drinking water, as the pipe leading down to the closet may
when the cistern is accidentally empty carry noxious effluvia into the
cistern. A separate flushing cistern capable of discharging two to
three gallons of water should be provided for each closet.
With a constant supply of water, cisterns are only required for
water-closets and for hot-water apparatus (see pages 168 and 164).
=Constant and Intermittent Services.=—With an intermittent service
of water, during the intervals of supply, water is only obtainable
from cisterns, water-butts, etc. The objections against this system
are that—(1) The cisterns required are expensive, and liable to get
out of order and become foul. (2) Their overflow pipes may improperly
communicate with the soil pipe or with some other part of the drainage,
instead of opening into the external air. (3) Putrid gases, from
neighbouring ventilating-pipes or other parts of the drainage system,
are liable to be absorbed by the stagnant water in the cistern. (4) The
chief objection to an intermittent supply is that, during the intervals
in which the water-mains are empty, foul air and liquids from the
contiguous soil and drains are liable to be sucked through imperfect
joints into the pipes. (5) In case of fire, the supply of water in the
system is insufficient. In certain towns rates of insurance against
fire have been reduced on replacing an intermittent by a constant
service of water.
On the other hand it is urged that more expensive fittings are required
for a constant service; and that, when taps are left open or pipes
burst, the waste of water is much greater than with a cistern supply.
The balance is decidedly in favour of a constant supply without
storage cisterns. Where storage cisterns are in use, the taps for
drinking-water should be connected with the “rising-main,” before it
supplies the cistern.
=The Advantages of the Constant Service= may be thus summarised:—
(1) Owing to the absence of cisterns, the risks connected with stagnant
water, and with improper arrangement of overflow pipes, are obviated.
(2) The risk of suction into supply mains of external contaminations is
reduced to a minimum, since the pipes are never empty.
(3) The pipes are less liable to rust. Air in the presence of a little
moisture, causes rapid corrosion.
(4) There is an abundant supply of water in case of fire.
Of course, when there is a temporary stoppage of supply, as for
repairs, some of the dangers incurred by an intermittent supply will
arise.