=Sewers=
=Definitions.=--A sewer is a conduit or pipe intended for the passage
of sewage, waste, and rain water.
A _House Sewer_ is the branch sewer extending from a point two feet
outside of the outer wall of the building to its connection with the
street sewer, etc.
=Materials.=--The materials from which sewers are manufactured is
earthenware "vitrified pipes."
Iron is used only for pipes of small diameter; and as most of the
sewers are of greater diameter than six inches, they are made of other
material than iron.
Cement and brick sewers are frequently used, and, when properly
constructed, are efficient, although the inner surface of such pipes
is rough, which causes adherence of sewage matter.
The most common material of which sewers are manufactured is
earthenware, "vitrified pipes."
"Vitrified pipes are manufactured from some kind of clay, and are
salt-glazed inside. Good vitrified pipe must be circular and true in
section, of a uniform thickness, perfectly straight, and free from
cracks or other defects; they must be hard, tough, not porous, and
have a highly smooth surface. The thicknesses of vitrified pipes are
as follows:
4 inches diameter 1/2 inch thick
6 " " 1/16 " "
8 " " 3/4 " "
12 " " 1 " "
The pipes are made in two- and three-foot lengths, with spigot, and
socket ends." (Gerhardt.)
Sewer pipes are laid in trenches at least three feet deep, to insure
against the action of frosts.
=Construction.=--The level of the trenches in which sewers are laid
should be accurate, and a hard bed must be secured, or prepared, for
the pipes to lie on. If the ground is sandy and soft, a solid bed of
concrete should be laid, and the places where the joints are should be
hollowed out, and the latter embedded in cement.
=Joints.=--The joints of the various lengths must be gas-tight, and
are made as follows: into the hub (the enlargement on one end of the
pipe) the spigot end of the next length is inserted, and in the space
left between the two a small piece, or gasket, of oakum is rammed in;
the remaining space is filled in with a mixture of the best Portland
cement and clean, sharp sand. The office of the oakum is to prevent
the cement from getting on the inside of the pipe. The joint is then
wiped around with additional cement.
=Fall.=--In order that there should be a steady and certain flow of
the contents of the sewer, the size and fall of the latter must be
suitable; that is, the pipes must be laid with a steady, gradual
inclination or fall toward the exit. This fall must be even, without
sudden changes, and not too great or too small.
[Illustration: FIG. 15.
A BRICK SEWER.]
The following has been determined to be about the right fall for the
sizes stated:
4-inch pipe 1 foot in 40 feet
6 " " 1 " " 60 "
9 " " 1 " " 90 "
12 " " 1 " " 120 "
=Flow.=--The velocity of the flow in sewers depends on the volume of
their contents, the size of the pipes, and the fall. The velocity
should not be less than 120 feet in a minute, or the sewer will not be
self-cleansing.
=Size.=--In order for the sewer to be self-cleansing, its size must be
proportional to the work to be accomplished, so that it may be fully
and thoroughly flushed and not permit stagnation and consequent
decomposition of its contents. If the sewer be too small, it will not
be adequate for its purpose, and will overflow, back up, etc.; if too
large, the velocity of the flow will be too low, and stagnation will
result. In the separate system, where there is a separate provision
for rain water, the size of the sewer ought not to exceed six inches
in diameter. In the combined system, however, when arrangements must
be made for the disposal of large volumes of storm water, the size of
the sewer must be larger, thus making it less self-cleansing.
=Connections.=--The connections of the branch sewers and the house
sewers with the main sewer must be carefully made, so that there shall
be no impediment to the flow of the contents, either of the branches
or of the main pipe. The connections must be made gas-tight; not at
right angles or by T branches, but by bends, curves, and Y branches,
in the direction of the current of the main pipe, and not opposite
other branch pipes; and the junction of the branch pipes and the main
pipe must not be made at the crown or at the bottom of the sewer, but
just within the water line.
=Tide Valves.=--Where sewers discharge their contents into the sea,
the tide may exert pressure upon the contents of the sewer and cause
"backing up," blocking up the sewer, bursting open trap covers, and
overflowing into streets and houses. To prevent this, there are
constructed at the mouth of the street sewers, at the outlets to the
sea, proper valves or tide flaps, so constructed as to permit the
contents of the sewers to flow out, yet prevent sea water from backing
up by immediately closing upon the slightest pressure from outside.
=House Sewers.=--Where the ground is "made," or filled in, the house
sewer must be made of cast iron, with the joints properly calked with
lead. Where the soil consists of a natural bed of loam, sand, or rock,
the house sewer may be of hard, salt-glazed, and cylindrical
earthenware pipe, laid in a smooth bottom, free from projections of
rock, and with the soil well rammed to prevent any settling of the
pipe. Each section must be wetted before applying the cement, and the
space between each hub and the small end of the next section must be
completely and uniformly filled with the best hydraulic cement. Care
must be taken to prevent any cement being forced into the pipe to form
an obstruction. No tempered-up cement should be used. A straight edge
must be used inside the pipe, and the different sections must be laid
in perfect line on the bottom and sides.
Connections of the house sewer (when of iron) with the house main pipe
must be made by lead-calked joints; the connection of the iron house
pipe with the earthenware house sewer must be made with cement, and
should be gas-tight.
=Sewer Air and Gas.=--Sewer gas is not a gas at all. What is commonly
understood by the term is the air of sewers, the ordinary atmospheric
air, but charged and contaminated with the various products of organic
decomposition taking place in sewers. Sewer air is a mixture of gases,
the principal gases being carbonic acid; marsh gas; compounds of
hydrogen and carbon; carbonate and sulphides of ammonium; ammonia;
sulphureted hydrogen; carbonic oxide, volatile fetid matter; organic
putrefactive matter, and may also contain some bacteria, saprophytic
or pathogenic.
Any and all the above constituents may be contained in sewer air in
larger or smaller doses, in minute or toxic doses.
It is evident that an habitual breathing of air in which even minute
doses of toxic substances and gases are floating will in time impair
the health of human beings, and that large doses of those substances
may be directly toxic and dangerous to health. It is certainly an
error to ascribe to sewer air death-dealing properties, but it would
be a more serious mistake to undervalue the evil influence of bad
sewer air upon health.
=Ventilation.=--To guard against the bad effects of sewer air, it is
necessary to dilute, change, and ventilate the air in sewers. This is
accomplished by the various openings left in the sewers, the so-called
lamp and manholes which ventilate by diluting the sewer air with the
street air. In some places, chemical methods of disinfecting the
contents of sewers have been undertaken with a view to killing the
disease germs and deodorizing the sewage. In the separate system of
sewage disposal, where sewer pipes are small and usually
self-cleansing, the late Colonel Waring proposed to ventilate the
sewers through the house pipes, omitting the usual disconnection of
the house sewer from the house pipes. But in the combined system such
a procedure would be dangerous, as the sewer air would be apt to enter
the house.
Rain storms are the usual means by which a thorough flushing of the
street sewers is effected. There are, however, many devices proposed
for flushing sewers; e. g., by special flushing tanks, which either
automatically or otherwise discharge a large volume of water, thereby
flushing the contents of the street sewers.