CESSPOOLS AND MAIN SEWERS.
The terms =Sewer and Drain= are used somewhat confusedly. The term
_drain_ should be used to designate the pipes bringing the sewage from
the house into the street-sewer, or any pipes by which the subsoil is
drained; the term _sewer_ being confined to the trunk canals into which
the house drains empty their contents.
Where the water-carriage system of sewerage is adopted, involving the
use of water-closets as described in the last chapter, the sewage may
be carried from the house either into cesspools or into the main sewer.
=Cesspools= are only permissible in isolated country-houses supplied
with water-closets. They should always be situated a considerable
distance from the house, and should be emptied at regular intervals,
the sewage being placed in shallow trenches on the land.
The construction of the cesspool requires careful attention. Its walls
should be of brickwork set in cement, lined inside with cement, and
surrounded by clay puddle. The bottom should have a fall towards one
side, where a pump can be fixed, to remove the more liquid contents.
The depth of the cesspool should never exceed 7 feet. The drain
emptying into the cesspool should be trapped and ventilated, near
its junction with the cesspool; and the cesspool itself should be
ventilated.
In connection with many old houses in towns, cesspools still exist,
sometimes under the basement or near the house, and so built as
to allow soakage in every direction. The surrounding soil becomes
contaminated for a considerable distance, the water in any neighbouring
well is tainted, or leaky water-pipes receive the soakage. The
cleansing of cesspools is always a disgusting process, and even
dangerous to the workmen employed. They incur the risk of suffocation,
and are very subject to ophthalmia. To avoid these dangers a pump and
hose connected with a partially exhausted barrel is employed, but even
with this provision some nuisance arises. In the Bexley cart, which is
used for this purpose, a hose is used to connect the cesspool with an
air-tight cylinder in the cart, into which the contents of the cesspool
are pumped.
A modification of the cesspool system, called the =Pneumatic System=
has been proposed by Captain Liernur. In it the cesspool is not placed
under the house or the courtyard of the house, but under the street
at the angle of junction of several streets. It is made of cast-iron
and air-tight, and is connected with all the houses of several streets
by iron pipes. By means of a powerful air-pump worked by steam, the
cesspool is emptied into barrels in which it is sent directly to farms;
and the barrels being placed on ploughs of peculiar construction, the
manure is discharged from the bung-hole of each barrel and covered over
with earth in the progress of the plough. The pipes tend in this and
similar systems to get clogged with fæcal matter, and large quantities
of water are required to keep them clean, so that the system merges
into that of the use of water-closets, but without the thoroughness of
the latter.
Cesspools have been almost improved out of existence in some
continental towns, by the introduction of movable cesspools,—_fosses
mobiles_,—to which would correspond strictly the tubs and pails used
in some of our large towns. Such movable receptacles have been still
further “improved” by the adoption of _separators_, by which the liquid
parts are allowed to escape into the sewer, while the solid parts
remain comparatively inoffensive. But when this is done, the cesspool
may be as well abolished, as the foulness of the sewage is not greatly
increased by allowing solid as well as liquid excreta to enter it.
=Sewers= are built of glazed stoneware or of impervious brick laid
on a bed of concrete to prevent sinking of any part, the parts being
most solidly put together with cement. Iron and steel pipes are also
used especially when extra strength is required, as when there is some
danger of the pipes sinking. For most small streets circular stoneware
sewers suffice. Oval brick sewers are more suitable for main streets
in which the amount of flow varies greatly. The cross-section of these
should be an egg-shaped oval with the small end downwards, as this
ensures the most rapid current. Sewers should be laid in as straight a
line as possible, and with a fall which will ensure a flow of at least
2½ feet per second. The following rule gives approximately the fall
required for smaller sewers and drains:
A 4-inch pipe requires a fall of 1 in 40.
A 6-inch pipe requires a fall of 1 in 60.
A 9-inch pipe requires a fall of 1 in 80.
For further particulars as to the velocity of flow see page 187.
Where a town is very flat, and a proper fall of sewer impossible,
Shone’s ejectors are sometimes used to raise the sewage. The sewers
are laid in sections, each section falling to a certain point, from
which the sewage is raised by the ejector to a higher level and so
carried to the next section of sewers. Each section has a separate
system of ventilation. The provision of manholes for inspection of
an intercepting trap and of ventilation of the house drain near its
junction with the sewer has already been considered (page 179).
As sewers have commonly to carry away the rain-water in addition to the
waste matter from houses, their size must be regulated accordingly.
The rainfall being very various, the sewers may occasionally become
overcharged and flood the basements of houses. During heavy rainfall
large quantities of road grit are washed into the sewers, the
intercepting gully tanks at the road sides being insufficient to
prevent this.
In addition to the above disadvantages associated with discharging
storm water into sewers, the sewage owing to its increased bulk is more
difficult of disposal, whatever method of disposal be adopted. The size
of the sewers and of storm-outfalls into the nearest river or the sea
must be regulated so that they are equal to these sudden strains on
them; or the =Separate system=, by which the rain is carried in special
conducts to the nearest river, must, in the alternative be adopted. The
objections to this plan are that it necessitates a double system of
sewerage, and does not allow of the useful scouring effect of rain on
sewers. Where it is feasible, and particularly in small country towns,
its adoption is advisable. In such cases the old brick drains are used
for carrying off rain-water, while new _pipe-sewers_ are employed for
carrying the sewage. Such pipe sewers are not liable to become fouled,
and on account of the decreased dilution of the sewage can be made
smaller than brick-sewers.
Sewers being closed conduits containing sewage, it is highly desirable
that the gases resulting from decomposition should be freely diluted.
Such gases in unventilated sewers may find their way through
intercepting traps into the house drain. The danger from this source
has been considerably exaggerated. =Ventilation of the sewer= is,
however, desirable. This has been commonly accomplished by gratings
opening at intervals directly into the middle of streets. In narrow
streets, the stench from street-grids is occasionally a source of
complaint, and may cause malaise and ill-health. Charcoal traps placed
below sewer grids to intercept offensive gases have been found to
be of little use. The best plan is to do away entirely with surface
ventilators, and carry up iron shafts or brick shafts lined with
stoneware pipes above the level of all neighbouring houses. The only
difficulty in adopting this plan is the difficulty in securing premises
to erect such shafts up houses, although no danger attaches to the
practice.
Ventilating shafts should be erected at intervals and particularly at
the highest points of the sewage system of a town, the upper end of
these shafts being remote from the windows of any dwelling-house. When
sewers are laid with too steep a gradient, they act as chimneys, the
gases mounting to the higher part of the town, and frustrating attempts
at ventilation on lower levels. Various attempts have been made to
ventilate sewers by artificial means, as by the aspirating effect of
street lamps, etc., but these efforts have not been successful, as the
effect of the up-current only influences a short length of sewer. The
usual method of combined up-shafts and street grids answers fairly
well, but when any complaint of smell from street-grids occurs they
should be replaced by up-shafts. In Bristol and a few other towns
no provision is made for sewer-ventilation, and no ill effects have
apparently resulted. There can be no question that the importance of
sewer-ventilation has been exaggerated. If the sewer has a sufficient
gradient, and is properly laid, and efficiently flushed, so that no
offensive deposits occur, the provision of up-shafts at favourable
points is all that is necessary. Sewer effluvia have been credited with
causing enteric fever, diphtheria, and other diseases. These diseases
rarely if ever owe their origin to this cause. The microbes discovered
in sewer-air have always been those of the outside atmosphere, and not
derived from the sewage. Sewer effluvia might, however, predispose to
such diseases, if exposure to them were frequent or protracted, by
lowering the powers of resistance of the constitution.
=Flushing of Sewers= is required at intervals, in order to remove any
deposits of grit or other solid matter. Flushing is effected by filling
special flushing shafts placed at intervals in the sewers with clean
water and then suddenly releasing this.
_Whenever there is stagnation, a foul odour_ is certain to be emitted.
The cardinal rule with regard to sewage is to keep it in rapid onward
motion, until it has passed the outlet of the sewer. _The introduction
into a sewer of hot water_ or waste steam is an occasional cause of
nuisance.
=The Outfall= of a sewer requires to be large and perfectly free in
order that the progress of the sewage may not be impeded. When the
sewage is discharged into the sea above the low water level, it becomes
backed up in the main sewers when the tide is high. The same condition
of things has occurred when the outfall is into a river below the water
line, or into a tank out of which the sewage has to be pumped. In all
these cases the ventilation of the sewers requires to be perfect, and
great precautions taken to prevent obstruction of the outflow.
In low-lying sewers where the outfall is impeded, mechanical aids are
required to prevent blocking of the sewers. This may be obtained by
pumping at the outfall, to enable sewage to escape at all conditions
of the tide, or to raise the sewage on to land for irrigation. In
the =Shone system= the sewage is raised to the required height by
means of compressed air. In this system the sewage is received into
“ejectors.” These are cylindrical reservoirs, in which is a float on
a counterpoised lever. When a certain quantity of sewage has entered,
a valve opens admitting the compressed air, which forcibly raises the
sewage into a higher length of sewer or to the outfall.