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Report of the Medical Officer of Health.
141
place the ventilation should be independent of the subsidiary
sewers and it would be advisable that they should be divided into
sections, so that each portion might be separately dealt with.
This could be done at intervals of, say, one mile by constructing
chambers upon the sewer in such a way that the greater part of the
sewer gas generated in any section could be cut off from the
section above by means of a hinged flap which would offer little
or no obstruction to the flow of sewage in times of flood; each
chamber should be provided with a vertical shaft for ventilation,
of course providing a sufficient number of surface gratings and
upcast shafts between the chambers.
The branch sewers might be similarly cut off from the mains
in the manholes usually provided at the junctions. This has been
done in connection with the London County Council's estate at
Totterdown, and it will be interesting to notice the result when the
houses are occupied.
When the main drainage of London was designed, every
house using the sewers contributed towards the ventilation of
them, as the interceptor, introduced many years later was
unknown.
By the adoption of interceptors, the original ventilation of
the sewers has been seriously interfered with and no other adequate
means have been substituted.
The usual condition of sewer and house drains are as
follows:—
A house drain, as now constructed, is made water-tight
and cut off from the sewers by means of an interceptor, a
fresh air inlet with mica flap is provided, often near the front
door or window and a ventilator carried above the eaves of
the house at the rear, great pains are taken to see that the
ventilator is gas and water-tight. The assumption is that
the inlet, the drain and the upcast shaft at the rear act as a
syphon, that only fresh air is admitted in front and that all
the foul air ascends the pipe at the highest point of the drain.
As a matter of fact, after a very short time, the mica flap is
of no use whatever, and when a volume of water is discharged
into the drain it draws in fresh air down the tall shaft
and discharges foul air from what theoretically should be the
fresh air inlet.