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81
With respect to the point as to the interference with traffic, so far as we have
been able to ascertain, the number of times it has been necessary to examine interceptors
under footways has amounted to about sixteen during the past four years.
The average has probably not exceeded four times a year, and even in these cases
it has not always been found that the interceptors were at fault.
With respect to the third paragraph, which deals with the flushing of drain traps,
it is true that it usually requires more than one flush with an ordinary cistern to clear
an intercepting trap. The trap has not only to be cleared, but also to be kept charged.
It is therefore very probable that on inspection, an interceptor will be found to contain
water contaminated with excreta. It is quite possible also that if left long enough the
contents of the interceptor may make some very slight addition to the effluvia in a
drain, but we very much question whether the use of the interceptor materially adds
to the stink of a sewer. The whole of the drain along which fœcal and offensive
matter and liquid are more or less constantly passing is contaminated and liable to
be offensive. The interior of the drain, however, presents such a very much larger
surface for the evolution of offensive gases than the comparatively small surface of the
fluid in the interceptor and consequently bears such a much larger share in the production
of drain effluvia, that the argument of the interceptor adding to the gases in
a house drain does not appear to us to be of any weight against the use of interceptors.
Moreover, even supposing the interceptor to make a material addition to the
drain gases, no harm is done if the drain is efficiently ventilated as is required by the
bye-laws. As a matter of fact, in ordinary use, the contents of the intercepting trap
are renewed with sufficient frequency to prevent such fermentative changes as may be in
progress, producing any marked effect in the evolution of drain gases.
What is stated in the fourth paragraph as regards gas from sewers bubbling
through the water in the intercepting traps is only likely to' happen during storms,
when the sewers are charged with storm water. At such times a very limited amount
of gas, chiefly generated in the portion of the drain between the interceptor and the
sewer, is by the pressure of the contents of the sewer, liable to be forced through the
interceptor. At such times, also, as is Well known, there may be, when the conditions
are favourable, flooding of basements with sewage. Moreover, other traps in
pipes leading to the sewers are liable to be forced, even the gullies in the roads.
However, supposing the small quantity of gas above referred to be forced through the
interceptor, the result can be of no practical significance if the drains are efficiently
ventilated as they ought to be.
That rats can dive through intercepting traps at will may be a fact, but experience
shows that it is not the rule for them to do so. It has only occasionally been observed,
and the use of the intercepting trap is one of the measures most relied upon for preventing
rats infesting premises.