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requirement as to provision of an intercepting trap, the controversy regarding the
necessity and usefulness of the intercepting trap in house drains, is one of very
long standing
The intercepting trap was originally employed in modern drainage systems in
order to prevent the escape of sewer air into dwelling houses and the consequent injury
to the health of the occupants, and those who favour the trap claim that in order to
accomplish this it is essential to intercept the aerial connection between the public
sewer and the private house drain, and that any disadvantages involved are mainly
matters of detail, and capable of remedy. Further, in view of the importance of
excluding sewer air from the drain, they advocate the endurance of any minor
irremediable disadvantages should they exist.
On the other hand, opponents of this trap, while agreeing that sewer air should
not be allowed to escape into the dwelling, contend that this desirable end is not
attained by use of this trap, and moreover the disadvantages attendant on its use are
of a most serious nature.
The results of the experiments conducted on behalf of the Commission are given
in the Report, which is divided into eight parts, and five appendices.
Part I. dealing with the disadvantages of the intercepting trap shows :—
1. That the appliance is always an obstacle to the free flow of the sewage.
2. That it retains anything up to 79 per cent. of the solids of the sewage.
3. That it is exceedingly liable to blockage.
4. That any blockage may exist for some considerable time quite unrecognised.
5. That the fresh air inlet necessitated by the provision of an intercepting trap
is a source of danger to health.
6. That the trap complicates and increases the cost of construction of
house drains.
When a trap is introduced it is suggested that iron drains, 3 inches in diameter,
should be used, and the channels in the manholes be covered and not left open as
obtain in present practice. If this be done, blockages of the drain can then be brought
to the notice of the householder earlier than would be the case if the manholes were
left free to act as a reservoir for a considerable quantity of sewage.
The advantages claimed for this intercepting trap are set out in Part 2. These
advantages are two in number viz.:—
1. That they act as barriers to rats.
2. That they prevent the passage of sewer air into the house drains.
These advantages, however, appear to be discounted by the result of the
experiments, which show that rats can and do negotiate traps, and that, providing the