This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
Where the drain is in proper working order, there can be no reasonable objection to this system.
In a previous chapter, the principles of drain-disconnection and the reasons for it, were fully described. It is now requisite to point out the various forms of trap which may safely be adopted for the purpose. These are made in a great variety of patterns and shapes, both for use upon the drain and for insertion in a manhole. The old-fashioned syphon-trap, shown in Fig. 364, through which the sewage flows in a continuous stream, is still largely used.
Fig. 364 - Section of Syphon-trap.
Fig. 306 -Syphon trap with Hand hole.
The one great objection to this form of trap is that frequently the amount of seal allowed is entirely inadequate to make proper provision against loss by evaporation; the depth of seal should never be less than 2 1/2 inches in any case. A modification of the plain Byphon-trap, shown in Fig. 365, is effected by the insertion of a hand-hole for cleaning out the trap. This is a particularly objectionable trap, as the obstruction caused by the hand-hole creates a rapid accumulation of sediment around it, which increases and hardens, and as there is not usually sufficient discharge of sewage from the house to remove it, the final result is a complete blockage of the drain.
ome people object entirely to the use of intercepting traps, looking upon them as a kind of miniature cesspool, and while their arguments may be good when applied to some forms of intercepting trap, it is certain that a right type of intercepting trap is of great advantage to a drainage-system. Undoubtedly in many instances the trap of the house-drain is found to l>e blocked with decomposing matter, but this would never have remained there, had there been systematic Hushing of the drains. Most complaints as to foul odours from air-inlets adjacent to intercepting traps are made during dry seasons, when little or no rain enters the drains for Hushing purposes, the only flush l>eing from the 2-gallon cistern in connection with the w.c, which is quite inadequate, not only to carry solid matter through the drain, but even to clear the intercepting trap. With a proper system of Hushing, there is no reason why intercepting traps cannot be kept constantly clean. An ideal condition of things can never be attained in the working of a drainage-system, and a certain amount of grease, and other solid matter difficult to move, is hound to come into the drain. It is essential, therefore, that the intercepting traps should be of such a form as will offer the least resistance to the matters which have to pass through them, and their shape should be such that they will in practice be self-cleansiiig; further, they should not hold more water than would be contained in the ordinary flush of a single water-closet, so that their contents may he frequently changed. A trap, to be effectual, should be set level, and should have a good solid flat base, so as to enable it to be firmly and securely fixed in its proper position. Where these points are attended to, intercepting traps will be found to work satisfactorily for any number of years.
The Buchan intercepting: trap is well known, and is shown in Fig. 366. It has a straight drop of 2 inches from the inlet to the water-level in the trap, but the one objection to it is that opposite to the inlet there is a straight wall, - the design being very similar in this respect to the wash-out w.c, - so that water coming into the trap at a high velocity comes in contact with this wall, and its flushing force is diminished, which lessens the chance of the water in the trap being completely changed.
It is important to make the fullest use of the incoming water for flushing purposes, and as far as possible to avoid its change of direction, and to concentrate it upon that part of the trap where it is most wanted. It is impossible to show every variety of intercepting trap which fulfils these conditions, but two good patterns are shown in figs. 367 and 368. An extension to the intercepting trap is shown in Fig. 369, which consists of a sudden drop of 4 indeat the outlet of the trap forming a weir, which gives a cascade motion to the water. This is known as Stidder's Drop Trap. Another trap which may be used without a manhole-chamber is the one known as Winser's Patent Villa Detector, which provides means for efficiently testing the drain by simply dropping a plug into the seating below the inlet of the trap. These traps can be fixed at any depth by simply extending the shaft as required. Fig. 370 is a section of the trap and shafts connected with it. The late Mr. Slagg introduced a trap, Fig. 371, which has a contracted throat, and for which he claimed that its capacity is large enough to pass the greatest quantity of water coming to it, and small enough to compel the water and sewage to pass through with twice the velocity it can possibly have when the sectional area is the same as the drain itself. The adoption of this form of trap necessitates the air-inlet shaft being constructed of brickwork, as part of the appliance consists of a short inverted shoot, marked a, from the drain to the trap having a fall in its length of 4 inches.
Fig. 366 - Section of the Buchan Intercepting Trap.
Fig. 367 - Section of the " Cerus" Intercepting Trap.
Fig. 368 - Section of Oates and Green's Intercepting Trap.
Fig. 369 - Section of Stidder's Drop Trap.