THE HONGKOng governmeNT GAZETTE, 18TH OCTOBER, 1890.
L. Rain-water should be diverted from house-drains to the full extent that possible by the use of surface-channels. As a general rule, when surface-channels cannot be used to divert rain- water, it must be admitted to the house-drain. Hence, in the majority of cases, the amount of rain- water to be carried off determines the size of the house-drain. The table, hereunto attached, gives the area from which 4" of rain, falling in one hour, will be carried off by pipes of various sizes, laid at different inclinations. In providing for the removal of rain-fall, from a given surface, it is to be remembered that it is not sufficient to provide a pipe of ample size to carry off the rain-fall. Traps and gratings must be provided in sufficient numbers and of sizes to admit the rain-water freely to the drain and the surface must have a fall to carry it to the openings. In many cases neglect of these precautions has caused flooding. This has been put down to the size of the pipes, whereas the in- lets were really at fault.
Rain-water cannot be looked upon as a legitimate agent for cleansing house-drains from deposit. Firstly, there should be no deposit to remove from well constructed and properly managed drains; and secondly, because there are long intervals in which no rain falls so that this flushing agent fails when most wanted.
M. The irregularity and uncertainty of the flow of sewage, which precludes an accurate determination of size, makes it impracticable, to calculate definitely the proper inclination of house-drains. Experience shews that, under the normal conditions of use, an inclination of one in thirty is desirable to ensure a self-cleansing drain; especially when the drain conveys the sewage from a single trap or inlet. An inclination of one in thirty, therefore, should, as a rule, be given to house-drains. Drains laid at much flatter gradients can be made to work perfectly, with care, and provided that the flow of sewage is copious i e. sufficient to fill the pipe nearly half full. The designer of house-drains having no control over their subsequent usage, will do well to give them, in every case, the greatest practicable fall.
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N. The ordinary "trap' or syphon is not a perfect preventive against the escape of sewage- tainted air from house-drains, although it is the only appliance available for the purpose. The water which seals the trap may evaporate during disuse, or a piece of fibrous matter may act like a a syphon and draw off the water sufficiently to leave a free passage for the escape of sewage- tainted air. Again, if there be a series of inlets connected to a drain which is unprovided with any free outlet; a sudden rush of water through one trap will force the air contained in the drain through the other traps or unseal them by drawing off their contents, and open a passage for the escape of sewage-tainted air into the dwelling, or into some confined space in its neighbourhood. Consequently, house-drains must be "ventilated," or rather provided with vents, to prevent the accumulation of sewage-tainted air therein, and to secure the efficiency of the traps. To effect this thoroughly, the drain must have, at or near its extremities, openings giving a free escape to the air within it, and between these openings, there should be no trap or other obstruction to the free passage of air.
that any sewage-tainted air which may With this object the inner ventilating- This is usually done by carrying up a
0. These ventilating-openings should be so placed escape from them shall be as little offensive as possible. opening should be placed above the roof of the dwelling. pipe, having a free opening at the top, above the roof; and at a distance from any window. The second opening is usually made on the house-side of the trap which severs the direct communica- tion between the drain and the public sewer. If, however, there be no trap between the house- drain and the sewer then there is no necessity for a second opening, the sewer itself serving the purpose. With a well constructed system of self-cleansing street-sewers this arrangement may be adopted with safety, even with advantage. Then every house-drain ventilator serves as a sewer ventilator also; and when this is dotie universally, no accumulation of sewer-tainted air can take place in any part of the system.
P. Care must be taken in selecting the position of the lower ventilating opening, because the elevated ventilating pipe by no means secures a permanent upward current of air. According to the relative temperature of the air within and without the drain, to the direction and force of the wind, so an upward or downward current will obtain.
Q. Complete ventilation is especially important when any part of the house-drain system is in communication with the interior of the dwelling; or where the system is extensive and has numerous inlets. It is essential where water-closets are used, or where the drains receive excre- mentitious matter. In the case of a short length of drain, leading from a single inlet, placed in an open space, such as a back-yard, though desirable, it is less necessary and may be omitted with- out serious danger.
R. Any direct communication between the interior of the house and the house-drain should be avoided. Pipes carrying water from baths or sinks, within the dwelling, should therefore deliver their effluent above trapped gulleys, in the open air. The effluent from baths or sinks, on upper storeys should, whenever practicable, beceived by open-topped pipes, delivering freely above trapped gulleys at or a little below the ground-level. If, as in the case of water-closets, a direct cómmunication has to be made between a fitting inside the house and the drain, then there must be complete ventilation by means of a pipe carried up outside and to the top of the building.
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