ORDINANCE No. 13 of 1901.
Public Health.
In the case of street-sewers draining large districts, from which the flow of sewage is comparatively regular and capable of estimation, the proper size of sewer may be calculated with some approach to precision. In the case of house-drains, however, each decision is impracticable. The rate of flow is irregular and the quantity of sewage depends upon the habits of the inmates. Were the size of a house-drain calculated to carry off even a most liberal water supply, which is, for practical purposes, the measure of the sewage, it would be found, in the vast majority of cases, to be much less than that which experience has shown to be applicable in practice. It has been found that a house-drain less than a certain size is inconveniently liable to stoppages, caused by extraneous matters, such as rags, paper and the like, which occasionally find their way into the best managed house-drains.
The minimum size of house-drain is usually fixed at from four to six inches in diameter. A four-inch drain, constructed in the usual manner, is probably somewhat small as a minimum; and a six-inch somewhat large. A diameter of five inches, were this size readily obtainable, would be a better minimum.
J. Whenever the minimum size will suffice it should be used. Indeed, subject to the limit laid down in the preceding paragraph, and to certain conditions to be set forth later on, the smaller the house drain the better. The use of unnecessarily large house-drains amounts to an abandonment of the self-cleansing principle. If a drain is so large that the normal flow of sewage cannot fill it to a sufficient depth to establish a self-cleansing velocity, deposit takes place and goes on, either till the pipe is choked and the sewage escapes, through leaks, to the sub-soil, or until the deposit has accumulated sufficiently to reduce the area of the sewage-way to that which will establish a current sufficiently rapid to prevent further deposit.
A. In order that the size of house-drains may be reduced to the minimum practicable, in each instance, rain-water should be excluded from them as far as possible. Rain-water cannot, however, be wholly excluded from house-drains. Many uncovered surfaces, such as back-yards, receive slops and foul-water in the course of daily use and this must be carried off by the house-drains. A duplicate set of drains for such areas, one for sewage and the other for rain-water, would be a costly complication and there would be no security that each set would be used for its proper purpose only. Some sewage would almost inevitably find its way into the rain-water drain, which, being of large size, would be imperfectly flushed and therefore offensive.
L. Rain-water should be diverted from house-drains to the full extent that is 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 volume from which 4 inches 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 inlets 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.
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 shows 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.
V. 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 absorbent matter may act like 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 seal 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.
6. These ventilating-openings should be so placed that any sewage-tainted air which may escape from them shall be as little offensive as possible. With this object the upper ventilating opening should be placed above the roof of the dwelling. This is usually done by carrying up a 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 communication 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 also as a sewer ventilator; and when this is done universally, no accumulation of sewer-tainted air can take place in any part of the system.
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.
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.
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