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THE HONGKONG GOVERNMENT GAZETTE, 18TH OCTOBER, 1890.
badly constructed or badly kept house-drain is, therefore, not only a source of danger to the inmates of the house that it drains, but a public nuisance also. Unless house-drains are well made and properly used, no system of street-sewers, however perfect, can work in a satisfactory manner.
D. Water being the agent which cleanses the house-drains, its liberal use by the inmates of the dwelling, is essential to the proper maintenance of house-drains. The sewage must be well diluted. Nevertheless the quantity of water necessary for the proper cleansing of house-drains, is not excessive. The water normally used by the inmates of a dwelling for washing and cooking, is sufficient for this purpose, provided that it is readily obtainable at all times, either in the dwelling, or in close proximity thereto.
E. The principal point to be attended to in the design of house-drains, is so to arrange matters that the sewage, as producel, shall flow through them in the most rapid current practicable; so that all suspended matter shall be swept away at once and completely.
F. The speed of a stream flowing through a pipe or channel, of given size and shape, depends upon the following conditions:-
current.
(a.) The inclination of the channel.
(6.) The smoothness of its surface.
(c.) The volume of the stream.
Toe steeper the slope and the smoother the sides of the channel, the swifter will be the The greater the volume of the stream, incl natin being the same, the greater the speed. Thus; if a 12" and a 3" pipe have the same inclination the velocity in the 12" pipe would be about twice as great as in the 3" pipc, provide 1 that both were half full. But to fill the two to this extent, the quantity of water passing through the 12" pipe, would be about thirty-two times pipes that passing through the 3" pipe. But if the same quantity flowed through both pipes then the current in the 3" pipe, being more concentrated, would flow more rapidly than that in the partially filled, 12" pipe.
G. These considerations would lead to the conclusion that the best size, to be used for any house-drain, would be that which would just suffice to carry off the sewage with the pipe not less than half full. Within certain limitations, this is the case.
II. In the case of street-sewers draining large districts, from which the flow of sewage is comparatively regular aui capable of estimation, the proper size of sewer may be calculated with some approach to precision. In t e case of house-drains, however, such precision is impraticable. 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 calulated to arry off even a m st liberal water supply, which is, for practical purposes the measure of the sewage, it would be found, in the vast majori y of cases, to be much less than that which experience has shewn, 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.
I. 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 con litions 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 escapos, 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
K. 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. 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.