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THE HONGKONG GOVERNMENT GAZETTE, 27TH SEPTEMBER, 1890.
967
20. To prevent casual obstruction, it is found that certain minimum sizes of sewers should be used, irrespective of the duty that they have to perform; say 9" for principal sewers, and 6′′ for branches. To provide for new buildings also, sewers must be made somewhat larger than the size which would be in accordance with the flow of sewage from existing tenements. Nor, in the case of pipes, is it possible to adjust the sizes with any minute accuracy. The graduation of sizes in the market does not permit one to do so.
21. For these reasons, it will be advisable to provide means of flushing the sewer, by discharging, suddenly, from time to time, a considerable volume of water, so as to cause a rapid current through it. This may be done, either by collecting the sewage from higher levels in a tank; or by water derived from some other source such as the water-works or perennial springs. In the present instance, numerous natural springs will afford an ample supply for flushing. It is The suddenly preferable, whenever practicable, that the sewer should keep itself clear of deposit, without flushing. discharged flush displaces an equal volume of air, which must escape somewhere, and so produce smells from the vents, which will be aggravated by the fact that deposit has taken place, temporarily, which is stirred up by the flush.
22. It will not be possible, wholly to exclude rain-water from house-drains and from sewers. To do so, absolutely, would complicate house-drainage. If back-yards were provided with two sets of drains, there would be no means of preventing sewage from being poured down the storm-water channel. Again, the rain-water which flows off court-yards, stable-yards and the like, is practically, as foul as sewage; particularly at the commencement of a shower. It will, however, be seen that the sewers, even when reduced to the minimum practicable size, in accordance with the principles laid down above, will yet have a margin of carrying-capacity, sufficient to provide for a slight rainfall. House-drains, on the other hand, may have to carry off a considerable amount of rain-water. The sewers need not be enlarged, on this account. The proposed system crosses and re-crosses natural water-courses, at frequent intervals. At these points, over- flows should be constructed, by which the contents of the sewer can escape. Then the sewer will carry as much as it can; and in heavy storms, the excess will escape by the storm-overflow, to the nearest water-course, down which, by that time, a roaring torrent will be flowing.
23. No hard-and-fast rule can be laid down, as to the amount of rain-water which may be admitted to house-drains. It should, however, be excluded, as far as possible; and carried off, by surface channels, to the storm-water system. As a rule, the rain-water from courts and yards only, should be admitted: places on which foul matter may accumulate. The advantage of this arrangement, as regards the householder, is great, for it facilitates the maintenance of the house- drain. It obviates any necessity for house-drains of the prodigious diameters, which are but too common in Hongkong, and which can only be kept clear of deposit by a lavish use of water. The common argument in favour of these huge drains is "make them big enough and they will not choke up--: thus assuming the existence of deposits as a necessary incident in the working of a drain. Now if a drain is well laid and provided with proper traps, any solid object that can pass through the trap, will be carried away by a reasonable flush. Of course an object large enough to obstruct a small pipe will not choke a large one, but will remain in it, and form a nucleus of deposit. What happens in drains of immoderate size is that deposit goes on accumulating, perhaps for many years. The drain becomes a cess-pool. Ultimately, it either chokes up altogether, and the sewage escapes by leaky joint; or the deposit goes on accumulating, until the sectional-area is diminished to that proper for the flow, and a passage is maintained by it, permanently, or till some casual obstruction takes place.
24. A case once came under my notice, where the architect most certainly provided a house-drain of ample size to prevent it from becoming choked. It was six feet high, and four feet wide. It did not choke for upwards of a century; and might have gone on for another hundred years, but for an out-break of typhoid fever, in the house that it drained: and a consequent examination of the drains. It was found that this ample channel was full of black deposit, up to the crown of the arch. Along this however there was a narrow water-way, equivalent in section to a six-inch pipe, the size of pipe ultimately used to drain this large mansion (Spencer House).
25. Having thus described the principles, which should be observed in the design of a system of sewerage, not only for the High-level District but generally, I will now proceed to the examination of the plans, prepared by Mr. Cooper, for the sewerage of this district. I find that the design is in strict accordance with the principles laid down, in the previous paragraphs of this report, which were fixed before the examination of the plans was undertaken.
26. A main-sewer runs from along the northern or lower boundary of the Caine Road, Bonham Road, and conducts the sewage to an out-fall at Slaughter-House point. This position of the out-fall is the natural one, assuming that this district is to be kept separate, from those below it. It is the point to which the formation of the ground, conducts the effluent of the district.
27. Wherever the Praya sea-wall has its foundation in deep water, which is now the case from Kennedytown to Peddar's Wharf; and will be, even more so, when the Praya Reclamation is complete, there is no important reason which gives the preference to any one point, as an out-fall, over any other, provided always that the sewage is conducted out to a point well below low water-mark; and into a strong run of tide. Salient points are preferable to re-entering angles, on this account. It will be well to limit the number of out-falls, simply to reduce the cost of construction; and to con