966
THE HONGKONG GOVERNMENT GAZETTE, 27TH SEPTEMBER, 1890.
sewer, except those at the top and the bottom. To the upper opening, he attached a ventilating fan, which exhausted the air from the sewer, sufficiently rapidly to remove its whole contents, every five minutes. Nevertheless, whilst the fan was at work, his apparatus, at the lower opening, continued to register a steady downward flow, just as it did before the fan commenced to work. The whole of the air, removed by the fan, as well as that which floved out below, must have entered by some un-trapped house-drain or other undiscovered opening. Indeed it may safely be asserted that the ventilation of a system of sewers, in the sense of producing a steady flow of air through it to some one point has never been effected. Indeed with the materials and appliances, at command, does it seem likely to be effected. If a fan or furnace were erected, for withdrawing the air from a large system of sewers at one point, and producing an indraft at another, it would have to be so powerful as to create a vacuum, near to it, sufficient to un-trap house-drains. Even if it did not, any house-connection which might become un-trapped, owing to leakage, or from evaporation, would admit enough air to destroy the current in the more remote parts of the system.
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14. In a well-constructed and well-managed system of sewers; and house-drains, no such heroic system of ventilation is required. Nor does it seem to be even desirable. If the sewer-tainted air will remain in the sewer, it is at least as harmless there, as anywhere else. What has to be done, is to provide a sufficient number of openings, more properly in called "vents' than "
ventilators," which, if the air, contained in the sewer, tends to escape; will allow it to do so, the open air, where it is relatively harmless, rather than by house-drains, into the interior of dwellings, where it is deadly. 15. The above stated facts and principles lead to the following conclusions as to the best way of abating the nuisance caused by escapes of sewer-tainted air from openings. It has been shewn that a small amount of sewage will taint a large volume of air. Clearly therefore, the less the quantity of air, exposed to the tainting-action of sewage, the better; thus leading to the conclusion that the size of sewers should be reduced to the minimum size practicable.
16. Now a sewer, which has also to act as a storm-water channel, must have a section, many times larger than if had to convey sewage only; and it will contain a larger volume of air, tainted with sewage, than a sewer, properly constructed, for sewage only. When, from any cause, an escape takes place, the large combined sewer will give forth a larger volume of tainted air, and produce a greater and more extended nuisance, than of the smaller volume escaping from the small sewer. The stream of sewage in the large sewer, exposes a greater area capable of imparting contamination to the contained air. Its damp sides, above the normal level of dry-weather-flow, are coated with low forms of fungoid growth, moulds, and the like, which doubtless contribute to the contamination of the air; and to the establishment of putrefactive action. The large sewer, therefore, is a large store-room of sewer-tainted air. The motion of this air is produced by causes, un- certain and beyond our control; and it will, from time to time, pour forth in large quantities, whenever and wherever the forces acting on the contained air cause it to issue forth. It is clear, therefore, that the section of sewers should be reduced to the minimum, and they should not be so designed as to convey large amounts of rain-water as well as sewage.
17. The manner of determining the minimum size, will be discussed later on, as well as the conditions under which rain-water may be admitted, and to what extent. The true remedy, however, for stenches from sewer-vents, is to make the sewage, passing through the public sewers, as little offensive as possible. To this end, it should be discharged into the sewers, fresh, before putrefaction has commenced, and it should be freely diluted with water. In other words, the remedy for the principal inconveniences, experienced in connection with sewers, consists mainly in good house-drains, properly made and properly used; in an ample water-supply; not lavished or wasted, but well-applied. An abundant water-supply is, even now, possessed by many, and in a short time, will be within the reach of all the inhabitants of Victoria (provided always that its distribution is well administered). The Government may assist the house-holder in obtaining good house-drains. But the people themselves have the prevention of nuisances, mainly in their own hands. If they will take care of their house-drains, the sewers will look after themselves. Unless the public, generally, co-operates with the Government in sanitation, by looking after the interior arrangements of their dwellings, by insisting on the drains being well constructed and decently maintained; all expenditure on sewers will be, if not useless, very much reduced in value, as regards the public health. If they do so co-operate the task of the Government in main- taining the sewers becomes a simple onc, certain success.
18. The area, the sewerage of which is about to be discussed, is almost entirely inhabited by Europeans, and principally by the wealthier classes of that community. It is scarcely to be anticipated that such a population will neglect to take the simple precautions, necessary for the proper management of their house-drains. Their habits are such as lead to an abundant consumption of water. Any reasonably well constructed system of sewers is certain to be successful. If any failure does take place it will be the fault of the inhabitants of the district, and not of the Government, or of the designers of the system of sewers.
19. In determining the size of sewers, the following principles should be kept in view. The normal dry-weather- flow should fill the sewer, to about one-third of its diameter. That is to say the sewage, proper, with any sub-soil water that may be admitted, should, during the hours of maximum flow, fill the sewer to one-third of its diameter. The gradient of the sewer should be such, that with this amount of water passing through it, the velocity shall, on no account, be less than 2 feet per second, preferably more. This velocity is the least that will prevent deposit; and will scarcely serve to remove it, when once formed. During some part of each day considerably greater than 2 feet per second should obtain.