976
THE HONGKONG GOVERNMENT GAZETTE, 27TH SEPTEMBER. 1890.
The low level sewer must be constructed with great care. It must be thoroughly water-tight. The whole flow having to be pumped out, the sewage proper should not be augmented, by infiltration from the subsoil. Indeed, I believe that such work could only be executed rapidly and economically, by departmental work, and that a large European staff, perhaps even European workmen, would be required for the purpose.
19. It will therefore be most desirable to avoid the construction of the low level sewer. This may be done in several ways. Instead of concentrating the pumping machinery at one point, pumps may be erected at each of the direct out-falls, raising the sewage into a high level out-fall, which may be of cast-iron. Or, the pumps may pump, from one to the other, till the final station; whence it could be forced to the out-fall by a more powerful pump. It would not be necessary to have a separate steam-engine, at each separate station.
20. Appliances are available whereby the scattered pumping apparatus may be worked by power, provided at a central station, and distributed by means of water under high pressure, compressed air, or by air under reduced pressure. In either of these systems the apparatus is so arranged as to regulate the rate of pumping, automatically, to the rate of sewage-flow, so that the constant presence of an attendant at each station is not required.
21. A system of pumping in detail is quite feasible, but in the present case, the desired end may be attained directly; by the use of a loug cast-iron syphon, with several branches. Though the principle of the syphon has been known from remote antiquity, it has not been applied extensively in sewerage works. The difficulty which has hitherto attended its use is that a syphon must be "charged," that is, completely filled with water, before it will work. If the syphon discharges the water more rapidly than the rate of supply, the water in the inlet reservoir falls below the level of the ascending leg, and the air enters, and the syphon must be recharged before it will again work.
This difficulty may, however, be obviated in a very simple manner. It is only necessary to prolong the inlet or ascending-leg downwards, below the level of the out-let of the descending leg, and to turn the latter up, at its discharging orifice. This done, if the water is removel more rapidly from the receiving well or reservoir than delivered by the syphon, it simply ceases to work, but no air can enter; because the level of the water in the collecting or inlet-wells cannot fall below the level of the upturned out-let, at the receiving-well; and at this level, the mouths of the ascending legs, will be- under water. The syphon therefore remains in a condition to work again, as soon as the supply increases. It therefore regulates its rate of working according to the work to be done. The attendant at the pump, which removes the sewage from the receiving well, has only to regulate the rate of pumping so as to keep the surface of the sewage, at some desired level. The syphon will bring it to him as it comes from the town; and even if he were to lower the level, below the upturned lip of the descending-leg, no evil result would take place. This simple arrangement has been adopted by Colonel WARING at Norfolk Va. U.S.A. with complete success.
22. To apply the syphon to the present case; it would be necessary to sink wells, at or near to each of the direct
out-falls.
The bottoms of these wells should be closed water-tight, with concrete, at a level of about - 16 OD., or about 27 feet below the Praya level. The diameter of these wells will be no larger than is necessary for convenient construction; indeed, cast-iron cylinders 3′ 0" diameter might be used, if this method of construction is economical. These wells will receive the sewage from the collecting sewers; and must be perfectly water-tight.
23. The syphon would consist of a cast-iron pipe, laid along the Praya, about 3′ 6′′ under ground, and laid and jointed, after the manner of gas or water pipes, the only difference being that regular gradients must be established, with definite summits at some point or points. The diameter of this pipe should be six inches at its western extremity then twelve inches, and lastly fifteen inches. At each of the collecting-wells, five in number, a branch-pipe should be carried down from the syphon, to the bottom of the wells. These will form the ascending legs of the syphons, and their openings would be at or about - 14·5 OD. but all at the same level exactly. The branches will be provided with sluice valves, whereby they may be shut off from the main-pipe if required.
24. At the pumping-station, a larger well should be constructed, having its bottom at the same level as that of the collecting wells. The syphon-pipe would be prolonged downwards to the bottom of the well, to form the descending-leg of the syphon, and its end would be turned upwards, bringing the orifice to about — 140 OD., or six inches above the inlets to the ascending branches. The pumping-well would contain the suction-pipes of the pumps, which are to raise the sewage
and force it to its out-fall.
25. The syphon would be charged, in the first instance, by filling up the wells, above the level of the horizontal pipe. When the air is completely expelled through cocks provided for the purpose, these cocks would be closed and the pump; set to work. The water in the pump-well will be lowered and the syphon will draw from the collecting wells and deliver into the pump-well. The water may rise in the collecting-wells to + 2 on datum, or indeed to + 5:36 at times. The level of the out-let of the descending branch is 15; consequently there will be a head to drive the sewage through the ripe, of 17 or 20 feet as tho ense may be. I' the pump removes the sewage more quickly than it comes in, all that can happen is that the water in the receiving-woll will be lowered below the inlet to the pump, but the upturned leg of the syphon will prevent the entry of air.
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