THE HONGKONG GOVERNMENT GAZETTE, 27TH SEPTEMBER, 1890.
977
26. The engine-driver would have to regulate the speed of his engines so as to keep the surface of the sewage in the pump-well, at some definite level. Then the syphon would bring it to the pump, as fast as it flows from the sewers to the receiving-wells, in which the sewage will rise to the height necessary to force it through the syphon.
27. A syphon, of the proposed dimensions, would deliver 1,000 gallons a minute or 1,440,000 gallons a day. But, as the maximum daily rate of flow of sewage is about twice the average rate, the syphon would be able to discharge 720,000 gallons of sewage in 24 hours; or at the rate of 12 gallons per head on a population of 60,000. The probable sewage flow of the district is estimated at the rate of about 360,000 gallons a day, whereas the syphon can discharge at the rate of 1,440,000 gallons in that time. There is therefore a margin of 1,080,000 gallons in 24 hours, to provide for rain-fall. The contributing area is 10,900,000 square feet, and therefore the syphon is capable of discharging about " of rain in 24 hours. But a very considerable part of the contributing area consists of gardens, and other uninhabited places, the rain falling ou which will flow to storm-water channels. The intercepting syphon, therefore, will be capable of removing a much larger
amount from the true urban area.
28. At each receiving-well there would be a storm over-flow, arranged as in the Central and Western Districts, with its edge at the level of 5'7; or high-water ordinary spring-tides. The sea water would be excluded from the wells and syphons, except for a short time, at high spring-tides. When the tide is above the storm over-flow, it would be the moment for flushing the syphon. The engines would then be worked to their full power, and the various branches will be shut and re-opened, in succession, so as to cause a strong current through the whole syphon. Sluices should also be provided for admitting sea water, at any time.
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29. An iron basket should be suspended in the receiving-wells, under the mouth of the sewer, to catch any solids which might choke the syphon or interfere with the pumps. A 1 inch wrought-iron pipe, connected with the water-mains, might be carried down to the bottom of the well. From time to time, the water might be turned on, and the issuing jet would act as an agitator, and stir up any sediment at the bottom of the well.
The mouth of the receiving-wells might be closed with a cast-iron cover, so that there will be no effluvium from them; and obviously, no effluvium can escape from the cast-iron pipe.
30. Ås some bubbles of air may be carried into the syphon by the sewage falling down the receiving-wells, which, in the course of time, might accumulate to an extent sufficient to reduce the carrying capacity of the syphon, it will be well to provide at summits the means of exhausting the air. This may be done by ejectors, worked from time to time, by water from the water-mains.
31. From the pumping station to the out-fall at North Point, the sewage would be carried, in an ordinary cast-iron main, 15" in diameter. To overcome the friction of this main, a head of about 10 feet will be required, and the sewage must be delivered at the level of high-water extreme springs; or + 7·18. The height to which the sewage must be raised at the engine-house will be 7·18 + 100 ≈ + 17·18 OD., and it will have to be pumped from a depth of 15 OD., or to a height of 33 feet in all. The maximum quantity to be raised per minute being 1,000 gallons the requisite nett H.P. will be 1,000 × 10 x 32
10 H.P. nearly. Allowing 20% for friction, this becomes 12 H.P. 33,000
32. The details of the steam-engines need not at present be discussed. It would be well to provide one compound condensing pumping engine of 12 H.P., constructed in the most complete manner, with regard to fuel economy, and two steam pumps, of the simplest type; and of a slightly larger capacity. The compound engine would do the ordinary daily work, and the simple engines would serve as stand-by engines, to replace the compound when stopped for repairs: or to assist it when flushing or during rain.
33. One or other of these engines would work night and day, as there is no storage for the sewage at night. To construct the necessary storage-reservoir, which must have a capacity of at least 500,000 gallons, would be very costly. Such a reservoir would be very apt to create a nuisance, for it could never be cleaned out thoroughly; and the putrifying matter, adhering to its walls, would promote fermentation in the sewage. No nuisance need be anticipated from the proposed receiving well; as it would be ventilated to the ash-pit of the boilers: so that all effluvia will pass through the boiler-fire.
34. It will be well to consider whether the water of the Mint Dam might not, after the completion of the water-mains, be used to work a hydraulic motor, during the night at least, and thus obviate the necessity for keeping steam up night and day.
35. The probable cost of the proposed sewage works, for the Eastern District may be estimated as follows:-
Street sewers and branches with man-holes, ventilators, and out-falls complete, Intercepting syphon, wells, building, pumping engine and pumping main,
.$ 42,500.00
70,000.00
Total,.......
Less, if the intercepting syphon be constructed concurrently with the sewerage, the value of 6 out-falls,
Remains,
.$112,500.00
6,000.00
.$106,500.00