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removed from the second portion, and thus there is not sufficient to carry off the sedi. mentary matter, which would be done when the two systems are combined.

18889. The position and size of the drains having been settled, the fall has to be arranged. It has been proved beyond a doubt that matters easily carried away by the increased velocity gained by using a small drain, remain as an obstruction in a large drain. A velocity of 2 feet per second is the least which will keep sewers clear of all ordinary obstructions; while house drains and small pipes require a velocity of 3 feet per second to keep them clear (Hurst). A fall of from 2 inches or 3 inches in 10 feet will be found quite sufficient for all practical purposes. A fall of 1 in 30 is considered by many to be a good fall, and not always to be obtained. Pipes half full, with a velocity of 3 feet per second require the following falls :-4 inch pipes, 1 in 100 ; 6 inch, 1 in 150; 9 inch, 1 in 225; 12 inch, 1 in 300; 15 inch, 1 in 350; 18 inch, 1 in 450 ; 24 inch, 1 in 600; 30 inch, 1 in 700. With a velocity of 2 feet per second, 4 inch pipes require a fall of 1 in 200 ; 6 inch, 1 in 300; 9 inch, 1 in 450 ; 12 inch, 1 in 600; 15 inch, 1 in 700; 18 inch, 1 in 900; 24 inch, 1 in 1200; 30 inch, 1 in 1400 (Sears).

1888h. Hence also the advantage of fushing a drain. One person has urged that his ten-roomed house and outbuildings have not, in the course of many years, ever been inconvenienced by the use of a 3-inch drain, whilst other houses of similar size, having 6-inch and even 9-inch drains, have been seriously affected. Much depends on the fall, and on the careful laying of the pipes, and something on the quantity of water used for household purposes. Where a water closet is placed at or near the head of a drain, a stoppage of its pipe often occurs; while grease from the kitchen sink incrusting in the pipe, for want of occasional flushing with hot water, is another frequent cause. Sewers also occasionally require assistance by flushing them from their head. One of the best arrangements proposed is that of an iron tilting cistern, to hold about 90 gallons, inserted in a brick pit at the head of a pipe sewer. This cistern, with its brass bearings and plates, brickwork, stone cover, and water tap, costs about nine pounds, and if one were placed at the head of each pipe sewer in a town, and all were turned off at the same time, a material assistance in keeping the main line also clear, would be found. The "self-acting syphon flush tank” is now much used for such purposes. Rogers Field's patent consists of two concentric tubes, the outer one being closed at the top and steadied by radial ribs projecting from the inner tube. The annular space between the tubes constitutes the ascending or shorter leg, and the inner tube the descending or longer leg; of the syphon (Builder, 1879, xxxvii. 1,002). There is another arrangement patented by him, combined with a grease intercepter. (See WATER WASTE PREVENTER.) A somewhat similar one is put forward as * Adams' patent flushing syphon.” Another by Banner, as in The Sanitarian's Companion.

1888i. The house drain should be effectually cut off from aërial connection with the common sewer, or any other house drain ; also, the house should be cut off from aërial connection with all soil and waste pipes; and all these external pipes and the house drain should be so formed and so connected that they shall at all times be freely flushed with fresh air, and all contribute to their mutual purification. “House drains," writes Mr. Honeyman, " as usually laid at present are not ventilated. A 4-inch drain, as recommended by Sir Robert Rawlinson (Trans. of Sanit. Inst., rol. vi., p. 72) and others, cannot be ventilated by merely leaving openings at each end of it. The friction in such a pipe would neutralise a considerable amount of energy, and there is no energy. The morement of the air is sometimes in one direction, sometimes in the other, and the quantity which gains admission is just about sufficient to promote fermentation and the propagation of organisms, and to allow the escape of abominably polluted air at either end, or into the house if it have the chance. My advice is to increase the size of the drain, to confine the sewage in a narrow channel, and to keep the whole clean. I am not prepared to say that even a well-ventilated house drain would be superior to one absolutely without ventilation, from which atmospheric air is entirely excluded; but it appears to me to be indisputable that there must be either thorough ventilation or none, and that in this case the usual via media is the very worst course that can possibly be adopted.”

1888k. A system has lately (1887) been patented by Mr. H. R. Newton, architect, whereby he shows the absolute necessity for the total enclosure of sewage from air in all ways, to prevent exhalations arising, and to absolutely control the method for their suppression. He points out the injurious influence of forcing air into fouled water in any way, or of allowing fouled water to have any contact with air; drains and sewers, he maintains, should be always full, instead of empty.

18881. Various arrangements are advertised for obtaining access to drains for inspection without the necessity for breaking into them, or for clearing stoppages. At the end of the drain next the sewer (and perhaps at other places) should be formed a manhole or “inspection ” chamber, having a syphon trap in it, or between it and the sewer. It may be formed of bricks in cement, sometimes set on a concrete bed, and is usually 3 feet 6 inches by 2 feet 6 inches in the clear, and finished with an air-tight cover, as by :

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