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attainable only for the adjacent lands of low level, unless it be forced up to the higher districts by mechanical means. The water of streams which are tributary to rivers is applicable for superior levels, and may, by judicious diversion and extension through artificial channels, be made widely useful. Wells are generally available by mechanical agency, and in some cases without it, provided a subterranean reservoir exists, and is subject to sufficient pressure from a higher source. All water at our command for practical use is more or less impure. Thus, rain-water contains ammonia, and seawater a variety of salts. The water from rivers, springs, &c., contains several kinds of impurities. These impurities are dispelled only by a compound process, or rather series of processes, by which such matters as are mechanically suspended in the water are allowed to subside, or are arrested by filtering media, and the chemical impurities are absorbed and withdrawn by suitable agents. A brief notice of several varieties of filtering apparatus concludes this section of our first Division.

48. The filters already described, which, acting by the spontaneous percolations of the water through the apparatus, may be termed self-acting, have been further improved by adding means for their self-cleansing. The arrangements introduced for this purpose at Paisley, and other places, by Mr. Robert Thom, are illustrated, in principle, by the adjoining figures 5, 6, and 7. Fig. 5 is a plan, and figs. 6 and 7 sections taken at right angles to each other through the filter. In these filters, which are provided with layers of gravel and sand, the foul water is admitted at the top, and descends through these strata to undergo filtration; but the con struction also admits of an occasional forced ascent of the water through these media, by which the foul par

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ticles are raised, deposited on the upper surface of the sand, and eventually carried away through a foul-water drain. The Paisley filter is 100 feet long and 60 feet wide, arranged in three compartments, each of which may be used separately while the others are cleansing. They are excavated, on a level site, to a depth of 6 or 8 feet, surrounded with retaining walls built in cement, and puddled behind. The bed is puddled 1 foot thick, and cemented pavement is laid upon it. It is then covered with fire-bricks laid on edge, and with spaces These are covered with

inch wide between them. flat tiles, perforated with holes

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the drains thus formed, six layers of gravel, each 1 inch deep, and of finer particles than the one below, are evenly spread, and overlaid with 2 feet depth of clean, sharp, fine sand, the upper 6 inches of which are mixed with ground animal charcoal. The water is admitted through a stone pipe, ▲, and vertical iron pipes, B B, each having an upper and lower outlet to the filter. These pipes are fitted with valves, by which either of these communications is opened and the other closed. The clean water passes from the bottom of the filter through openings c c, fitted with stop-cocks, into a drain, D, and thence into the clean water basin, E. When the cleansing is going on, these connections are shut off, and access is given to the foul matter through holes, F F, drain, G. The cost of this filter was less than 6007., and the quantity of clean water produced every 24 hours, on average, is 106,632 cubic feet. Trap rock, from the hills above Greenock, has since been substituted by Mr. Thom for the charcoal with perfect success, and considerable economy: one part of the charcoal was mixed with eight or ten parts of the sand.

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The charcoal is sometimes laid in deep layers, without mixture, and is then worth reburning for a second use.

49. In the use of charcoal as a filtering agent, an attempt is made to effect something more than the mere mechanical clearing of the water by absorbing some of the gases with which it is chemically adulterated. How far this expedient is valuable is, however, very questionable. The power of charcoal to act in this manner is well known to depend upon its being thoroughly and recently burnt and dry. Moisture diminishes this absorbing power, and in a short time the chemical action of the charcoal ceases. Some difference, doubtless, exists, in this respect, between animal and vegetable charcoal, but neither of them can be admitted as an effective chemical agent in the purification of water, without requiring a costly rapidity of renewal quite impracticable upon an extended scale.

50. With a view to promote the mechanical action of filters, many arrangements of internal partitions have been suggested. One of the best of these is exhibited in fig. 8, and was successfully applied in Switzer

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land, by Sir Henry Englefield, upwards of forty years ago. This filter is divided into chambers by parallel partitions, A B, which admit the passage of the water alternately above and below them. The intermediate spaces be filled with filtering materials of uniform quality. The course of the water must evidently be in the direction of the arrows; and the effect of this arrangement is, that the floating impurities are retained on the surface, while the heavier particles sink to the lower level.

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51. An apparatus for close filtering, within an iron water-tight box, has been introduced by M. Maurras; and its principal novelty consists in interposing the strata of fine sand between flat iron cases, perforated with holes and filled with sand of particles larger than the holes in the cases, with an arrangement of sluicecocks, &c.; the process of cleaning was effected by sudden and violent currents of water. A machine of this kind, 5 ft. 6 in. by 5 ft. 6 in., working under a head of water of 12 ft. 6 in., is said to have filtered an average quantity of 150,000 imperial gallons in 24 hours. A filter of this kind was tried for four months at the works of the New River Water Company, but the experiment does not appear to have been prosecuted, or the invention adopted.

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52. In the year 1841, the Council of Health of Paris reported upon several processes which had been tried for filtering the waters of the Seine. The two principal plans noticed were those known as Fonveille's" and "Souchon's." The apparatus used in the first of these consists of several layers of sponge, sand, and charcoal, disposed alternately in a close vessel. The filtration is accelerated by a considerable pressure upon the water. This arrangement was found to produce the most clear and least impure water; but, although this superiority

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