Here it deposits coarse suspended matter, and gradually rising in height penetrates the filter bed from below, and is drawn, filtered, into the factory from the surface of the bed by a pipe built into the lower portion of the wall. For purifying the filter it is simply necessary to allow the current to pass in the opposite direction - that is, the stream is permitted to flow from the reservoir upon the filter bed, which it penetrates and frees from impurities. The settling basin is cleansed by permitting a current of water to flow through it from the reservoir into the river by the lowest opening in the right wall. If such cleansing take place every 3 weeks there will be no need for a renewal of the filter bed except at rare intervals. The Telocity of filtration is about 1 1/2 cub. yd. per minute. For the decoloration of peaty water, the addition of 1 part of alum to 8000 of water has been found, in Graningen, an efficient means. The water and the right amount of alum solution are pumped together through a pipe into a clarifying basin, where they remain for 8 hours, and are then passed through a filtering basin, when the residue of suspended matter is given up.
Water thus purified remains unaltered in taste, but its hardness and residue, on evaporation, increase by a minute.
The application of sand as a filtering medium has received a curious modification in Hyatt's filter. The N.. 1 form, shown in Fig. 151, is especially adapted to houses, small team boilers, laundries, etc, and wherever the quantity of water to be filtered is supplied through a 5/8-in.-pipe under a pressure of 5 or 6 atmosphere?, or leu. Its operation is as follows; The water is admitted by the compound cook a, and passes through the valve b, into the sand. The course of the water, during the operation of filtering, is indicated by arrows. A portion of the water passes upward from the valve 6 entirely through the sand by the aide of the filter to the top, and then descends to the discharge pipes. Other portions traverse the sand from the side at various heights, between the top and bottom, all escaping through the perforated discharge tubes c d. The upward current of water entering from the valve b, loosens up the sand and keeps it in a state of mild ebullition for a distance laterally something less than 1/4 of the diameter of the filter.
The sand is loosened the most and has the greatest motion next to the side of the filter, while farther away it gradually moves slower, and becomes closer as the distance increases from the side, until motion ceases, and the sand compacts together more and more by the pressure of the water passing through. By this plan, in the first part of the filtering operation, the coarsest impurities in the water are retained in a distributed condition by the portion of sand which is in a loosely moving state; the next finer impurities are arrested a little farther away, where, the current of water being slower, the sand is not so much disturbed; finer particles again are stopped farther away by the still denser sand; and so the process goes on by gradations, till the water comes into land which is motionless and compact. In this compact sand, adjacent to the outlet, the fine and last remaining impurities are obstructed, and pure water passes through the tubes c d into the outlet pipec.
This description applies to each of the 3 varieties of Hyatt filters. It permits a larger amount of water to be filtered by a given quantity of sand than is possible where the silt and impurities are permitted to accumulate in a dense stratum upon the motionless surface of a filter bed. At the same time the sand is in condition to be more easily cleansed, the impurities being loosely distributed among the particles of sand instead of adhering together in a more or less tenacious mass.
The filtering process having thus been explained, the method of cleansing the sand from the accumulated impurities will be described. As a rule the sand in a filter should be thoroughly washed at least once a day, although this depends upon the character and amount of impurities which the water contains. In warm weather, especially, cleansing should be done frequently to prevent decomposition of the organic matter remaining in the sand, which makes filters which are only cleansed at long intervals fountains of filth instead of purity.
In washing Hyatt filter No. 1, the handle of the compound cock a is turned to the left as far as it will go. This shuts off the water from the valve b, and permits it to enter through the small valves /, which are distributed at regular intervals in the bottom of the filter bed. From these valves the water rushes upward through the sand, loosening and carrying with it all the silt and impurities that have been retained in the sand while filtering, and discharging them through the central pipe g, from which these issue by one of the openings in the compound cock a, into the waste pipe h; 5 or 10 minutes for washing is usually quite sufficient. If this be done regularly each day, the filter will be kept in order, and will do its work for a practically indefinite period, as there is no waste of sand, and the filter is constructed of bitu-minised iron and has no working parts liable to get out of order. After washing, the handle is turned to the right until it stops, and filtering is at once resumed.
Some of these filters are arranged for the introduction of the unfiltered water over the sand instead of at the bottom. It is then filtered downward, and discharged through perforated metal below. In a filter of the form and capacity of house filter No. 1, this arrangement will give finer filtration but a less quantity of water. The plan of washing the sand is, however, as just described.
All kinds of charcoal, but especially animal charcoal, are useful in the construction of filters, and have consequently been much used for that purpose. Charcoal, as is well known, is a powerful decolorising agent, and possesses the property in a remarkable degree of abstracting organic matter, organio colouring principles, and gaseous odours from water and other liquids. It has been shown that it deprives liquids, for example, of their bitter principles, of alkaloids, of resins, and even of metallic salts, so that its usefulness as a medium through which to pass any suspected water is undoubted. The one point to be observed is that it does not retain its purifying power for any great length of time, so that any filter depending upon it for its purifying principle must either be renewed or the power of the charcoal restored from time to time, and this the more frequently in proportion to the amount of impurity present in the water. A combination-filter of sand or gravel and granulated charcoal is a good one; but the physical, or chemico-physical, action of such compound filters, or of the other well-known filter, composed of a solid porous carbon mass, differ in no respect from that of the simple substances composing them; that is to say, such combinations or arrangements are much more a matter of fancy or convenience than of increased efficiency.