of the methods employed for this purpose consists of a broad, shallow box, which is set in an inclined position. The dirty sand is thrown into this box and a jet of water played upon it from a hose. The water overflows from the lower end of the box and carries the dirt with it. This is continued until the water runs off clean. The more elaborate methods employ mechanical means to force the water through the sand. Drum-washers, operated by horse or steam power, are largely used in Germany. They are set in an inclined position, and the sand, with streams of water playing upon it, is forced from the lower to the upper end by means of revolving spiral blades. Various other methods more or less on the same principle are employed. Fig. 15 shows the sand-washer used at Hudson, N. Y. The dimensions are shown on the figure. The dirty sand is shoveled onto the stage, from which it is washed by a hose jet into the revolving cylinder. From the lower end of the latter it falls into a wooden trough one foot square in section and closed at both ends. The screw forces the sand into a pit at one end, from which it is elevated by buckets to the floor of the sand storehouse above. The water overflowing the box carries the dirt with it. The "Ejector" washer is probably the most efficient of all the methods employed. It consists of a series of conical hoppers arranged in a row. At the bottom of each hopper is an ejector through which a stream of water passes under a pressure of fifteen or twenty pounds. The dirty sand is thrown into the first and largest hopper. From this it is ejected through a vertical pipe into a trough, from which it falls into the next hopper. Here the same thing occurs, and the process is repeated until the water, which is continually overflowing from the hoppers, comes off clear. The whole arrangement must be inclosed in a masonry pit, from which the dirty water is conducted by drains. Six or eight hoppers are required for each machine, which will have a capacity of from five to six cubic yards per hour. Sand-washers of this type are used in the new filters at Hamburg (Fig. 16), and are to be used in the plant now under construction at Albany, N. Y. Details of the latter are given in "Engineering News," February 10, 1898. The volume of water required in sand-washing varies, according to the method used, from twelve to twenty times that of the sand, the ejector machines apparently requiring the most. question of cost will be referred to under maintenance. The INTERMITTENT FILTRATION. The operations which have been described in the foregoing pages are those connected with the carrying on of what is known as continuous sand filtration; and in determining what methods produce the best result, our only test has been the degree of bacterial purification effected. The reason of this is, as we have already seen, that in waters at all likely to be used as public supplies, the actual amount of organic matter is relatively so small as to be of little sanitary significance. Nevertheless, there is a certain degree of chemical purification effected by this process. Analyses of the effluents show a reduction of the dissolved organic matter of from 30 to 60 per cent. This is brought about by the action of the bacteria, which, though existing under adverse conditions, are yet capable of producing this result in the presence of the free oxygen in the water, the amount of which is usually quite sufficient for the purpose. Now, in the case of sewage, which is only very highly polluted water, the amount of free oxygen is very small in comparison with the organic matter present. And it was found, in making experiments on the purification of sewage by passing it through beds of sand, that if air were artificially introduced a very complete reduction of the organic matter would be effected by the bacteria. This was accomplished by working the bed intermittently; that is to say, at regular intervals of time-say twenty-four hours-the bed was allowed to drain, and fill its pores with the air drawn in after the sewage. After taking this breath the bed rested for a day; then the sewage was again turned on to the surface, preventing the escape of the air which was necessary to provide oxygen for the next twenty-four hours' purification. The same method used in connection with water is what is termed intermittent filtration. The first filter of the kind was built at Lawrence, Mass., by H. F. Mills, C. E., member of the State Board of Health. Since then small plants on the same principle have been built at Mt. Vernon, N. Y., and Grand Falls, North Dakota. The results do not seem to indicate any necessity for their use, not being at all superior to those of continuous filters, while the method of operation is not suited to cold climates, and either requires a greater area of bed or a higher rate of filtration. A description of the Lawrence filter may be found in "Trans. Am. Soc. C. E.," 1893, p. 350. GENERAL ARRANGEMENT. From what has been said, it will be evident that where the water is at any time liable to turbidity, a settling basin, capable of holding from twelve to fourteen hours' supply at least, must be provided. Also, in order that the filter may be able to work continuously at a uniform rate, a clear-water basin will be necessary of a capacity sufficient to cover the maximum fluctuations in the con FIG. 17.-Sketch showing the relation of the parts of a filtration system. sumption. Fig. 17 indicates roughly the general arrangement of the parts of a complete system. If the supply is from a storage reservoir, the filters are placed below the dam, and are, of course, supplied by gravity. (See Fig. 18). But even when the supply is from a river or lake, the topography of the ground often admits of the same economical arrangement. If this is not possible, the water must be pumped into the settling basin by a separate pump of the low lift variety. The extra expense of two pumpings may be almost eliminated if the same station, boiler plant, etc., can be made to serve for both pumps. The total area of filter beds required depends in the first place upon the maximum rate adopted; and, second, upon the area out of use while being scraped and refilled. The higher the rate of filtration the less the total area, and therefore the first cost of the plant. The principal item of expense connected with the operation of this plant is that for scraping; and it is found that the amount scraped for any given quantity of water filtered is independent of the rate. Also, the allowance for the area out of use will not vary with the rate to any extent. Hence an increase in the rate will not by any means produce a proportionate reduction in the cost of filtration. A rate of 3,000,000 gallons per acre of bed in use will give results entirely satisfactory from the standpoint of efficiency, and at a cost which is usually by no means excessive. The size of the individual beds will depend in part upon the extent of the total area, the smaller plants having necessarily to use smaller beds. A large bed costs less per unit of area than a small one, on account of the proportionately greater length of wall in the latter case. With a large bed it is, however, probably more difficult to obtain a uniform rate of filtration over the whole area. During the winter of cold climates the cost of maintenance is considerably increased by the expense of removing the ice which forms in the bed. It is also difficult to avoid injuriously disturbing the surface of the sand. Beside this when the water is drawn down, the surface sometimes freezes before it can be scraped. On account of such disadvantages as these filter beds should be covered in all cold climates. The best method of constructing these roofs has already been referred to. The proper number, shape and area of the beds of a system can only be determined for any particular case by careful study of the local conditions, and by making comparative estimates of the different items of cost of construction, maintenance, etc. There will be opportunities for the exercise of considerable ingenuity in the general laying out of the system, the relative placing of its parts, the arrangement of the piping, drains, etc., in order that convenience and economy may be happily combined. THE COST OF CONSTRUCTION. This will, of course, depend on the local circumstances and the kind of materials used. As in all hydraulic work, great care is required in the construction, and the best quality of materials must be used. In the main, it is the same class of work as is required in the building of distributing reservoirs. |