IN a recent issue we commented on the importance of an efficient system of filtration in connection with the water-works of cities and towns as well as in industrial establishments, public institutions and hotels. The accompanying illustration represents the National Filter, which, with Professor Alfred R. Leeds’ system of aeration, is controlled by the National Water Purifying Company of New York.

Its operation is as follows:

The water to be filtered enters at the top and right of the filter at A, as wash the filter more than once a day, unless the and impure. After it has been in use several days, however, it should be washed from the bottom, by sending a reverse current of water through the lower series of pipe valves, shown in the bottom of filter (after first washing the top of the bed), in order to break up the passages made by the water in filtering through the bottom part of the bed. Ordinarily once a week will answer to wash the lower part of the bed.

The principal advantages claimed for the National Filter are: Cheapness in first cost and economy in service, simplicity and durability, quickness and thoroughness in cleansing, and economy in water for cleansing the filter bed. The manufacturers also offer to furnish, when the condition of the water requires chemical treatment, all the appliances for automatically precipitating the clay or vegetable stain free of additional cost.

It has been demonstrated beyond question that the thorough purification of drinking water can be accomplished by combining filtration, precipitation and aeration, the last of which processes, as practiced by this company in purifying the supplies of cities and towns, is both simple and inexpensive, preventing alike all vegetable growth in reservoirs, and odor in the mains ; in brief, revivifying the water.

For city or other service, water can be pumped directly through the shown in the cut, and passes down through the bed of fine sharp sea sand (or coke and sand mixed), and out through the pipe valves, G, at the bottom of the filter. These valves are so arranged as to allow the filtered water to pass freely, but will not permit any of the filtering material to escape with the filtered water.

II is the precipitating device, which can be opened or closed at will, and is arranged to give a certain amount of the alum, or other chemical used, to the water as it flows into the filter, without obstructing its pressure or flow, and can be close entirely when washing the filter. In its operation the chemical used to precipitate sewage, vegetable stain, etc., is deposited with the impurities at the top of the bed and thrown out when the filter is washed, no trace of the chemical being found in the filtered water.

The filter is cleansed or washed by first closing the inlet valve, A, then opening the waste valve, II, at the top and left of the filter, also opening the valve, C, to the washing pipe, F, shown in the cut (under the top of the bed of filtering material) which sends a reverse current through the top or surface of the filter bed. Five minutes’ time will generally wash out all the filth and impurities taken from the water during five hours, even when the water being filtered is very bad, so that it is not necessary to National Filter into the mains or a reservoir. Finally, it is claimed that a great saving of labor can be effected by applying this system to open filter beds, in which it will wotk equally well as in the closed top pressure filters, the pressure of the water in washing the filter beds by this process accomplishing in a few minutes what would, under the old methods, be a day’s work for a number of men.

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