FILTERS Filters are of two classes. One class is designed to be attached to the end of the faucet or to special connection for drawing directly for use. The other is for 'use in the general house service, and filters all the water that passes through the main service for whatever pup pose. In the former class, sand, free stone, or unglazed potter's clay is used as the filtering medium. Ordinary fillings become foul through out the mass, and require cleansing or renewing. The clay (unglazed porcelain) of which the Pasteur filter is an example, permits nothing to enter the filtering medium that the pores of this material will strip out. With such, therefore, it is necessary only to remove the tubes and cleanse the surface with which the unfiltered water comes in contact. Any porous filter plate depends for its efficiency upon the minuteness of the pores through which the water passes; and there is a real danger that after a prolonged period of use, these pores may become enlarged by wear from the flowing stream to a size sufficient to allow the passage of bacteria which at the first would have been retained upon the surface of the filter plate. Porous clay filters, however, are exceedingly slow in operation; and it is necessary to employ a multiplicity of tubes, and to collect the filtered water in a reservoir, in order to be able to get enough at once to serve ordinary cooking needs.
The filters are supplied with as many tubes as desired, together with the necessary reservoir, all complete excepting connections for the water pipe.
Large filters for service interposition depend upon animal char coal, beach sand, and coagulating processes—usually the last-men tioned feature in conjunction with one of the other two. A sand filter, for instance, will be made to favor the subsidence of foreign material by the water taking an upward course through the mass of filling, a portion of the water being passed through an alum chamber so as to impregnate the supply sufficient to coagulate impurities which sand alone would allow to pass. When dissolved and carried away, the alum must be replaced. The filling is discarded and new sand put in its place from time to time; and periodic cleansing of the filling is done by reversing the flow of water and flushing out through a waste connection at the bottom. The means of thus keeping the filter in good order are provided for in its construction, in a way to make the cleansing and renewing of the material as easy and convenient as possible.