Frequently and with good success filters of so-called plastic coal are used, but filtration proceeds very slowly; and it is therefore necessary to employ several of these kind of filters, also the efficiency of the plastic coal decreases in use and must be renewed at times. Plastic coal is a combination of wood, charcoal, sawdust, tar and asphalt, heated under exclusion of air and afterwards pressed in different forms - sheets or blocks. For a large water supply several of these forms are combined to stative as shown in the appended cuts, and suspended or adjusted in the filter, which might be made of a barrel, or a wooden or galvanized iron tank. The water filters through the porous mass and finds its way to the pipe leading from within the sheet or circular block of plastic coal to the faucet attached to it. The porous mass, especially the outside, which soon gets filled with the impurities of the water and clogged up, ought to be cleansed every week. This can be done by slightly heating it or grinding off the outside. However, after some time the coal must be renewed. There is a widespread opinion thattplastic coal acts both chemically and mechanically in purifying the water. This is an error. It acts best mechanically, removing suspended matters, and does not remove organic or inorganic matters which are held in solution.

Fig. 42.   Plastic Coal Filter

Fig. 42. - Plastic Coal Filter.

Fig. 43.   Plastic Coal Filter Tank

Fig. 43. - Plastic Coal Filter Tank.

The next home-made filter is illustrated in the annexed engraving. This filtering apparatus consists of 3 vessels a, b, c, made of stoneware, or they can be had in all sizes and adjusted with tube connections. The connecting tubes may be of glass, tin or rubber. The tubes between a and b, to prevent their being clogged up by the filtering material, are protected by linen, cotton or fibrous asbestos, easily covered, and the protecting substance is secured by a few heavy pieces of the filtering medium. Then cover the bottom of vessel a with a small layer of coarse sand carefully washed, put over this a layer of coarse but fresh wood, or, better, animal charcoal, but not higher than about half the size of the vessel. On this put a sheet of linen and then another layer of clean gravel. In vessel b put first a thicker layer of clean coarse sand, put on top a piece of felt closely fitting the sides in the vessel all around, and hold. it in its position by laying a few clean stones or some coarse sand over it. The second vessel is half the size of the first.

"When vessel a is filled with water, continuously or at intervals, it filters through the sand and coal in a and b, and collects in vessel c, which is empty, from where it is drawn off by the cock.

This arrangement furnishes also an excellent opportunity for filtration. The exerted pressure in vessel b is infinitesimal, and the bulk of suspended and dissolved impurities remains in vessel a. The thickness of the layers of the filter-mediums may be approximately taken from the cut.

Fig 44.   Sectional View of Stoneware Filters

Fig 44. - Sectional View of Stoneware Filters.