FILTRATION. A process for freeing liquids from particles held in suspension in them, by causing them to percolate through various porous substances, which intercept the insoluble matter, but allow a passage to the liquids, which are thereby rendered clear and transparent. The purpose to which filtration is most ex tensively applied, is the purification of water for domestic purposes ; and from the importance of pure water as regards the preservation of health, and from the general complaints of the impurities abounding in the water supplied by the different companies, the subject has of late excited much attention, and a variety of filtering apparatus have been offered to the public, some few of which we propose to describe.
The first oithese machines which we shall notice is Messrs. White and Aveline's "artificial spring," in which the water is made to filtrate upwards by its pressure against the under side of a stone, the quantity filtered depending upon the area of the stone, and the height of the reservoir from which the water descends ; but with a head of 35 feet, which can be obtained in most houses in London, a stone of 10 inches square will filter nearly thirty gallons per hour. The en graving on the opposite page exhibits a vertical section of the apparatus. a is the cistern which receives the water in its impure state ; it has a ball float and lever to keep a constant head of water over the pipe b, and likewise to prevent any air passing down it. The pipe b b is shown broken off, that the space between may be considered as of any required length. To the lower end of the pipe there is a nozle c through which the pipe passes, which causes the water to shoot up against the under surface of the filtering stone f. Through this stone the water oozes with great rapidity, leaving the animalcules and other impurities in the lower part or basin e of the machine, from whence they are drawn off occasionally by the cock g, and carried away by the waste pipe h. When the filtered water rises in the reservoir above k to a certain height, the filtration is stopped by the rising of the float 1, which by its lever or rod n, shuts a cock o in the supply pipe. When the stone has become charged with a deposit on its under surface, it is capable of being cleansed by the scraper r which is turned round by means of a handle shown at the bottom of the reservoir k, the axis passing through the stone ; provision is thus made for reviving the filtering properties of the stone whenever required, and with very little trouble.
A very old contrivance for filtering water, but which has been the origin of most of the more recent apparatus for the purpose, consists in nearly filling the two legs of a pipe, formed either of metal as in Fig. 1, or of wood as in Fig. 2, with washed sand, leaving merely a space at b and e to receive the turbid water, and another at c or f for the filtered water to run off by. The chief objection to these machines is, that they soon become foul, and consequently useless, until restored by cleansing, and this task, as generally performed, is such a laborious, tedious, and slopping one, that these filters are usually abandoned in a short time. This objection seems to be obviated in the arrangement shown in the cut on the following page. a is a barrel capable of being turned round, but rendered stationary by pins passing through the extremities of their bearings at b lo; c is a bed of sand occupying about one-third of the cask ; d is the supply pipe or hose, (any flexible tube,) which conducts the turbid water from a reservoir above, into the cask ; at e is a union joint and nozle piece, containing a sponge, which serves three purposes : it prevents the grossest impurities of the water from entering among the sand ; it prevents the column of water from forcing up the bed of sand ; and it pre vents the sand from falling into the pipe. The filtered water is drawn off at f. When the sand requires cleaning, the pins at the bearings of the axis are taken out, and the winch turned so as to bring the union joint to the top of the cask, previous to which the pipe should be detached by unscrewing. The sponge being now removed, the water may be let on freely at top, and the barrel turned by the winch g, by which means the sand is expe ditiously washed, the water being let on and run off as often as necessary, which it is obvious may be effected with so much facility, that the filtering powers may be at any time renewed in a few minutes.