176. In all varieties of filters the velocity of the water passing through them should be low enough to permit the finest sediments to deposit themselves upon the surface of the beds of filtering material. Otherwise, in treating muddy water, it will retain a muddy color.

The velocity of the water passing through a filter bed of bone charcoal, should also be low, so that the water may be in contact with the charcoal as long as possible, the chemical changes in the impurities thereby being more complete. The beds of filtering material become gradually clogged by the accumulation of refuse upon the surface of the bed and upon the grains of sand or charcoal; the flow of water is checked and the usefulness of the apparatus is greatly impaired. This can be remedied by reversing the direction of the flow of water, at suitable intervals. Thus, the accumulations can be washed away and be run to waste, and the filter can be operated almost continuously.

A filter which cannot be thus reversed should not be employed if possible to avoid it, because the care and trouble which will be required to keep it in good working order, will be so great as to lead almost certainly to neglect. A filter which is neglected is likely to become foul, and thus give rise to the very danger that it was intended to prevent.

There are so-called filters which are made to screw upon the nozzle of an ordinary faucet. They consist of a cup having a filling of bone charcoal or other filtering material, and they operate only as strainers, to hold back the insoluble impurities which are carried by the water. They do not purify the water except in a mechanical way. The bone charcoal has no purifying effect upon it, because it passes through far too rapidly for any chemical effect to take place.

177. Filters should be kept full of water. They should not be allowed to become dry, nor to be exposed alternately to water and to air. Alternate wetting and drying of putrescible matter greatly hastens putrefaction and increases the growth of disease germs, etc. A filter which is thus operated is liable to become a source of poison instead of a protection against it.

In cities and towns having a water supply which is liable to become muddy at times, dwellings should be supplied with a filter, located in the basement. All of the water which enters the house should pass through the filter. This will prevent the kitchen boiler from filling up with mud, and will insure clean water throughout the building.

178. The mode of constructing an ordinary filter, suitable for rain water, etc., is shown in Fig. 64.

The body of the filter is built of brick, laid in mortar composed of Portland cement mixed with an equal volume of clean, sharp sand, and it is divided into two chambers by means of a partition slab a of slate or flagstone. The bottom of the chamber A is provided with a low place or pocket d in which may gather sediment, and from which it may be removed by the garden pump or other convenient means. The chamber B is fitted with a perforated bottom b upon which is placed a course of gravel, then clean sand, nearly up to the level of the discharge pipe f. It is then topped with gravel. The rain water enters chamber A through the pipes c, c, and deposits any solids that may accompany it into the pocket, as shown at d. It then flows upwards through the sand in chamber B, which clarifies it. Chamber A is also provided with an overflow pipe e, so that if the filter becomes choked with dirt, the water will not acquire sufficient head to force the dirt through the filter; the pipe also acts as an overflow for the cistern into which f delivers.

Filters 69

Fig. 64.