In the United States, neither filtration nor covered reservoirs are in use —except, perhaps, in isolated eases. In some instances, however, artiRcial aeration bv one means or another is employed with beneficial results for preventing the development of aquatic vegetation, to which, during the hot weather, the water in open reservoirs is very susceptible. The objec tion urged against the adoption of the system of filtration so extensively practised in England is its cost. The subject of the purity of the water supply is nevertheless attracting g-rowing- attention.
Artificial Filtration is more frequently resorted to, large watertight basins of masonry being founded for the purpose. Besides sand and grav el, sponge, coke, animal and wood charcoal, pumice-stone, pounded brick, carbide of iron, spongy iron, and many other materials, have been used for this purpose, but except fine sand there has yet been found no practically available material for filtration on a large scale.
Filters for Domestic the filters designed for use on a large scale, there are smaller appliances for domestic service, manufac tories, etc. The requisites of an effective filter of this class are that it shall free the water from mechanically-suspended impurities, and shall pass it constantly, automatically, and rapidly enough for practical purposes, and that with the least amount of labor it shall permit the ready and thorough cleansing of the filter-bed. The forms of such devices are legion. One, invented bv Fonville for filtering the surface-water, is shown in Figure 9 (pi 52). it consists of a wooden chamber divided by horizontal partitions of perforated wood, with three compartments. These compartments con tain well-washed sponge, coarse river-sand, and vegetable charcoal, proceed ing from above downward in the order given. The pipe at the right conducts the unfiltered water to the apparatus with the pressure of the service-mains; the pipe at the left conducts the filtered water away. The eight valves with which the filter is provided may be so adjusted as to ex clude the water from any compartment or to cause the water to flow for ward or backward in any of them; so that whenever the contents of anv of the filtering compartments become clogged or require cicansing a brief reversal of the flow is generally sufficient to restore it to proper working order. The charcoal requires renewal once a week.
In certain of these devices it is sought to effect the rapid removal of the very fine sediment by the nse of such coagulating substances as alum, by which it is very thoroughly separated, and remains on the filter en tangled in the flocculent alumina. The quantity of alum required to ac
complish this object is extremely small. In some forms of the apparatus it is admitted in regulated quantities predetermined by the condition of the water and thc capacity of the filter, and the quantity remaining un decomposed in the filtered water is so infinitesimal that it affects neither the taste nor the qualities of the water. In other forms the filtration, either simple or aided by chemical agencies as just described, is supple mented by artificial aeration, whereby the palatability and general whole someness of the water are notably improved.
the water-supply of Jarg-e towns and cities there are usually provided elevated reservoirs whose principal object is to preserve the equilibrium between the water consumed and that supplied. The larg-er portion of the consumption takes place during the daytime, but even then it is not regular, while the influx of water from flowin,g springs or a gravity supply is constant during the twenty-four hours. Con sequently, without reservoirs for storage, the portion of the water flowing in during the night would run to waste. Where the supply is pumped from rivers or lakes, the presence of a large reservoir in which a consid erable volume of water may be stored is a safeguard in case of an accident to the pumping-machinery, for without this provision the stoppage of the pump would at once stop the supply. In large cities there are usually several pumping-stations; so that in the event of a break-down of one of the pumps communication between the disabled plant and its proper reser voir is shut off by closing a valve in the pumping-main, and connection is established between the reservoir and the pumps of another station.
Cofiacily ono' Classes of Rescrzyirs.—To be really serviceable, the capacity of these reservoirs should be very large; as a minimum, they should be capable of holding one-third of an entire day's supply. To facilitate their cleansing—which is periodically necessary—ther should be separated into several compartments by suitable division-walls. Reser voirs may be classed, according to the manner of their location, (r) those whose water-level is below the surrounding ground-level, and (2) those whose water-level is elevated to a greater or less extent. The first are large substantial excavations with floor and walls of brick, masonry, or concrete. The second often require costly foundations and very strong retaining-walls, which are usually great embankments of earth. All such reservoirs should be covered, to keep the stored water clean and cool.