A cistern-filter, the filtering medium consisting of animal charcoal only, and fulfilling all the foregoing requirements, was, after years of experiment, after having been most satisfactorily tested, proved to be practicable.

To explain the principles on which this filter is constructed, it is necessary in the first place to remark that, in filtering water, the impurities must remain in the crevices and pores of the filtering material, and that ultimately these will be filled up, and the whole become clogged, so as not to allow the water to pass. In order to lessen this inconvenience as much as possible, it is necessary first to prevent such foreign matters from entering as can be got rid of otherwise, and secondly, to afford the greatest facility for removing impurities which may be intercepted within the filter. These two points are most essential in the construction of a filter; so much so, indeed, that much of its real value depends upon them. The choice of materials is by no means a matter of indifference, but it is a decided mistake to believe that it is the only needful consideration, as it is frequently represented.

The first of these essentials is to be obtained by precipitating the suspended impurities separately outside the filter, whilst those only which are held in solution actually pass into the filtering material, the water being purified from these in the act of ascension. The second essential is attained by an arrangement for permitting the filtering material to be easily removed, and after cleansing to be as easily replaced.

The tendency of suspended objects in water to be precipitated ought, under all circumstances, to be taken advantage of; and cisterns for storing water (whatever may be said against the intermittent system of supplying water), have, on this system, their advantage in separating the solid matters in suspension. Of this there can be no better proof than that the floors of cisterns filled by intermittent supplies are generally covered with a layer of mud and slimy matter, accumulated during the intervals of rest. These gross impurities would, on the system of continuous supply, have remained in the water, and have been consumed by the inhabitants. Cisterns ought to be constructed so as to favor the precipitation of solid matter, and, above all, they should be readily got at for examination and cleaning; but builders appear, in many cases, to have imagined that the most proper place for a cistern is in the most out-of-the-way and inaccessible position in the building, where the cleansing is rendered most difficult, whilst the accumulation of extraneous matters, and the imbibition of very offensive gases, are unfortunately much facilitated.

Any cistern, where organic or inorganic impurities by the aid of alum, limewater, permanganate of potassium, etc., are precipitated, may be connected with an air compressor and a filter, and thus purification improved.

The illustrations on the following page show two cistern-filters, manufactured by leading English manufacturers.

This filter is simple in construction and can be either connected with a cistern or attached to the main service pipe. It requires no attention beyond an occasional opening of the cleansing tap, and will deliver a supply of purified water at the rate of 50 to 1,000 gallons per hour, according to size. It is easily fixed, and the cistern can be cleansed without disturbing it. Layers of loose charcoal and carbon blocks form the filtering medium. By an arrangement of the taps, either cylinder can be washed out backwards with the filtered stream from its companion, so that when working the most impure water, the filter can be effectually and instantaneously cleansed. When the impurities of the water, organic or inorganic, are by means of alum, lime water, permanganate of potassium, etc., previously precipitated in the cistern, and afterwards the water is filtered through these filters to perfect purification, a water in a high grade of purity may be obtained.

Fig. 31.   Cistern Filter

Fig. 31. - Cistern Filter.

Fig. 32.   Double Cistern Filter

Fig. 32. - Double Cistern Filter.