Where sand deposits do not occur at a point suitable for the location of the disposal plant, but where sand may be procured at a reasonable cost, the beds may be formed artificially similar to the natural sand beds heretofore described, but should not be less than three feet deep. It is generally necessary in the case of artificially constructed sand filters to provide underdrains as described below.
Two views of such an artificial sand filter are shown by Figs. [41] and [42]. In Fig. [41] the settling tank and siphon chamber may be seen, situated between two of the four beds composing the filter. In Fig. [42] is shown a nearer view of one of the beds with the distributing trough and its branches on the surface of the bed. This bed, of the four composing the filter, was not in operation at the time the photograph was taken.
Fig. 40.—Portion of Distributing Troughs for Sand Filters.
In Fig. [38] is shown a sand filter layout with three beds. In this drawing are shown the sewer leading from the house, the settling tank, the siphon chamber, in which are placed two siphons, the effluent sewers, and the diverting manhole, from which three pipe lines convey the sewage to the filter beds. In Fig. [39] is shown also a view of the three filter beds, one of the beds being shown in section. Figs. [43] and [44] show a plan and view of the diverting manhole.
Where sand must be carted in to form the filters, the embankments to retain the sand should generally be formed by excavating for a depth of two feet the whole area upon which the beds are to be placed. The material thus excavated will usually be sufficient to form the embankments. The embankments should usually be at least two feet wide on top and should have side slopes of one and a half to one; that is, the bottom width of the embankment should be two feet plus three times the height. In clay soils the pits for the filter beds may be excavated with the sides vertical, or nearly so. The bottom of each bed, as it is prepared for the placing of the sand which is to compose the filter, should slope slightly from the sides toward the centre line of the bed.
Fig. 41.
Where the character of the underlying strata of soil or the presence of ground water requires that sand filters, whether natural or artificial, should be underdrained, this may be accomplished by laying a longitudinal main drain through the centre of the bed at a depth of at least three or four feet below the surface, with branches each way at intervals of about fifteen feet. The main underdrain should be six inches in diameter, of agricultural tile or of vitrified sewer pipe, laid with open joints, and should have a fall of at least six inches per hundred feet. The branches may be of three-inch agricultural tile.
