When it is possible to place the filter floor on or within 2 feet of the surface of the ground, the method illustrated in [Fig. 60] is the design most commonly adopted. Sometimes the arrangement of the floor shown in [Figs. 61] and [62] is preferred. There is, however, still another type of floor which is applicable to this case. This is illustrated in [Fig. 63], from which it will be seen that the whole of the floor slopes in one direction, and the effluent is thus discharged over one-half of the circumference of the filter, with the result that the effluent channel is only one-half the length of that required in the case of [Fig. 60].
Floors of Filters.—In all cases the floors for percolating filters should be of a substantial character. They are usually constructed of cement concrete. The thickness of the concrete will depend upon the nature of the subsoil, but in any case it should be increased at the centre in order to provide a safe foundation for the revolving sprinkler. The surface of the floor should be smooth, so as to facilitate the flow of the effluent, and any suspended solids it may contain, to the outlet. From the preceding illustrations it will be noted that the slope of the floor may be in three directions: (a) from the centre to the circumference, (b) from the circumference to the centre, or (c) from one side of the filter to the other, a uniform slope in one direction along the diameter of the filter. It has been suggested by some that the floor should consist of a series of alternate V-shaped ridges and furrows, with slopes at right angles to the general slope of the floor, but this has the disadvantage that it causes difficulties in arranging the floor tiles and placing the filtering material in position, and further it increases the cost of construction without providing any real compensating advantages.
Fig. 63.
Fig. 64.—Floor Tile.