Sewage is sometimes applied directly to the beds without treatment in settling tanks, generally, in such cases, after having been screened to remove the larger suspended matters, but it is decidedly preferable in the case of the smaller plants under discussion to pass the sewage first through settling tanks, as in the method of sub-surface irrigation. Therefore, the areas of beds given in the table are for sewage which has been passed through settling tanks. It is even necessary, in the case of sand filters for institutions where considerable grease and soaps are contained in the sewage, to provide grease traps through which the sewage must pass before it reaches the settling tank. The effluent from the tank should be discharged intermittently by means of a dosing chamber and siphon and should be distributed quickly over the surface of the bed as uniformly as possible. This is generally accomplished in the case of the larger beds by laying on the surface of the bed, wooden troughs, with short branches, as shown in Figs. 38 and 39. A detail of a portion of these distributing troughs is given in Fig. 40. This view shows the hinged gates which are used to effect a proportionate division of the flow of the various branches of the main trough. The view also shows the slots in the sides of the troughs which allow the sewage to flow out onto the bed. [Illustration: FIG. 38.—Layout for Intermittent Sand Filtration.] If the ground-water level is within three feet of the surface at any time, or if the sand is very fine and contains a slight proportion of clay, underdrains should be laid at depths of four feet to prevent the beds from becoming waterlogged. [Illustration: FIG. 39.—Intermittent Sand-filtration Beds.] 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. [Illustration: FIG. 40.—Portion of Distributing Troughs for Sand Filters.]