The construction of sprinkling filters as compared to contact filters differs principally in the size and depth of filtering material, in the means provided for distributing the effluent over the beds, and in the arrangements for draining the filter. The depth of filtering material in a sprinkling filter is usually from five to ten feet, preferably not less than eight feet. The material for the filter is the same as that used for contact filters but the fragments should be from one to three inches in diameter. Instead of a system of tile underdrains on the floor of the filter, a false floor of perforated tile, rectangular in section, or of half tile, circular in section, with drainage holes cut out along the sides, should be laid over the entire floor of the filter. As the settling-tank effluent is sprayed on the filter it passes downward between the spaces of the filtering material, and reaching the floor of the filter is collected in a main drainage channel, through which it passes to the outfall sewer, and thence to the final settling tank or into the stream. As with contact beds and sand filters, intermittency of application of the effluent to the filter is essential for proper action of the filter, and this is accomplished as in the other types of filters by means of automatic siphons placed in dosing tanks. These tanks, however, are of special design in the case of sprinkling filters. Each siphon discharges into a main carrier of iron pipe which extends sometimes over the surface of the filter, but generally along the floor of the filter. The main carrier has branch pipes of smaller diameter extending at right angles nearly to the sides of the filter. On these branches vertical riser pipes spaced about ten feet apart are connected, and these riser pipes extend a few inches above the surface of the filter. Nozzles are fitted to the ends of the riser pipes by means of which the sewage effluent, under pressure, is sprinkled, at short intervals, in the form of a fine, umbrella-shaped spray, over the surface of the filter. This results in a thorough aëration of the effluent before it reaches the filtering material, and makes this form of filter very effective. [Illustration: FIG. 47.—View of Sprinkling Filter at Danville, Pa., in Winter.] In Fig. 47 is shown a view of a sprinkling filter at Danville, Pa., operating when the temperature was 14° below zero. It is generally believed, however, that in cold climates it is advisable to house small sprinkling filters.