In many cases the local conditions are such that contact beds or sprinkling filters may be constructed more easily or more economically than sand filters and, at the same time, the lesser efficiency of the contact bed or the sprinkling filter, owing to the fact that the stream is not used for water supply, may not preclude the adoption of these latter types of plants. However, where natural deposits of sand of not too finely divided particles occur or where such sand may be readily procured, intermittent sand filters are most satisfactory for the final treatment of sewage. INTERMITTENT SAND FILTERS The agencies employed in purifying sewage by intermittent sand filtration involve its oxidation, or nitrification by bacterial action, while the mechanical straining effected by its passage through the sand plays a very small part in its reduction. Where natural deposits of sand of suitable quality occur, sand filters are constructed by levelling off definite areas of sand and making embankments eighteen inches high to enclose these areas, the embankments being generally formed of the surface loam and subsoil which must usually be removed in order to expose the sand layer. There should be from three to five beds prepared in order to provide for alternating the discharge of the effluent from the settling tank over different portions of the filtration area and thus to provide resting periods for each bed while in operation. Also, the preparation of several equal areas permits discontinuing the use of any single area for several days or a week at a time in order to allow it to dry out and permanently retain its filtering capacity. In Fig. 37 is shown a view of a set of sand-filter beds arranged in terraces on sloping ground, the embankments being formed by the material excavated to uncover the natural sand layer. [Illustration: FIG. 37.—View of Sand-filter Beds for Village in Massachusetts.]