It has been found that a tank, in order to do successful work in separating solids and in eliminating as much as possible of the sediment, needs to be of a capacity to equal about one day's flow of the sewage, and this is a good basis for computation. Here, again, the fact that the sewage from a single house is considerably fresher than the sewage from a city must be remembered, since, while many cities build tanks holding only one third or one fourth of their daily flow with good results, in the case of a single house this is not possible, and the tanks, if built at all, ought to hold at least the full day's flow. Ten persons, at 25 gallons each, furnish 250 gallons per day or 33 cubic feet. The tank, then, must be large enough to hold this volume, and suitable proportions generally require that the tank be at least 5 times as long as wide. A certain allowance must always be made for deposit in the bottom and for the accumulation of scum on the top, so that an extra foot or more of depth is desirable. The tank, then, to furnish the required 33 feet, might be made 3 feet wide, 3 feet deep, and 5 feet long, and probably in no case would a tank much smaller than this be used. [Illustration: FIG 72.--Section of a septic tank with syphon chamber.] There are two or three details of tank construction which may be suggested, although almost any kind of tank will answer the purpose. It is desirable in order that the surface scum may not be disturbed, and in order that the inflowing sewage may distribute itself as uniformly as possible across the tank, to attach an elbow to the entering pipe so that the sewage enters about halfway between the top and bottom of the tank (see Fig. 72). Similarly, at the outlet or weir an elbow should be provided because it is not desirable to allow the floating matter of the surface to be carried onto the bed, and a pipe taking off liquid, open halfway between top and bottom, will carry away but little of either the surface scum or bottom sediment. Such a tank must be built of concrete or masonry or timber, although the latter is not to be recommended because of its short life. The walls of an ordinary tank may be built 6 inches thick at the top and 12 inches to 18 inches thick at the bottom, the latter being necessary if the depth is over 8 feet. The tank should have 6 inches of concrete on the bottom, and the roof may be made of flagstone or of concrete slabs in which some wire mesh has been buried.