It is economical to arrange that the level of the sewage in the dosing tank, at the time when that tank discharges, shall be at the level of the sewage in the settling tank, since then no head is lost. It is better still to arrange the mechanism in the dosing tank so that the level of the sewage there at the time of its discharge will be from four to eight inches higher than the normal level in the settling tank. The effect of this is to back up the sewage and raise the general level in the settling tank, and when the dosing tank discharges there is drawn off not only the sewage in that tank, but also an amount in the large settling tank equivalent to that which is above the normal level of the sewage there. The advantage of this is plain in that it reduces the necessary volume of the dosing tank by that of the back water in the settling tank, and, while it was thought at one time that such a frequent variation in the level of the main tank might affect injuriously the scum which forms there, and perhaps also the bacterial action going on in the tank, there seems to be no real reason why this method may not be used with considerable advantage in economy of construction. The bottom of the dosing tank, which is preferably made of concrete, should have a slope toward the point from which the outlet pipe leads, thus enabling the outward rush of sewage to carry off any material which would otherwise settle in the bottom and perhaps decompose there. [Illustration: FIG. 11.—Sludge Valve for Side Wall of Tank.]