──────────┬───────────┬───────────┬───────────┬───────────┬──────────── │ │ │ │ │ Height of │Diameter of│ Total │ │ │ the │Tank at the│ Discharge │ Average │ │ Discharge Diameter │ Discharge │ for One │ Rate of │Diameter of│ Line above of Siphon │ Line in │ Flush in │ Discharge │ Sewer in │the Edge of in Inches │ Feet │ Gallons │in Sec.-ft.│ Inches │ the Bell ──────────┼───────────┼───────────┼───────────┼───────────┼──────────── 4│ 3│ 60│ 0.35│ 4 to 6│1 ft. 2 in. 5│ 3│ 100│ 0.73│ 6 to 8│1 ft. 11 in. 6│ 4│ 240│ 1.06│ 8 to 10│2 ft. 6 in. 8│ 4│ 280│ 2.12│ 12 to 15│2 ft. 11 in. ──────────┴───────────┴───────────┴───────────┴───────────┴──────────── When flush-tanks are placed at the upper end of laterals provision should be made for inspecting and cleaning the sewer by the construction of a separate manhole, or by combining the features of a manhole and a flush-tank in the same structure. Such a combination is shown in Fig. 38 from a design by Alexander Potter. Except under unusual conditions flush-tanks are used only on separate sewers. They should be placed at the upper end of laterals in which the velocity of flow when full is less than 2 to 4 feet per second. The capacity of the tank or the volume of the dose is dependent on the diameter and slope of the sewer. The most effective flush is obtained by a volume of water traveling at a high velocity and completely filling the sewer. A large volume allowed to run slowly through the sewer will have but little if any flushing action. Data on the quantity of flushing water needed are given in Table 24.[38] As the result of a series of experiments conducted by Prof. H. N. Ogden on the flushing of sewers,[39] the conclusion was reached that the effect of a flush of about 300 gallons in an 8–inch sewer on a grade less than 1 per cent would not be effective beyond 800 to 1,000 feet, but that on steeper grades much smaller quantities of water would produce equally good results. [Illustration: FIG. 38.—Automatic Flush-Tank and Manhole. Miller-Potter Design. Pacific Flush Tank Co. ] TABLE 24 GALLONS OF WATER NEEDED FOR FLUSHING SEWERS