In Northern States, particularly in exposed situations, it is desirable to have the top of the tank 1 to 2 feet underground, thus promoting warmth and uniformity of temperature in the sewage. In Southern States this feature is less important, and the top of the tank may be flush with the ground. Every tank should be tightly covered, for the reason above stated and to guard against the spread of odors, the transmission of disease germs by flies, and accidents to children.
Fig. 22.—Sewer-cleaning tools—how to use them. A, Ordinary 1½ or 2 inch auger welded to a piece of ¾-inch extra-strong wrought pipe about 5 feet long; the stem is lengthened by adding other pieces of pipe with screw couplings, and is fitted with a pipe handle; all cleaning work should proceed upstream; B, twist or open earth auger; C, ribbon or closed earth auger; D, spiral or coal auger; E, ship auger; F, root cutter; G, sewer rods with hook coupling, usually of hickory or ash 1 or 11 inches in diameter and 3 or 4 feet long; H, gouge for cutting obstructions; I, scoop for removing sand or similar material; J, claw, and K, screw, for removing paper or rags; L, scraper; M, wire brush for removing grease, drawn back and forth with a wire or rope; N, home-made wire brush (for a 5-inch sewer use a 1½-inch wooden pole to which is securely tacked a piece of heavy rubber, canvas or leather belting or harness leather 5½ by 8 inches, spirally studded, as shown, with ordinary wire nails 1½ inches in length).
Considerable latitude is allowable in the design and construction of septic tanks. No particular shape or exact dimensions can be presented for a given number of people. One family of 5 persons may use as much water as another family of 10 persons; hence the quantity of sewage rather than the number of persons is the better basis of design. Exact dimensions are not requisite, for settlement and septicization proceed whether the sewage is held a few hours more or a few hours less. As to materials of construction some form of masonry, either brick, building tile, rubble, concrete, or cement block, is employed generally. Vitrified pipe, steel, and wood have been used occasionally.
A plant for use all year round should have two chambers, one to secure settlement and septicization of the solids and the other to secure periodic discharge of the effluent by the use of an automatic sewage siphon. The first chamber is known as the settling chamber, the second as the siphon or dosing chamber. The siphon chamber is often omitted and the effluent is allowed to dribble away through subsurface tile, as illustrated in figures [17] and [18]. The latter procedure is not generally advised, but may be permissible where the land slopes sharply or has long periods of rest, as at summer houses and camps.
The septic tanks shown in this bulletin are designed to satisfy the following conditions:
1. Water consumption of 40 gallons per person per day of 24 hours.
2. A detention period of about 24 hours; that is, the capacity of the settling chamber below the flow line is approximately equal to the quantity of sewage discharged from the house in 24 hours.
3. Where a siphon chamber is provided, its size is such that the dose of sewage shall be approximately equal to 20 gallons per person; that is, the capacity of the siphon chamber between the discharge and low-water lines is roughly equal to the quantity of sewage discharged in 12 hours.
