Siphons.—Reference has already been made to the vital importance of air in sewage filtration. If the spaces within a filter or soil are constantly filled with water, air is excluded, and the action of the filtering material is merely that of a mechanical strainer with its clogging tendency. The purpose of a siphon is twofold: (1) To secure intermittent discharge, thus allowing a considerable period of time for one dose to work off in the soil and for air to enter the soil spaces before another flush is received; (2) to secure distribution over a larger area and in a more even manner than where the sewage is allowed to dribble and produce the conditions of the old-fashioned sink drain—namely, a small area of water-logged ground.
Fig. 22.—Two-chamber septic tank, simple and inexpensive, constructed of 24-inch vitrified sewer pipe, size suitable for 125 gallons of sewage daily (nominally 3 persons). A, House sewer; B, settling chamber, made of double T branch and one length of straight pipe, each 3 feet long and 2 feet in diameter, supported by 4 inches of concrete, all joints made water-tight; C, submerged outlet, consisting of a metal T slipped into the sewer-pipe branch; D, wire screen ¼-inch mesh; E, siphon chamber made of one T branch 3 feet long and 2 feet in diameter; F, siphon; G, 3-inch overflow; H, sewer to distribution field; I, tight cover with lifting ring; J, concrete protection around sewer-pipe hubs
Fig. 23.—Typical two-chamber concrete septic tank. (See table for dimensions and quantities for different sizes)
Three types of sewage siphon are shown in [Figure 24]. In all, the essential principle is the same: A column of air is entrapped between two columns of water; when the water in the chamber rises to a predetermined height, called the discharge line, the pressure forces out the confined air, destroying the balance and causing a rush of water through the siphon to the sewer. The entire operation is automatic and very simple. The siphons shown are commercial products made of cast-iron; they have few parts and none that move, and the whole construction is simple and durable. The table ([fig. 24]) lists stock sizes adapted to farm use. Manufacturers furnish full information for setting their siphons and putting them in operation. For example, take type 2, [Figure 24]: (1) Set siphon trap (U-shaped pipe) plumb, making E (height from floor to top of long leg) as specified; (2) fill siphon trap with water till it begins to run out at B; (3) place bell in position on top of long leg, and the siphon is ready for service. Do not fill vent pipe on side of bell.
| Type 2 INCHES | |||||
| Diameter of siphon | A | 3 | 3 | 4 | 4 |
| Diameter of outlet | B | 4 | 4 | 4 | 4 |
| Drawing depth | C | 13 | 15 | 14 | 17 |
| Depth to floor | D | 16¼ | 18¼ | 17¼ | 20¼ |
| Height above floor | E | 7¼ | 9¼ | 8¾ | 11¾ |
| Clearance under bell | F | 2 | 2 | 2 | 2 |
| Inside bottom of outlet, to discharge line | G | 20½ | 22½ | 22¾ | 25¾ |
| Discharge line, to top of wall | H | ||||
| Depth of outlet sump | I | ||||
| Length and width of outlet sump | J | ||||
| Diameter of carrier (R), and minimum fall (S) in feet per 100 feet
| R | 4 | 4 | 4 | 4 |
| S | 2 ft. | 2 ft. | 7 ft. | 8 ft. | |
| R | 5 | 5 | 5 | 5 | |
| S | 1½ ft. | 1½ ft. | 2 ft. | 2½ ft. | |
| R | 6 | 6 | 6 | 6 | |
| S | 1 ft. | 1 ft. | 1 ft. | 1 ft. | |
