The overhead siphon, type 3, [figure 26], may be installed readily in a tank already built by addition of an outlet sump. If properly set and handled, sewage siphons require very little attention and flush with certainty. Like all plumbing fixtures they are liable to stoppage if rags, newspaper, and similar solids get into the sewage. If fouling of the sniffing hole or vent prevents the entrance of sufficient air into the bell to lock the siphon properly, allowing sewage to dribble through, the remedy is to clean the siphon. Siphons are for handling liquid; sludge if allowed to accumulate will choke them.
Submerged outlet.—The purpose of a submerged outlet is to take the outflow from a point between the sludge at the bottom and the floating solids or scum. The outlet in [figure 25] may be readily made of sheet metal by a tinsmith. Wrought iron or steel pipe with elbows or light lead pipe may be used, the pipe being set in the concrete and left in place. Sometimes a galvanized wire screen (¼-inch mesh) is fitted over the inner end to prevent large solids leaving the settling chamber and possibly clogging the siphon or distribution tile. If a screen is used it should be easily removable for cleaning.
Fig. 27.—Home-made reinforced concrete covers. (1) Slabs placed crosswise permit uncovering the whole tank for cleaning, but as inspection is somewhat difficult, cleaning is the more likely to be neglected; (2) manhole, 18 inches square; cover, 22 by 22 by 3 inches thick, easy to make and to slide or lift from the opening.
Manhole frame and cover.—The frame and cover shown in [figure 25] are stock patterns made of cast iron and weighing about 250 pounds per set. The cover is 21 inches in diameter; it is tight and, on account of its weight, is unlikely to be disturbed by small children. The frame or rim is about 7 inches high and 31 inches in longest diameter. If desired, light cast-iron cistern or cesspool covers obtainable from plumbing supply houses, home-made slabs of reinforced concrete (see [fig. 27]), or wooden covers (see [fig. 23]) may be used.
Overflow.—The purpose of an overflow is to pass sewage to the distribution field should the siphon stop working. The overflow (fig. 25) is a 3-inch riser pipe with top 3 inches above the discharge line and the bottom calked or cemented into the side outlet of a T-branch. The run of the T-branch should correspond with the size of the sewer from the tank to the distribution field. If this sewer is 4-inch pipe, a 4 by 3 inch T-branch is used, the 4-inch spigot end of the siphon being calked or cemented into the branch, as shown in [figure 25]; if the sewer is 5-inch, a 5 by 3 inch T-branch is used and connected to the siphon with a 5-inch to 4-inch reducer (in vitrified specials the equivalent is a 4-inch to 5-inch increaser); if the sewer is 6-inch, a 6 by 3 inch T-branch is used and connected to the siphon with a 6-inch to 4-inch reducer.
Concrete work.—Before excavation for the tank is begun, two wooden forms should be built for shaping the inside of the settling and siphon chambers. In most instances the ground is fairly firm, so that the lines of excavation may conform to the outside dimensions of the tank, the back of the walls being built against the earth. The forms may be made of square-edged boards, braced and lightly nailed, as shown in [figure 28]. The forms should have no bottom. If it is desired to lay the sides and covering slab in one operation, the top of the forms must be boarded over. All pipe and manhole openings should be accurately placed and cut. The faces of the forms may be covered with paper or smeared with soap or grease to facilitate removal later.
The ground should next be excavated to the proper depth for placing the floors in both chambers. The settling chamber floor, being the lower, should be placed first. Effort should be made to secure water-tight work, a feature of especial importance where leakage might endanger a well or spring. A concrete mixture of 1:2:4 is generally preferred (1 volume cement, 2 volumes sand, 4 volumes stone). The ingredients should be of best quality and thoroughly mixed. The concrete should be poured promptly and worked with a spade or flat shovel to make the face smooth and eliminate pockets or voids within the mass.[10] Before the settling chamber floor has hardened the form should be set upon the floor and the concrete work continued up the sides. The pipe form for the submerged outlet should be set. When the side walls of the settling chamber have reached the bottom of the excavation for the siphon chamber, the siphon trap with its connecting branch and short piece of pipe should be set to proper line and grade and blocked in position. The floor of the siphon chamber should now be poured and the form for that chamber placed thereon, leaving a 6-inch or 8-inch space (according to the thickness of the division wall) between the ends of the two forms. Pouring of all side walls and the top slab should continue without stop, making the entire structure a monolith.
[10] See footnote, [p. 17]. For more detailed information on form and concrete work the reader is referred to U. S. Department of Agriculture Farmers' Bulletin No. 481, "Concrete Construction on the Live-Stock Farm."