When a body of water sufficiently large to nearly fill a pipe is discharged into it, it drags air along with it, and exhausts the air in all branch-pipes to a greater or less extent. When this exhaustion amounts to a diminution of the pressure in the pipe of about 1/350, the water in an ordinary trap (with a one-inch seal) will be forced out of it by the pressure of the external atmosphere, or in other words will be sucked out of the trap into the pipe, leaving no water-seal in the trap, and, therefore, allowing free exit for sewer-air. This effect is known as the “siphoning” of the trap, and is most likely to occur when the main pipe is not open at its upper extremity (is not ventilated), and when the branch wastes are of the same size with the main one. This fault is remedied by the ventilation of the traps, and of the main pipes, as described in the regulations above quoted (17 and 33).
2. To defects in the pipes. (a.) Drains.—Bad odors in cellars often come from broken earthenware or tile drains, cracked by settling, or from rat-holes in cemented drains. Earthenware or brick drains inside a house should be replaced by iron ones.
(b.) Joints.—Cement or putty joints are often cracked. Water-closets sometimes become loosened from the floor and leak fluids and gases. The covers of hand-holes of traps sometimes get loose and leak gas. Sometimes trap-screws, instead of being at the bottom of the trap, where they are covered by water, are at the top; if these are loose, gases may escape. All such defects should be repaired.
(c.) Pipes.—Both lead and iron pipes are corroded and in time perforated by sewer-air. Such corrosion is most likely to occur at points where gases come in contact with a dry part of the pipe, viz., at the crowns or domes of traps, and in ventilating-pipes. For this reason, ventilating-pipes should never be made of thin iron (galvanized iron), and should always be protected by a tar-coating. The crowns of lead traps (especially of water-closets) are often bitten into by rats in search of water. In all cases when the crown of a trap is perforated, there is rarely any leakage of fluid to indicate it, but offensive gases escape. All corroded traps and pipes should be replaced by new ones, and not patched up. Holes in iron pipes may be closed by iron bands surrounding the pipe, and fastened with red-lead putty and screw-bolts (Fig. 32). A new joint may be made by means of a saddle-hub (Fig. 33) carefully secured in a similar manner. Lead pipes are sometimes protected against rats by being encircled by tin, or packed in pounded glass.
Fig. 32.—Bands.
Fig. 33.—Saddle-hub.
3. To carelessness. (a.) Evaporation.—If fixtures are not in constant use, the water soon evaporates from the traps and gives exit to sewer-air. This often happens in warm weather, and when a house is vacant for a time. To prevent evaporation, a little oil may be poured into the trap, to protect the surface of the water-seal, or it may be made the special duty of a servant to keep the traps full of water when the fixtures are not in use.
(b.) Dirt.—The inside of a pipe often becomes coated with slime, consisting largely of grease, especially in kitchens. This filthy lining rapidly decomposes, and that portion above the trap may give out bad odors. The pipes may be cleansed with a strong hot solution of soda.