Cesspools, Septic Tanks and City Sewer Systems
152. Relative Value of Cesspool and Septic Tank. Sewer pipes for private water systems usually drain into cesspools or septic tanks (Figs. 81, and 81-a). The waste goes thru a process of decomposition before passing out into the soil. Sewage should both liquify and oxidize before entering into the soil. Oxidation purifies liquid sewage so that it is not contaminating. If oxidation is not brought about in the cesspool or septic tank, sewage, which is fresh, should be run onto the surface of the ground where the air and bacteria for oxidation can be found. Cesspools are not as good as septic tanks because there is not the surety of sewage being oxidized in them, as there is in the septic tank. They lack oxidizing chambers.
Fig. 81. Septic tank and tile.
Unoxidized liquid sewage being in a condition to flow readily thru the earth, is more dangerous than fresh sewage because it is more likely to seep into wells.
Fig. 81-a. Septic tank.
153. Construction of the Septic Tank. The septic tank is composed of two chambers—one the liquefying chamber and the other the oxidizing chamber. Both are water-tight (Fig. 82). The fresh sewage comes into the liquefying chamber thru a pipe placed near the top of the tank. Here it stands and liquefies, which is a process of decomposition. The solids fall to the bottom as they come into this chamber, and the liquid formed rises to the top and flows into the oxidizing chamber (B, Fig. 82), when it reaches a point a little below the height of the inlet pipe. It either does this by flowing over a partition or thru a pipe leading from one compartment to the other.
The second compartment is usually slightly smaller than the first. Here the sewage is held until the process of oxidation takes place, which renders it less dangerous. When the sewage in the second chamber reaches a certain height, it siphons out into a tile which distributes it over a plot of ground (Fig. 81).