[Illustration: Fig. 3.--How an apparently good well may draw foul drainage. Arrows show direction of ground water movement. _A-A_, Usual water table (surface of free water in the ground); _B-B_, water table lowered by drought and pumping from well _D_; _C-C_, water table further lowered by drought and heavy pumping; _E-F_, level line from surface of sewage in cesspool. Well _D_ is safe until the water table is lowered to _E_; further lowering draws drainage from the cesspool and, with the water table at _C-C_, from the barn. The location of well _G_ renders it unsafe always.] [Illustration: BPR-RE 1385 Fig. 4.--An insanitary, poorly drained barnyard. (Board of Health, Milwaukee.) Liquid manure or other foul drainage is sure to leach into wells situated in or near barnyards of this character.] HOW SEWAGE DECOMPOSES. When a bottle of fresh sewage is kept in a warm room changes occur in the appearance and nature of the liquid. At first it is light in appearance and its odor is slight. It is well supplied with oxygen, since this gas is always found in waters exposed to the atmosphere. In a few hours the solids in the sewage separate mechanically according to their relative weights; sediment collects at the bottom, and a greasy film covers the surface. In a day's time there is an enormous development of bacteria, which obtain their food supply from the dissolved carbonaceous and nitrogenous matter. As long as free oxygen is present this action is spoken of as aërobic decomposition. There is a gradual increase in the amount of ammonia and a decrease of free oxygen, the latter going to support bacterial life. When the ammonia is near the maximum and the free oxygen is exhausted the sewage is said to be stale. Following exhaustion of the oxygen supply, bacterial life continues profuse, but it gradually diminishes as a result of reduction of its food supply and the poisonous effects of its own wastes. In the absence of oxygen the bacterial action is spoken of as anaërobic decomposition. The sewage turns darker and becomes more offensive. Suspended and settled organic substances break apart or liquefy later, and various foul-smelling gases are liberated. Sewage in this condition is known as septic and the putrefaction that has taken place is called septicization. The odor eventually disappears, and a dark, insoluble, mosslike substance remains as a deposit. Complete reduction of this deposit may require many years. [Illustration: BPR-RE 1388 Fig. 5.--Poor relative locations of privy, cesspool, and well. (State Department of Health, Massachusetts.) Never allow privy, cesspool, or sink drainage to escape Into the plot of ground from which the water supply is taken.] IMPORTANCE OF AIR IN TREATMENT OF SEWAGE.