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.]