TREATMENT OF CITY SEWAGE. 83 
bacteria per c.c. Among these bacteria are always large numbers 
of the various decomposition, bacteria, ready to seize upon the 
organic material and decompose it. Such sewage, if left to itself, 
will undergo a rapid and quite complete decomposition, which 
results in reducing large quantities of matter to a gaseous state. 
Other parts are rendered perfectly soluble and are completely 
dissolved in the water, so that the water of the sewage is left free 
from putrescible matter. 
To bring about this result two different methods of treatment 
have been adopted, sometimes used together and sometimes sepa- 
rately, each of which has several modifications. 
The Septic Tank. This is a method of making use of the anaerobic 
bacteria which decompose products rapidly, but incompletely. The 
septic tank is a large closed chamber, perhaps below the surface of 
the ground, and closed upon all sides and the top, with simply a vent 
pipe extending from the top to allow the escape of gases. The 
sewage is passed into one end of the tank in a somewhat slow but 
constant stream, and the cavity of the tank is so divided by partitions 
as to insure a slow uniform passage of the sewage through the tank, 
and a final exit at the other end by an effluent pipe. The flow is 
regulated so that each particle of sewage remains in the tank from 
twenty-four to forty-eight hours. 
During this slow flow through the tank bacterial action is 
vigorous, and it is chiefly the anaerobic bacteria that develop, 
since the closed tank allows little oxygen to enter. Furthermore, 
a heavy scum usually grows on the surface that prevents the excess 
of oxygen. In these anaerobic conditions, therefore, decomposition 
proceeds rapidly, the organic bodies becoming partly broken down. 
Gas is evolved in quantity and bubbles up through the liquid to 
find exit from the tank by special vents. The gases represent the 
partial destruction of the organic matters in the sewage, and as 
fast as they are evolved the organic ingredients in the sewage 
disappear and the sewage becomes clearer. When it leaves the 
outlet, after flowing through the tank, it is much purer than when 
it entered, and may then be discharged into streams without 
greatly contaminating them, if the process has been efficient. The 
