go THE MANURE HEAP AND SEWAGE 
the organic matter to be acted on is generally in a soluble or partly 
dissolved condition, and very easily acted upon by bacteria. 
Secondly, the great abundance of water facilitates the action, for 
bacteria require an abundance of water for their best growth. 
Thirdly, the bacteria are present in extreme abundance. All 
sewage contains bacteria in large numbers, although naturally 
the number varies. A common sewage contains from 7,000,000 to 
10,000,000 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 decomposi- 
tion, 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 
separately, 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 incom- 
pletely. 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 some- 
what 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 access 
of oxygen. In these anaerobic conditions, therefore, decomposi- 
