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 



