Aerobic and Anaerobic Agencies 117 



fiber, like straw, chaff, paper and fragments of the 

 wooden pavements. Here, then, is an important differ- 

 ence between aerobic and anaerobic action. In the case 

 of the latter, organic matter is almost completely de- 

 stroyed in the septic tank with the formation of a very 

 slight amount of sediment. 



In view of these facts, a sewage-purification plant 

 that provides for the activities of both classes of bac- 

 teria will allow more complete decomposition of the 

 organic matter than can be accomplished by either the 

 septic tank or the aerobic filter alone. Furthermore, 

 the septic tank will dispense with the preliminary 

 chemical treatment for the removal of the suspended 

 matter, since the latter not only settles out in the tank, 

 but is also liquefied by the anaerobic bacteria. 



Separation of bacterial activities. Thus the gradual 

 development of sewage-purification methods, as based 

 on bacterial activities, may be noted. The clearer un- 

 derstanding of the chemical reactions caused by the 

 various microorganisms and of the part played by at- 

 mospheric oxygen in these reactions, made possible the 

 differentiation of the aerobic and anaerobic changes. 

 Following this differentiation came the septic tank, 

 reserved for the activities of anaerobic germs alone, and 

 the aerobic filters, reserved almost entirely for the 

 aerobic bacteria. While in the contact beds the two 

 processes were somewhat mixed, the conditions were 

 so adjusted as to favor one or the other set of 

 changes. 



In more recent installations, greater heed has been 

 paid to the separation of the anaerobic and aerobic 



