100 



SEWAGE AND ITS PURIFICATION 



In ordinary bacteria beds these reactions are often reversed 

 and confused, according to the periods of filling or rest, which 

 allow the different bacteria to act in the same filter. iVlthough 

 it is practically impossible to confine the bacterial action to 

 one species, by seeding or otherwise, in view of the immensely 

 varied character of the organisms that are present — nor would 

 such a proceeding be advantageous — yet the best conditions are 

 attained when a sewage plant is so arranged as to afford separate 

 areas favourable to groups of organisms which work similarly. 

 This principle will be further discussed in Chapter XI. We 

 have seen that the disappearance of pathogenic bacteria in 

 cultivation beds, through the crowding out of these special 

 forms by the more numerous harmless varieties which thrive 

 at ordinary temperatures, is also an important part of natural 

 purification. 



Symbiosis and Antagonism. 



Organisms growing together either antagonize each other's 

 development or, more rarely, encourage it, or even are necessary 

 to one another. The former is " antagonism," the latter 

 "symbiosis." In mixed cultures certain species develop 

 rapidly, to be supplanted later by those of slower growth, and 

 the more vigorous organisms are not always the most useful. 

 Some species prevent the growth of others by (i) exhausting 



