THE BACTERIOLOGY OF SEWAGE 353 



purification of sewage by natural or biological means with- 

 out the addition of any chemicals whatsoever, and to render 

 it fit to be run into rivers or streams without causing 

 pollution. 



It is only of recent years that the precise changes that 

 occur in the passage of a water from pollution to subse- 

 quent purification have been correctly understood, and 

 when these changes were still in obscurity it is not sur- 

 prising that the methods intended to bring about purifica- 

 tion were initially faulty, and did not aim in the right 

 direction. 



In this connection it will be of assistance if we first 

 consider the general composition of town sewage, and it 

 will be at once apparent that the earlier methods of sewage- 

 disposal (precipitation) could at best only accomplish 

 clarification, which, though an essential point, is neverthe- 

 less only a step in the successful purification of sewage. 

 The constituents of ordinary sewage may be roughly 

 classified as follows : 



1. Inorganic matter in suspension. 



2. Inorganic matter in solution. 



3. Organic matter in suspension. 



4. Organic matter in solution. 



1. The Inorganic Matter in Suspension is chiefly sand and 

 road grit, clay, etc., the greater part of which will settle 

 out by simple subsidence if the sewage is allowed to remain 

 at rest. 



2. Inorganic Matter in Solution. — There is no great 

 increase in the inorganic matter in solution as compared 

 with unpolluted water, with the exception of a small 

 amount of phosphates, which are, however, one of the chief 

 causes of sewage fungus. 



3. Organic Matter in Suspension (such as faecal matter, 



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