70 



SEWAGE AND ITS PURIFICATION 



The staining and microscopical examination of the various 

 bacteria occurring in sewages require great care and experience, 

 and beyond the rough outHnes which I have given we cannot 

 attempt to enter into the different processes. 



The size of organisms is recorded in micro-miUimetres = T^V(T 

 of a milHmetre (about -^^^uij of an inch), commonly abbreviated 

 /JL. In the absence of a scale,. a comparison may be made with 

 bodies of known size, such as red blood- corpuscles. 



Sterile water, which is required in large quantities, can be 

 obtained by a Pasteur - Chamberland or Berkefeld filter. 

 Apparatus, cover-glasses, etc., must be carefully freed from 

 grease and dust, and all vessels for cultures are sterilized before 

 use by heating for some hours above ioo° C. Perishable 

 articles, like rubber corks, are soaked in a i per cent, solution 

 of formalin, and then thoroughly rinsed with hot sterile water. 



The enumeration of all the bacteria occurring in a sewage 

 would, of course, be impossible, and a bacteriological examina- 

 tion is usually confined to the following items : 



1. Number of organisms per c.c. capable of growing at room 

 temperature — (a) aerobic, (6) anaerobic. 



2. Number of organisms per c.c. at blood heat — {a) and (b). 



3. Number of organisms per c.c. that liquefy gelatine. 



4. Special tests for spores and their number per c.c. 



5. Identification of important species and number per c.c. 



6. Special tests for pathogenic forms. 



