65 
Exercise 32. The Bacteriological Examination of Sewage 
MATERIALS: 
7 tubes of sodium caseinate agar 
7 sterile Petri dishes 
Case of sterile 1 cc pipettes 
3 9 cc. sterile water blanks 
3 99 cc. sterile water blanks 
Sample raw sewage 
1. Prepare dilutions of the sewage according to the scheme in Fig. 20. Pre¬ 
pare duplicate plates with 1/100,000 and 1/1,000,000 cc. Make one control. 
2. Melt the agar and cool it to 45° C. Pour the plates, and when the agar 
FIG 20—MANNER OF MAKING SEWAGE DILUTIONS 
is solid, incubate them at room temperature for one week. 
3. Count the plates and record the data as in Table XIII. 
TABLE XIII.—Bacterial Content of Sewage 
Dilution 
Colonies 
on plate 
Bacteria 
per cc. 
Kind of colonies 
1/10,000 cc. 
I 
II 
1/100,000 cc. 
I 
II 
1/1,000,000 cc. 
I 
| 
II 
(a) How does the bacterial content of sewage compare with that 
of water? 
( b ) What kinds of organisms predominate in raw sewage? 
(c) What chemical changes take place in sewage? 
