74 SOIL AND SEWAGE 



of B. entcritidis sporogenes and the number of streptococci. 

 Compared with water the B. ent. sporogenes determination is 

 more valuable and that for streptococci less valuable. 



Great initial dilution of the sample is required before the 

 different fractions are removed for examination, the dilutions 

 to be used depending upon the degree of probable dilution of 

 the effluent Dilutions varying from O'l to O'oooooi c.c. should 

 be examined from unknown samples in routine cases. 



It must be remembered that the bacterial content of both 

 sewage and sewage effluent samples varies from hour to hour. To 

 get representative results samples would have to be taken every 

 hour and in amount proportional to the flow, kept in ice until 

 the end of the 24 hours, pooled and a representative mixed 

 sample taken for bacteriological examination. It is only in 

 very rare cases that it is worth while taking all this trouble. 

 As a rule all that is necessary is to show that samples 

 of effluent taken at different times contain these indicator 

 organisms in large numbers, often not materially reduced from 

 their numerical presence in the crude sewage, and that there- 

 fore it is a reliable deduction that the sewage effluent in 

 question is not safe to discharge into any stream which itself 

 may ultimately be used for drinking purposes or which will 

 wash food material watercress, shellfish, etc. to be used for 

 human food. 



CHAPTER V 



SHELLFISH 



It has now been conclusively proved that shellfish may 

 and do convey disease to man, typhoid fever in particular, but 

 probably also other diseases. This fact has directed considerable 

 attention to the bacteriology of shellfish and to their bacterio- 

 logical examination to detect and measure the extent of their 

 bacterial contamination. 



