166 



MICRO-ORGANISMS IN WATER 



Number of Bacteria found in 1 grm. of Sand taken at Various 

 Depths of Tank No. 1 on Different Dates 



In general there is, therefore, a very marked de- 

 crease in number from the top to the bottom, there 

 being in the bottom inch but o per cent, of the number 

 found in the top inch ; but the decrease is most rapid 

 in the first few inches at the top. These results were 

 confirmed by several similar investigations made on 

 different sand filters, and are in accordance with Piefke's 

 examinations of the bacterial contents of the sand at 

 different depths in waterworks filters (see p. 158). 



The number of experimental tanks was upwards of 

 twenty, and the investigations on each of these included 

 innumerable chemical analyses of the raw sewage and 

 of the effluent, besides the bacteriological examination 

 of a very large number of samples. 



In summarising the results obtained by the inter- 

 mittent filtration of sewage through very coarse sand, 

 as well, as through sand which was practically fine 



