QUANTITATIVE BACTERIOLOGICAL ANALYSIS. 35 



The Board employed the term "hygienic efficiency" to 

 indicate the percentage removal by filtration of the applied 

 bacteria capable of producing disease. The hygienic efficiency 

 of the Lawrence filter beds, tested with the B. typhosus, was as 

 follows : 



Material of 

 filter. 



S.VU! 1 . 



5-25 feet 

 5 feet 



2 feet 

 1 foot 



Loam. 



2 inches 



I inch 



Percentage of B. 



removed. 



99-95 



99-00 



100-00 



100-00 



100-00 



97-20 



100-00 



99-16 



99-00 



Bacterial efficiency appears to depend largely on the rate of 

 filtration. Low rates are safer than high rates ; but up to a 

 certain limit the rate does not exert much influence. At Altona 

 Kiimmel obtained the following; results : 



Kate of filtration. 



2 inches per hour (4 feet per day) 

 4 , (8 ., ) 



Bacteria per c.c. in the effluent. 

 . . . 11-97 

 , . - 5-79 



7-72 



Kiimmel did not regard four inches per hour as beyond doubt 

 the maximum rate of safe filtration, but considered that the 

 danger of passing pathogenic organisms was more unlikely at the 

 lower than at the higher rates, and that the best velocity was 

 not the same for all waters. Differences in the mineral, vege- 

 table, and animal admixtures were of great importance in this 

 question. 



The Massachusetts Board of Health believed that the factor 

 which caused the rate of filtration to become practically nil was 

 the age of the filter. Increasing the rate of filtration ten to 

 twenty per cent, might cause a temporary increase in the number of 

 bacteria if the filter were new and composed of coarse grains ; but 

 with ordinary ripe filters no increase in the bacteria was noticed. 

 Stopping the filtration for thirty-six hours, and then increasing 

 it, had no effect on old filters, but on new filters with coarse 

 grains it had a temporary effect. Lowering the rate and then 



