FILTRATION OF WATER 



69 



various causes ; surface-polluted water oozing into the well is probably 

 the commonest, but decaying animal or vegetable matter might also 

 raise the number of micro-organisms present almost indefinitely. 

 Koch's proposal for such a polluted well was to fill it up to its 

 highest water level with gravel, and above that, up to the surface 

 of the ground, with fine sand. Before the well is filled up in this 

 manner it must, of course, be fitted with a pipe passing to the 

 bottom and connected with a pump. This simple procedure of filling 

 up a well with gravel and sand interposes an effectual filter-bed 

 between the subsoil-water and any foul surface-water percolating 

 downwards. Such an arrangement yields as good, if not better, 

 results than an ordinary filter-bed, on account of there being 

 practically no disturbance of the bed nor injury done to it by frost. 

 The evidence that filter-beds remove pathogenic bacteria has not only 

 been demonstrated by experiment but by actual experience. At 

 Lawrence, Hamburg, Mount Vernon, and other towns, a marked 

 decline in water-borne typhoid fever has occurred as a result of 

 filtration. 



The effect of filtration upon the number of bacteria was demon- 

 strated in the results which Sir Edward Frankland arrived at in his 

 investigation of London waters so long ago as 1887.* 



Mean of Monthly Examinations for the Year. 



In 1899 the Massachusetts Board of Health found that by 

 continuous filtration through 45 inches of sand (size 0'23 mm.) 

 99-49 per cent, of the bacteria were removed ; and by intermittent 

 filtration 99'08 per cent, of the organisms were removed. In 1902 

 the intermittent filter removed 98'7 per cent, of the total bacteria, 

 99-9 per cent. B. coli, and 100 per cent. B. typhosus. The continuous 



* Report on the Metropolitan Water Supply, 1887. 



