PURIFICATION OF WATER FOR DRINKING PURPOSES 135 



centimetre, whilst the number in the filtered water 

 ranged from 98 to 16, and averaged 36 in the same 

 volume. The length of time which had elapsed since 

 last cleaning varied between 3 and 62 days, but there 

 was no connection discoverable between the age of the 

 filters and efficiency of the filtration. 



On October 3, 1892, the unfiltered water before 

 and after storage, as well as the filtered water from 

 seven filter beds, was submitted to examination. 



Inasmuch as all the filter beds are supplied with 

 water which has passed through at least one storage 

 reservoir, and in some cases two, the number of micro- 

 organisms in the water actually undergoing filtration 

 may be taken as from 177 to 318 per cubic centimetre, 

 whilst the numbers in the filtered water ranged from 

 4 to 18 in the same volume and averaged 11. The ages 

 of the filter beds varied from 15 to 65 days, but in 

 no case was the efficiency affected by the age of the bed. 



Southwark and Vauxhall Waterworks, Battersea, August 19, 1892 



Filtered Water 



No. of Colonies 



No. of Filter Bed Age of Filter Bed in days from 1 c.c. 



of Water 



Nos. 3, 4, 5 . Cleaned 14 days before, . . 70 



charged since, but only 



began filtering 6 hours 



previously 



No. 6 .... 21 .... 3 

 From main ... .... 31 



Unfiltered Water 



No. 3 Bed, representing Nos. 3, 4, and 5 Beds . 394 . 

 No. 6 Bed .346 



Unfortunately these works are ill adapted for an 

 investigation of this kind, as the water coining from 

 the individual filter beds is not in most cases accessible. 



The first sample represents the united filtrate of 

 three beds which had only been in operation six hours 



