142 MICIiO-ORGANISMS IN WATER 



"wark works) were filter beds examined on the first day 

 after cleaning, and on botli of these occasions there was 

 evidence that the water issuing from such new beds 

 was not as free from micro-organisms as that coming 

 from older beds at the same works. It appears, how- 

 ever, from the results obtained with beds which had 

 been recently cleaned that normal efficiency is attained 

 within two or three daj^s. On the other hand, there 

 was no evidence of the efficiency of beds being reduced 

 through prolonged use. 



(5) Of the small number of micro-organisms found 

 in these filtered waters, it appears that only a very 

 insignificant part of them can be derived from the 

 nnfiltered water, for the numbers in the filtered bear 

 no relationship to those in the unfiltered waters ; the 

 majority of the bacteria in the filtered water are, there- 

 fore, attributable to post-filtration sources. It must 

 be remembered in this connection that the filterino- 



a 



materials are not sterile themselves, nor are the 

 channels, culverts, well, &c., into which the filtered 

 water passes. 



Berlin Water Supply^ River Spree and Lake Tegel. — 

 The water supply of Berlin is derived from the Eiver 

 Spree and Lake Tegel, both of which are submitted 

 to sand-filtration before distribution. Plagge and Pros- 

 kauer, Wolff'hiigel and others, have made a large number 

 of examinations to ascertain the bacterial contents of 

 the water supplied from these two sources before and 

 after filtration. 



The following table furnished by Plagge and Pros- 

 kauer ^ shows the redaction in the number of micro- 

 organisms effected by the treatment to which the water 

 has been subjected at the waterworks. 



^ Zeitsclirift filr Hygiene, vol. ii. 1887, p. 401. 



