144 



MICEO-ORGANISMS IN WATER 



the results of sand-filtration. on river waters containing 

 a large number of micro-organisms with those obtained 

 when microbially very pure water has to be dealt with. 

 For this purpose the water-supply of Zurich has been 

 selected. The source from which it is derived is the 

 Lake of Zurich, and before delivery it is subjected to 

 sand-filtration. 



In the report^ on the water-supply of Zurich for 

 1889 details are given of the conditions under which 

 some of the filters were working, and of the manner in 

 which these conditions afiected the microbial purity of 

 the filtered water. The results are brought together in 

 the following table : — 



Special Investigations 07i the Working of Particular Filter Beds in 

 Zurich from July 6 to September 7, 1889 



Filter Beds 



Condition 



Age of working in months . 

 Thickness of filtering sand in cm. 

 Bacteria per c.c. . . . , 



Miters 1.5 IT., IV. and V. were old ones, having 

 been constructed from two to four years previously ; of 

 these Nos. I. and V. had had the surface sand scraped 

 off from time to time, and the residual layer of sand had 

 thus become considerably reduced in thickness ; Nos. 

 11. and IV. had respectively had their fine sand com- 

 pletely renewed 6'6and 7"5 months previously; whilst 

 No. III. had been totally reconstructed from top to 

 bottom 0'6 month before the aboA^e investigation was 

 made. The results recorded are the means of eight 



^ Jahreshericht ilher die Wasserversorgiing von Zurich und Um- 

 gehung, 1889. 



