146 



MICRO-ORGANISMS IN WATER 



times, those of the uncovered ones nine times in the 

 year ; the average thickness of sand removed in scrap- 

 ing being 20 mm. each time. 



On renewing the sand in a filter, it was found that 

 it was only after from 3^ to 36^ days that it got into 

 an efficient working condition and yielded a satisfactory 

 nitrate. The time required for such conditioning of 

 a filter is of course exceptionally great in a case like 

 this, in which a comparatively clear water is being 

 filtered. 



In the following table the bacterial composition 

 of the water is given before and after filtration for the 

 year 1891 l :- 



Unfiltered Water from the Lake of Zurich, 1891 

 Pumping Station 



Filtered Water taken from the Filtered Water Beservoir, 1891 



In consequence of the extreme cold the lake was 

 frozen in this year, and some special investigations were 

 made to ascertain the effect upon the microbial condi- 



Licht- und Wasserwerke Zurich.' Jahresbericht fur 1891. 



