80 BACTERIOLOGY FOR NURSES 



food for rotifers and Crustacea. Dilution plays an important 

 role in that a small amount of infection in a lake or river is soon 

 so diluted as to practically become lost. A slow-moving river 

 is purified much in the same way that snow clears the air: the 

 particles of mud which are constantly settling enmesh the bac- 

 teria in their fall and carry them down to the bottom, where they 

 soon die. 



Boiled Water. So far as water-borne infections are concerned 

 boiling renders water safe. Typhoid and dysentery bacilli and 

 cholera spirilla are killed even at a lower temperature. Holding 

 for twenty minutes at 60 C. or a few minutes at 70 C. is sufficient 

 to destroy them. 



The principal methods employed for the purification of water on 

 a large scale are (1) storage, (2) filtration, (3) addition of a chemi- 

 cal. In some cities two or even all the methods are combined. 



Storage. Several of Nature's methods are applied in this form 

 of purification; namely, time, oxidation, dilution, sedimentation, 

 etc. The growth of algae and decomposition of organic matter 

 sometimes gives to water stored in an open reservoir a disagree- 

 able taste and odor. That may be obviated, however, by the use 

 of a closed reservoir. 



Filtration. Two forms of filters are in general use for public 

 water supplies : slow sand filters and mechanical filters. 



A slow sand filter consists of a large shallow reservoir with 

 underdrain pipes and containing five or six feet of filtering material 

 of graded size, beginning at the bottom with broken stone or gravel 

 and finishing with an upper layer of fine sand. The water passes 

 through the filter very slowly from above downwards, and in its 

 passage almost all the bacteria and fine particles are strained out. 

 The process is not merely a simple straining ; its efficiency is due 

 rather to bacterial activity. The spaces between the finest sand 

 are enormous compared to the size of bacteria, and yet 99 per cent 

 of bacteria do not pass beyond the upper layer. What really 

 happens is that the microorganisms resting upon the surface grow 

 and form gelatinous masses which adhere to the particles of sand 

 and gradually close up the interstices. This continuous carpet- 



