348 WATER BACTERIOLOGY 



1. Evaporation and condensation which gives the purest of 

 natural waters. Millions of gallons of water are annually evapo- 

 rated from the surface of the globe. Thus, we have an enormous 

 natural still by which water is constantly being purified in Nature. 



2. The self-purification of running streams which although 

 important is often hard to estimate quantitatively. It is due to 

 many factors, chief among which are: (a) Chemical the oxidation 

 and reduction of organic and inorganic constituents of the water 

 with the formation of simple substances which are not- well suited 

 to the maintenance of life and growth of many forms of bacteria, 

 and the germicidal influence of sunlight which is an important but 

 very variable factor. (b) Biological the death of microorgan- 

 isms through various not well-understood conditions grouped under 

 the heads of symbiosis, antibiosis, time and various other means. 

 (c) Physical of which dilution and sedimentation are the more 

 important. 



3. The storage in lakes and ponds which through the prolonging 

 of the time of action greatly intensifies those factors at work in 

 the natural purification of running streams. 



4. The combined physical, chemical and biological action of 

 soil upon water which filters through the soil. This is one of 

 Nature's greatest purifying agents and stands second to evapora- 

 tion and condensation in effectiveness. 



Artificial Purification. Those methods which are so effective in 

 the purification of water under natural conditions are usually the 

 methods which are made use of in the artificial purification of 

 water. Only a few of the best known can be briefly considered 

 here. The student who is more deeply interested in the subject 

 is referred to any of the many comprehensive works on this subject. 



The slow sand filter frees water from impurities through the 

 interaction of sedimentation, filtration, and the biological destruc- 

 tion of organic matter and bacteria. It has been extensively used 

 for over one hundred years, but a great impetus was given to this 

 measure when Koch, in 1893, showed that the proper filtration of 

 the water from the Elbe River saved Altona from an epidemic ot 

 cholera which devastated Hamburg which was using unfiltered 

 water. 



The method consists in causing water to pass through a layer 

 of sand of such fineness and thickness that the requisite removal 

 of suspended substances is accomplished. The filter as usually 

 constructed is a basin having a water-tight concrete base on the 

 surface of which are laid perforated tiles or pipes. These are 

 covered with about a foot of gravel graded in size from 25 to 3 mm. 

 in diameter from bottom to top. Over this is placed three or four 

 feet of sand which acts as the real filter. The water passes through 

 this and is conveyed to the mains by the underlying pipes. The 



