BACTERIA OF WATER AND SEWAGE 301 



In water analyses, an important part is the collection and 

 transportation of the samples. The sample must be collected 

 in sterile bottles and in such a way as to avoid contami- 

 nation, and examination must be made either immediately 

 or the sample must be kept packed in ice so as to prevent the 

 changes which otherwise take place. 



The Purification of Water. As the population of the 

 country increases, the necessity of purification of water 

 supplies for certain portions of the population, at least, be- 

 comes imperative. The purification of water is at the bottom 

 a bacteriological problem. It may be accomplished in a num- 

 ber of different ways; by sedimentation, which may take place 

 naturally in some waters, or as a result of the addition of 

 chemicals in others. It is secured also, and most frequently, 

 by filtration methods. This is accomplished by what are 

 known as continuous sand filters, used largely in Europe, 

 by means of mechanical sand filters, developed and widely 

 used in America, and by intermittent sand filters, developed in 

 America and very successfully used on grossly polluted waters. 

 In the continuous sand filter the efficiency depends upon the 

 formation of a slime layer on the surface of the filter, so that 

 this filter is really a living filter. In the mechanical filter, 

 the efficiency is due to a precipitate produced on the sand by 

 the addition of chemicals to the water. Its advantage over 

 the continuous filter comes from the fact that it works much 

 more quickly, and hence occupies less space. In the inter- 

 mittent sand filter, the purification depends largely upon the 



