STERILISATION. 19 



lower layers of sand will be found to contain few bacteria. 

 Thus it is the matter contained in the water itself that converts 

 the upper part of the bed of sand into a filter. The necessary 

 condition for satisfactory working is that the water shall flow 

 slowly, in order that the bacteria and other particles may 

 have time to settle on the slimy grains of sand, and also to 

 prevent the skin breaking or the formation of channels through 

 the bed of sand. The rate of flow must depend on the nature 

 of the supply. If rich in bacteria, it should not sink more 

 rapidly than 2 to 3 inches per hour. For the same reason 

 the water level must be retained within certain limits. During 

 the slow passage of water, the bacteria embedded in the upper 

 part of the sand are able to retain some of the dissolved organic 

 matter in the water, so that when it leaves the filter it should 

 be free from fermentative and putrefactive components. A 

 stage is reached at last when the pores are so completely filled 

 with bacteria that the capacity of the filter is greatly reduced, 

 and it is then renewed by removing the top inch of sand. 

 This process can be repeated until a layer of sand 16 to 20 

 inches thick is left; the layer must then be restored to its 

 original thickness. / 



As many different factors condition the activity of the filter, 

 such as the changing biological contents of the water, altera- 

 tions in temperature, etc., its activity must undergo many 

 variations from a biological point of view, and this necessitates 

 a continuous bacteriological control. Completely sterilised 

 water cannot, of course, be obtained from such a filter, but 

 it has been shown that in a good filter only one out of every 

 1,000 or more germs is transmitted. 



Considerable use is also made of " rapid " filters. They 

 may be constructed of sand, wood charcoal, etc., so as to 

 allow of the passage of large quantities of water. The filtration 

 is combined with the use of chemical precipitants, whereby 

 the greater part of the slimy particles and organisms are 

 separated from the water. 



The treatment of water with ozone is dealt with in a 

 subsequent section. 



The exact method of sterilisation of liquid and solid sub- 

 strata by means of heat is determined by their chemical and 



