CHAPTER V. 



QUANTITATIVE ANALYSIS continued. 



THE RELATION OF A QUANTITATIVE BACTERIOLOGICAL ANALYSIS 

 TO FILTRATION OF WATER THROUGH SAND. 



THE purification of water by filtration through sand is chiefly a 

 biological process, the object being to remove micro-organisms 

 from the supply. Piefke was the first to show that sand alone 

 could not remove micro-organisms from water ; filters made of 

 sterilised sand were found to increase the number of bacteria in 

 the filtrate during the first few days of working, but, with the 

 formation of a gelatinous layer on the surface, the true filtering 

 action commenced. This gelatinous layer consists of zooglcea of 

 bacteria combined with suspended materials in the water ; it is 

 extremely friable and readily broken by excessive pressure on the 

 surface or disturbance of the body of the filter bed. The degree 

 of fineness and the uniformity of the sand grains are also of 

 importance in securing a good filtrate. By using fine sand the 

 current of water passing through the bed is rendered more 

 uniform, and the walls of the lacunar spaces are approximated, 

 permitting molecular action to take place, and giving greater 

 firmness to the gelatinous layer. Hence the work of a sand 

 filter is partly mechanical and partly vital. By the growth of 

 bacteria in the bed food material is used up, and products of 

 bacterial life, which have a powerful elFect in arresting the 

 growth of bacteria through the filter, are eliminated. It must 

 not be supposed that a sand filter will arrest all the micro- 

 organisms contained in a water applied to its surface. At one 

 time it was considered that a filter bed consisted of two super- 

 imposed systems, viz., an upper system which arrested all bac- 

 teria, and a lower system formed by the inferior layers of the 

 bed which contained a few bacteria clinging to the materials. It 



