EXAMINATION OF FILTERS 407 



coating will reduce the velocity with which the water passes, 

 and at the same time some of the bacteria will tend to grow 

 downwards into the lower strata of the filter, and, if the 

 process were continued long enough, would be washed 

 through into the filtrate, and ultimately become more 

 numerous there than in the unfiltered water. The in- 

 creasing resistance to the passage of water would also make 

 it necessary for the pressure to be increased, which would 

 in this class of filter assist the passage of organisms. It is 

 therefore necessary in the working of sand-filters to run the 

 filtrate of each bed to waste, or to permit a body of water 

 to stand on the filter without filtration, until a sufficient 

 coating has been formed to arrest organisms ; to stop 

 filtration when the deposit has increased to such extent as 

 to threaten the renewed passage of organisms ; and to 

 remove the upper or filtering layer, and permit a fresh 

 deposit to be formed. The indication for scraping usually 

 adopted is that the filter-bed no longer passes the required 

 quantity of water under the maximum permissible head.. 

 The sufficiency of this practice has not been clearly shown.. 

 No general rule can be given for the depth to which the 

 top layer must be removed, as it varies with the nature of 

 the water and sand, temperature, etc. 



The ordinary rules for the selection of the epochs for 

 starting and arresting the filters, and the operation of 

 removing the upper surface, require considerable experience 

 and judgment ; and it frequently happens that through 

 carelessness or unavoidable mistake the filtration is im- 

 perfect. Thus, in 1894 the filters at Nietleben, Altona, 

 Hamburg, and Stettin, being over 10 per cent, of the total 

 sand-filters in use in Germany, where great attention has 

 been given to the subject, passed the cholera organism, 

 and permitted epidemics in their towns. In the same year 

 the typhoid organism, of which the detection was difficult 



