WATER FILTRATION. XIX. 



that the first cost of the work would be increased propor- 

 tionately. 



Sand Filtration. 



Filtration through sand is straining water through fine 

 insoluble media. The flow being continuous, the interstices 

 in the sand become clogged with the coarser particles in 

 the water, the interstitial space being reduced, the finer 

 particles are in turn intercepted, until in rotation the 

 bacteria are intercepted by the slimy coating thus formed. 

 The bacteria increase in number until the filter becomes 

 blocked and scraping of the top layer has to be carried out. 



In order to maintain a high degree of efficiency several 

 important points have to be observed, and the regulations 

 issued by the German Government, drawn up under the 

 supervision of Dr. Koch, are as interesting as they are 

 valuable : — 



1. In judging the quality of a filtered surface water the following 

 points should be specially observed : (a) The operation of a 

 filter is to be regarded as satisfactory when the nitrate con- 

 tains the smallest number of bacteria, not exceeding the 

 number which practical experience has shewn to be obtain- 

 able with good nitration at the works in question. In those 

 cases where there are no previous records shewing the possi- 

 bilities of the works and the influence of the local conditions, 

 especially the character of the raw water, and until such 

 information is obtained it is to be taken as a rule that a satis- 

 factory filtration shall never yield an effluent with more than 

 about 100 bacteria per cc. (6) The filtrate must be as clear 

 as possible, and in regard to colour, taste, temperature and 

 chemical composition must be no worse than raw water. 



"2. To allow of a complete and constant control of the bacterial 

 efficiency of filtration, the filtrate from each single filter must 

 be examined daily. Any sudden increase in the number of 

 bacteria should cause a suspicion of some unusual disturbance 

 in the filter, and should make the superintendent more atten- 

 tive to the possible causes of it. 



