117 



CHAPTEE V 



THE PURIFICATION OF WATER FOR DRINKING PURPOSES 



Sand Filtration. It is obvious that, with the informa- 

 tion which the bacteriological investigation of water 

 has furnished us, the subject of water-purification must 

 be approached from an entirely novel point of view ; 

 that, whereas formerly the chemical standard was the 

 only one which could be appealed to as a guarantee of 

 the suitability or not of a water for domestic supply, 

 we have now a far more delicate test as to the efficiency 

 of the purification processes employed in the biological 

 examination to which a water can be submitted. Thus, 

 perhaps, a concrete example will most clearly illustrate 

 how the purification of water must now be regarded. 

 Supposing that a water, derived from a source which 

 is altogether unimpeachable as regards contamination 

 with animal matters, is yet so highly impregnated with 

 vegetable constituents as to be unpalatable, the question 

 will arise how this water may be treated so as to free it 

 from this blemish and render it suitable for .drinking 

 purposes. In a case of this kind it is obvious that 

 chemical purification will be of paramount importance, 

 whilst the removal of organic life from the water will 

 be of less pressing consequence. On the other hand, 

 if water which is known to have received sewage 

 matters (and the entire exclusion of such from supplies 

 drawn from rivers is practically impossible) is to be 

 supplied for dietetic use, and if this water, as is so often 



