BACTERIA IN WATER 73 



ARTIFICIAL PURIFICATION OF WATER 



Sedimentation and Precipitation. Naturally, we see this 

 factor in action in lakes or reservoirs. For example, the 

 water supply of Glasgow is the untreated overflow from 

 Loch Katrine. Purification has been brought about by 

 means of subsidence of impurities. Nothing further is 

 needed. Artificially, we find it is this factor which is 

 the mechancial purifier of biological impurity in such 

 methods as Clark 's process. By this mode " temporary 

 hardness," or that due to soluble bicarbonate of lime, is 

 converted into insoluble normal carbonate of lime by the 

 addition of a suitable quantity of lime-water. Carbonates 

 of lime and magnesia are soluble in water containing free 

 carbonic acid, but when fresh lime is added to such water 

 it combines with the free CO 3 to form the insoluble carbon- 

 ate, which falls as a sediment : 



CaCO 3 + CO 2 + CaH 2 O 2 (lime-water) = 2 CaCO 3 + H 2 O. 



As the carbonate falls to the bottom of the tank it carries 

 down with it the organic particles. Hence sedimentation 

 is brought about by means of chemical precipitation. It is 

 obviously a mechanical process as regards its action upon 

 bacteria. Nevertheless its action is well-nigh perfect, and 

 300 or 400 m.-o. per cc. are reduced to 4 or 5 per cc. We 

 shall refer to this same action when we come to speak of 

 bacterial purification of sewage. Alum has been frequently 

 used to purify waters which contain much suspended mat- 

 ter. Five or six grains of alum are added to each gallon of 

 water, with some calcium carbonate by preference. Precipi- 

 tation occurs, and with it sedimentation of the bacteria, as 

 before. But, as Babes has pointed out, alum itself acts 

 inimically on germs; in such treatment, therefore, we get 

 sedimentation and germicidal action combined. 



As a matter of actual practice, however, sedimentation 



