282 PUEIFICATION OF PUBLIC WATER SUPPLIES. 



the removal of all suspended matter and the great differ- 

 ence in the results of analysis after filtration, compared 

 with those of the water before filtration (the analysis of 

 the filtered water showing a high degree of purity), are 

 most emphatic proofs of the benefits conferred by this 

 process. It seems to be admitted by the ablest oppo- 

 nents of this method of filtration that coloring matter 

 can be removed from water more readily by this process 

 than by any other. 



The quantity of alum used is generally stated at about 

 one-half grain per gallon of water filtered and to vary 

 from 3 10 to 7-10 grains, according to the condition of 

 the water treated. There are waters, however, which re- 

 quire very much more. 



The water of the Hudson is of that class. In the spring 

 of 1886 the water of the river was very turbid with clay 

 and our filter beds were unable to clarify it. One of the 

 mechanical filter companies sent a small filter here and 

 erected it at our jDumping station and operated it for a 

 month or more. Its mechanical operation was appar- 

 ently satisfactory and it was possible to render the water 

 as clear and bright as the purest spring water. When I 

 enquired as to the quantity of alum used, the attendant 

 stated that he did not know and had no means of deter- 

 mining it. From experiments made by myself, with 

 baiium chloride and hydrochloric acid, I found it neces- 

 sary to add at the rate of about five grains of alum per 

 gallon to the water of the river to produce the same re- 

 action that was obtained with the water as it came from 

 the small filter. I concluded, therefore, that more than 

 five grains of alum per gallon were required to produce 

 the results obtained. In other conditions of the river 

 less alum would, of course, be necessary. 



With a very small filter and slower rate of filtration 

 and less clay in the applied river water, I have found 

 four grains of alum to produce excellent results. 



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