1922] Chemistry in Relation to State Water Supplies 95 



the towns grow the number of purification plants are rapidly in- 

 creasing. 



Almost the universal method for purification is the so-called me- 

 chanical or rapid sand filtration involving the coagulation of the water 

 with aluminum hydrate and the preparation of a film of this chem- 

 ical as a medium through which the water is filtered. In conjunction 

 with this is chemical sterilization with chlorine. These processes 

 are basically chemical ones, and difficulties encountered in their use 

 must be largely solved through chemistry. The mechanical features 

 in both cases have been very highly developed, and such lack of 

 efficiency as is found is chiefly due to the chemical features. 



When mechanical filtration first began to be utilized aluminum 

 sulphate (commonly called filter alum) was used and the hydrate 

 serving as coagulent prepared by adding soda ash sufficient to react 

 with the aluminum sulphate required and to furnish sufficient ex- 

 cess to insure complete precipitation of the alum. 



While the process was in general recognized as a success, different 

 waters reacted differently using the same amount of chemicals, and 

 on the same type of water poor coagulation was frequently expe- 

 rienced and in many cases the reaction involved did not complete it- 

 self until after passing the filters with precipitation in the water 

 mains. These troubles were experienced particularly in the case of 

 waters with considerable vegetable stain such as are found in eastern 

 North Carolina. 



About the year 1915 several engineers, including the writer, called 

 attention to the fact that what was involved in the whole process 

 was so-called ' ' colloidal chemistry. ' ' It was well recognized that 

 aluminum hydrate was precipitated in colloidal condition and its 

 coagulation followed the known action of such physical condition. 

 The finely divided clay and organic matter was obviously in colloidal 

 condition, and Thorndike Saville, working in the Harvard Laboratory, 

 proved that the vegetable coloring matter was in colloidal suspen- 

 sion. It was also determined that the clay was negatively charged 

 and aluminum hydrate and coloring matter positively charged. It 

 was suggested that the proper application of aluminum sulphate and 

 the ratio of this to alkali present was one of ionization balance. 



Due largely to the fact that "colloids" and "ions" represented 

 something surrounded by impenetrable mystery to the average engi- 



