PURIFICATION OF WATER SUPPLIES 171 



if present in excessive amounts, renders the water unfit . for 

 drinking. 



There is no entirely satisfactory method of treating these 

 waters. A certain amount of relief is obtained in some cases, 

 however, by adding calcium sulphate in the form of gypsum or 

 plaster of Paris. This neutralizes, to a greater or less extent, 

 the sodium carbonate giving rise to the objectionable alkalinity. 



Algae. At times, the growth of algae, including the so-called 

 slimes as well as a number of more minute organisms, gives rise to 

 highly objectionable odors and tastes in waters stored in ponds 

 and reservoirs. Ground waters so stored often seem to be par- 

 ticularly susceptible to such growths. 



Most of the organisms producing the objectionable features 

 in the water are very sensitive to copper in solution, and by the 

 use of the latter in amounts that, while fatal to the organisms, are 

 harmless to those using the supply for drinking purposes, the 

 odor and taste may be largely removed. 



Copper sulphate is the compound used. About I ounce of 

 the chemical is added to 30,000 gallons or 4000 cubic feet of water 

 when the algous growth is very pronounced ; where slight, I ounce 

 to 75,000 gallons or 10,000 cubic feet is sufficient. When the 

 waters are high in carbonate of lime, more of the copper sulphate 

 will be required, since a certain portion of the sulphate reacts 

 with the carbonate. 



The chemical is weighed, and then either placed in cloth bags 

 and towed over the surface of the reservoir until dissolved, or it 

 is dissolved in water and added through a perforated pipe at the 

 inlet. Some time may be required to destroy the organisms, 

 and a week may elapse before the dead algae, which often have an 

 offensive odor, settle to the bottom. For the latter reason, many 

 persons consider the copper-sulphate treatment of more benefit if 

 applied as a preventive before the algae have formed than as a 

 remedy after they have developed. Although not all the offen- 

 sive organisms are killed and the process is, moreover, open to 

 the other objections mentioned, it may be said that, in general, 



