WATER SUPPLY 



below the point where the Missouri River joins it. 

 Recently a part of an additional purification plant has 

 been completed on the Missouri River, thirty-seven 

 miles from the mouth and about twenty miles west 

 of the city. 



At the Chain of Rocks plant, water is taken from 

 the Mississippi River through two intake towers and 

 pumped to a delivery well on a higher level. From 



Courtesy Board of Water Supply, New York City 



FIG. 104. AERATION OF WATER 



this point the water flows by gravity through the 

 rest of the plant. Heavy grit and other suspended mat- 

 ter are removed in the grit chambers. After the water 

 leaves the grit chambers a solution of milk of lime is 

 added and thoroughly mixed in more than a mile and 

 a half of conduit. 1 As the water leaves the mixing 

 conduits and enters the settling basins a charge of 

 iron sulphate is added to it. You will recall from the 

 experiment that from the chemical interaction of milk 

 of lime (calcium hydroxide or limewater) and iron 

 sulphate, a green precipitate of iron hydroxide is 

 formed. 



calcium hydroxide (soluble) 



+ 

 iron sulphate (soluble) 



H 



iron hydroxide (insoluble) 



+ 

 calcium sulphate (soluble) 



The water remains in the settling basins on the 

 average about thirty hours, during which time the 

 iron hydroxide precipitate settles to the bottom, 

 carrying with it much of the larger suspended ma- 

 terial. The settling basins are cleaned from time to 

 time to remove the settled matter. 



The water is collected from the settling basins in 

 a conduit, and passes through meters where it is meas- 

 ured and a charge of aluminum sulphate (alum) is 

 added. From the experiment you will recall that when 

 limewater and aluminum sulphate react chemically 

 an insoluble substance (precipitate) of aluminum hy- 

 droxide is formed. 



1 Conduit, a channel, as a pipe, for conveying water. 



aluminum sulphate (soluble) \ 

 calcium hydroxide (soluble) / 



aluminum hydroxide (insol- 

 + uble) 



calcium sulphate (soluble) 



As the white aluminum hydroxide precipitate 

 forms rather slowly, the water is run into another 

 settling basin where the reaction is completed. Some 

 of the precipitate settles out, carrying part of the re- 

 maining fine, suspended matter to the bottom. The 

 water is then taken to the rapid sand filter where it 

 passes through thirty inches of sand. Much of the 

 aluminum hydroxide is caught on the filters, and thus 

 their efficiency is increased to the point where even 

 many of the larger bacteria are removed. 



After filtration the water is sterilized with chlo- 

 rine and sent into the citv mains. 



Courtesy St. Louis Deft, of Public Utility 



FIG. 105. SETTLING BASIN, CHAIN OF ROCKS PLANT 



In a single year 15,800 tons of lime, 1,900 tons of 

 iron sulphate, 4,000 tons of aluminum sulphate, and 

 110 tons of liquid chlorine were used. It cost on an 

 average of $9.15 to purify each million gallons, from 

 which were removed 17,214 pounds of mud and 

 27,252,000,000,000 bacteria. The efficiency of the puri- 

 fication in the removal of bacteria is shown by the 

 following table. These data are for 1929. 



Bacteria in each cubic centimeter of river water 15,000 

 Bacteria in each cubic centimeter of settled water 1,100 

 Bacteria in each cubic centimeter of water to filter 190 

 Bacteria in each cubic centimeter of water to main 18 



The new plant recently completed on the Missouri 

 River makes use of about the same purification 

 methods as the older plant. It is an excellent example, 

 however, of how modern engineering is able to speed 

 up processes. 



Water is taken in from a shore intake and pumped 



