HOW COMMUNITIES OBTAIN PURE WATER 



67 



to the rising well. Can you find this in the diagram 

 shown in Figure 106? From this point water flows by 

 gravity through the rest of the plant. It first flows 

 to a pre-sedimentation basin where heavier grits are 

 settled out. Lime is next added and rapidly mixed 

 with the water as it flows past a series of baffles. After 

 the iron sulphate is added, another rapid mixing oc- 

 curs in a series of circular chambers. The water is then 

 run into two settling basins where it remains for 

 about two hours and then passes into two larger 

 basins where the iron hydroxide continues to settle, 



souri River level, water is distributed to the city by 

 gravity. 



Tucson, Arizona, located in the midst of the great 

 western desert country, has a different problem of 

 procuring a water supply from that of New York or 

 St. Louis. To provide water for a population of 32.000 

 in desert country is a distinct problem. 



With no rivers, lakes, or large watersheds close at 

 hand, the city has drilled eighteen deep wells in the 

 Santa Cruz valley, from which a clear, cool water 

 without taste or odor is obtained. This water is 



CL EV. OF WATER IN SYSTtN 

 L.SPUNPIN6 55 MGD 



FILTRATION 35 MSD. 



Courtesy St. Louis Deft, of Public Utility 



FIG. 106. DIAGRAM OF MISSOURI RIVER PLANT, ST. LOUIS WATER SUPPLY SYSTEM 



carrying much suspended matter to the bottom with 

 it. Both of these basins are equipped with continuous 

 sludge-removing devices. Hence, it is unnecessary to 

 drain them for cleaning. 



After about thirteen hours of settling, the water 

 is conducted to another basin. Here carbon dioxide, 

 which is recovered from the smokestack gases and 

 purified, is bubbled through the water. This tends to 

 make the water softer and prevents scale from form- 

 ing on the filters and in boilers which use this water. 

 A charge of aluminum sulphate is next added to the 

 water, mixed with it, and all is then taken to the rapid 

 sand filters. Following filtration the water is sterilized 

 with chlorine and conducted to an underground reser- 

 voir of five-million-gallon capacity. From this point 

 the water is pumped to a large one-hundred-million- 

 gallon reservoir about nine miles away. From this 

 reservoir, about three hundred feet above the Mis- 



pumped directly into the distribution system without 

 the necessity of chemical treatment. 



During the mild, warm winter days in this city, the 

 average consumption of water is about 75 gallons per 

 person per day, while during the hot summer months 

 it rises to about 250 gallons per person per day. 



In point of area Los Angeles is the largest city in 

 the United States. Its boundaries enclose 441 square 

 miles. Water is supplied at the rate of 160,000,000 

 gallons a day in this area all the way from sea level 

 to an elevation of 1,400 feet. These two factors of area 

 and elevation make the problem of water supply dif- 

 ferent from those of the other cities you have studied. 



The Los Angeles River has been a source of water 

 supply for the city for more than 150 years. Today it 

 supplies about forty square miles of the city's area. 

 Water is obtained from wells and from galleries 

 placed below the river bed into which the water flows 



