484 WATER SUPPLY AND SEWAGE DISPOSAL 



requiring large amounts of water, as much as 100 or even 

 200 gal. per capita are required. To furnish such immense 

 amounts of water, elaborate pumping and distributing systems 

 are necessary. Figure 295 shows a modern pumping station for 

 a small city. The pump used is a CENTRIFUGAL PUMP; it is 

 driven by a gas engine (page 574) which uses producer gas as 

 fuel (Art. 119). 



THE CENTRIFUGAL PUMP. In principle, the centrifugal pump 

 resembles the fan or blower of the plenum system of heating 

 and ventilation (Art. 367 and Fig. 237). 

 Within the circular casing of the pump 

 are rapidly revolving fan-like vanes 

 (Fig. 296). The water enters the casing 

 at its center and is thrown violently out- 

 ward by CENTRIFUGAL FORCE (Art. 622 

 and Ex. 102). The casing prevents its 

 escape at all points except through the 

 discharge pipe. Centrifugal pumps may 

 be so constructed as to handle large 

 quantities of water at low pressure or smaller quantities at 

 high pressure. 



567. Sources of City Supply. It is often a serious under- 

 taking for a city to secure an adequate supply of water of such 

 a degree of purity that it may be used safely for drinking pur- 

 poses, that is, in its unboiled, or "raw," state. Many of our 

 larger cities are located on rivers or lakes where an abundance 

 of water is obtainable. But the purity of such water is fre- 

 quently not such as to warrant its use without purification. 

 Sometimes sufficient purification is secured by pumping the 

 water into a SETTLING TANK where most of the sediment is 

 removed and then passing it through sand filters where most 

 of the finer suspended matter is removed. Sometimes it is 

 found necessary to treat the water chemically in addition to 

 filtering it. This is most frequently the case when a city gets 

 its water supply from a river into which other cities nearer its 

 source have emptied their sewage. 



