360 SEWAGE AND SEWAGE DISPOSAL 



ing of bacilli out of soil in which they may have lain dormant for 

 many months. The persistence of typhoid fever around certain 

 habitations may be plausibly explained on the supposition of an 

 extensive soil infection. There is no doubt that the practice of 

 using human excrement for manuring vegetable gardens entails 

 a danger no less real because often unrecognized." 



Necessity of Sewage Disposal. Sewage is obnoxious to the senses 

 because of its appearance, and especially because on decomposing 

 it yields malodorous compounds. It is usually considered that 

 hydrogen sulphid is the main offender, but indol, skatol, cadaverin, 

 mercaptan and some other compounds are considerably more 

 repulsive and exist in sewage. 



More important still is the fact that sewage contains bacteria 

 which have come from persons sick with typhoid fever, dysentery, 

 tuberculosis and other diseases, as these may reach the food or 

 water of healthy individuals and thus give rise to epidemics. Statis- 

 tics show that the abandonment of privies and the substitution 

 of a good sewerage system have greatly reduced the general death- 

 rate in many a city. 



What Should be Accomplished in Sewage Disposal The sanitary 

 engineer attempts to dispose of sewage as rapidly as possible, with 

 the least nuisance to the smallest number of people, with the least 

 damage to health or property, and at the smallest cost. Sewage 

 can be made entirely harmless only by the complete destruction 

 of its organic matter and bacteria. A complete purification is 

 not attempted normally as the plant required for such would be 

 so elaborate and too expensive. Moreover, practical experience 

 has shown this to be unnecessary. 



Methods of Disposal. The method selected for sewage disposal 

 will vary with the district, location and means at the disposal of 

 the sanitary engineer. However, in all cases he must keep in 

 mind convenience and public health. In rural districts the well- 

 constructed cesspool may of necessity be used, whereas in the 

 urban district this may not be tolerated. One of the readiest 

 methods, and the one which until the last few years has been uni- 

 versally used in this country, is to allow the sewage to flow without 

 treatment into the nearest stream, lake or harbor. This is very 

 successful as long as the quantity is not excessive, the dilution 

 great, and the receiving water is not used by other communities 

 for drinking and culinary purposes. Where this method is used 

 the dilution should be great. The Chicago drainage canal was 

 designed on the basis of 3.3 cubic feet per second for 1000 people. 

 The efficiency of purification, however, varies with the nature of 

 the sewage. The presence of trade wastes, especially those of an 

 oily nature, which float on the surface, may form scums which 

 interfere with the absorption of oxygen from the air. Rapidly 



