364 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



kitchens, laundries, and bathrooms of residences, hotels, hospitals, 

 schools, and business establishments. This liquid is quite frequently- 

 augmented by the liquid wastes, usually called industrial wastes, from 

 various industries, such as dairies, laundries, slaughterhouses, and 

 food-processing plants. 



The character and the strength of sewage will generally depend 

 upon the water consumption and the amount of industrial wastes 

 present. Where a city uses a large quantity of water, it often dis- 

 charges it into the sewers and the strength of the sewage is diminished. 

 On the other hand, if large quantities of industrial wastes are disposed 

 of into the sewers, the strength of the sewage is usually increased. 



The type of sewerage system in a city will affect the character of 

 the sewage. There are two types of sewerage systems in general use : 

 the separate system and the combined system. A separate system con- 

 sists of two distinct groups of sewers, one called sanitary sewers and 

 the other called storm sewers or storm drains. Sanitary sewers re- 

 ceive the discharge from bathrooms, kitchens, and laundries in resi- 

 dences and business establishments and the industrial wastes from 

 food-processing plants, manufacturing plants, etc. This discharge, 

 called sewage, may flow either into a watercourse without treatment 

 or to a sewage-treatment plant. The storm sewers or storm drains 

 receive the storm flow from streets, sidewalks, roofs, lawns, and unde- 

 veloped areas. The discharge from storm drains generally flows 

 untreated into nearby watercourses. 



The second system of sewers, which is in more general use, at least 

 in medium-size and large cities, is the combined system of sewers. This 

 system receives both the sewage from dwellings, industrial plants, and 

 other sources, and the rain water and surface discharge from high- 

 ways, roofs, and similar areas. Where the sewage from these two types 

 of sewerage systems has to be treated, some differences in the design 

 of the sewage-treatment works are necessary. 



Sewage has an appearance not unlike that of dishwater, in which is 

 suspended a wide diversity of materials such as fruit skins, pieces of 

 paper, match sticks, and fecal matter. One useful measure of the 

 strength of sewage is the amount of suspended solids in it. The quan- 

 tity of suspended matter in the sewage from cities in the United States 

 will generally average from 0.02 to 0.03 percent by weight and the 

 quantity of water will exceed 99.9 percent. Although the amount of 

 suspended solids when evaluated on a percentage basis is small, the 

 total daily amount from a large city can be considerable. For ex- 

 ample, the suspended solids in the sewage from Washington, D. C, 

 amounts to about 113 tons a day. Sewage also contains organic and 

 inorganic solids in solution, together with vast numbers of viruses and 

 bacteria. A useful measure of the strength of sewage, called the bio- 



