306 IMPROVEMENT OF MAN'S ENVIRONMENT 



near-by shores. Chicago dumps sludge, untreated, into the 

 drainage canal and the Illinois River. Sludge from small commu- 

 nities can be buried or spread on some flat area to dry, but for large 

 cities the best way to dispose of it seems to be by means of sludge 

 tanks, where bacteria are allowed to work on it and decompose it. 



American Museum of Natural History. 

 Protection against typhoid infection. 



The fluid sewage, after the solid matter is taken out, is usually 

 run over filter beds composed of coarse sand, or over trickling 

 filters composed of small stones. In these filters bacteria oxidize 

 the remaining organic matter of the sewage, so that the hquid 

 which flows off is harmless and odorless. StiU another method is 

 to let the sewage flow over sandy land, later using this land for the 

 cultivation of crops. This method is used by many smaU Euro- 

 pean cities. It is possible for use only in rather small cities. 



The Work of the Department of Street Cleaning. — Another city 

 problem is the disposal of refuse and garbage so that it will not be 

 a menace to the health of the citizens. The city streets, when dirty, 

 contain countless millions of germs which have come from decaying 

 material, or from people and animals more or less diseased. In 

 most large cities a department of street cleaning not only cares for 

 the removal of dust from the streets, but also has the removal of 

 garbage, ashes, and other waste as a part of its work. The disposal 



