506 MAN CONTROLS HIS ENVIRONMENT FOR HEALTH 



as well as the water supply of many of our cities, is becoming a 

 serious problem. 



The best way to avoid the pollution of rivers is by proper sewage 

 disposal, even if this method is expensive. Sewers for large cities 

 are planned so that the dilute sewage is carried to a sewage dis- 

 posal plant, usually situated a short distance outside of the com- 

 munity. Here the solid wastes are screened out, and then the 

 smaller particles are precipitated out. The disposal of the solid 

 material, called sludge, is still a serious problem. In some cities 

 this sludge is dried, treated, and used as fertilizer. The fluid 

 sewage, after the solid matter is taken out, is usually run over 

 filter beds composed of coarse sand. In these filters bacteria 

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

 liquid which flows off is harmless and odorless. But such water 

 is never used until it is first treated with chemicals, such as 

 chlorine, in order to kill any harmful germs that may be left. 



Practical Exercise 33. Report on an up-to-date method of sewage disposal 

 in some city or community. Compare the conditions of this city with those 

 existing in your community. 



The work of the department of street cleaning. Another city 

 problem is the disposal of refuse and garbage. The city streets, 



Culture A was exposed to the air in a well-cleaned and watered street in a residential 

 section of a city. Culture B was exposed to the air in a crowded street in a business section 

 of the same city. How do you account for the differences and results ? 



when dirty, contain countless millions of germs which have come 

 from decaying material or from people and animals more or less 



