222 Introduction to the Study of Science 



The disposal of sewage is the most important problem in provid- 

 ing a safe water supply. 



Sewage disposal plants are used by some cities. 



In rural sections a concrete septic tank is the most effective means 

 of disposal for a single household. 



The three main groups of bacteria found in sewage are the aerobic, 

 anaerobic, and pathogenic. 



Typhoid bacilli come from human beings who are typhoid patients 

 or typhoid carriers. 



The effluent from a public or private septic tank may be treated 

 with chemicals to insure absolute safety. 



Storage in still reservoirs increases the safety of water through 

 sedimentation, the activities of beneficial bacteria, and the weakening 

 or destruction of pathogenic germs. 



The presence of algae in a reservoir indicates sources of danger 

 on the watershed. 



Sand filters, such as are used by municipalities, are effective in 

 removing pathogenic germs through the action of beneficial bacteria, 

 and are inexpensive to operate. 



Water for the home may be made safe by boiling. 



REVIEW QUESTIONS 



1. What is the general belief as to the transmission of disease by 

 water? 2. What evidence is there in support of this belief ? 3. What 

 agencies are known to be involved in the transmission of typhoid 

 fever? 4. Give in brief form an account of the method employed in 

 tracing a typhoid epidemic to its source and in bringing the disease 

 under control. 5. What is the lesson from the experience of such 

 cities as Hamburg and Altona, Germany, Pittsburgh and Chicago, 

 U. S. A., in dealing with water-borne disease? 6. What famous 

 scientist was associated with the perfecting of the sand filter ? 7. For 

 what other discoveries is he renowned? 8. What general conclusion 

 can be drawn from the facts presented in the table, page 209, as to 

 the relation of water to health? 



9. Is water safe for drinking merely because it is clear, odorless, 

 tasteless, and cold? Explain your answer. 10. Mention five diseases 

 which may become epidemic through the imperfect or careless dis- 

 posal of human waste matter and sewage. 11. What are the common 

 methods of sewage disposal in urban and rural districts? 12. Why 

 is the septic tank better than the cesspool? 13. Describe the plan 

 and operation of the septic tank. 14. What kind of bacteria does it 



