SEWAGE DISPOSAL 111 



without the production of objectionable odors or without 

 injury to the water life. If the amount of sewage is large 

 in proportion to the water, the organisms will soon exhaust 

 the oxygen, and the decomposition will not be complete. 

 Products having offensive odors and an injurious action on 

 the higher animal forms of the water will be produced. 



The application of sewage to the land increases its fer- 

 tility. On the sewage farms great crops of grasses and 

 roots may be raised. The addition of sewage to a stream 

 will, of course, produce an increase in the number of bac- 

 teria. This will cause an increase in the forms that live 

 on bacteria, such as the protozoa. The greater number of 

 these low animal forms will cause an increase in the crus- 

 tacea that serve as food for fish. Hence the addition of 

 not too large quantities of sewage to a body of water us- 

 ually results in an increase in the number of fish. If these 

 are consumed as food, some portion of the organic matter is 

 again made use of by man, and is not wholly lost. The farm 

 home can make use of this method for the disposal of its 

 sewage if a body of water of some size is available. In 

 case any city draws its water supply from this source, it 

 should not he used for the disposal of household sewage, 

 because of the danger of spreading typhoid fever, as will be 

 explained later. 



If the sewage is allowed to undergo partial or complete 

 decomposition before it is discharged into a body of water, 

 a much greater amount of sewage can be added to the water 

 without injuring it in any way. Since it is not easy to 

 establish conditions so that the decomposition can be car- 

 ried out by aerobic organisms, as in the soil, the larger part 

 of the decomposition is allowed to take place under anaer- 

 obic conditions in large tanks, called septic tanks because 

 the processes are carried on by bacteria. The tanks are so 

 arranged that the sewage flows slowly through them; the 



