24 HUMAN AND ANIMAL WASTES 



SEWAGE 



Disposal of sewage and other home wastes is one of the important 

 sanitary problems of men living in industrial and residential centers. 

 Haphazard methods of disposing of sewage not only lead to conditions 

 most unpleasant to human habitation but they are dangerous from the 

 standpoint of infectious diseases. 



Sewage abounds in microorganisms that originate not only from hu- 

 man excreta but also from other household and industrial wastes. The 

 various saprophytic bacteria present in sewage rapidly attack the or- 

 ganic constituents and bring about their gradual mineralization. The 

 destructive action of saprophytic organisms greatly reduces the number 

 of pathogens (342, 343). Activated sludge, for example, has been 

 shown (882) to possess a definite and consistent bactericidal action 

 against the colon bacteria. In addition to antagonistic organisms, active 

 bacteriophages against nearly all types of intestinal bacteria are present 

 in sewage. The destruction of pathogens by bacteriolysis thus readily 

 finds a place in the activated-sludge method of sewage purification. 



Dissolved oxygen is generally present when sewage is diluted with 

 water. As the destruction of the organic matter proceeds rapidly, the 

 oxygen becomes depleted, so that none is left after a few hours. The 

 predominant bacterial flora of the water may then become anaerobic, 

 with the result that the chemical processes of decomposition are com- 

 pletely changed J hydrogen sulfide, mercaptans, and other foul -smell- 

 ing substances are then formed. This is accompanied by a typical 

 anaerobic breakdown of carbohydrates, leading to the formation of vari- 

 ous organic acids, carbon dioxide, hydrogen, and methane. The nitro- 

 gen in the protein and urea is transformed to ammonia and various 

 amines. When sewage is aerated, the anaerobic processes gradually give 

 way to aerobic processes, as the oxygen diffuses into the liquids or as the 

 sewage is diluted with water containing dissolved oxygen. 



When sewage is freed from solids by sedimentation before discharge, 

 or when it is aerated sufficiently to maintain the concentration of dis- 

 solved oxygen, decomposition proceeds rapidly without the production 

 of the bad odors usually associated with the anaerobic breakdown. The 

 destruction of the pathogenic bacteria results largely through the ac- 



