26 HUMAN AND ANIMAL WASTES 



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- 

 tivities of the saprophytes (772, 980). For the purpose of promoting 

 the development of aerobic bacteria, processes employing the use of 

 intermittent sand filters, broad irrigation, contact beds, trickling filters, 

 and activated sludge are applied. 



The modern methods of sewage purification are based on the long- 

 known fact that the soil is a destroyer of offensive wastes. In early days, 

 in fact, the soil handled all sewage problems. Sewage disposal plants in 

 modern cities are so operated that microorganisms found to be so effi- 

 cient in the soil are able to act under optimum conditions, resulting in 

 rapid purification. Sewage freed from most of its organic constituents 

 can be discharged into a stream and will not deplete the water of its dis- 

 solved oxygen. Chlorine is frequently employed in the final treatment 

 to assure the complete destruction of the pathogens. 



GARBAGE 



The processes involved in the disposal of garbage from the home 

 are similar to those utilized in the disposal of stable manure rather than 

 of sewage. At present, garbage usually is destroyed by burning, which 

 results in great economic waste, or is dumped outside cities, thus creat- 

 ing centers of infection and unpleasant appearance. More logical and 

 less wasteful processes are based upon the principle of composting. Sev- 

 eral of these processes are now utilized in India and China, where eco- 

 nomic pressure is greatest. By proper handling, a product is formed that 

 is free from injurious insects, parasitic worms, and bacteria, and that has 

 conserved all the valuable elements essential for plant growth. 



