Biological Sewage Disposal 247 



to 15% of nitrates, is practically odorless, and harbors large numbers 

 of nitrogen-fixing bacteria. Such sludge, therefore, serves as a 

 valuable fertilizer. When starting a new tank of activated sludge, 

 it is customary to inoculate the tank with "ripe" sludge from a 

 previous batch. This inoculum provides desirable organisms re- 

 quired to decompose sewage into carbon dioxide and water, and 

 thus cuts down on the length of time required for breakdown of the 

 organic matter. 



If the aerated sludge is not to be used as fertilizer, anaerobic 

 decomposition is permitted to take place, and sewer gas ( methane ) 

 is liberated during this reaction. Since methane is combustible, it 

 can be collected and employed to warm the sludge tanks and speed 

 up microbial activity. Solids remaining after anaerobic decom- 

 position of sludge have little or no value as fertilizer, but they can 

 be burned, be used as fill, or even be employed as soil conditioners 

 in sandy regions. 



DISEASES POTENTIALLY TRANSMISSIBLE THROUGH 

 SEWAGE 



Statements are frequently found in the literature to the effect 

 that pathogenic bacteria do not multiply in sewage. Careful de- 

 teiTninations probably have not been made to support this conten- 

 tion, but it is reasonable to assume that the high biological competi- 

 tion in sewage does not favor the development of pathogens. It 

 has been reported, however, that Salmonella ttjphosa can be 

 isolated from sludge after several weeks of storage. Perhaps these 

 microbes have remained in a dormant state, even though active 

 multiplication of their numbers may not have taken place. 



In some European countries, and particularly in the Far East, 

 a process called sewage farming is commonly practiced. Raw 

 sewage is spread out on the land and crops are grown in these 

 areas, but it is not advisable to raise vegetables on these farms, 

 especially the types of vegetables that are normally eaten raw. 

 Serious outbreaks of enteric diseases have been traced to the con- 

 sumption of small crops harvested from sewage farms and con- 

 sumed without previous cooking. Grain and hay are frequently 



