98 



SEWAGE AND ITS PURIFICATION 



The further important change necessary for the complete 

 destruction of the organic matter involves the essential that 

 free or available oxygen, either from the air or oxidized com- 

 pounds, shall be present. The following table shows the weight 

 required to oxidize some typical organic compounds completely 

 to their final stable products : 



Fatty acids require a higher amount of oxygen. Thus, a 

 molecule of stearic acid, CisH3(302 = 284, requires 832 of O, or 

 nearly three times its weight. But most of these are insoluble, 'j 

 and none of them are putrescent or dangerous. 



The action which takes place in filter-beds and in rivers, 

 as well as beneath the surface of sewage farms, is sometimes^ 

 referred simply to " nitrification " ; but since organic carbon is 

 oxidized along with the nitrogen in these processes, the more 

 general term '' oxidation " should be employed as indicative of 

 both the carbon and nitrogen purification. 



The experiments of the Massachusetts Board of Health and 

 of the London County Council at Barking have been directed 

 almost entirely to the second and final stages in sewage treat- 

 ment. The Barking i-acre filter-bed only dealt with sewage 

 which had been chemically precipitated. It is also important 

 to recollect that the anaerobic or hydrolytic change takes place 

 very rapidly under favourable conditions, and that it is not 

 unusual to find, especially in towns in which the sewers are 

 old and tortuous, a crude sewage in which the preliminary dis- 

 integrating changes have taken place to a very considerable 

 extent. 



Anaerobic fermentation is called by the Germans true putre- 

 faction (Faulniss), while aerobic is termed mouldering {Verwe- 



