282 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



(ii.) SOLUBLE SALTS : 

 Nitrates. 

 Phosphates. 

 Sulphates. 

 Chlorides. 



(iii.) Insoluble residual matters conveniently termed 

 ' Humus.' l 



It is possible to transform sewage into these harmless 

 products by direct oxidation, through the agency of the 

 requisite organisms, in presence of air. 



Thus, if a sample of sewage be shaken in a bottle with an 

 excess of water saturated with air, and allowed to stand a 

 sufficient time, under conditions which allow of an excess of 

 oxygen being always present, it will be gradually transformed, 

 and eventually nothing will be left in the bottle but a solution 

 of the above salts, with some brown particles of ' humus,' 

 and some carbon dioxide in solution. 



Although it can be shown by careful analysis that the 

 sewage suffers a regular sequence of changes, yet at no point 

 are offensive gases evolved under these conditions and 

 neither marsh gas nor hydrogen is produced. 



In the above case the sewage is purified under strictly 

 aerobic conditions. In practice such conditions are met with 

 when sewage is discharged into a stream or body of water, of 

 such a volume that an excess of dissolved oxygen is always 

 present, over that necessary to oxidise the sewage. 



But it is rare to find conditions under which it is possible 

 to deal with sewage in this way, by what may be termed the 

 dilution method. A favourably situated outfall must admit 



1 Strictly speaking the term ' humus ' should be reserved for organic 

 residual matter of special chemical characteristics. For the sake of 

 brevity the term is used here to include organic matters of somewhat 

 indefinite composition which remain undecomposed at the end of the 

 ordinary processes of purification of sewage. They are generally associated 

 with a fair proportion of mineral matter, especially phosphates, and lime, 

 and alumina compounds. 



