158 BACTERIAL TREATMENT OF SEWAGE 



plant life finds nutriment. That the carbon is converted into COj, 

 the hydrogen into water (H^O), and the " lost " nitrogen refixed in 

 the soil, we have already seen. 



Now just as soil contains these Economic Organisms, whose role 

 is to complete the cycle of nature, removing the dead remains of 

 plants and animals, and assimilating them in such a way as to add 

 to the fertility of the soil and recommence the cycle of life, so also 

 in sewage we have all the required organisms normally present, whose 

 business it is to render soluble the solid matters, and to split up the 

 organic compounds into their simple elements, and then as a final 

 stage in the process to oxidise these elements and so produce an 

 effluent free from putrescible matter, but containing nitrates and 

 other mineral substances.* For practical purposes these two main 

 groups of bacteria, the breakers-down and the builders-up, are looked 

 upon as anaerobic or aerobic. The former are active in the absence 

 of air, and their activity effects a decomposition of complex organic 

 matter and allied substances. The aerobes are most active in the 

 presence of oxygen, and part of their business is to convert urea into 

 ammonia and ammonia into nitrate. 



From this brief recital of the functions of many of the sewage 

 bacteria we learn that they have important operations to perform, 

 and that their presence in sewage, even in very large numbers, is 

 not matter for regret, but far otherwise. We see also a remarkable 

 adaptation of those fermentations discovered by Schloesing and 

 Miintz, in 1878, to be of such inestimable economic value in soil. 



We are now in a position to consider the treatment, especially 

 the biological treatment, of sewage. 



The Biological Treatment of Sewage 



Almost from time immemorial there has been adopted one of 

 three great methods of disposal of sewage : — 



1. Disposal without purification. 



2. Mechanical and chemical separations. 



3. Biological methods. 



It may be convenient to add here that the complete purification 

 of sewage involves three processes : — First, the process of clarifica- 

 tion, that is to say, the removal of suspended solid matters ; secondly, 

 an alteration of the chemical constitution of organic putrescible 



* The following have been considered as the general conditions which an effluent 

 ought to fulfil : (a) It must contain practically no solids in suspension ; (6) it must 

 not contain in solution a quantity of organic matter sufBcient to seriously absorb the 

 oxygen from the stream water into which it is discharged ; (c) it must not be liable 

 to putrefaction or secondary decomposition ; {d) it must contain nothing inimical to 

 microbial growth and activity, therefore it must not be treated with strong anti- 

 septics ; (e) it must not contain pathogenic organisms. 



