THE SEWAGE DISPOSAL PROBLEM 181 



(a) Excretory substances — i.e. solid excreta and urine. 



(b) Household waste — washings, etc. 



(c) Manufacturing waste products of all kinds. 



(d) Rain- and storm-water. 



(e) Grit, sand, gravel, etc. 



Bacterially, sewage should be an ideal nidus for organic life, 

 owing to the large amount of organic matter present. There 

 is but one reason against this, and that is the struggle for 

 existence, which must be exceptionally keen. The source of 

 the organisms is principally the dejecta, but the air and extra- 

 neous substances also contribute. Various figures have been 

 given by observers as to the number of organisms present 

 per c.c. of sewage — e.g. Laws and Andrewes found that London 

 crude sewage contained from 2,781,690 to 11,216,666 micro- 

 organisms per c.c. Not only are these numbers incredibly large, 

 but in addition there is an extensive representation of species, 

 including saprophytes and parasites, pathogenic and non- 

 pathogenic. "Just as the superficial layers of the earth contain 

 economic organisms whose role it is to complete the cycle of 

 nature, removing the dead remains of animals and plants and 

 assimilating them in such a way as to add to the fertility of the 

 soil, and so 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 organic compounds into their simple elements and so 

 produce effluents free from putrescible matter. 



For all practical purposes these organisms are divisible into 

 two main groups : first, the " breakers down," the liquefying or 

 anaerobic organisms, as they are called ; they are most active 

 in the absence of air and light ; and second, the " builders up," 

 or aerobic organisms, whose activity is greatest when the con- 

 ditions permit of an abundance of light and air. No strict line 

 of demarcation can be drawn as to where one group ends and 

 the other begins. It is a complete co-operation shared in by 

 a variety of organisms roughly classified into the above two 

 groups. 



The chief valuable ingredients of sewage are the phosphates, 

 salts of potash, and the different forms of combined nitrogen. 

 The money value of these constituents has been calculated by 

 various authorities, but such theoretical calculations are pro- 

 bably very far from representing the real values available for 



