152 BACTERIAL TREATMENT OF SEWAGE 



similar physical conditions.* Moreover, the sewage itself is con- 

 stantly undergoing rapid changes owing to fermentation, and the 

 competition of micro-organisms and the effect of their products. It 

 is clear that they are the chief agents in setting up fermentative and 

 putrefactive changes, for if sewage be placed in hermetically sealed 

 flasks and sterilised by heat it will be found that these changes do 

 not occur. Hence it will be at once apparent that no exact or hard- 

 and-fast formula can be laid down. Eespecting the chemical con- 

 dition, with which we have but little to do here, we may shortly say 

 that the chief characteristic of sewage is its enormous amount of 

 contained organic matter (yielding saline and albuminoid ammonia, 

 etc.) in suspension or in solution. But there are in addition various 

 inorganic substances, and hence it is customary to subdivide the 

 chemical constituents into (a) organic matter in suspension ; excreta, 

 etc. ; (b) organic matter in solution ; (c) inorganic matter in suspen- 

 sion, such as sand, grit, street and road washings, gravel, etc. ; and 

 (d) inorganic matter in solution, which is not great in amount, 

 but includes phosphates, one of the favouring agencies of sewage 

 fungus. "We may summarily classify the constituents of sewage as 

 follows : 



(a) IZxcretory substances, composed of solid excreta and urine. 

 The former consist of nitrogenous partly-digested matter, together 

 with vegetable non-nitrogenous residues of food. The former are 

 easily broken . down ; but the latter are only gradually attacked 

 (chiefly by the anaerobic bacteria), and broken down into soluble 

 compounds foetidly odorous, and into small black masses, which 

 slowly deposit as black sludge. 



(b) Household waste, solid substances, washings, etc. 



(c) Rain and storm water of varying amount, according to season. 



(d) Grit, gravel, sand, etc. 



(e) Manufacturing waste products in certain localities. 



Turning to the bacterial content, it will at once occur to us that 

 such a large quantity of organic matter as sewage contains, and in 

 which decomposition is constantly taking place, will afford an almost 

 ideal nidus for micro-organic life. There is, indeed, but one reason 

 why such a medium is not absolutely ideal from the microbe's point 

 of view, and that reason is that in sewage the vast numbers of 

 bacteria present make the struggle for existence exceptionally keen. 

 The source of the organisms is most largely the organic dejecta chiefly 

 constituting the sewage, but there are in addition the organisms of 

 the air and extraneous fluids and substances found in sewage. The 

 result of Jordan's f investigations into sewage gave an average of 

 708,000 living bacteria per c.c., his highest result being 3,963,000 



* Analyst, 199, xxiii., 1898. 



f Report of State Board of Health, Massachusetts, 1890. 



