82 REPORTS ON THE STATE OF SCIENCE.—1918. 
(iv) Length of travel of outfall sewer. 
(v) Rainfall, contour of district, nature of subsoil, and 
character of sewers, &c. 
In industrial areas the character of the sewage is altered to an 
extent dependent on the nature and relative volume of the trade 
wastes admitted to the sewers. 
In certain districts, e.g., in woollen districts (Bradford, Hud- 
dersfield, &c.), or centres of chemical industry (Manchester, Bir- 
mingham, &c.), these are such as to alter very materially the 
character of the sewage, and consequently to vary the problem 
of its purification. 
The main constituents of domestic sewage have been sum- 
marised by Fowler as follows :— 
1. Matters in actual suspension : 
(a) Sedimentary matters, such as silt and sand, &e. 
(b) Floating and finely divided suspended matters 
(paper, rags, faeces, animal and vegetable 
debris, &c.). 
2. Colloidal matters in pseudo solution or emulsion : 
Products of faecal emulsion, soaps and fatty matters, &c. 
3. Matters in true solution: 
Ammonium salts resulting mainly from the hydrolysis 
of urea. Nitrogenous substances—urea, products 
of the decomposition of albumen—peptone like 
bodies, «carbohydrates; mineral — salts—chiefly 
sodium chloride, with phosphates mainly derived 
from urine. 
It will be understood that on account of its complex character 
and the unstable nature of its organic contents it is practically 
impossible to differentiate with anything lke a high degree of 
accuracy the various states of the organic matter present in 
sewage which under physical and biological action is subject 
to transition from one form to the other. 
O’Shaughnessy has shown that the colloid matter in pseudo 
solution is derived mainly from the faecal matter present, and 
that although there is a limit to the amount that can be taken up, 
the actual quantity (as measured by the 4 hours’ oxygen absorp- 
tion test) varies with the volume and character of the diluting 
water, the time of contact and the amount of agitation. 
it is thus evident that the amount of organic ‘‘ hydrosols ”’ 
varies very considerably in different sewages. For an average 
domestic sewage it may be taken that roughly one-half of the 
soluble organic matter is present in pseudo solution. 
Sewage 1s such a heterogeneous mixture of complex and varying 
character that apart from such determinations as chloride con- 
tent, ammonium salts, organic nitrogen, total suspended and 
dissolved (true and pseudo solution) solids, its chemical examina- 
tion is usually confined to certain empirical tests such as 
‘“‘albuminoid ammonia ’”’ resulting from the distillation of the 
sewage with an alkaline solution of permanganate of potash 
after removal of the ammonium salts, which is taken as a measure 
se Te. ee 
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