Composition of Sewage 111 



substances in suspension and in solution. At night, the 

 volume of sewage is diminished and its composition like- 

 wise modified. Different sewers in the same town may, 

 likewise, show well-marked differences in the character 

 of their sewage as affected by the density of population 

 and its mode of life. Tlje quantity of sewage produced 

 in any town or city is affected by the water- supply, as 

 well as by the nature and extent of their industries. 



From the sanitary standpoint, the composition of 

 sewage is to be considered both chemically and bac- 

 teriologically. The solid matter held in suspension and 

 solution is potential food for bacteria and other micro- 

 organisms. It may undergo putrefaction and become 

 offensive to sight and smell and may lead to the destruc- 

 tion of large quantities of fish in inland waters. From 

 the bacteriological standpoint, it may act as a carrier 

 of disease germs, and may be the cause of serious out- 

 breaks of disease, even when diluted with a large volume 

 of bacteriologically pure water. 



The chemical examination of sewage serves to indi- 

 cate the extent of possible pollution when it is added to 

 a given volume of surface water. It serves still another 

 purpose when employed in connection with the various 

 methods of sewage-purification. It becomes a means, 

 then, for the gauging of the efficiency of these methods. 

 The analyses performed at the different stages of every 

 process show clearly to what extent the objectionable 

 materials are removed or rendered harmless. This 

 procedure finds an analogy in the bacteriological ex- 

 aminations in which the numbers and kinds of certain 

 groups of bacteria are determined. 



