CONDITIONS. 
As the population of a city increases, the difficulties of obtaining a 
sufficient water supply which is free from contamination by sewage be- 
comes more and more difficult. It is now a well established fact that 
sewage-contaminated waters are to a considerable extent the cause of 
summer complaint and other bowel troubles, besides the more dangerous 
disease, typhoid fever. ‘The extensive death rate among children is in 
some measure chargeable to impure water. 
There are very few large cities that are able to obtain a ground water 
of satisfactory quality and quantity. We are therefore driven to the use of 
surface water. 
OBJECT OF SEWAGE PURIFICATION. 
Large volumes of sewage are discharged into the White, the Wabash 
and Ohio rivers and their branches, also into Lake Michigan, by the cities 
situated near them. In order to maintain a stream in a condition ap- 
proaching normal purity, methods for the purification of sewage are ap- 
plied, so that the resulting effluent discharged into a stream is purer. This 
purification is obtained by some method of oxidation which will remove 
the putrefactive material or highly organized food on which pathogenic 
bacteria live. 
Sewage purification is a relative matter, and absolute purity of the 
efiuent is practically impossible and generally is unnecessary. The prob- 
lem, then, is to adapt available means to the conditions in order to eco- 
nomically defend the people against water-born diseases. 
Dilution may be considered a process of purification, and therefore 
the larger the volume of pure water available in a stream the lower the 
percentage of purification required, for wherever there is running water 
not already contaminated, oxygen is present and some purification takes 
place; vegetation and sedimentation also assist. 
The old theory that a stream would purify itself in a flow of ten miles 
Was a dangerous one, because it depended distinctly on conditions. 
