PAPERS ON CHEMISTRY AND PHYSICS 149 



I want to invite your attention, first, to the raw material 

 and the final product which must be produced; second, 

 to typical factory layouts in which the process may be 

 carried out ; and third, to some of the chemical reactions 

 which take place. 



1. Raw materials. A general idea of the raw mater- 

 ials may be gained from the statement that the amount of 

 sewage is very close to the cities' total water consump- 

 tion. This ranges from 75 to 150 gallons in American 

 cities per capita per day. In Chicago, due to the enor- 

 mous waste of water, the consumption is over 300 gallons 

 per capita per day. When the wastes from one person, 

 even including his share of factory and trade wastes, are 

 diluted with such a large volume of water it is apparent 

 that the amount of organic matter per gallon of sewage is 

 comparatively small. And yet it is exceedingly objec- 

 tionable. 



Sewage is most concentrated and the flow is greatest 

 during the day. It drops off both in concentration and 

 flow, reaching a minimum about midnight to 1 :00 or 2 :00 

 o'clock A. M. In addition to the hourly variation in con- 

 centration and flow, sewage also varies with the days of 

 the week and the seasons of the year. In fact, so great 

 is the variation in sewage that it is impossible to make 

 a sufficiently reliable laboratory analysis of it. Where 

 any considerable sewage treatment work is contemplated 

 it is necessary to erect an experimental plant to try out 

 various methods on a sufficient scale to allow for the 

 effect of the numerous variables. 



(2) Typical factory layouts. Although each plant 

 must be designed to handle the particular local problem, 

 there are certain steps that are usually made use of in 

 purifying sewage. Time will permit us to mention only 

 the two most important methods. 



The new plants being built at Decatur and Urbana- 

 Champaign may be taken as typical Imhoff tank and 

 sprinkling filter installations. The sewage flows first 

 through bar screens which remove larger debris of more 

 than 2 to 3 inches in size. Xext it passes through grit 

 chambers in which large gravel, cinders, and grit parti- 

 cles settle out. Xext it may be passed through some sort 



