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Theodore Sedgwick Johnson 
sediment. Travis used this idea by suspending many plates in 
the sedimentation basin. 
A certain portion, therefore, of the solid content of sewage 
will settle to the bottom of the Imhoff tank in a relatively short 
time, while a larger portion will settle in three hours. After 
that, no appreciable increase in amount of sediment will take 
place, and the amount deposited at the end of six or eight hours 
will be little larger than that found at the end of three hours. 
Dr. Imhoff, in discussing this question, divides solids in sewage 
in four classes: 
1. Settling solids, or those heavier portions which would settle 
out of still sewage in two hours’ time. 
2. Finely divided matter, or matter which would remain as 
residue after filtration through paper. 
3. Colloidal matter, still finer, which would pass the filter 
paper but remain on a dialyzing membrane. 
4. Matter that is in true solution. 
It is only the solids in Class 1 that can be affected by the oper- 
ation of plain sedimentation, though, of course, the length of 
time allowed — ^two hours — or the division line between classes 
one and two, might be uncertain. Dr. Imhoff claims, however 
that to rightly judge the efficiency of sedimentation processes, 
only the matter that comes under Class 1 should be included, as 
those capable of deposition. Any currents or velocity of flow in 
the tank would tend to decrease the amount of matter that 
would settle out of a given sewage, and efficiencies can therefore 
rightly be calculated on that basis. That means, then, that in 
Imhoff tanks a period of detention in excess of two hours is 
unnecessary. In existing plants the detention periods are as 
follows : 
Plant Time 
Atlanta, Georgia 3 hours 
Winters, California 3 hours 
Philadelphia, Pennsylvania (experimental plant) 1 hour 
Philadelphia, Pennsylvania (experimental plant) (deeper tank) 2 hours 
Erie, Pennsylvania 40 mins. 
Recklinghausen, Germany 1 hour 
Essen-North 1 hour 
From the character of the sewage to be treated in this instal- 
lation, an average period of two hours will be assumed. For the 
