A Comparative Study of Imhoff Tanks 
345 
the invert be below ground-water level. This sewage, there- 
fore, will be almost entirely domestic in composition, and will 
be very much diluted at first. Authorities differ in the amount of 
sewage per capita and the variations in it. 
Amount of sewage. In the Emscher district the following val- 
ues obtain: Recklinghausen has a daily flow of 83 gallons, Essen- 
North about 90, and two smaller towns 125 to 145 gallons per 
capita daily. Kinnicutt, Winslow and Pratt, in statistics of 
English towns, show values ranging from 35 to 95, and state that 
the average for English towns is 49.1 gallons per capita, for large 
towns about 100. In the Emscher District the maximum hourly 
flow is about one-eighteenth of the daily. Raikes says in English 
practice the minimum is one-half the average and the maximum 
twice the average. 
Ogden says the maximum is 50 per cent more than the mean. 
He advocates also an indirect method, by finding the water con- 
sumption, taking 75 per cent of this for the average, and adding 
100 per cent to the latter for a maximum. Fuller, in tables 
from Massachusetts State Board of Health report for Gardner, 
Massachhsetts, in 1898, and in reports for Columbus, shows that 
the maximum rate is about 125 per cent of the average. 
For the purposes of this design, a flow of 75 gallons per capita 
daily and a maximum flow of one and one-half times the mean 
will be assumed. 
This means, then, a flow of 2500 X 75 gallons or 187,500 gal- 
lons per day, or an average hourly flow of 7810 gallons. Assum- 
ing a maximum flow of one and one-half times the mean, we 
would have to provide for a maximum hourly flow of 11,715 
gallons. 
Detention period. After obtaining the quantity of sewage to 
be treated, the next factor in the design is the determination of 
the best period of detention or the flowing through time. 
Opinion and practice differ in regard to what may properly be 
called solids capable of removal by sedimentation. The funda- 
mental proposition of sedimentation is that every solid particle 
will settle in water at a velocity depending upon its size and 
weight and upon the viscosity of water. It is also found^ to de- 
pend upon the area of the bottom surface exposed to receive 
2 Hazen’s Transactions of A. S. C. E., voL liii, p. 45. 
