A Comparative Study of Imhoff Tanks 
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fied, according to the manner of disposal of the sludge or settled 
solid matter. If the sludge remains in the bottom of the tanks 
until partly liquiefied by bacterial action, the tank becomes a 
septic tank. 
H. W. Clark, at the Lawrence Experiment Station, under the 
direction of the Massachusetts State Board of Health, in 1899 
suggested the idea of a separate compartment for the settled out 
portions into which they could be occasionally flushed for further 
digestion and bacterial action, and in accordance with this idea 
a tank was constructed in November, 1899. 
In England, Dr. W. O. Travis and his assistants, acting upon 
the favorable results of the Lawrence experiments, developed a 
type of hydrolytic or septic tank, which they patented and began 
to use in service in 1903. The improvement over the Lawrence 
idea lies in the fact that both compartments are parts of the 
same tank. 
In the work of the Emschergenossenschaft, under the direction 
of Dr. Karl Imhoff and Mr. Chas. Saville, the Travis hydrolytic 
tank was experimented on, beginning in 1906, and modifications 
were soon made, which had for their specific purpose the pre- 
vention of any disturbance of the settled out solids, by the sew- 
age flow through the sedimentation chamber above. Continued 
experiments have been and are being made, looking toward the 
improvement of the tank treatment. 
Briefly, the advantage of the tank lies in the stability of the 
clarified sewage, and the ease with which it can be subjected to 
later purification process; the freedom from objectionable odors; 
the absence of any dissolved oxygen in the digested sludge; the 
ease of manipulation and favorable character of the sludge, as 
compared with the sludge from the ordinary single story septic 
tank. The tank will be found to be more expensive to build, but 
gives evidences of operating results, which are worth the added 
expense. 
The Imhoff tanks are built in three distinct types: (1) the 
radial flow tanks; (2) horizontal flow tanks, with rectangular di- 
gestion chambers, and (3) horizontal flow tanks, with circular 
digestion chambers. This thesis will discuss the comparative 
values of the first two types. 
