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Theodore Sedgwick Johnson 
many in 1906, its use in America is comparatively new, there 
probably being to date no more than one score of installations. 
Experiments at Philadelphia and Chicago have proved favorable, 
and at least one large city — xltlanta, Georgia — has installed them 
as adjuncts to disposal plants. 
This thesis attempts an economic design of two such tanks as 
applied to the disposal plant of a small community, and makes 
comparisons of rectangular and circular tank, with special re- 
gard to costs and clarification efficiency. 
Historical Sketch of Development of Clarification 
Methods 
At this point a brief digression into the history of the Imhoff 
tank may be of value. 
In 1810 the water-closet was reinvented and its use in the 
cities of the Continent and in England rapidly increased. Until 
1815 the drains of London, as they were called, were not in- 
tended to carry any but surface wastes, and it was not until 
that time that the members of English communities realized the 
need of sanitary precautions and ordered all water-closets to be 
connected to the drains or sewers, as they really began to be. 
This led, of course, in the thickly populated districts of Europe, 
to a fearful state of river pollution, which continued until 1858 
in England, when, as a result of a Sanitary Commission’s report, 
further pollution of the rivers was prevented. In Germany the 
same conditions were generally true, finding their worst condi- 
tion in the Emscher district. This district was placed under the 
control of the Emschergenossenschaft in 1906, though steps had 
been taken to improve the wretched sanitary conditions of the 
valley as early as 1889. 
When river dilution was prohibited, various stages of disposal 
processes were gone through, tending chiefly in the direction of 
purification by infiltration. In any of the early disposal proc- 
esses, serious trouble was caused by the suspended solid material, 
partly material and partly organic in nature. To remove this 
suspended matter, various types of sedimentation processes were 
resorted to. Sedimentation may be either plain or chemical, 
and both types may be further classified as grit chambers and 
true sedimentation tanks. Again, the latter may be re-classi- 
