Hydraulic Characteristics of
Circular Sedimentation Basins
JAMES R. VILLEMONTE, Professor of Civil Engineering
and
GERARD A. ROHLICH, Professor of Sanitary Engineering
University of Wisconsin
Madison, Wisconsin
INTRODUCTION
A rational approach to the design of settling basins is of comparatively
recent origin, with the result that a majority of the existing structures has
been designed largely from field experience. A review ofthe literature
indicates, also, that most of the present design criteria have been developed
from studies made on rectangular basins. Although many circular basins
have been constructed and are in satisfactory operation, the extent to which
rectangular basin criteria can be applied to circular basins is not fully understood, mainly because of the paucity of data on circular basin characteristics.
In view of these facts, a research project was initiated in September, 1959,
by the Hydraulics and Sanitary Laboratories of the University of Wisconsin,
with support from the NIH.   The main objectives of the program were:
1. To determine the hydraulic characteristics of circular basins, for
several types of inlets and outlets and flow-thru conditions.
2. To compare the hydraulic characteristics of circular and rectangular
basins.
3.
basins.
To investigate the principles of similitude which apply to circular
EXPERIMENTAL APPARATUS
To meet the program objectives it was necessary to construct two sizes
of model basins of one-eighth in. thick thermoplastic transparent material.
The first was six ft in diameter and three ft deep, as illustrated in Figure 1.
It contained an adjustable false bottom which would permit a minimum
depth of 1.0 ft. The second basin was one-fourth geometric scale model of
the first, except that its maximum depth could be made equal to the basin
diameter, instead of one-half the diameter as in the first (See Figure 2). A
flow diagram is given in Figure 3, which also illustrates the various flow
measuring equipment and appurtenances.
Three basic inlets were used, as shown in Figures 4 and 5. Inlet "A"
was a diffusion type with screen and orifice baffles and was located at the
center of the basin with the flow directed vertically downward. Types "B"
and "C" are also located at the center, but bring the influent in at the bottom with an upward flow in a 2. 38 in. diameter riser. Four slots then delivered the flow to a shroud either eight or 11 ins. in diameter. The third
basic type was the Stengel jet as shown in Figure 5. Four two in. diameter
jets were located at the one-quarter points as shown. The concave targets
were spaced 1. 0 ft from the end of the tube for the purpose of distributing the
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