Tracer Studies in
Circular Sedimentation Basins
K. L. MURPHY, Assistant Professor
Department of Civil Engineering
McMaster University
Hamilton, Ontario
Although the use of sedimentation basins for the separation of solids and
liquids may be traced back to 260 B.C., one of the first attempts at a theoretical analysis of the process was by Seddon in 1889 (1). Subsequently, many publications, including those of Hazen (2), Camp (3,4), and Fitch (5,6), have added
to the theoreticalknowledge of the process. However, most of the hypotheses developed have been based on certain simplifying assumptions.
One of the most universal of these assumptions has been the concept of a uni -
form velocity distribution within a basin. Unfortunately, a completely uniform
velocity distribution, in both magnitude and direction, is seldom, if ever, encountered. Fluid viscosity, density differentials, uneven fluid distribution and
the momentum of the fluid tend to produce deviations from the "ideal." Anderson (7) found bottom currents in final settling basins following the activated sludge
process. He stated the velocity to be approximately seven fpm, but this would
be affected by inlet velocity and basin proportions. By changing the effective
depth, through varying the depth of the sludge blanket, he reported that the velocity varied inversely with the depth of flow. Gould (9,10) and Kin (11) verified the existence of these currents. Although Anderson believed these currents
were confined to final basins, Rohlich (12), Townend and Wilkinson (13) and Burgess, et al (14,15,16,17,18), found similar currents existed where density differentials caused by high solids concentrations were not present. Apparently, poor
inlet conditions produced this non-uniform velocity distribution. Variations in
temperature, likewise, have caused short-circuiting. A three or four C differential has proved sufficient to completely alter the flow pattern in the basin (Lip-
tak(19), Ingersoll (20).)
Such deviations from assumed conditions influence basin performance and
cast doubt on the validity of the design criteria employed. Escritt (21) in 1946
emphasized this point in his statement that the details of design were important
factors in solids removal than were the actual basin capacities.
The extent to which non-uniformity of velocity, short-circuiting, or mixing
exists within a basin may be measured by dispersion-testing techniques. One of
the early reports concerning the use of chemical tracers was the determination of
the residence time of a trickling filter at the Lawrence Experiment Station (22).
The tracer, sodium chloride, was applied to the influent at a known, constant
rate and the time of residence was taken to be equal to the time interval between
the start of dosing and the attainment of the influent concentration of tracer in
the effluent. Clifford (23,24,25) used a momentary dosage technique and defined the residence time in a trickling filter as being equal to the interval between the time of dosing the influent and the time to the center of gravity of the
area under the concentration curve of the tracer in the effluent. He demonstrated
the correlation of both methods, but observed that the interpretation used at the
Lawrence Experiment Station actually measured the time of the slowest particle.
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