Comparison of Flotation and
Sedimentation in Treatment of
Industrial Wastes
A.  A.  KALINSKE  and  R.  R.  EVANS
Director of Research and Development and Manager Waste Treatment
Department, Infilco, Inc.
Tucson, Arizona
The most widely used equipment to affect separation of solids from
liquids or liquids of different densities depend on the force of gravity
to provide the separation. The separation may occur due to settling of
the heavier than water material or it may float due to its buoyancy.
Conventional clarifiers, settling basins, gravity oil separators, etc., are
examples of such equipment.
In order to assist in the speed of removal, many times artificial aids
are used, either to increase the specific gravity so as to hasten settling,
or to decrease the effective specific gravity so as to speed up the flotation
process. This can be accomplished by designing special equipment which
will produce denser and larger precipitates to speed up settling, or by
using various artificial weighting agents. Flotation can be greatly
accelerated by attaching gas bubbles to the particles or droplets of
foreign matter to be removed.
It is the purpose of this paper to analyze and evaluate the applicability and limitations of the process of settling and flotation in the
removal of suspended matter from liquid wastes.
Clarification by Gravity Settling
Extensive literature exists giving the general fluid dynamic theory
relating to the settling of particles in water, and also how this theory can
be applied to the design of various shapes of settling basins. There is
no need to review all this at present. It is known that basically the
sizing of any sedimentation basin will depend on the settling velocity
of the particles being removed. This settling velocity may be that of
the individual particles, or of a suspension composed of the particles,
depending on how concentrated the material is that is being removed,
and the general hydraulic design of the clarification basin (1). In any
case, such settling velocities can be readily determined by simple laboratory tests.
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