Foam Fractionation —
The Effect of Salts and Low Molecular
Weight Organics on ABS Removal
ASHIS K. SENGUPTA, Research Associate
W. O. PIPES, Associate Professor of Civil Engineering
The Technological Institute
Northwestern University
Evanston, Illinois
INTRODUCTION
Foam fractionation is a process by which a solution containing a surface active solute is separated into two fractions, the foam fraction which has a higher
concentration of the surface active solute than the original solution and a drain
fraction depleted of surface active material. The method of effecting the separation is to cause a foam to be produced by vigorous aeration and then to separate
the foam from the bulk of the solution by mechanical means. When the foam is
initially produced, the surface active solute is preferentially absorbed on the air
water interfaces of the bubbles and thus the foam is richer in surface active material than the residual bulk solution. If some of the water held by the foam is allowed to drain back into the residual bulk solution, further enrichment of the
foam fraction is obtained.
The foam fractionation process has many potential applications as a method
of purification, including removal of surface active materials from sewage and
industrial wastes. A recent review by Rubin and Gaden (1) presents the theoretical
basis of the process and describes many of its applications. The removal of alkyl
benzene sulfonates (ABS) from and the reduction of chemical oxygen demand
(COD) in sewage effluents have been reported by Rubin and Everett (2) and by
Eldib (3). Grieves and Wood (4) have reported the application of the foam fractionation process to treatment of petrochemical wastes.
It is a common observation that most of the foam on an activated sludge
aeration tank is produced near the effluent end of the tank. The usual explanation
of this phenomenon is that the high concentration of organic matter near the influent end of the tank suppresses foaming but after much of the organic matter has
been removed by the activated sludge organisms, the detergents are released to
produce foam. However, there has not yet been reported quantitative studies by
which this explanation can be evaluated.
It is also usually observed that the foam produced by aeration of sewage effluents is quite stable; so stable, in fact, that it may be carried several hundred
yards by the wind without breaking up. Yet a study of the foam fractionation of
aqueous solutions of pure ABS, the major component of the detergents present in
sewage effluents, produced the observation that ABS and distilled water did not
produce a particularly stable foam (4).
The research reported herein was undertaken with the objective of explaining
the two observations just noted. It was postulated that organic matter in solution
suppressed the production of foam from aqueous solution of ABS and that the concentrations of salts and organic matter in the effluent from biological waste
treatment processes increased the stability of the foam produced.
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