The Foam Separation Process:
A Model for Waste Treatment Application:
ROBERT B. GRIEVES, Associate Professor of Civil and Chemical Engineering
DIBAKARBHATTACHARYYA, Research Engineer
Department of Civil Engineering
Illinois Institute of Technology,
Chicago, Illinois
INTRODUCTION
Continuous foam separation has a great deal of potential as a sanitary engineering separation technique; it has been used successfully in a number or waste
treatment applications. Rubin (1,2) and Eldib (3) have reported on the foam
separation of alkyl benzene sulfonate and of organics analyzed as chemical oxygen
demand from secondary sewage effluents. Tertiary treatment by foaming provides a method for the concentration of organics which are difficult to decompose
biologically; the concentrated foam may then be recycled and combined with the
influent to the secondary process. The removal of radioactive metal ions from
waste streams by the use of foaming agents has been investigated by Schoen
(4,5), Schnepf (6), and Schonfeld (7). These applications involve rather dilute
solutions; in addition, the process may be applied to wastes containing higher
concentrations of surface-active agents, including the direct treatment of laundry
wastes. Grieves and Wood (8) have proposed foaming for the removal of specific
surfactants from industrial wastes, in particular from refining and petrochemical
wastes.   Eldib (9) has successfully foam separated phenol from refining wastes.
Foam separation employs the tendency of unsymmetrical organic molecules
to accumulate at an interface between a gas phase and the aqueous solution phase.
Foam provides the means for the efficient generation and collection of gas-liquid
interfaces. Several excellent reviews of the process have been compiled (9,10,
11). A schematic diagram of a foaming column is presented in Figure 1. The
process is continuous with a steady liquid feed of rate, L ml/min and composition,
xLmg/ml and with a gas rate, G ml/min. The steady rates of the effluent streams
are F (collapsed foam) and B (bulk), of composition yc and xg, respectively. A
list is presented of the significant independent variables controlling the process,
and wherever possible, the corresponding designations are made on the diagram.
The dependent variables, xn and F, are to be minimized for maximum efficiency:
it is desirable to reduce the concentration of surfactant in the bulk stream to a
minimum, at the same time providing a concentrated foam stream of low flow
rate. Once the response of xg andF to changes in the independent variables is determined, yp and B may be calculated from the overall and component material
balances.
Although foam separation has been the subject of considerable study, little
effort has peen devoted to understanding the fundamentals of the continuous,
steady-state process, particularly including a quantization of the effect of each of
the numerous independent variables. Some preliminary work has been done by
Grieves, et al. (12,13,14,15) on the influence of Xl« L, G, t, Hp, and Hg upon
the foaming of aqueous surfactant solutions; however, most of the conclusions
have been qualitative. The overall objective of the present study is the development of a model describing the foam separation process: starting with basic
965 -