OUTLOOK Foam Separation Foaming by intention There seems little doubt that foam separation processes for the removal of organic contaminants will become an essential part of any total waste water pollution abatement system
Lately, much attention has been focused on several new methods of removing complex and continually varying mixtures of organic contaminants introduced into municipal waste streams by foam-forming detergents. One response to the challenge of detergent removal has been the development of a foam separation process. The process has demonstrated inherent simplicity and, hence, implicitly low operating costs. Also, its use effects appreciable flotation removal of suspended solids. Conventional biological sewage treatment processes are unable to decompose hurd detergents such as alkyl benzene sulfonate (ABS). So, until the development of a soft or completely biodegradable detergent, foam removal from water systems was an essential but difficult task. One system that involves the physical separation and removal of foam by deliberate foaming of waste waters-foam separation-appears to offer the best method of removing the hard detergents or other foamers from waste streams. In the foam separation process, a sparger producing small bubbles disperses a gas, usually air, in a tank containing the waste liquid. The gas bubbles rise through the liquid and adsorb surface-active solutes and collect suspended solids. As the bubbles accumulate at the liquid surface, they form a foam which is then forced out of the foamer and collapsed to yield a waste concentrate. General foam separator types
There are two general types of foam separators: column and trough. In the column type, feed liquor is intro-
116 Environmental Science and Technology
duced at some point along the length of a vertical column and allowed to flow downward through the column and discharge at the bottom. Gas spargers, arranged near the bottom of the column, assure effective dispersion of the gas bubbles, which rise countercurrent to the downward liquid flow. The foamliquid boundary is maintained at some position above the feed point in the column. The foam, generated at this boundary, is carried upward by the airstream through a foam delivery tube into a mechanical foam breaker, and, finally, into the foamate collector. To use the foaming principle under conditions of such large volumes of flow as are encountered in treatment of municipal waste streams, employment of trough-type foamers has been found to be particularly effective. In a troughtype foamer the waste stream flows horizontally through a covered trough. Air bubbles emerging from the spargers along the bottom of the trough generate a foam, as in the vertical column, at the foam liquid boundary. The foam phase, which is maintained in the space between the liquid surface and the trough cover, moves to a discharge opening by the drag of the air stream as it leaves the enclosure. A vertical baffle holds the liquid back while the foam spills over into a chamber from which the foamate is discharged. Material balance
To initiate a system for establishing the design parameters to effect the removal of surface-active materials, a theoretical material balance for a single-solute system may be made, assuming the following conditions:
complete mixing of liquid in the foamer, sufficient depth of liquid to reach maximum solute adsorption at the gasliquid interface, constant liquid density, no bubble rupture in the foam phase, and negligible volume of the liquid layer containing the surface excess of solute. The material balance equation is:
where CP and CBare feed and bottoms product concentrations in mg./l.; G is volumetric gas rate in l./min.; F is liquid feed rate in mg./l,; r B is solute surface excess corresponding to CB in mg./sq.cm.; and S is specific surface of bubbles in foam phase in sq. cm./cc,. To extend the use of this equation to multicomponent systems, it is necessary to sum over all components. Such an extension requires complete surface excess relationships for all components and knowledge of any interactions among components with respect to surface excess. However, because the materials found in waste streams are similar in type, workers in this field generally assume that they exhibit, as a group, a surface excess directly dependent on their equilibrium solution concentration CB-'BS. To a n unknown extent, the surface excess is inversely dependent on the concentration of other surface-active organics that tend to displace ABS from the surface. Neglecting this effect of other organics, a first and rough approximation for B.'Bs at low ABS concentration involved is klCB-'Bs. Substituting this relationship in the previous equation gives:
The specific bubble surface, S, is affected by the sparger type, possibly by the gas rate through the sparger, and by many other design and operating variables, including the properties of the solutions. Gas-liquid interface
While adsorption can take place at many different types of interfaces, only adsorption at gas-liquid interfaces is of interest in foam separation. In quantitative descriptions of this adsorption, the most general and popular relationship is the Gibbs adsorption isotherm. Other equations, applicable under special conditions, as well as simplified versions of the Gibbs relationship have also been developed. The general foam fractionation method of removal of organic compounds from waste water has proved to be effective. At volume flow ratios of air to liquid feed (G/F) 2 3, the chemical oxygen demand is reduced 25 and ABS concentrations are reduced 5070%. Specifically, the use of 1.5 liters of air per milligram of ABS in secondary effluent may, in a broad sense, be expected to produce a residual ABS concentration of 0.4 p.p.m. This 0.4 p.p.m. concentration is significant in that 0.50 p.p.m. has been set in USPHS standards as the maximum allowable ABS con-
centration in water used for drinking purposes. Other studies (Grieves, Crondall, and Wood-1964) have shown that the efficiency of the system is affected by the position at which the feed is introduced into the system. Optimum removal of ABS comes when the feed stream is introduced at the midpoint of the column. Gassett, Sproul, and Atkin (1965) have shown that ABS removal increases with an increase of pH. But as temperature increases, ABS removal decreases. Contaminant removals from municipal waste waters by foaming can be improved by adding selected cationic polyelectrolytes to the feed. On the basis of investigations comparing the effects of different kinds of particulates on foam separation of surfactants, Dr. Robert B. Grieves at the Illinois Institute of Technology shows that the presence of particulates gives substantial increases in foam volumes with tin(1V) oxide and ferric oxide (C &EN, Sept. 19, 1966, page 22). Pseudomonas jluorescens, kaolin, and potassium dichromate, on the other hand, give substantial decreases. The two oxides are hydrophobic colloids, P. fluorescens is hydrophilic, and dichromate particles are probably strongly hydrated. The manner of adsorption (chemical or
physical) at specific sites (as in the case of bacteria) and the resulting orientation of the surfactant are apparently the key factors in foam stability and drainage. The success of the foam separation process, in general, depends on the possibility of making a material foam; it does not depend on detergent concentration. The degree of foamability required for effective foam separation is of a low order of magnitude. Effluents that do not cause any foaming problems at waste treatment plants can be treated by the foaming process, and effective contaminant removal still can be obtained. There is only a very small amount of foam separation data available for effluents containing predominately linear alkylate sulfonate-type (soft) detergents. Tests, however, indicate that such detergents have sufficient foamability to permit at least some degree of foam separation. According to Brunner and Stephen (1965), in one sample where straight-chain detergents were introduced on a test basis, a methylene blue detergent concentration of only 0.6 mg./l. was detected. When this sample was processed through a foam separation apparatus and the detergent concentration was reduced to 0.4 mg./l. the soluble organic content was reduced only slightly-by 6 %. Qualitative ob-
N e w technology effects f o a m removal f r o m water
FOAM
w
FEED
F
COLLAPSED FOAM
Y
LfQUlD
Column foam fractionator
I 4I
FOAMEDPRODUCT
:ER
rrough foam fractionatov
PRODUCT
COLLPSED FOAM
servations of the foam and its stability suggest that significant foam fractionation removal of soft detergents can be achieved.
QUOTE . . .
Process costs
The cost of a foam separation process depends on all of the aforementioned variables-plus the extent of demands for purity of the final product. Brunner and Stephen have calculated foam separation costs, with the foamate disposal not included, to be as follows: ikfillions of gallons per. day 1 10 100
Cents per thousand gallons
3.6 1.9 1.4
These estimates are based on the modular design for the large-capacity units. The technique for removing soluble organic compounds from waste water streams has been sufficiently developed that such methods are now technically and economically feasible. Already pilot plants have been built that effectively remove this class of compounds. There seems little doubt that the use of foam separation processes in the treatment of waste water will become a necessary part of any total waste water pollution abatement system. In the establishment of such a system for water treatment (using a systems approach to the problem), the foam separation process will depend to a large measure on the source of the waste water being handled, the concentrations and kinds of organic materials, and whether the feed liquor (effluent) is primary or secondary. Decisions will have to be made, in analyzing the system for employing foam separation, regarding the advisability of controlling feed p H and temperature-or on the need to add polyelectrolytes to the feed. SELECTED REFERENCES
Brunner, C . A,, and Stephen, D. G., Ind. Eng. Chem. 57 ( 5 ) , 40-8 (1965). Gassett, R. B.. Sproul, 0. J., and Atkin, P. F., Jr., J . Water Pollution Control Federalion 37 (4), 460-70 (1965). Grives, R. B., Crondall, C. J.. and Wood, R. K. (Northwestern University, Evanston. Ill.), Air Water Pollution 8 (8/9), 501-13 ( 1964). Rubin, E., et al., “Contaminant Removal from Sewage Effluents by Foaming,” PHS Pub. 999-WP-5, Cincinnati, Ohio (1963). Summary Report, “Advanced Waste Treatment Research,” PHS Pub. AWTR-14, April 1965. 118
Environmental Science and Technology
ONE E N V I R O N M E N T
. . . We have long been accustomed t o t h e idea t h a t the solution t o almost any physical problem is t o b e found in our technology. This is a state of m i n d t h a t m u s t be broadened because the problem of water pollution control is not only technological. It is also an economic problem directly affecting every industry and every consumer or user of the products of industry and agriculture. It is a social problem affecting t h e development of our cities and t h e patterns and practices of our daily lives. It is a political problem, a legislative problem, a health problem-and n o less importantly, it is an education problem. Many important decisions and choices m u s t b e made before we have finally solved t h e problems of water pollution control. However, neither t h e number of the problems nor the complexity of the problems should b e used t o postpone action where t h e need for action is both obvious and immediate.. . . . . . Technology is not one of t h e obstacles t o water pollution control. Technological solutions already are available for every major pollution problem at a price. B u t can our pollution problems b e solved plant by plant and municipality by municipality? I don’t believe so. The fact of t h e matter is that water pollution control m u s t begin with the concept of a n entire stream or a river. To think i n smaller terms is t o lose sight of t h e complexity and magnitude of t h e problem. If we look for solutions through technology alone, we are failing t o recognize that t h e water pollution control problem is as m u c h a philosophical problem as a technical problem. If I were t o categorize t h e main challenges before us, I would name three as t h e most important [economics, knowledge of the scope of pollution problems, and awareness of conservation practices and goals]. Not t h e least of these is t h e matter of economics. To insist upon clean water is meaningless until we decide how m u c h cleanliness we want and are willing t o pay for. Cleaning u p is a program t h a t will cost many billions of dollars over t h e next 10 years and each of us will pay his share of t h e expense-in addition to city, state, or federal taxes, higher water prices, and higher prices for t h e products of industry and agriculture.. . . . . . From this point on, there is one environment. It is n o longer city, rural, a n d wilderness. We cannot discuss water pollution, air pollution, land pollution, or the use a n d misuse of our resources, outside t h e bigger context of our total land environment.. . . Edgar G. Paulson M a n a er Hall Laboratories Division Ealgon Corp,, before t h e Soil C o k e r v a t i o n Society of America, Albuquerque, N.M., Aug. 17, 1966
