ELIEZER RUBlN
water pollutants such as synthetic detergents, Among pesticides, and other petrochemicals which resist "
waste treatment methods and natural stream purification, alkylbenzene sulfonate (ABS) is perhaps the most prevalent. Even though its significance is often overemphasized, it does create a nuisance by foaming in streams and sewers. However, foaming which now creates a nuisance may be put to work as a promising technique for separating ABS from sewage plant effluents. Other dissolved contaminants and suspended solids may be removable as well. In the work described here, concentrations of ABS in sewage plant effluents were reduced below the maximum allowable concentration in drinking water (from 2.0 to 3.2 p.p.m. to 0.3 to 0.5), using volumetric flow ratios of air to effluent of 3 to 7. Moreover, a correlation exists between residual ABS concentration and volume of air available per unit mass of ABS in the effluent. Chemical oxygen demand (COD) in the effluent (averaging 118 p.p.m.) was decreased by 10 to 459;b, and evidence was found that COD removal increases as surfactant concentration in the effluent decreases. However, a minimal surfactant concentration is required to generate foam which provides the physical means for removing COD-bearing materials. Volumetric reductions (flow rate ratios of feed to collapsed foam) up to 1000 were obtained. On the negative side, total solids and chloride concentration were not affected by foaming, nor was con44
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
taminant removal enhanced by addition of surfaceactive and/or surface-inactive materials. The theory underlying this technique is simple. Surface-active solutes such as ABS tend to collect at gas-liquid interfaces, and foaming provides an efficient means of generating and collecting such gasliquid interfaces. Thus, as the foam is enriched with these solut,es, their content in the residual bulk liquid is depleted. The foam can be collected and collapsed to produce a so1ute;rich product. Mathematical relationshipsare available for describing quantitatively this assqrption at a gas-liquid interface (6), the most popular of which is the Gibbs adsorption equation (3,5). Successful separation deperrdp on adsorption characteristics of the system as well as'.+e nature of the foam produced. Stability and drainability are twoimportant foam properties--good drainabilik permits liquid to flow from the lamellae separating individual gas bubbles, when acted on by -gravity and sudtion a t Plateau's borders; stability enablesthe foam to withstand without rupturing ( 5 6 )lamellar thinn'kng.fromdrainage. Because concentration of ABS and other surfactants in sewage effluents is relatively low (I& than lOp.p.m.), foam bubbles have weak and unstable lamellae. Therefore only a small degree of drainage is needed for these bubbles to rupture and deposit their surfactant inventory near the liquid level and thereby establish a concentration gradient in the bulk liquid. If the surfactant deposits faster than it diffuses back into the
+
R I C H A R D E V E R E T T , J R
bulk liquid, it accumulates near the liquid level until its concentration is sufficient to produce a pocket of relatively stable foam. This cycle may be repeated until eventually a thick layer of stable foam appears. Also, dissolved COD-bearing materials can be removed. Two mechanisms are involved: dissolved surface-active organic compounds may be concentrated in the foam by adsorption at the gas-liquid interface; and dissolved surface-inactive organic compounds can be concentrated by combining with the surface-active solutes. Furthennore, suspended solids content can be reduced, but this is attributable to a froth flotation mechanism rather than to foam separation. Some surfaceactive solutes, in addition to concentrating at gasliquid interfaces, also act as flotation collectors and thus remove particulate matter. Experimental Procedure
Secondary sewage effluent samples were obtained regularly from the 26th Ward Sewage Treatment Plant, Brooklyn, N. Y., which processes mixed domestic and industrial wastes. All samples were used on the day of receipt, and their composition varied considerably. High solids and chloride concentrations, caused by back-up of sea water into the sewage system, were encountered from time to time, especially after storms. In preliminary investigations to check feasibility of foaming, batch experiments with filtered effluent were used. About 2.2 liters were passed through Whatman
ABS COD Chloride
volatile solids Total solids
PH
Rnngs
AD.
1.1-4.7 85 -195 102-1884 103-628 4584249 6.9-7.8
2.9 118 547 224 1425 7.3
I:
Carl*fUrnf
ABS Q
COD Total and volatile aolids
Method Methylene blue Volhard KIQ.~O,H&OIAgtS01(catalyst)
blue method is subject to interference. Therefore the graphical data correlations presented can reflect only gross trends. Batch Experiments. Filtered effluents gave an average ABS removal of 86% with a range of 67 to 92%. Average COD removal was 31% withii a range of 3 to 80%. Because ABS has a theoretical COD of about 2 mg. per mg., and an average of 2.4 p.p.m. of ABS or 5 p.p.m. COD was removed, constituents other than ABS must have been removed. That chloride ion concentration and total solids content were not affected markedly is to be expected because most do not exhibit surface activity and therefore are not amenable to removal by foaming. Also, filterable colloidal material which may be removed by a froth flotation mechanism, constitutes a negligible fraction of the total solids. Organics which volatilize at the 103' C. drykg temperature used for total solids determinations are not detectable, but it is conceivable that some of these compounds were removed during foaming. D u e to lack of precision in the volatile solids determination, no real evidence exists to indicate whether organics which volatilize at 600' C. were temoved. Of total solids in the effluent, volatile components averaged 18% with a range of 14 to 22%. Continuous Feed. Steady state conditions in bottoms A B S and COD concentration are reached usually within 1 to 2 residence times (the ratio of air-effluent contacting volume V to volumetric feed flow rate, F). Samples were taken for analysis after 5 to 10 residence times. ABS removal depends primarily on gas-to-feed flow ratio and concentration in the feed as shown in Figures 2 and 3. In Figure 4 which shows general relationship of bottoms ABS concentration us. volume of air available per unit mass of ABS in the feed, 1.5 liters of air per mg. Rimre 2. Efed of gar-fmdJow mk ralro on cowenfrdron of ABS in bottoms. Cmumlrdion of ABS m feed, 2.0 and 3.2 p . p m
Drying.
0
FILTEUED FEED
e UNFILTEUED FEW
Results
Because the etRuent was subject to microbiological action, fresh samples were used for each experiment. Thus, sample composition for each experiment was presumably different. Also, analyses, such as COD, and total and volatile solids are nonspecific, and the methylene
--
AUTHORS Eliezm Rubin is Project Director and Richard
Everett, Jr., a Chemical Enginccr at Radiation Applications, Inc., 36-40 37th St., Long Island City, N.Y. Work performd undcr contract with the Aduanced Waste Treatment &search program, U.S.Public Health Service. 46
INDUSTRIAL A N D ENGINEERING CHEMISTRY
e
0.001 0.0
1
.
1
8
8
8
1
,
8
I
'
6.0 8.0 10.0 6AS/FEED FLOW ME MND, DIMENSIONLESS
2.0
4.0
3
5
of 5.0 p.p.m. of dodecylamine hydrochloride does the percentage of COD removal approach the value obtained by foaming the effluent alone. These experiments relegate foam stability to a position of lessa importance in contaminant removal. They indicate that selection of the additive or combination of additives is more important and can do more than merely provide a physical medium for removal. In a limited number of continuous-feed foaming experiments where surface-inactive and/or surface-active substances were added, COD removal was inferior to or at best comparable to that obtained without additives. However, several important additives remain to be evaluated.
“I
Conclusions
and bulk ABS concentration. Interrelationship of these factors is not simple, however, especially for the unstable foams encountered with secondary effluents. Here also, different foamability of the samples affects results, and more investigation is needed.
Surfactants may be added to aqueous systems to impart surface activity to surface-inactive solutes. This technique is used extensively in foam separation for removing cations from dilute aqueous solutions (7, 8). For low concentrations of an anionic surfactant such as dodecylbenzene sulfonate and low concentrations of metal ions, the foam is enriched with metal ions. This phenomenon is attributed to a combination of the anionic part of the surfactant molecule with the cationic metal ion to form a molecule which has surface activity, and therefore yields to collection at a gas-liquid interface. Surface-active and surface-inactive additives may be employed to improve the froth flotational characteristics of the foaming process. Here frothing agents, collecting agents, and coagulants may prove beneficial. Also, addition of surfactants to increase foam stability and thereby improve contaminant removal from sewage effluents may prove a promising technique. However, some disadvantages are involved: COD and total and volatile solids are necessarily increased. Even though inorganic chemicals such as aluminum sulfate and ferric chloride do not contribute to the COD, they add substantiallyto total solids content. To study effect of additive concentration on COD removal from unfiltered effluent by continuous-feed foaming, dodecylamine hydrochloride, a cationic surfactant was selected (4). Only at a concentration S T R I A L AND ENGINEERING C H E M I S T R Y
Activated sludge sewageplant effluents can be purified, at least partially, by foaming. Akylbenzene sulfonate and other refractory organic compounds, measured by COD, can be removed. For continuous-feed foaming of both filtered and unfiltered secondary effluent, a correlation exists between volume of air available per unit mass of ABS in the feed and the residual ABS concentration. Specifically, 1.5 litem of air per mg. of effluent ABS generally leaves a 0.4-p.p.m. residual concentration. This is significant because according to USPHS 1961 standards, the maximum allowable concentration in drinking water is 0.5 p.p.m. For unfiltered effluent a relationship exists between per cent COD removal and the dimensionless ratio obtained by multiplying the air-to-feed flow rate ratio by the COD-to-ABS concentration ratio in the feed. When the dimensionless ratio is 150, a maximum of about 40% COD removal is obtained. In the limited number of experiments where surface-inactive and/or surface-active substances were added, COD removal was either inferior to or at best comparable to that obtained without additives. Whae volume reduction is defined as the ratio of the feed flowrate to the collapsed-foamflow rate, an average value of 127 was obtained within a range of 4 to 1163. This factor depends on column design and operation. LITERATURE CITED (1) American Public Health h., Inc., ‘‘Standard Mahods for
Examination of Water and Waste Water,” New York, 1960. (2) Bikerman, J. J., “Foams, Theory, and Industrial Applications,” Reinhold, New York, 1953. (3) Gibbs, J. W., “Collected Works,” Vol. 1, Longmaas, Green and Co.,New York, 1928. (4) Hansen, A. H., Gotaas, H. B., Scurogc Work J. 15,242 (1942). (5) Moilliet, J. L., Collie, B., Black, W., “S& Activity,” S p n Ltd., 1961. (6) Rubin, E., Gaden, E. L., Jr., “Foam Separation,” in “New Chemical Engineering Separation Techniques” (H. M. Schom, ed.), Intersdcnce, New York, 1962. (7) Rubin, E., Schonfeld, E., Everett, R., Jr., “Removal of Metallic Ions by Foaming Agents and SUS&OM: Laboratory and Engineering Studies,” R.41-104, October 1962. (8) Schonfcld, E., othm, “Runoval of Strontium and Cesium from Nudear Waste Solutions by Foam Separation,” NYO9577, July 1960.
