.
.
,
C. A. Brunner
D. 6. Stephan
Recent development efforts to apph foam fractionation to remove surfactants from mun@ipal waste streams have led to a potential answer to the detergent problem in water treatment as well as a practical design approach f o r this process. ncreased admission of foam-forming detergents to
I municipal . ' waste streams by our expanding population could present a real need for detergent-removal processes despite the use of more biodegradable detergents. Attention has turned to foam fractionation as one possible response to the challenge of detergent removal from waste 40
INDUSTRIAL A N D ENGINEERING CHEMISTRY
streams which contain complicated and continually varying mixtures of organic contaminants. Inherent simplicity and hence implied low operating and investment costs, plus the fact that its use effects appreciable flotation removal of suspended solids, has spurred the pilot-scale development of a foam-fractionation detergent-removal process. The positive results of this development effort have confirmed the process's applicability to removal of alkyl benzene sulfonate (ABS) detergents, and possible applicability to the removal of recently introduced h e a r akylate sulfonate ( U S ) detergents from waste streams. In addition, this effort has demonstrated the practicability of a fundamental approach to foam fractionation process development even for a system where highly complex variables Thus new light is shed on this separation , are i m p e d . pmess.
Fractionation Theory
'
3
In foam separation, a gas, normally air, is dispersed in a liquid by a sparger to produce small bubbles, which rise and adsorb surface-active solutes and collect 8u5 pended solids. As they accumulate at the liquid surface, these bubbles form a foam which is subsequently forced out of the foamer and collapsed to yield a waste concentrate. There are two general types of foam separators. I n one, the column type (Figure l), feed liquor is introduced at some point along the length of a vertical column and allowed to flow downward by gravity through the column and discharge at the bottom. Gas spargers are placed near the column bottom to assure dispersion of the gas bubbles, which rise countercurrent to the downward liquid flow. By use of a simple level-control device, the foam-liquid boundary is maintained at some paition above the feed point in the column. The foam is generated at thii boundary and flows upward through a bend and out of the column into a foam collapse chamber. I n the second configuration, the trough type (Figure 2), the feed is introduced at one end of a horizontal covered trough. It flows through the trough and exits near the opposite end. Gas spargers are placed at intervals along the trough bottom, and the foam is maintained in the space between the liquid level and the trough cover. A vertical baffle is used to hold the liquid back while the foam spills over into a chamber fromwhich the foamate is discharged. As in the column-type apparatus, the foam is transported as a bulk by the drag of the gas stream as it leaves the equipment. In selecting the design parameters of systems which affect 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 gas-liquid 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: C, and C, are feed and bottoms product concentrations, mg./l.; G is volumetric gas rate, l./min.; Fis liquid feed rate, l./min. ; F a is solute surface excess corresponding to C,, mg./sq. cm.; and S is specific surface of bubbles in foam phase, sq. cm./cc. For solutions of a single surfactant of sufficient concentration to prevent foam breakage, it has been shown (7) that Equation 1 holds quite well with Fs being a function only of C., In order to extend the use of Equation 1 to multicomponent systems, it is necessary to sum over all components. This would require complete surface excess relationships for all components and knowledge of any interactions among components with respect to surface excess. Unfortunately, Iittle such basic information exists, and the chemical identity of the total
'FOAM lW
I
I
FOAMED PIODUCI
FOAM MEm Figurc 2.
Trough-type foam froclionator
surfactant complex in any municipal waste is not known. A relation similar to equation 1 can be written for the removal of suspended matter during foam separation. The excess of suspended material found at the gas-liquid interface is not, however, necessarily only a function of the amount of suspended material in the liquid. Surface properties, size distribution, particle shape, density, etc., all play roles in establishing the equilibrium surface concentration. Owing to the complex and continually varying nature of both the soluble and suspended contaminants in municipal waste water, it is not presently possible, and probably never will be practical, to analyze routinely a waste b stream for any other than a few gross characteristics, such as chemical oxygen demand (COD), total organic carbon, and total suspended solids. There is, therefore, little possibility of finding r, values and essentially no VOL 57
NO. 5 M A Y 1 9 6 5
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possibility of a completely rigorous approach to the design of foam separators for municipal waste-treatment service. There is, in addition, great difficulty in determining accurate values for S, because the foam produced using municipal waste water is relatively unstable and undergoes considerable breakage. This results in effective S values that are lower than those that would be estimated from liquid-phase bubble size determinations. About all that can be theoretically predicted for the degree of gross organic removal by surface adsorption is that it should at least depend on G/F, the type of sparger because of its effect on S, and possibly the foam residence time because of its relationship to foam breakage and, therefore, to S. However, for a small group of highly surface-active materials, detectable by the methylene blue t&st for anionic detergents (ABS), some use can be made of Equation 1. Because such materials are similar in type, it is assumed that they exhibit, as a group, a surface excess directly dependent on their equilibrium solution concentration, CBABS.To a presently 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 rBAssat the low ABS concentrations involved is kl CBABS. Substituting this relation in Equation 1 gives:
The specific bubble surface, S, is affected by the sparger type, possibly by the gas rate through the sparger, and by a number of other design and operating variables including the properties of the solution. While relationships could be established for simple, synthetic systems, development of such information for the actual waste-water systems of interest is not practical. For laboratory-scale glass foaming columns using sintered glass spargers, therefore, solution properties appear to be the most important factors. Because of the lack of basic information in this area, only the two major types of feed concerned, primary effluents and secondary effluents, are distinguished. Applying this reasoning to Equation 2, one obtains, simply: C,*BS/C,*BS=
f
(g,
type of effluent
)
(3)
laboratory-Scale Studies
Continuous runs on municipal primary effluents from seven treatment plants and secondary effluents from eight treatment plants were carried out in laboratory-scale glass foam fractionation columns. Samples were taken from Cleveland, Ohio; Little Ferry, N. J., Nassau County, N. Y .; Whittier Narrows, Calif. ; Pomona, Calif.; Bowery Bay, N. Y . ; and 26th Ward, N. Y . , plants. Description of some of the apparatus and procedural ,details have been reported by Rubin et al. ( 2 ) . Briefly, the feed point to the foaming column was 42
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
somewhat above the middle of the liquid-filled section. For most runs, column diameter was about 1 in., but columns from '/z to 4 in. diam. were sometimes used. Height of liquid above the sparger was generally at least 2 ft. No specific attempt was made to determine effects of liquid depth. The spargers used were coarse-porosity sintered-glass filter sticks except for the runs with Little Ferry secondary effluent, for which medium-porosity sintered glass was used. Air was used as the foaming gas in all experiments. Both gas and liquid rates were varied during early runs. However, liquid residence times as short as 5 min. had no significant effect on removal results; therefore, a constant liquid rate was used for most subsequent runs and only the gas rate was varied. The chemical oxygen demand (COD) and the methylene blue detectable or anionic detergent (ABS) concentrations were determined in the feed, in the bottom product, and for some samples in the collapsed foam. The effect of G / F on gross organic removal from example primary and secondary effluents is shown in Figure 3. Note that COD removal improves markedly with increasing G/F for G / F < 3. If bubble size could be held constant over the total range of gas flow rate, a more significant influence of G / F would probably be found at the higher gas rate. With glass spargers, however, the fact that bubble size increases with gas rate at high rates probably accounts for the poorer removals found a t G/F = 10 than at intermediate values. COD-removal curves corresponding to Figure 3 for all effluents studied do not clearly show the influence of G / F because of a large amount of scatter in the data from one effluent sample to another. However, since most of the data were taken at G/F values between 3 and 10, where G/F has relatively little effect, gross organic removals for effluents were averaged. The fraction of COD removal for primary effluents gave an average of 24% with a range for particular plants of 18-38%; the fraction of COD removal for secondary plants gave an average of 25% with a range of 12-41y0 for particular plants. In every case, significant amounts of organic contaminants are removed, but with considerable variation in performance. Because gross organic removal was the parameter of prime interest in these laboratory-scale studies, no attempt was made to distinguish between soluble and insoluble contaminant removals. Likewise, no determinations of the readily biodegradable portion of the organic material removed were made, which portion would exert a biochemical oxygen demand (BOD) in the receiving body of water. The relationship suggested by Equation 3 may be utilized to present laboratory ABS-removal data. In Figure 3 the fraction of ABS remaining in the product is plotted against G/F for both primary effluents foamed and all secondary effluents foamed. While these data are quite scattered, it should be remembered that results from eight different waste treatment plants are included, and that at any single plant effluent foaming characteristics change significantly by the
season, day, and even hour. The curves indicate that average ABS removals of a t least 50% may be expected when primary eWuents are foamed at G/F 2 3. For secondary effluents, ABS removals of at least 70% are anticipated at the same G/F. This improvement in removal &ciFncy, although not quantitatively predictable, is in accord with the earlier contention that the
presence of otlier non-ABS organics should adveraely affect ABS removal. In the laboratory experiments no c o d a t i o n of foam volume reduction (volume of feed/volume of collapsed foam) to operating variables was developed. For any one feed sample, however, volume reduction quite clearly increased with both foam height and column diameter. While volume reduction ratios as low as 5 to 10 were measured on a very few occasions, by far most of the volume reductions were greater than 100, with the average for all runs beiqg about 250. Volume reduction increases as the foam drains, and total drainage is the result of liquid flow due to both thinning of bubble walls and foam breakage. Drainage due to thinning of bubble walls is almost entirely beneficial since it serves to decrease the volume of waste concentrate without significantly reducing the amount of contaminants within the foam; the bulk liquid in the bubble walls is therefore highJy dilute with respect to the liquid a t the interface. Drainage of liquid from broken foam lamellae, on the other hand, is detrimental because part of the actual concentrate returns to the liquid. As mentioned earlier, most waste water foams are inherently unstable and undergo considerable breakage. An adverse effect of volume reduction on ABS removal was observed; this effect on gross organic removal as volume reduction is increased was not observed. This probably results because much of the COD removed is suspended matter that does not readily drain from the foam even though bubble breakage dws OCCUI'. Piio(.Scale Studies
Figurs 3. Effectof G/F on COD r m d (two top lines from Bowny Bay primmy eflumt (&am&), and from B o w Bay secondmy eflumt (squares); ABS rmouds (bottom two lints) from pri'ma.y c@nts (triangIcs) and sccondary elffucntr (circles) using datafrom 7 and 8 heafmcntplants, rcspeclim[y
IWRUENT
c
I
I
i
:, ,
:I
;l
I l
I
l
l
Under a joint water renovation research study with the Los Angeles County Sanitation Districts, a 500,000gal./day pilot-scale foam separator was built at the Pomona, Cali., Water Pollution Control Plant. The design of this equipment, shown in Figure 4, is based largely on the laboratory results described in the preceding section. Provision was made for varying impor-
i.llmEs (14 on I N N . CENTERS1
I i i i'
.&..,
I
ror
TOP DIFFUSUI 1451
..... .....
DIRUSERS
MlDW
MIDDLE DIFFUSER5 (50)
MllOM DIFFUPM (501
DIFFUPM
FOM
O I nOM DIFFUSER5
F i p e 4. S c h &
dagram of pilot foam frmtiowkw und at
P H S L s Angelus rcstarch project, Pamow, C& VOL 57
NO. 5
M A Y 1965
43
tant factors such as F,G,and foam height. The details of the equipment design, procedures, and a complete presentation and discussion of experimental results are given by Whitt and Zoltek (4). The length of this unit was 15 ft., and its overall height was 9.8 ft. An important basic consideration in pilot plant design was the ability to obtain information that could most directly be “scaled up” to equipment of 10 to 100 million gal./day size. Experience indicates that removal efficiencies are generally higher for column-type than for trough-type foamers. For laboratory applications, columns are used almost exclusively. Unfortunately, largescale use of the column-type foam-fractionator presents a number of design difficulties, mostly associated with the uniform distribution of feed liquid, inlet gas, and foam take-off points over a large horizontal projected area. For these reasons, the pilot equipment is of the trough type. Three novel features are: (1) the necking down of the upper part of the liquid-filled section to promote maximum possible concentration of contaminants in the region of the liquid-foam boundary, (2) horizontal, lateral foam take-off with downward discharge to allow minimum foam residence time when desired to promote maximum removal efficiencies when foam stability is low, and (3) foam collapse by recycling previously collapsed foam through a series of flat fan-type spray nozzles onto the just-generated foam. I t should also be noted that the nominal 500,000 gal./day capacity could be doubled simply by adding a second, mirror-image chamber in parallel with the first. Even though a trough foamer was selected for the first pilot unit, it is believed that column foamers are inherently superior for foam separation, especially for multicomponent feeds such as municipal waste water. To explain, in a column foamer a generally countercurrent contact occurs between liquid and gas. Virgin gas bubbles adsorb small traces of even weak surfactants from the most highly purified liquid just before it is discharged as product. These same air bubbles then rise through the down-flowing liquid adsorbing more surface active molecules and, more importantly, picking up both strong surfactants and other molecules present in the feed that act to generate a foam sufficiently stable to be removed. In the trough-type foamer, this process does not occur since the air bubbles that do strip organics from the liquid just prior to its discharge do not have the opportunity to pick up the stable foam builders in the feed. Several schemes for achieving this type of contact in trough foamers are being explored. Also of importance here are the numerous observations giving strong evidence of a positive foaming synergism among contaminants present in waste waters. Similar synergisms can be created artih5ally in the laboratory (for example, ABS removal is increased in the presence of egg albumin) and undoubtedly exist in many, if not all, multicomponent surfactant solutions. This positive synergism with respect to foamability, when present, enhances the performance advantage of the column foamer. To date, secondary effluents have been used as feed in all pilot plant NnS. For most runs, a feed rate of about 44
INDUSTRIAL AND ENGINEERING C H E M I S T R Y
,
Fig : (top 1 in pilot foam frcrcrionorion with nir sryop[icd ad bottom of uni:
r I
Figure 6. pilor &a show effect of foam height on r d s : bottom lines damk ABS i n o w l and top lines COD r d , circles h k &a wi>h 2.5-ft. betwem wcir wjan and lip2 mfmx, m ‘ q h &e: weir heighl of 7 f t . OT less
Figura 7. Hot da& show
fled of lipuid law1 height about air b o t h data me ABS rmovals, top a h COD removals; circles ma data taken wilh 50% of air feed through top and bottom hfiers, rcsptctimly; triangles am data t d m Wirh So% of air feed through m i M e and bottom dfiers; liws arc data taken wirh all air through bottom di&strs &&ms:
325 gal./min. was used and the gas rate was varied to obtain a range of G / F ratios. A few runs were carried out at liquid rates from 150 to 600 gal./min. Feed and product were routinely analyzed for COD and ABS. For a limited number of runs, suspended solids and the COD contribution of the suspended solids were determined in both feed and product. Longitudinal and vertical concentration profiles for ABS and COD in the liquid were also investigated. The pilot unit, although not exhibiting a significant vertical liquid concentration gradient, did show a measurable longitudinal gradient. There was enough mixing, however, that, even near the inlet, contaminant concentrations were considerably lower than in the feed. In one run, for example, the COD near the inlet was 4% higher than at the outlet and the ABS concentration was 12% higher. Vertical baffles in the water would reduce longitudinal mixing. Because of the feed-to-outlet concentration gradient, contaminant removal does not all take place from liquid of close to the product composition, as i i essentially true in laboratory columns. R e moval results, therefore, do not as closely relate to point removals as in the laboratory work. Figure 5 shows the effect of G / F on COD removal when air is introduced through diffusers near the bottom of the foamer. Data scatter is unexpectedly small, considering that test results were obtained over a 5-month period. The data are consistent enough, for example, to show clearly a “threshold” G / F value of roughly 0.5, below which no separation is achieved. This is probably the G / F required to form enough stable foam to be physically transported over the h a m overAow weir. A strong effect of G / F at G / F < 5 is apparent. At G / F 2 5, about 35% of the gross organic contaminants were removed. The high removal in comparison to most of the laboratory runs can be explained partly by the higher ABS concentrations that, in many runs, accounted for 20% of the COD in the feed. To obtain a rough compositional picture of the gross organics removed, a short study wa8 made of the portions of the COD removed as suspended solids, ABS, and other soluble organics. The suspended solids in feed and product samples were filtered off through glass fiber filter paper and their COD’S determined. The COD contribution of the ABS removed was estimated by doubling the difference in ABS concentration between feed and product, since the theoretical COD/weight ratio for ABS is about 2. For the runs examined, suspended solids and ABS each contributed about 40% of the total COD removed. I t was concluded, therefore, that roughly 20% of the total organic material foam separated was soluble non-ABS organics. One noticeable difference from the laboratory results was that, while ABS constituted almost 40% of the total organic matter removed by the pilot foamer, ABS generally constituted less than 20% of the total in the laboratory runs. ABS removals for runs in which air was supplied by the bottom diffusers are represented in Figure 5. Here also, C./C, correlates quite well with G / F for runs conducted over a 5-month period. As in the case of the
I -
Effmt o f ) !eight t tuxmjoam mq¶ow nndliguidnrrjacc is: 0.6 f t . (solidcirclcs), 1.0 j t . ( h i q k s ) , 1.5jt. (smicirclcs), 2.0 j t . (opan circls), and 2.5 jt. (opm triangles)
Figurr 8.
VOL 5 7
NO. 5 M A Y 1 9 6 5
45
COD results, a threshold G / F is evident and the fraction of ABS remaining after foaming is strongly affected by G / F for ratios less than 5 . Above this G / F an average A B S removal of about 70% can be expected. In comparison with the laboratory results largely for feeds from the New York area, the pilot foamer exceeded laboratory performance with respect to total organic removal and produced equivalent percentage ABS removals. To attain these levels, however, G/F values of at least 5 were required, while in the laboratory, values of 3 gave maximum attainable removals. This difference is attributed to the obviously larger average bubble size observed in the pilot equipment and the consequent lower specific bubble surface. Figure 5 shows COD and ABS removal data for all runs; in these, the distance between the static liquid level and the foam overflow weir vaned from 0.6 ft. to 2.5 ft. Figure 6 shows removal results plotted with foam height as the parameter. It is clear that some of the scatter in the data of Figure 5 is due to differences in foam residence time which is a function' of foam height, and therefore, to degree of foam drainage. The effect of foam drainage upon COD removal is quite definite in contrast to results from the laboratory. To assess the effect of diffuser depth upon contaminant removal, runs were carried out with air being introduced through diffusers other than those near the bottom. Figure 7 shows the results of two modes of operation investigated. In both cases,a portion of the air was introduced through diffusers at the bottom of the foaming tank; another portion was fed through diffusersat either middepth (approximately 3 ft.) or quite near the liquid surface (
